Nicolas Maudoux , Eric Tan , Yuya Hu , Thierry Roisnel , Vincent Dorcet , Jean-François Carpentier and Yann Sarazin

Aluminium, gallium and indium complexes supported by a chiral phenolato-prolinolato dianionic ligand

De Gruyter | Published online: November 26, 2016

Abstract

Congeneric complexes (S)-{OˆO}MCH2SiMe3 (M=Al, 3; Ga, 5; In, 6) of the triel metals supported by an enantiomerically pure phenolato-alkoxo {OˆO}2− dianionic tridentate ligand derived from prolinol, along with their chloro derivatives, have been prepared and characterised. The aluminium-alkyl species (S)-{OˆO}AlMe and 3 form four-coordinate complexes with slightly distorted tetrahedral geometries, whereas the geometry in the Lewis acidic five-coordinate (S)-{OˆO}GaCl·THF is a distorted trigonal bipyramid. These alkyl complexes do not react cleanly, if at all, with protic sources. The indium(III) compound 6, which is for instance inert towards iPrOH or BnOH even after hours at 70°C, catalyses at 95°C the controlled, immortal ring-opening polymerisation of racemic lactide (up to 1000 equivalents) in the presence of excess BnOH as a chain transfer agent. It affords atactic, monodisperse polylactides with predictable molecular weights.

Introduction

Dianionic tetradentate bis(phenolate)s derived from salen (Jacobsen, 1993; Atwood and Harvey, 2001; Cozzi, 2004; Katsuki, 2004; Larrow and Jacobsen, 2004; Baleizao and Garcia, 2006; Gupta and Sutar, 2008; Wezenberg and Kleij, 2008), salan (Atwood, 1997; Matsumoto et al., 2008) and salalen (Matsumoto et al., 2007) proteo-ligands (A, B and C, respectively, in Figure 1) constitute ubiquitous ligand platforms that are readily amenable to the tuning of their steric and electronic properties. As a result, they have been used in the past decades to generate a plethora of well-defined main-group and transition metal complexes. These have in turn been utilised for a variety of purposes, and have for instance demonstrated overall excellent performances as homogeneous catalysts in metal-mediated asymmetric organic transformations such as olefin epoxidation and polymerisations (Bellemin-Laponnaz and Dagorne, 2014). Of particular interest, unsymmetrical salen ligands, where the two salicylidene moieties bear different substituents, can feature original catalytic behaviour (Kleij, 2009). On the other hand, surprisingly little has been reported on the stabilisation of discrete complexes using mixed phenol/alcohol proteo-ligands bridged by amino or imino moieties (Figure 1D).

Figure 1: Examples of salen, salen, hemisalen and mixed phenol/alcohol proteo-ligands.

Figure 1:

Examples of salen, salen, hemisalen and mixed phenol/alcohol proteo-ligands.

In 2009, our group reported (Alaaeddine et al., 2009) on aluminium compounds bearing an unsymmetrical fluorinated alkoxo-phenolato-diimino dianionic ligand that competently mediated the isoselective (Pm=0.87) ring-opening polymerisation (ROP) of racemic lactide (Figure 2E). During the course of the work presented here, it was shown (Qian et al., 2014) that an ytterbium(III) ‘ate’ complex supported by two chiral phenolate/α,α-diphenylalcoholate ligands {ArOˆNˆCPh2ˆOAlc} promoted the asymmetric epoxidation of α,β-unsaturated ketones with ees up to 99% (Figure 2F); excellent results were also obtained by mixing in situ the proteo-ligand with [(Me3Si)2N]3Ln(μ-Cl)Li·(THF)3 (Zeng et al., 2015). Moreover, related chiral tridentate, dianionic ligands {ArOˆNˆOAlc} have afforded zirconium precatalysts (Figure 2G) which display good performances in the asymmetric hydroamination of terminal aminoalkenes, with ee values reaching up to 94% (Zhou et al., 2015).

Figure 2: Examples of complexes bearing mixed phenolate/alcoholate dianionic ligands.

Figure 2:

Examples of complexes bearing mixed phenolate/alcoholate dianionic ligands.

We have long maintained a keen interest in the implementation of the triel metals aluminium, gallium and indium in homogeneous catalysis. The ROP of cyclic esters is a field where aluminium-salen (Le Borgne et al., 1993; Ovitt and Coates, 1999, 2002; Nomura et al., 2002; Zhong et al., 2002, 2003; Majerska and Duda, 2004; Hormnirun et al., 2006; Nomura et al., 2007), -salan (Hormnirun et al., 2004; Alaaeddine et al., 2009; Du et al., 2009; Maudoux et al., 2014; Pang et al., 2014; Press et al., 2015; McKeown et al., 2016), -salalen (Whitelaw et al., 2011; Hancock et al., 2013; Pilone et al., 2015) and -hemisalen (Darensbourg et al., 2011; Normand et al., 2013; Li et al., 2014) complexes, and also more recently indium (Douglas et al., 2008; Peckermann et al., 2009; Normand et al., 2012; Yu et al., 2012; Aluthge et al., 2013; Maudoux et al., 2014) and gallium (Horeglad et al., 2010, 2012; Bakewell et al., 2013; Hild et al., 2013; Maudoux et al., 2014; Horeglad et al., 2015) ones, have been prolific (Dagorne et al., 2013). Aluminium complexes in particular still are the precatalysts of choice for the isoselective ROP of racemic lactide (Stanford and Dove, 2010; Dijkstra et al., 2011). They have also proved useful catalysts for a range of asymmetric reactions, e.g. the Meervein-Pondoorf-Verley reduction of prochiral ketones (Ooi et al. 1998; Campbell et al., 2001, 2002), the cyanosilylation of ketones affording chiral cyanohydrins (Hamashima et al., 1999; Baeza et al., 2003; Alaaeddine et al., 2008), aldol reactions (Evans et al., 2001), and ketone and aldehyde hydrophosphonylations (Saito and Katsuki, 2005; Saito et al., 2007). As part of our ongoing program aimed at developing triel complexes in molecular catalysis, we embarked on the preparation of Al, Ga and In complexes supported by the chiral ligand {ArOˆNˆCPh2ˆOAlc} derived from (S)-proline. Their syntheses, characterisation and performances in ROP catalysis are presented here.

Results and discussion

The enantiomerically pure proteo-ligand (S)- {ArOˆNˆCPh2ˆOAlc}H2 (labelled (S)-{OˆO}H2 hereafter) was selected as a platform for the production of chiral complexes of aluminium, gallium and indium. Our attempts to synthesise it on a multi-gram scale following a reported three-step procedure (Shen et al., 2004) were unsuccessful. It was instead obtained as a colourless powder in a seven-step protocol (overall 29% yield, Scheme 1) taking advantage of the synthesis of α,α-diphenyl-(S)-prolinol described by Schore and co-workers (Price et al., 2002). X-ray quality single-crystals of (S)-{OˆO}H2 were grown by slow evaporation of a methanol solution, and its molecular structure was determined (Figure 3); it shows that the configuration of (S)-proline and α,α-diphenyl-(S)-prolinol is retained at C14 in the proteo-ligand. Its NMR data recorded in CDCl3 essentially matched those reported elsewhere (Shen et al., 2004).

Scheme 1: Seven-step synthesis of (S)-{ArOˆNˆCPh2ˆOAlc}H2 (≡(S)-{OˆO}H2). (i) Boc2O, NEt3, CH2Cl2, 0°C, 3 h, 95%; (ii) MeI, K2CO3, DMF, 25°C, 12h, 99%; (iii) PhMgBr, Et2O, 3 h, 25°C, 50%; (iv) KOH, MeOH, DMSO, 65°C, 4 h, 78%; (v) NaBH4, EtOH, 0°C, 1 h, 99%; (vi) SOCl2, CH2Cl2, 25°C, 1 h, 100%; (vii) NEt3, CH2Cl2, 25°C, 12 h, 79%.

Scheme 1:

Seven-step synthesis of (S)-{ArOˆNˆCPh2ˆOAlc}H2 (≡(S)-{OˆO}H2). (i) Boc2O, NEt3, CH2Cl2, 0°C, 3 h, 95%; (ii) MeI, K2CO3, DMF, 25°C, 12h, 99%; (iii) PhMgBr, Et2O, 3 h, 25°C, 50%; (iv) KOH, MeOH, DMSO, 65°C, 4 h, 78%; (v) NaBH4, EtOH, 0°C, 1 h, 99%; (vi) SOCl2, CH2Cl2, 25°C, 1 h, 100%; (vii) NEt3, CH2Cl2, 25°C, 12 h, 79%.

Figure 3: ORTEP representation of the molecular solid-state structure of the enantiomerically pure proteo-ligand (S)-{OˆO}H2, showing only the main component of the disordered p-tBu group. H atoms other than those on oxygen atoms omitted for clarity. Ellipsoids drawn at the 50% probability level.

Figure 3:

ORTEP representation of the molecular solid-state structure of the enantiomerically pure proteo-ligand (S)-{OˆO}H2, showing only the main component of the disordered p-tBu group. H atoms other than those on oxygen atoms omitted for clarity. Ellipsoids drawn at the 50% probability level.

Reacting AlEt2Cl with (S)-{OˆO}H2 in toluene at 70°C yielded the chloro complex (S)-{OˆO}AlCl (1) as a colourless solid in 92% yield upon double protonolysis and release of two equivalents of ethane (Scheme 2). By contrast, attempts to react AlCl3 with (S)-{OˆO}Li2 or (S)-{OˆO}Na2·THF (which was structurally characterised; upon recrystallisation from THF, the sodium salt forms the tetranuclear [(S)-{OˆO}Na2·THF]2, with a distorted cubane Na4O4 central core featuring bridging Ophenolate and Oalcoholate atoms, see Figure 4) failed to deliver 1.

Scheme 2: Syntheses of mononuclear aluminium (1–3), gallium (4–5) and indium (6) complexes supported by the chiral ligand (S)-{OˆO}2−.

Scheme 2:

Syntheses of mononuclear aluminium (1–3), gallium (4–5) and indium (6) complexes supported by the chiral ligand (S)-{OˆO}2−.

Figure 4: ORTEP representation of the molecular solid-state structure of [(S)-{OˆO}Na2·THF]2.Ellipsoids drawn at the 30% probability level. H atoms and tBu groups omitted and THF molecules and C6H5 substituents in shaded tone for clarity. Selected bond lengths (Å): Na1-O2=2.376(6), Na1-O3=2.252(5), Na1-O4=2.421(6), Na1-O81A=2.347(8), Na2-O1=2.380(6), Na2-O3=2.522(6), Na2-O4=2.246(5), Na2-O91A=2.437(7), Na3-O1=2.280(5), Na3-O2=2.296(6), Na3-O4=2.297(6), Na4-O2=2.307(5), Na4-O1=2.324(6), Na4-O3=2.256(6).

Figure 4:

ORTEP representation of the molecular solid-state structure of [(S)-{OˆO}Na2·THF]2.

Ellipsoids drawn at the 30% probability level. H atoms and tBu groups omitted and THF molecules and C6H5 substituents in shaded tone for clarity. Selected bond lengths (Å): Na1-O2=2.376(6), Na1-O3=2.252(5), Na1-O4=2.421(6), Na1-O81A=2.347(8), Na2-O1=2.380(6), Na2-O3=2.522(6), Na2-O4=2.246(5), Na2-O91A=2.437(7), Na3-O1=2.280(5), Na3-O2=2.296(6), Na3-O4=2.297(6), Na4-O2=2.307(5), Na4-O1=2.324(6), Na4-O3=2.256(6).

The identity of 1 was established based on its NMR spectroscopic data recorded in THF-d8 and combustion analysis. The 1H NMR data for 1 feature two doublets at 2.75 and 3.88 ppm for diastereotopic hydrogen atoms in the methylene unit bridging the aromatic and prolinol moieties (AB spin system, 2JHH=12.2 Hz). The colourless methyl complex (S)-{OˆO}AlMe (2) was isolated in quantitative yield following the reaction of (S)-{OˆO}H2 with stoichiometric amounts of Al2Me6; the diagnostic resonance for [Al]-CH3 hydrogen atoms was located at δ1H=–0.44 ppm in the 1H NMR spectrum of this complex. The equimolar reactions of (S)-{OˆO}H2 with M(CH2SiMe3)3 in toluene at 80°C afforded the corresponding alkyl complexes (S)-{OˆO}MCH2SiMe3 (M=Al, 3; Ga, 5; In, 6) in excellent yields upon consecutive protonolysis reactions and release of two equivalents of SiMe4. These three complexes were obtained as analytically pure, colourless powders. For each of them, the resonances in the 1H NMR spectra for the two diastereotopic [M]-CH2 SiMe3 hydrogen atoms appeared as a pair of doublets (2JHH=12.0–12.6 Hz) in the region δ1H=0.0 to –1.0 ppm. GaCl3 and (S)-{OˆO}Li2 reacted smoothly in THF at room temperature to give (S)-{OˆO}GaCl·THF (4·THF). The presence of coordinated THF in this complex was unambiguously confirmed by spectroscopic, combustion and crystallographic analyses. Attempts to prepare the analogous, THF-free complex by carrying out the synthesis in hydrocarbons returned intractable mixtures. As a result of the binding of the Namine atom onto the metal centre M, both the Namine and M atoms become chiral in 1–6, taking the total to three optically active centres in these complexes. Yet, the simple 1H and 13C{1H} NMR spectra for 1–6, with single sets of narrow resonances, were consistent with the existence of a single diastereisomer in solution, and this agreed with the crystallographic data. For instance, the examples of the 1H NMR spectra of the in situ generated 1 and 5 (reactions in J-Young NMR tubes) are shown in Figures 5 and 6.

Figure 5: 1H NMR spectrum (THF-d8, 500.1 MHz, 298 K) of (S)-{OˆO}AlCl (1) generated in situ upon reaction of (S)-{OˆO}H2 and AlEt2Cl at 343 K for 3 h. *THF-d8.

Figure 5:

1H NMR spectrum (THF-d8, 500.1 MHz, 298 K) of (S)-{OˆO}AlCl (1) generated in situ upon reaction of (S)-{OˆO}H2 and AlEt2Cl at 343 K for 3 h. *THF-d8.

Figure 6: 1H NMR spectrum (benzene-d6, 500.1 MHz, 298 K) of (S)-{OˆO}GaCH2SiMe3 (5) generated in situ upon reaction of (S)-{OˆO}H2 and Ga(CH2SiMe3)3 at 343 K for 3 h.

Figure 6:

1H NMR spectrum (benzene-d6, 500.1 MHz, 298 K) of (S)-{OˆO}GaCH2SiMe3 (5) generated in situ upon reaction of (S)-{OˆO}H2 and Ga(CH2SiMe3)3 at 343 K for 3 h.

Attempts to prepare the aluminium alkoxide(S)-{OˆO}Al(OiPr) by treatment of (S)-{OˆO}H2 with stoichiometric amounts of {Al(OiPr)3}4 or AlMe2(OiPr) only returned mixtures of species which could not be purified or characterised. The reaction of 2 with iPrOH did trigger the rapid release of methane (vigorous bubbling was observed); the 1H NMR spectrum of the crude product indicated that the starting material 2 had been consumed, but it was not consistent with the formation of a putative (S)-{OˆO}Al(OiPr) species. By contrast, the indium complex 6 remained entirely unreacted in the presence of BnOH or iPrOH, even after several hours at 25°C or 70°C.

Single-crystals of 2, 3 and 4·THF suitable for X-ray diffraction crystallography were grown by recrystallisation of the purified products, and their structures were established. The 4-coordinate aluminium complex 2 crystallises in the orthorhombic P212121 space group. Upon coordination of the Namine atom (N18), the geometry about the metal forms a distorted tetrahedral (Figure 7), with the geometry index τ4=0.87 (Yang et al., 2007). As a result, the aluminium atom lies only 0.34 Å above the mean plane defined by the O1, O2 and C34 atoms. The asymmetric unit contains a single stereoisomer (together with one molecule of non-interacting toluene), with the configurations S, S and R at C14, Al1 and N18, respectively; this is consistent with the NMR spectroscopic data for this complex. The Al-O1 (1.747(2) Å) and Al-O2 (1.763(2) Å) bond lengths to the Oalkoxide and Ophenolate atoms in 2 resemble those in a five-coordinate aluminium chloride supported by a alcoxo-phenolato-diimino {(3,5-tBu2-1-OC6H4)CH=N(trans-1,2-cyclo-C6H10)N=C(Me)CH2C(CF3)2O}2− ligand (1.763(2) and 1.791(2) Å, respectively) (Alaaeddine et al., 2009), and in the four-coordinate aluminium-β-oxy enolate{BHT}AlEt{O=C(tBu)CH2C(Me)(tBu)O} (1.720(5) and 1.732(5), respectively) (Power et al., 1990). The Al-C bond distances in this complex (1.962(9) Å) and in 2 (1.950(3)) are also very similar.

Figure 7: ORTEP representation of the molecular solid-state structure of (S)-{OˆO}AlMe (2).Ellipsoids drawn at the 50% probability level. H atoms and non-interacting toluene lattice molecule omitted for clarity. Selected bond lengths (Å) and angles (°): Al1-O1=1.747(2), Al1-O2=1.763(2), Al1-C34=1.950(3), Al1-N18=2.0082; O1-Al1-O2=116.69(10), O1-Al1-C34=120.01(13), O2-Al1-C34=112.89(12), O1-Al1-N18=89.48(10), O2-Al1-N18=99.07(9), C34-Al1-N18=113.95(11).

Figure 7:

ORTEP representation of the molecular solid-state structure of (S)-{OˆO}AlMe (2).

Ellipsoids drawn at the 50% probability level. H atoms and non-interacting toluene lattice molecule omitted for clarity. Selected bond lengths (Å) and angles (°): Al1-O1=1.747(2), Al1-O2=1.763(2), Al1-C34=1.950(3), Al1-N18=2.0082; O1-Al1-O2=116.69(10), O1-Al1-C34=120.01(13), O2-Al1-C34=112.89(12), O1-Al1-N18=89.48(10), O2-Al1-N18=99.07(9), C34-Al1-N18=113.95(11).

The molecular structure of 3 depicted in Figure 8 matches closely that of 2. It crystallises in the monoclinic space group P21. Each asymmetric unit contains two independent and very similar molecules, with four coordinate aluminium atoms sitting in a distorted tetrahedral arrangement (τ4=0.86 and 0.89; Yang et al., 2007). Each molecule presents S, S and R configurations at the chiral carbon, aluminium and nitrogen atoms, respectively, and therefore a single stereoisomer is found in the crystalline material. The main metric parameters in 3 compare very well with those for 2. In the molecule represented in Figure 8, the metal atom Al2 rests 0.38 Å above the mean plane defined by O51, O73 and C91. The Al2-C91 bond length of 1.937(5) Å in 3 is somewhat shorter than in [(Me3SiCH2)2AlOCH2CH2OMe]2 (1.983(1) and 1.969(1) Å) (Westerhausen et al., 1999), in Al(CH2SiMe3)3·NMe3 (1.984(3) Å) (Feighery et al., 1994) and in the enantiomerically pure (1,2)-diphenylethylene-salen complex {(R,R)-(MeONNOMe)}AlCH2SiMe3 (1.976(3) Å) (Maudoux et al., 2014).

Figure 8: ORTEP rendering of the molecular solid-state structure of (S)-{OˆO}AlCH2SiMe3 (3).One of the two independent molecules in the asymmetric unit is represented. Ellipsoids drawn at the 50% probability level. H atoms and non-interacting benzene lattice molecules omitted for clarity. Selected bond lengths (Å) and angles (°): Al1-O23=1.742(3), Al1O1-1.749(3), Al1-C41=1.949(5), Al1-N17=2.007(5), Al2-O73=1.731(4), Al2-O51=1.765(3), Al2-C91=1.937(5), Al2-N67=1.989(5); O23-Al1-O1=117.32(16), O23-Al1-C41=115.7(2), O1-Al1-C41=112.7(2), O23-Al1-N17=89.28(17), O1-Al1-N17=98.36(17), C41-Al1-N17=120.6(2), O73-Al2-O51=116.95(17), O73-Al2-C91=116.9(2), O51-Al2-C91=112.8(2), O73-Al2-N67=90.17(18), O51-Al2-N67=100.05(17), C91-Al2-N67=116.9(2).

Figure 8:

ORTEP rendering of the molecular solid-state structure of (S)-{OˆO}AlCH2SiMe3 (3).

One of the two independent molecules in the asymmetric unit is represented. Ellipsoids drawn at the 50% probability level. H atoms and non-interacting benzene lattice molecules omitted for clarity. Selected bond lengths (Å) and angles (°): Al1-O23=1.742(3), Al1O1-1.749(3), Al1-C41=1.949(5), Al1-N17=2.007(5), Al2-O73=1.731(4), Al2-O51=1.765(3), Al2-C91=1.937(5), Al2-N67=1.989(5); O23-Al1-O1=117.32(16), O23-Al1-C41=115.7(2), O1-Al1-C41=112.7(2), O23-Al1-N17=89.28(17), O1-Al1-N17=98.36(17), C41-Al1-N17=120.6(2), O73-Al2-O51=116.95(17), O73-Al2-C91=116.9(2), O51-Al2-C91=112.8(2), O73-Al2-N67=90.17(18), O51-Al2-N67=100.05(17), C91-Al2-N67=116.9(2).

Owing to the presence of the coordinated THF molecule, the five-coordinate gallium atom in 4·THF sits in a slightly distorted trigonal bipyramid environment (τ5=0.62) (Addison et al., 1984), with the OTHF and Namine atoms in the apical positions (∠O1-Ga1-N31=165.37(19)°), and the O11, O37 and Cl1 occupying the equatorial sites (Figure 9). The bond distances Ga1-O11 to the alkoxide and Ga1-O37 to the phenolate, 1.830(4) and 1.836(4), are almost identical, and much smaller than that to O1 (2.107(4)) belonging to the coordinated THF molecule. They are comparable to those in the five coordinate gallium complexes {Me2NCH2CH2O}2GaCl (1.841(2) and 1.842(3) Å) and {Me2NCH(CH3)CH2O}2GaCl (1.8373(12) and 1.8389(12) Å) (Basharat et al., 2008).

Figure 9: ORTEP representation of the molecular solid-state structure of (S)-{OˆO}GaCl·THF (4·THF). Ellipsoids drawn at the 50% probability level. H atoms omitted for clarity. Selected bond lengths (Å) and angles (°): Ga1-O11=1.830(4), Ga1-O37=1.836(4), Ga1-O1=2.107(4), Ga1-N31=2.113(5), Ga1-Cl1=2.206(2); O11-Ga1-O37=127.9(2), O11-Ga1-O1=85.69(17), O37-Ga1-O1=83.97(17), O11-Ga1-N31=94.59(18), O37-Ga1-N31=84.35(18), O1-Ga1-N31=165.37(19), O11-Ga1-Cl1=111.02(15), O37-Ga1-Cl1=120.26(15), O1-Ga1-Cl1=91.97(15), N31-Ga1-Cl1=101.53(15).

Figure 9:

ORTEP representation of the molecular solid-state structure of (S)-{OˆO}GaCl·THF (4·THF). Ellipsoids drawn at the 50% probability level. H atoms omitted for clarity. Selected bond lengths (Å) and angles (°): Ga1-O11=1.830(4), Ga1-O37=1.836(4), Ga1-O1=2.107(4), Ga1-N31=2.113(5), Ga1-Cl1=2.206(2); O11-Ga1-O37=127.9(2), O11-Ga1-O1=85.69(17), O37-Ga1-O1=83.97(17), O11-Ga1-N31=94.59(18), O37-Ga1-N31=84.35(18), O1-Ga1-N31=165.37(19), O11-Ga1-Cl1=111.02(15), O37-Ga1-Cl1=120.26(15), O1-Ga1-Cl1=91.97(15), N31-Ga1-Cl1=101.53(15).

Attempts to prepare cationic Al-In complexes

Well-defined cationic metal complexes associated to weakly-coordinating anions have proved very competent catalysts in a variety of catalysed organic transformations, for instance the hydroamination of alkenes (Zulys et al., 2005; Mukherjee et al., 2012) or the ROP of cyclic esters (Sarazin and Carpentier, 2015), and the role of cationic metallocenium in olefin polymerisation is also well established (Chen and Marks, 2000). Several cations of group 13 metals have been reported (Atwood, 1998; Dagorne and Atwood, 2008), and we were keen on probing whether complexes 1–6 would be amenable to cationisation.

Attempts to generate [(S)-{OˆO}M]+·[H2N{B(C6F5)3}2] (M=Al-In), where the metallic cation is paired with Bochmann’s amidodiborate anion (Lancaster et al., 2002), were carried out. The choice of this weakly coordinating anion was motivated by its usually excellent crystallisation properties, owing in particular to the presence of a dipolar moment (Bochmann, 2009; Sarazin and Carpentier, 2015). The equimolar treatment of 2, 3, 5 or 6 with [H(OEt2)2]+·[H2N{B(C6F5)3}2] did not give the expected cationic complexes, but returned instead intractable mixtures in which the starting charge-neutral complexes were by a large margin the main components. Similarly, the stoichiometric reaction of [(S)-{OˆO}H3]+·[H2N{B(C6F5)3}2], obtained quantitatively by treatment of the (S)-{OˆO}H2 with [H(OEt2)2]+·[H2N{B(C6F5)3}2], with the alkyl precursors M(CH2SiMe3)3 gave a mixture of compounds which could not be purified by standard washing, stripping or recrystallisation procedures. This lack of reactivity of 6 towards these strong acids can be related to the inertness of the complex towards alcohols (see above), and it contrasts with the reactivity observed for aminotroponimitate indium dialkyl complexes, which formed cations upon treatment with [PhNMe2H]+·[{B(C6F5)4] (Delpech et al., 2002). It testifies to the robustness and high degree of covalence of the remaining [In]-CH2SiMe3 bond in 6. The salt metathesis reactions between the chlorinated compounds 1 or 4·THF and [Na(OEt2)4]+·[H2N{B(C6F5)3}2] also failed to generate [(S)-{OˆO}M]+·[H2N{B(C6F5)3}2]. We thus sought to prepare triflate salts, reasoning that a more coordinating anion would perhaps encourage cleavage of the [M]-CH2SiMe3 bond. The reaction of the gallium alkyl 5 with triflic acid (TfOH) in Et2O at room temperature afforded a single species, the NMR data of which were fully consistent with the formulation [(S)-{OˆO}Ga]+·[TfO] or (S)-{OˆO}GaOTf: quantitative release of CH2(SiMe3)2 was observed in the 1H NMR spectrum, while all resonances for aliphatic hydrogens other than those belonging to tBu groups were substantially deshielded compared to the parent complex 5; a sole singlet was found at δ19F=–78.6 ppm in the 19F NMR spectrum. However, we did not manage to grow X-ray quality crystals of this product, and therefore the proposed formulation remains for now tentative.

Ring-opening polymerisation of lactide

The indium complex 6 catalysed competently the controlled ROP of racemic lactide under immortal conditions in toluene at 95°C (Ajellal et al., 2010; Sarazin and Carpentier, 2015), but without stereoselectivity as it only afforded atactic polylactides (Scheme 3). It converted 500 equivalents of monomer in the presence of 10 equivalents of BnOH as chain transfer agent in 12 h, affording a polymer with a low molecular weight distribution (Mw/Mn=1.12) and an experimental molecular weight (Mn,exp=8800 g mol−1 as determined by gel permeation chromatography) which matched reasonably well its theoretical value (Mn,exp=7000 g mol−1) calculated on the basis of the [lactide]0/[BnOH]0 ratio. The reaction was equally well controlled when 1000 equivalents of lactide were used (conversion=100% in 6 h, Mn,exp=12 000 g mol−1, Mn,theo=14 500 g mol−1, Mw/Mn=1.03). The polymerisation of the enantiomerically pure L-lactide was also catalysed by 6 in a controlled fashion, and occurred without epimerisation of the chiral centres. Considering the absence of reactivity between 6 and alcohols, it is very likely that these reactions follow an activated monomer mechanism (Ajellal et al., 2010). On the other hand, the gallium congener 5 did not afford controlled reactions, whereas the aluminium complex 3 required extremely forcing conditions to afford monomer conversion without suitable control over the polymerisation parameters; none of these systems displayed appreciable level of stereoselectivity in the polymerisation of racemic lactide.

Scheme 3: Immortal ROP of lactide catalysed by (S)-{OˆO}InCH2SiMe3 (6).

Scheme 3:

Immortal ROP of lactide catalysed by (S)-{OˆO}InCH2SiMe3 (6).

Conclusions

The synthesis of mononuclear aluminium, gallium and indium alkyl complexes supported by a bulky, enantiomerically pure aminophenolate-alkoxo dianionic ligand has been achieved. Whereas the aluminium complex are four-coordinate distorted tetrahedra, the complex of the large gallium is five-coordinate and rests in a highly distorted trigonal pyramidal geometry. The S configuration of the chiral centre in the proteo-ligand is retained in the resulting triel heteroleptic complexes 1–6. These compounds are fairly robust against hydrolysis, and do not react cleanly, if at all, with other protic reagents such as alcohols and Bochmann’s acid. The indium-alkyl complex 6 is, upon association with benzyl alcohol, an efficient catalyst for the controlled ROP of lactides, afforded monodisperse polylactides with defined molecular weight and good end-group fidelity, albeit without significant stereocontrol. Based on the inability of 6 to react with alcohol to yield an alkoxo complex, we surmise that the polymerisations mediated by this complex follow an activated monomer mechanism where benzyl alcohol is the true initiator (Ajellal et al., 2010; Normand et al., 2013; Maudoux et al., 2014). The congeneric gallium and aluminium complexes lead to lower activity and less controlled polymerisations.

One can also seek to take advantage of the chiral centres in these complexes in other homogeneous enantioselective reactions. For instance, in a preliminary test, the aluminium-methyl complex 2 (10 mol% metal loading) quantitatively catalysed the Meerwein-Pondorf-Verley reduction of acetophenone into enantiomerically enriched 1-phenyl-ethan-1-ol (ee=28%) in the presence of four equivalents of isopropanol in 17 h at 80°C (Ooi et al., 1998; Campbell et al., 2001, 2002). Future efforts should focus on taking advantage of these well-defined complexes for other enantio- or stereo- selective reactions.

Experimental section

General procedures

All manipulations were performed under inert atmosphere using standard Schlenk techniques or in a dry, solvent-free glove-box (Jacomex; O2<1 ppm, H2O<5 ppm) for catalyst loading. Racemic and L-lactides were purified by recrystallisation from a hot (80°C), concentrated iPrOH solution, followed by two subsequent recrystallisations in hot (105°C) toluene; after purification, they were stored at –30°C under the inert atmosphere of the glove-box. Isopropanol (Aldrich) was dried and distilled over magnesium turnings and stored over 3 Å molecular sieves. InCl3 (Strem), GaCl3 (Strem), AlBr3 (Strem), AlMe3 (2.0 m solution in toluene; Aldrich), AlMe2(OiPr) (Strem), AlEt2Cl (0.9 m solution in toluene; Acros Organics) and Al(OiPr)3 (Aldrich) were used as received. Solvents (THF, Et2O, CH2Cl2, pentane and toluene) were purified and dried (water contents below 8 ppm) over columns alumina (MBraun SPS). THF was further distilled under argon from sodium/benzophenone ketyl. All deuterated solvents (Eurisotop, Saclay, France) were stored in sealed ampoules over activated 3 Å molecular sieves and were thoroughly degassed by several freeze-thaw-vacuum cycles. Al(CH2SiMe3)3 (Beachley Jr. et al., 1982), Ga(CH2SiMe3)3 (Beachley Jr. and Simmons, 1980) and In(CH2SiMe3)3 (Beachley Jr. and Rusinko, 1979) were synthesised according to literature procedures.

NMR spectra were recorded on Bruker AM-400 and AM-500 spectrometers. All 1H and 13C{1H} chemicals shifts were determined using residual signals of the deuterated solvents and were calibrated vs. SiMe4. Assignment of the signals was carried out using 1D (1H, 13C{1H}) and 2D (COSY, HMBC, HMQC) NMR experiments.

Elemental analyses were performed on a Carlo Erba 1108 Elemental Analyser instrument at the London Metropolitan University by Stephen Boyer and were an average of a minimum of two independent measurements.

Size exclusion chromatography measurements were performed on an Agilent PL-GPC50 equipped with two PLgel 5 Å MIXED-C columns and a refractive index detector. The column was eluted with THF at 30°C at 1.0 mL·min−1 and was calibrated using 11 monodisperse polystyrene standards in the range of 580–380 000 g·mol−1. The molecular weights of all PLAs were corrected by a factor of 0.58 (Save et al., 2002).

(S)-{OˆO}AlCl (1):

(S)-{OˆO}H2 (350 mg, 0.74 mmol) was dissolved in toluene (4 mL) and added dropwise at –40°C to a solution of AlEt2Cl (0.83 mL of a 0.9 m solution in toluene, 0.75 mmol) in toluene (2 mL). The reaction mixture was warmed to room temperature over 30 min and was then heated at 70°C for 2 h. The volatiles were pumped off and the resulting solid was washed with pentane (2×2 mL) and dried under vacuum to afford 1 (365 mg, 92%) as a white powder. 1H NMR (THF-d8, 298 K, 400.16 MHz): δ=7.90 (d, 3JHH=7.4 Hz, 2H, CaromH), 7.62 (d, 3JHH=7.4 Hz, 2H, CaromH), 7.30–6.99 (m, 7H, CaromH), 6.75 (d, 4JHH=2.4 Hz, 1H, CaromH), 4.17 (m, 1H, NCH), 3.88 (d, 2JHH=12.2 Hz, 1H, Carom-CHHN), 3.13 (m, 1H, NCHHCH2), 2.75 (d, 2JHH=12.2 Hz, 1H, Carom-CHHN), 2.46 (m, 1H, NCHHCH2) 1.84 (m, 2H, NCH2CH2), 1.58 (m, 1H, NCHCHH), 1.48 (m, 1H, NCHCHH), 1.40 (s, 9H, C(CH3)3), 1.21 (s, 9H, C(CH3)3) ppm. 13C{1H} NMR (THF-d8, 298 K, 100.25 MHz): δ=155.9 (Carom-OH), 151.9 (i-C6H5), 150.8 (i-C6H5), 139.2 (Carom-C(CH3)3), 137.3 (Carom-C(CH3)3), 128.6 (CaromH), 127.9 (CaromH), 126.6 (CaromH), 126.2 (CaromH), 126.1 (CaromH), 126.0 (CaromH), 124.9 (Carom-CH2N), 123.8 (CaromH), 123.7 (CaromH), 77.1 (C(Ph)2), 74.5 (NCH), 62.0 (Carom-CH2N), 56.0 (NCH2CH2), 35.2 (C(CH3)3), 34.4 (C(CH3)3), 31.9 (C(CH3)3), 30.5 (NCHCH2), 30.3 (C(CH3)3), 21.1 (NCH2CH2) ppm. Elem. Anal. For C32H39AlClNO2 (532.09 g.mol−1): Calcd, C, 72.2; H, 7.4; N, 2.6%; Found, C, 72.0; H, 7.5; N, 2.7%.

(S)-{OˆO}AlMe (2):

(S)-{OˆO}H2 (500 mg, 1.06 mmol) was dissolved in toluene (8 mL) and added dropwise at –40°C to a solution of trimethylaluminium (0.53 mL of a 2.0 m solution in toluene, 1.06 mmol) in toluene (2 mL). Then the reaction mixture was warmed to room temperature over 15 min and then stirred at 70°C for 2 h. The volatiles were pumped off and the resulting solid was washed with pentane (3×3 mL) and dried under vacuum to give 2 (525 mg, 97%) as a colourless powder. Crystals suitable for X-ray diffraction crystallography were grown from a concentrated toluene solution. 1H NMR (C6D6, 298 K, 400.16 MHz): δ=7.88 (d, 3JHH=7.6 Hz, 2H, CaromH), 7.74 (d, 3JHH=7.6 Hz, 2H, CaromH), 7.50 (d, 4JHH=2.4 Hz, 1H, CaromH), 7.19–6.99 (m, 5H, CaromH), 6.88 (t, 3JHH=7.3 Hz, 1H, CaromH), 6.69 (d, 4JHH=2.4 Hz, 1H, CaromH), 4.08 (d, 2JHH=12.5 Hz, 1H, ArCHHN), 3.68 (m, 1H, NCHCH2), 2.75 (m, 1H, NCHHCH2), 2.42 (d, 2JHH=12.5 Hz, 1H, ArCHHN), 2.00 (m, 1H, NCHHCH2), 1.64 (s, 9H, C(CH3)3), 1.58–1.39 (m, 2H, NCH2CH2), 1.34 (s, 9H, C(CH3)3), 1.21–1.04 (m, 2H, NCHCH2), –0.44 (s, 3H, Al-CH3) ppm. 13C{1H} NMR (C6D6, 298 K, 100.25 MHz): δ=155.8 (Carom-OAl), 149.9 (i-C6H5), 149.7 (i-C6H5), 139.6 (Carom-C(CH3)3), 138.9 (Carom-C(CH3)3), 128.5 (CaromH), 128.3 (CaromH), 128.2 (CaromH), 127.9 (Carom-CH2N), 126.8 (CaromH), 125.9 (CaromH), 125.8 (CaromH), 125.0 (CaromH), 123.5 (CaromH), 79.3 (CHCO), 75.4 (NCHCO), 60.4 (ArCH2N), 54.8 (NCH2CH2), 35.4 (C(CH3)3), 34.3 (C(CH3)3), 32.1 (NCH2CH2), 32.0 (C(CH3)3), 30.2 (C(CH3)3), 20.4 (NCHCH2) ppm; Al-CH3was not detected. Elem. Anal. for C33H42AlNO2 (511.69 g·mol−1): Calcd, C, 77.5; H, 8.3; N, 2.7%; Found, C, 77.4; H, 8.2; N, 2.6%.

(S)-{OˆO}Al(CH2SiMe3) (3):

(S)-{OˆO}H2(245 mg, 0.52 mmol) and Al(CH2SiMe3)3 (150 mg, 0.52 mmol) were dissolved in C6H6 and the reaction mixture was stirred at 80°C overnight. The volatiles were removed under vacuum and the resulting solid was washed with pentane (3×2 mL) and subsequent drying under vacuum afforded 3 (282 mg, 93%) as a white powder. X-ray quality single-crystals were obtained by recrystallisation of the crude product at 25°C from a concentrated benzene solution. 1H NMR (C6D6, 298 K, 500.13 MHz): δ=7.86 (d, 3JHH=7.3 Hz, 2H, CaromH), 7.75 (d, 3JHH=7.3 Hz, 2H, CaromH), 7.48 (d, 4JHH=2.6 Hz, 1H, CaromH), 7.08–7.06 (m, 3H, CaromH), 7.02–6.99 (m, 2H, CaromH), 6.83 (t, 3JHH=7.3 Hz, 1H, CaromH), 6.67 (d, 4JHH=2.6 Hz, 1H, CaromH), 4.08 (d, 2JHH=12.5 Hz, 1H, ArCHHN), 3.72 (m, 1H, NCHCH2), 2.83 (m, 1H, NCHHCH2), 2.43 (d, 2JHH=12.5 Hz, 1H, ArCHHN), 2.04 (m, 1H, NCHHCH2), 1.63 (s, 9H, C(CH3)3), 1.53–1.47 (m, 2H, NCH2CH2), 1.33 (s, 9H, C(CH3)3), 1.26–1.22 (m, 2H, NCHCH2), 0.56 (s, 9H, Si(CH3)3), –0.57(d, 2JHH=12.5 Hz, 1H, AlCHHSiMe3), –0.97 (d, 2JHH=12.5 Hz, 1H, AlCHHSiMe3) ppm. 13C{1H} NMR (C6D6, 298 K, 125.75 MHz): δ=155.8 (Carom-OAl), 149.8 (i-C6H5), 149.7 (i-C6H5), 139.6(Carom-C(CH3)3), 138.8 (Carom-C(CH3)3), 128.6 (CaromH), 128.5(CaromH), 128.3(CaromH), 126.8(CaromH), 126.4 (CaromH), 125.9 (CaromH), 125.8(CaromH), 125.0(CaromH), 123.4 (Carom-CH2N), 79.3 (CHCO), 75.8 (NCHCO), 60.6 (ArCH2N), 54.9 (NCH2CH2), 35.4 (C(CH3)3), 34.3 (C(CH3)3), 32.1 (C(CH3)3), 32.0 (NCH2CH2), 30.3 (C(CH3)3), 20.4 (NCHCH2), 3.1 (Si(CH3)3)) ppm; the resonance for Al-CH2SiMe3 was not detected. Elem. Anal. For C36H50AlNO2Si (583.87 g·mol−1): Calcd, C, 74.1; H, 8.6; N, 2.4%; Found, C, 73.9; H, 8.5; N, 2.4%.

(S)-{OˆO}GaCl·THF (4·THF):

(S)-{OˆO}H2 (250 mg, 0.53 mmol) was dissolved in THF (20 mL) and nBuLi (0.3 mL of a 1.6 m solution in hexanes) was added dropwise at 0°C. The mixture was stirred at 0°C for 1 h, and it was then added to a suspension of GaCl3 (93 mg, 0.53 mmol) in THF (10 mL). The reaction mixture was stirred overnight at room temperature, and the volatiles were pumped off. The resulting colourless solid was dissolved in dichloromethane (10 mL), the solution was filtered by cannula to remove insoluble impurities. Removal of the solvent under vacuum afforded a colourless powder. Recrystallisation from a mixture of dichloromethane and pentane at –26°C afforded crystals of 4·THF (0.47 mmol, 89%) suitable for X-ray diffraction crystallography. 1H NMR (500.13 MHz, 298 K, CD2Cl2): δ 7.90 (d, 3JHH=7.3 Hz, 2H, CaromH), 7.69 (d, 3JHH=7.3 Hz, 2H, CaromH), 7.39–7.27 (m, 5H, CaromH), 7.22–7.15 (m, 2H, CaromH), 6.79 (d, 4JHH=2.5 Hz, 1H, CaromH), 4.39–4.25 (m, 1H, NCHCH2), 4.06 (m, 4H, OCH2CH2), 3.91 (d, 3JHH=12.3 Hz, 1H, ArCHHN), 3.35 (m, 1H, NCHHCH2), 2.97 (d, 3JHH=12.4 Hz, 1H, ArCHHN), 2.91–2.77 (m, 1H, NCHHCH2), 2.01 (m, 4H, OCH2CH2), 1.94–1.71 (br m, 4H, NCH2CH2 and NCHCH2), 1.43 (s, 9H, (CH3)3), 1.26 (s, 9H, (CH3)3) ppm. 13C{1H} NMR (CD2Cl2, 125.76 MHz, 298 K): δ=156.9(Carom-OGa), 149.5(i-C6H5), 139.1, 128.8, 128.3, 127.2, 126.6, 124.8, 124.5, 123.2 (all arom-C), 78.4 (CHCO), 73.7 (NCHCO), 69.6 (OCH2CH2), 60.7 (ArCH2N), 56.8 (NCH2CH2), 35.3 (C(CH3)3), 34.3 (C(CH3)3), 31.7 (C(CH3)3), 30.0 (C(CH3)3), 29.3 (NCH2CH2), 26.0 (OCH2CH2), 20.8 (NCHCH2) ppm. Elem. Anal. For C36H47ClGaNO3 (646.95g·mol−1): Calcd, C, 66.8; H, 7.3; N, 2.2%; Found, C, 66.6; H, 7.3; N, 2.1%.

(S)-{OˆO}Ga(CH2SiMe3) (5):

(S)-{OˆO}H2(245 mg, 0.52 mmol) and Ga(CH2SiMe3)3 (172 mg, 0.52 mmol) were dissolved in toluene and the reaction mixture was stirred at 80°C for 2 h. The volatiles were removed under vacuum and the resulting solid was washed with pentane (3×2 mL). Subsequent drying under vacuum to constant weight afforded 5 (283 mg, 87%) as a white powder. 1H NMR (C6D6, 298 K, 500.13 MHz): δ=7.96 (d, 3JHH=7.7 Hz, 2H, CaromH), 7.80 (d, 3JHH=7.7 Hz, 2H, CaromH), 7.49 (d, 4JHH=2.5 Hz, 1H, CaromH), 7.21–7.05 (m, 4H, CaromH), 6.99 (t, 3JHH=7.3 Hz, 1H, CaromH), 6.87 (t, 3JHH=7.3 Hz, 1H, CaromH), 6.69 (d, 4JHH=2.5 Hz, 1H, CaromH), 3.99 (d, 2JHH=12.0 Hz, 1H, ArCHHN), 3.77–3.72 (m, 1H, NCHCH2), 2.90–2.84 (m, 1H, NCHHCH2), 2.43 (d, 2JHH=12.0 Hz, 1H, ArCHHN), 2.20–2.12 (m, 1H, NCHHCH2), 1.64 (s, 9H, C(CH3)3), 1.54–1.46 (m, 2H, NCH2CH2), 1.35 (s, 9H, C(CH3)3), 1.24–1.11 (m, 2H, NCHCH2), 0.46 (s, 9H, Si(CH3)3), –0.22(d, 2JHH=12.9 Hz, 1H, GaCHHSi), –0.69 (d, 2JHH=12.9 Hz, 1H, GaCHHSi) ppm. 13C{1H} NMR (C6D6, 298 K, 125.76 MHz): δ=158.5(Carom-OGa), 150.7(i-C6H5), 149.7(i-C6H5), 139.4(Carom-C(CH3)3), 138.9(Carom-C(CH3)3), 126.8 (CaromH), 126.3 (CaromH), 126.2 (CaromH), 125.9 (CaromH), 124.9 (CaromH), 123.9(Carom-CH2N), 80.9 (CHCO), 74.8 (NCHCH2), 62.2 (ArCH2N), 56.0 (NCH2CH2), 35.5 (C(CH3)3), 34.2 (C(CH3)3), 32.1 (C(CH3)3), 30.4 (C(CH3)3), 29.0(NCH2CH2), 20.6(NCHCH2), 2.2 (Si(CH3)3), –3.9 (GaCH2) ppm. Elem. Anal. for C36H50GaNO2Si (626.61 g.mol−1): Calcd, C, 69.0; H, 8.0; N, 2.2%; Found, C, 68.9; H, 8.1; N, 2.3%.

(S)-{OˆO}In(CH2SiMe3) (6):

(S)-{OˆO}H2 (150 mg, 0.32 mmol) and In(CH2SiMe3)3 (122 mg, 0.32 mmol) were dissolved in toluene and the reaction mixture stirred at 80°C for 2 h. The volatiles were pumped off. The resulting solid was dried overnight under dynamic vacuum to afford 6 (210 mg, 98%) as a white powder. 1H NMR (C6D6, 298 K, 500.13 MHz): δ=8.02 (d, 3JHH=7.3 Hz, 2H, CaromH), 7.87 (d, 3JHH=7.3 Hz, 2H, CaromH), 7.51 (d, 4JHH=2.6 Hz, 1H, CaromH), 7.21–7.13 (m, 4H, CaromH), 7.02 (t, 3JHH=7.3 Hz, 1H, CaromH), 6.91 (t, 3JHH=7.3 Hz, 1H, CaromH), 6.75 (d, 4JHH=2.6 Hz, 1H, CaromH), 4.16 (d, 2JHH=11.8 Hz, 1H, ArCHHN), 3.74 (m, 1H, NCHCH2), 2.79 (m, 1H, NCHHCH2), 2.43 (d, 2JHH=11.8 Hz, 1H, ArCHHN), 2.14 (m, 1H, NCHHCH2), 1.67 (s, 9H, C(CH3)3), 1.54–1.45 (m, 2H, NCH2CH2), 1.39 (s, 9H, C(CH3)3), 1.21–1.02 (m, 2H, NCHCH2), 0.26 (s, 9H, Si(CH3)3), –0.02 (d, 2JHH=12.6 Hz, 1H, InCHHSi), –0.33 (d, 2JHH=12.6 Hz, 1H, InCHHSi) ppm. 13C{1H} NMR (C6D6, 298 K, 125.76 MHz): δ=160.8 (Carom-OIn), 152.0 (i-C6H5), 150.7 (i-C6H5), 139.5 (Carom-C(CH3)3), 137.9 (Carom-C(CH3)3), 128.3 (CaromH), 128.4 (CaromH), 126.8 (CaromH), 126.1 (CaromH), 126.0 (CaromH), 124.9 (CaromH), 124.8 (CaromH), 124.2 (ArCH2N), 80.0 (CHCO), 73.7 (NCHCH2), 60.0 (ArCH2N), 56.3 (NCH2CH2), 35.6 (C(CH3)3), 34.2 (C(CH3)3), 32.1 (C(CH3)3), 30.4 (C(CH3)3), 28.3 (NCH2CH2), 20.6 (NCHCH2), 2.1 (Si(CH3)3), –1.2 (InCH2) ppm. Elem. Anal. for C36H50InNO2Si (671.70 g.mol−1): Calcd, C, 64.4; H, 7.5; N, 2.1%; Found, C, 64.3; H, 7.6; N, 2.3%.

[(S)-{OˆO}Na2·THF]2:

(S)-{OˆO}H2(276 mg, 0.5 mmol) was dissolved in THF (5 mL), NaH (30 mg, 1.25 mmol) was added in with a bent finger. The mixture was stirred at 25°C overnight and the residual insoluble material was eliminated by filtration. The volatiles were removed under vacuum and the resulting solid was washed with pentane (3×2 mL) and subsequent drying under vacuum afforded the title compound (267 mg, 91%) as a yellow powder. The crude product was dissolved in pentane. Green X-ray quality crystals were obtained after 2 days at 25°C. 1H NMR (C6D6, 298 K, 500.13 MHz): δ=8.06 (d, 2H, 3JHH=7.5 Hz, CaromH), 7.40 (s, 1H, CaromH), 7.20 (s, 3H, CaromH), 7.09 (m, 4H, CaromH), 6.98 (m, 1H, CaromH), 6.89 (m, 1H, CaromH), 4.00(d, 1H, 2JHH=12.5 Hz, ArCHHN), 3.64 (m, 1H, NCHCH2), 3.40 (m, 4H, OCH2 CH2), 3.32 (m, 1H, NCHHCH2), 2.95 (d, 1H, 2JHH=12.5 Hz, ArCHHN), 2.44 (m, 1H, NCHHCH2), 1.46 (s, 9H, C(CH3)3), 1.53–1.47 (m, 2H, NCH2CH2), 1.34 (m, 4H, OCH2CH2), 1.30 (s, 9H, C(CH3)3), 1.05 (br s, 2H, NCHCH2) ppm. 13C{1H} NMR (C6D6, 298 K, 125.76 MHz): δ=164.6 (Carom-ONa), 157.8 (i-C6H5), 156.8 (i-C6H5), 135.4 (Carom-C(CH3)3), 129.1 (Carom-C(CH3)3), 127.6 (CaromH), 127.5 (CaromH), 126.6 (CaromH), 126.4 (CaromH), 126.2 (Carom-CH2N), 79.0 (CHCO), 68.6 (NCHCO), 67.9 (OCH2CH2), 54.2 (NCH2CH2), 34.5 (C(CH3)3), 34.0 (C(CH3)3), 32.4 (C(CH3)3), 29.9 (C(CH3)3), 29.1 (NCH2CH2), 25.6 (OCH2CH2) 20.3 (NCHCH2) ppm. Satisfactory, reproducible elemental analysis for C72H94N2Na4O6 (1175.52 g·mol−1) could not be obtained.

Typical Schlenk-scale polymerisation procedure

In the glove-box, the metal catalyst (ca. 5.0–15.0 mg) was placed in a Schlenk flask together with the monomer (ca. 0.1–1.0 g). The Schlenk flask was sealed and removed from the glove-box. All subsequent operations were carried out on a vacuum manifold using Schlenk techniques. The required amount of solvent (toluene) was added with a syringe to the catalyst and the monomer, followed when necessary by addition of alcohol (BnOH, 3–10 μL). The resulting mixture was immersed in an oil bath pre-set at the desired temperature and the polymerisation time was measured from this point. The reaction was terminated by addition of MeOH and the polymer was precipitated in methanol or a methanol/pentane mixture. It was washed thoroughly and reprecipitated from dichloromethane/pentane. The polymer was then dried to constant weight in a vacuum oven at 55°C under dynamic vacuum (<5×10−2 mbar).

X-ray diffraction crystallography

All crystals suitable for X-ray diffraction analysis were obtained by recrystallisation of the purified compounds. Diffraction data were collected at 150(2) K using a Bruker APEX CCD diffractometer with graphite-monochromated MoKα radiation (λ=0.71073 Å). A combination of ω and Φ scans was carried out to obtain at least a unique data set. The crystal structures were solved by direct methods, remaining atoms were located from difference Fourier synthesis followed by full-matrix least-squares refinement based on F2 (programs SIR97 and SHELXL-97) (Sheldrick, 1997a,b). Carbon- and oxygen-bound hydrogen atoms were placed at calculated positions and forced to ride on the attached atom. All non-hydrogen atoms were refined with anisotropic displacement parameters. The locations of the largest peaks in the final difference Fourier map calculation as well as the magnitude of the residual electron densities were of no chemical significance. Relevant collection and refinement data are collated in Table 1. Crystal data, details of data collection and structure refinement for all complexes structurally characterised (CCDC 1503970–1503973) can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Table 1:

Summary of the crystallographic data.

[(S)-{OˆO}Na2·THF]2 (S)-{OˆO}AlMe (2) (S)-{OˆO}AlCH2SiMe3(3) (S)-{OˆO}GaCl·THF(4·THF)
Formula C72H94N2Na4O6 C40H50AlNO2 C84H112Al2N2O4Si2 C36H47ClGaNO3
CCDC 1503973 1503972 1503970 1503971
Mol. wt. 1175.45 603.79 1323.9 646.92
Crystal system Monoclinic Orthorhombic Monoclinic Orthorhombic
Space group P21 P212121 P21 P212121
a(Å) 10.4386(12) 9.612(2) 10.7733(8) 9.0981(13)
b(Å) 17.390(2) 13.877(3) 26.8859(17) 13.1689(18)
c(Å) 19.033(3) 26.482(7) 14.0782(8) 27.314(5)
α(o) 90 90 90 90
β(o) 98.397(5) 90 90.026(4) 90
γ(o) 90 90 90 90
V(Å3) 3418.0(8) 3532.3(14) 4077.7(5) 3272.5(9)
Z 2 4 2 4
Density (g/cm3) 1.142 1.135 1.078 1.313
Abs. coeff. (mm−1) 0.093 0.091 0.112 0.958
F(000) 1264 1304 1432 1368
Crystal size, mm 0.6×0.3×0.27 0.6×0.59×0.22 0.43×0.17×0.12 0.22×0.09×0.05
θ range, deg 1.08–27.61 3.00–27.48 2.96–27.57 2.98–27.7
Limiting indices –13<h<13

0<k<22

0<l<24
–12<h<12

–16<k<17

–34<l<34
–13<h<13

–34<k<34

–18<l<18
–11<h<10

–15<k<17

–34<l<35
R(int) 0.0000 0.0781 0.0525 0.1511
Reflections collected 8100 26 115 30 477 27 522
Reflec. Unique [I>2σ(I)] 8100 8026 16 596 7478
Completeness to θ (%) 98.6 99.4 96.5 97.9
Data/restraints/param. 8100/61/736 8026/0/405 16596/1/818 7478/0/385
Goodness-of-fit 1.055 1.023 1.008 0.984
R1 [I>2σ(I)] (all data) 0.0796 (0.1630) 0.0604 (0.1007) 0.0625 (0.1064) 0.0721 (0.1325)
wR2 [I>2σ(I)] (all data) 0.1962 (0.2451) 0.1256 (0.1441) 0.1222 (0.1441) 0.1441 (0.1704)
Largest diff. e·A−3 0.469 and –0.466 0.546 and –0.328 0.296 and –0.31 0.434 and –0.581

Acknowledgments

The authors are grateful to Total Raffinage-Chimie and Total Co. (Ph.D. grant to N.M.) for funding. We thank Stephen Boyer (London Metropolitan University) for combustion analyses.

References

Addison, A. W.; Nageswara Rao, T.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen–sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′-yl)-2,6-dithiaheptane]copper(II) perchlorate. J. Chem. Soc. Dalton Trans.1984, 1349–1356. Search in Google Scholar

Ajellal, N.; Carpentier, J.-F.; Guillaume, C.; Guillaume, S. M.; Hélou, M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Metal-catalyzed immortal ring-opening polymerization of lactones, lactides and cyclic carbonates. Dalton Trans.2010, 39, 8363–8376. Search in Google Scholar

Alaaeddine, A.; Roisnel, T.; Thomas, C. M.; Carpentier, J.-F. Discrete versus in situ-generated aluminum-salen catalysts in enantioselective cyanosilylation of ketones: role of achiral ligands. Adv. Synth. Catal.2008, 350, 731–740. Search in Google Scholar

Alaaeddine, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. Aluminum and yttrium complexes of an unsymmetrical mixed fluorous alkoxy/phenoxy-diimino ligand: synthesis, structure, and ring-opening polymerization catalysis. Organometallics2009, 28, 1469–1475. Search in Google Scholar

Aluthge, D. C.; Patrick, B. O.; Mehrkhodavandi, P. A highly active and site selective indium catalyst for lactide polymerization. Chem. Commun.2013, 49, 4295–4297. Search in Google Scholar

Atwood, D. A. Salan complexes of the group 12, 13 and 14 elements. Coord. Chem. Rev.1997, 165, 267–296. Search in Google Scholar

Atwood, D. A. Cationic group 13 complexes. Coord. Chem. Rev.1998, 176, 407–430. Search in Google Scholar

Atwood, D. A.; Harvey M. J. Group 13 compounds incorporating salen ligands. Chem. Rev.2001, 101, 37–52. Search in Google Scholar

Baeza, A.; Casas, J.; Nájera, C.; Sansano, J. M.; Saá, J. M. Enantioselective synthesis of cyanohydrin O-phosphates mediated by the bifunctional catalyst binolam–AlCl. Angew. Chem. Int. Ed.2003, 42, 3143–3146. Search in Google Scholar

Bakewell, C.; White, A. J. P.; Long, N. J.; Williams, C. K. 8-Quinolinolato gallium complexes: iso-selective initiators for rac-lactide polymerization. Inorg. Chem.2013, 52, 12561–12567. Search in Google Scholar

Baleizao, C.; Garcia, H. Chiral salen complexes: an overview to recoverable and reusable homogeneous and heterogeneous catalysts. Chem. Rev.2006, 106, 3987–4043. Search in Google Scholar

Basharat, S.; Knapp, C. E.; Carmalt, C. J.; Barnett, S. A.; Tocher, D. A. Synthesis and structures of gallium alkoxides. New J. Chem.2008, 32, 1513–1518. Search in Google Scholar

Beachley Jr., O. T.; Rusinko, R. N. Preparation and properties of ((trimethylsilyl)methyl)indium(III) compounds. Inorg. Chem.1979, 18, 1966–1968. Search in Google Scholar

Beachley Jr., O. T.; Simmons, R. G. Preparation and properties of ((trimethylsilyl)methyl)gallium(III) compounds. Inorg. Chem.1980, 19, 1021–1025. Search in Google Scholar

Beachley Jr., O. T.; Tessier-Youngs, C.; Simmons, R. G.; Hallock, R. B. Attempted syntheses of low-oxidation-state organometallic derivatives of aluminum, gallium, and indium. A new synthesis of tris(trimethylsilylmethyl)aluminum (Al(CH2SiMe3)3). Inorg. Chem.1982, 21, 1970–1973. Search in Google Scholar

Bellemin-Laponnaz, S.; Dagorne, S. Coordination chemistry and applications of salen, salan and salalen metal complexes. In: The Chemistry of Metal Phenolates. Zabicky, J., ed. John Wiley and Sons: Chichester, 2014, Vol. 1, pp 263–309. Search in Google Scholar

Bochmann, M. Highly electrophilic main group compounds: ether and arene thallium and zinc complexes. Coord. Chem. Rev.2009, 253, 2000–2014. Search in Google Scholar

Campbell, E. J.; Zhou, H.; Nguyen, S. T. Catalytic Meerwein–Pondorf–Verley reduction by simple aluminum complexes. Org. Lett.2001, 3, 2391–2393. Search in Google Scholar

Campbell, E. J.; Zhou, H.; Nguyen, S. T. The asymmetric Meerwein–Schmidt–Ponndorf–Verley reduction of prochiral ketones with iPrOH catalyzed by Al catalysts. Angew. Chem. Int. Ed.2002, 41, 1020–1022. Search in Google Scholar

Chen, E. Y.-X.; Marks, T. J. Cocatalysts for metal-catalyzed olefin polymerization: activators, activation processes, and structure–activity relationships. Chem. Rev.2000, 100, 1391–1434. Search in Google Scholar

Cozzi, P. G. Metal–Salen Schiff base complexes in catalysis: practical aspects. Chem. Soc. Rev.2004, 33, 410–421. Search in Google Scholar

Dagorne, S.; Atwood, D. A. Synthesis, characterization, and applications of group 13 cationic compounds. Chem. Rev.2008, 108, 4037–4071. Search in Google Scholar

Dagorne, S.; Normand, M.; Kirillov, E.; Carpentier, J.-F. Gallium and indium complexes for ring-opening polymerization of cyclic ethers, esters and carbonates. Coord. Chem. Rev.2013, 257, 1869–1886. Search in Google Scholar

Darensbourg, D. J.; Karroonnirun, O.; Wilson, S. J. Ring-opening polymerization of cyclic esters and trimethylene carbonate catalyzed by aluminum half-salen complexes. Inorg. Chem.2011, 50, 6775–6787. Search in Google Scholar

Delpech, F.; Guzei, I. A.; Jordan, R. F. Cationic indium alkyl complexes incorporating aminotroponiminate ligands. Organometallics2002, 21, 1167–1176. Search in Google Scholar

Dijkstra, P. J.; Du, H.; Feijen, J. Single site catalysts for stereoselective ring-opening polymerization of lactides. Polym. Chem.2011, 2, 520–527. Search in Google Scholar

Douglas, A. F.; Patrick, B. O.; Mehrkhodavandi, P. A highly active chiral indium catalyst for living lactide polymerization. Angew. Chem. Int. Ed.2008, 47, 2290–2293. Search in Google Scholar

Du, H.; Velders, A. H.; Dijkstra, P. J.; Sun, J.; Zhong, Z.; Chen, X.; Feijen, J. Chiral salan aluminium ethyl complexes and their application in lactide polymerization. Chem. Eur. J.2009, 15, 9836–9845. Search in Google Scholar

Evans, D. A.; Janey, J. M.; Magomedov, N.; Tedrow, J. S. Chiral salen–aluminum complexes as catalysts for enantioselective aldol reactions of aldehydes and 5-alkoxyoxazoles: an efficient approach to the asymmetric synthesis of syn and antiβ-hydroxy-α-amino acid derivatives. Angew. Chem. Int. Ed.2001, 40, 1884–1888. Search in Google Scholar

Feighery, W. G.; Kirss, R. U.; Lake, C. H.; Rowen Churchill, M. Alkyl for hydride exchange between alane-trimethylamine and group IVB metal alkryls. Inorg. Chim. Acta1994, 218, 47–51. Search in Google Scholar

Gupta, K. C.; Sutar, A. K. Catalytic activities of Schiff base transition metal complexes. Coord. Chem. Rev.2008, 252, 1420–1450. Search in Google Scholar

Hamashima, Y.; Sawada, D.; Kanai, M.; Shibasaki, M. A new bifunctional asymmetric catalysis: an efficient catalytic asymmetric cyanosilylation of aldehydes. J. Am. Chem. Soc.1999, 121, 2641–2642. Search in Google Scholar

Hancock, S. L.; Mahon, M. F.; Jones, M. D. Aluminium salalen complexes based on 1,2-diaminocyclohexane and their exploitation for the polymerisation of rac-lactide. Dalton Trans.2013, 42, 9279–9285. Search in Google Scholar

Hild, F.; Neehaul, N.; Bier, F.; Wirsum, M.; Gourlaouen, C.; Dagorne, S. Synthesis and structural characterization of various N,O,N-chelated aluminum and gallium complexes for the efficient ROP of cyclic esters and carbonates: how do aluminum and gallium derivatives compare? Organometallics2013, 32, 587–598. Search in Google Scholar

Horeglad, P.; Kruk, P.; Pécaut, J. Heteroselective polymerization of rac-lactide in the presence of dialkylgallium alkoxides: the effect of Lewis base on polymerization stereoselectivity. Organometallics2010, 29, 3729–3734. Search in Google Scholar

Horeglad, P.; Szczepaniak, G.; Dranka, M.; Zachara, J. The first facile stereoselectivity switch in the polymerization of rac-lactide: from heteroselective to isoselective dialkylgallium alkoxides with the help of N-heterocyclic carbenes. Chem. Commun.2012, 48, 1171–1173. Search in Google Scholar

Horeglad, P.; Cybularczyk, M.; Trzaskowski, B.; Zukowska, G. Z.; Dranka, M.; Zachara, J. Dialkylgallium alkoxides stabilized with N-heterocyclic carbenes: opportunities and limitations for the controlled and stereoselective polymerization of rac-lactide. Organometallics2015, 34, 3480–3496. Search in Google Scholar

Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; White, A. J. P.; Williams, D. J. Remarkable stereocontrol in the polymerization of racemic lactide using aluminum initiators supported by tetradentate aminophenoxide ligands. J. Am. Chem. Soc.2004, 126, 2688–2689. Search in Google Scholar

Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; Pugh, R. I.; White, A. J. P. Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators. Proc. Natl Acad. Sci. USA2006, 103, 15343–15348. Search in Google Scholar

Jacobsen, E. N. In Catalytic Asymmetric Synthesis; 1st Ed. Ojima, I. Ed. VCH: Weinheim, Germany, 1993. Search in Google Scholar

Katsuki, T. Unique asymmetric catalysis of cis-β metal complexes of salen and its related Schiff-base ligands. Chem. Soc. Rev.2004, 33, 437–444. Search in Google Scholar

Kleij, A. W. Nonsymmetrical salen ligands and their complexes: synthesis and applications. Eur. J. Inorg. Chem.2009, 193–205. Search in Google Scholar

Lancaster, S. J.; Rodriguez, A.; Lara-Sanchez, A.; Hannant, M. D.; Walker, D. A.; Hughes, D. H.; Bochmann, M. [H2N{B(C6F5)3}2]: a new, remarkably stable diborate anion for metallocene polymerization catalysts. Organometallics2002, 21, 451–453. Search in Google Scholar

Larrow, J. F.; Jacobsen, E. N. Asymmetric processes catalyzed by chiral (salen)metal complexes. Top. Organomet. Chem.2004, 6, 123–152. Search in Google Scholar

Le Borgne, A.; Vincens, V.; Joulgard, M.; Spassky, N. Ring-opening oligomerization reactions using aluminium complexes of Schiff’s bases as initiators. Makromol. Chem. Macromol. Symp.1993, 73, 37–46. Search in Google Scholar

Li, L.; Liu, B.; Liu, D.; Wu, C.; Li, S.; Liu, B.; Cui, D. Copolymerization of ε-caprolactone and L-lactide catalyzed by multinuclear aluminum complexes: an immortal approach. Organometallics2014, 33, 6474–6480. Search in Google Scholar

Majerska, K.; Duda, A. Stereocontrolled polymerization of racemic lactide with chiral initiator: combining stereoelection and chiral ligand-exchange mechanism. J. Am. Chem. Soc.2004, 126, 1026–1027. Search in Google Scholar

Matsumoto, K.; Saito, B.; Katsuki, T. Asymmetric catalysis of metal complexes with non-planar ONNO ligands: salen, salalen and salan. Chem. Commun.2007, 3619–3627. Search in Google Scholar

Matsumoto, K.; Sawada, Y.; Katsuki, T. Asymmetric epoxidation of olefins catalyzed by Ti(salan) complexes using aqueous hydrogen peroxide as the oxidant. Pure Appl. Chem.2008, 80, 1071–1077. Search in Google Scholar

Maudoux, N.; Roisnel, T.; Dorcet, V.; Carpentier, J.-F.; Sarazin, Y. Chiral (1,2)-diphenylethylene-salen complexes of triel metals: coordination patterns and mechanistic considerations in the isoselective ROP of lactide. Chem. Eur. J.2014, 20, 6131–6147. Search in Google Scholar

McKeown, P.; Davidson, M. G.; Kociok-Köhn, G.; Jones, M. D. Aluminium salalens vs. salans: ‘initiator design’ for the isoselective polymerisation of rac-lactide. Chem. Commun.2016, 52, 10431–10434. Search in Google Scholar

Mukherjee, A.; Sen, T. K.; Ghorai, P. Kr.; Samuel, P. P.; Schulzke, C.; Mandal, S. K. Phenalenyl-based organozinc catalysts for intramolecular hydroamination reactions: a combined catalytic, kinetic, and mechanistic investigation of the catalytic cycle. Chem. Eur. J.2012, 18, 10530. Search in Google Scholar

Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. Stereoselective ring-opening polymerization of racemic lactide using aluminum-achiral ligand complexes: exploration of a chain-end control mechanism. J. Am. Chem. Soc.2002, 124, 5938–5939. Search in Google Scholar

Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Stereoselective ring-opening polymerization of a racemic lactide by using achiral salen– and homosalen–aluminum complexes. Chem. Eur. J.2007, 13, 4433–4451. Search in Google Scholar

Normand, M.; Kirillov, E.; Roisnel, T.; Carpentier, J.-F. Indium complexes of fluorinated dialkoxy-diimino salen-like ligands for ring-opening polymerization of rac-lactide: how does indium compare to aluminum. Organometallics2012, 31, 1448–1457. Search in Google Scholar

Normand, M.; Dorcet, V.; Kirillov, E.; Carpentier, J.-F. {Phenoxy-imine}aluminum versus -indium complexes for the immortal ROP of lactide: different stereocontrol, different mechanisms. Organometallics2013, 32, 1694–1709. Search in Google Scholar

Ooi, T.; Miura, T.; Maruoka, K. Highly efficient, catalytic Meerwein–Ponndorf–Verley reduction with a novel bidentate aluminum catalyst. Angew. Chem. Int. Ed.1998, 37, 2347–2349. Search in Google Scholar

Ovitt, T. M.; Coates, G. W. Stereoselective ring-opening polymerization of meso-lactide: synthesis of syndiotactic poly(lactic acid). J. Am. Chem. Soc.1999, 121, 4072–4073. Search in Google Scholar

Ovitt, T. M.; Coates, G. W. Stereochemistry of lactide polymerization with chiral catalysts: new opportunities for stereocontrol using polymer exchange mechanisms. J. Am. Chem. Soc.2002, 124, 1316–1326. Search in Google Scholar

Pang, X.; Duan, R.; Li, X.; Chen, X. Bimetallic salen–aluminum complexes: synthesis, characterization and their reactivity with rac-lactide and ε-caprolactone. Polym. Chem.2014, 5, 3894–3900. Search in Google Scholar

Peckermann, I.; Kapelski, A.; Spaniol, T. P.; Okuda, J. Indium complexes supported by 1,ω-dithiaalkanediyl-bridged bis(phenolato) ligands: synthesis, structure, and controlled ring-opening polymerization of L-lactide. Inorg. Chem.2009, 48, 5526–5534. Search in Google Scholar

Pilone, A.; De Maio, N.; Press, K.; Venditto, V.; Pappalardo, D.; Mazzeo, M.; Pellecchia, C.; Kol, M.; Lamberti, M. Ring-opening homo- and co-polymerization of lactides and ε-caprolactone by salalen aluminum complexes. Dalton Trans.2015, 44, 2157–2165. Search in Google Scholar

Power, M. B.; Apblett, A. W.; Bott, S. G.; Atwood, J. L.; Barron, A. R. Aldol condensation of ketones promoted by sterically crowded aryloxy compounds of aluminum. Organometallics1990, 9, 2529–2534. Search in Google Scholar

Press, K.; Goldberg, I.; Kol, M. Mechanistic insight into the stereochemical control of lactide polymerization by salan–aluminum catalysts. Angew. Chem. Int. Ed.2015, 54, 14858–14861. Search in Google Scholar

Price, M. D.; Kurth M. J.; Schore, N. E. Comparison of solid-phase and solution-phase chiral auxiliaries in the alkylation/iodolactonization sequence to γ-butyrolactones. J. Org. Chem.2002, 67, 7769–7773. Search in Google Scholar

Qian, Q.; Tan, Y.; Zhao, B.; Feng, T.; Shen, Q.; Yao, Y. Asymmetric epoxidation of unsaturated ketones catalyzed by heterobimetallic rare earth-lithium complexes bearing phenoxy-functionalized chiral diphenylprolinolate ligand. Org. Lett.2014, 16, 4516–4519. Search in Google Scholar

Saito, B.; Katsuki, T. Synthesis of an optically active C1-symmetric Al(salalen) complex and its application to the catalytic hydrophosphonylation of aldehydes. Angew. Chem. Int. Ed.2005, 44, 4600–4602. Search in Google Scholar

Saito, B.; Egami, H.; Katsuki, T. Synthesis of an optically active Al(salalen) complex and its application to catalytic hydrophosphonylation of aldehydes and aldimines. J. Am. Chem. Soc.2007, 129, 1978–1986. Search in Google Scholar

Sarazin, Y.; Carpentier, J.-F. Discrete cationic complexes for ring-opening polymerization catalysis of cyclic esters and epoxides. Chem. Rev.2015, 115, 3564–3614. Search in Google Scholar

Save, M.; Schappacher, M.; Soum, A. Controlled ring-opening polymerization of lactones and lactides initiated by lanthanum isopropoxide, 1. General aspects and kinetics. Macromol. Chem. Phys.2002, 203, 889–899. Search in Google Scholar

Sheldrick, G. M. SHELXS-97, Program for the Determination of Crystal Structures. University of Goettingen: Germany, 1997a. Search in Google Scholar

Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal Structures. University of Goettingen: Germany, 1997b. Search in Google Scholar

Shen, Y.; Feng, X.; Li, Y.; Zhang, G.; Jiang, Y. Asymmetric cyanosilylation of ketones catalyzed by bifunctional chiral N-oxide titanium complex catalysts. Eur. J. Org. Chem.2004, 129–137. Search in Google Scholar

Stanford, M. J.; Dove, A. P. Stereocontrolled ring-opening polymerisation of lactide. Chem. Soc. Rev.2010, 39, 486–494. Search in Google Scholar

Westerhausen, M.; Birg, C.; Nöth, H.; Knizek, J.; Seifert, T. Formation of calcium–carbon bonds from a Lewis acid-base reaction of calcium bis[bis(trimethylsilyl)amide] and tris(trimethylsilylmethyl)alane. Eur. J. Inorg. Chem.1999, 2209–2214. Search in Google Scholar

Wezenberg, S. J.; Kleij, A. W. Material applications for salen frameworks. Angew. Chem. Int. Ed.2008, 47, 2354–2364. Search in Google Scholar

Whitelaw, E. L.; Loraine, G.; Mahon, M. F.; Jones, M. D. Salalen aluminium complexes and their exploitation for the ring opening polymerisation of rac-lactide. Dalton Trans.2011, 40, 11469–11473. Search in Google Scholar

Yang, L.; Powell, D. R.; Houser, R. P. Structural variation in copper(I) complexes with pyridylmethylamide ligands: structural analysis with a new four-coordinate geometry index, τ4. Dalton Trans.2007, 955–964. Search in Google Scholar

Yu, I.; Acosta-Ramírez, A.; Mehrkhodavandi, P. Mechanism of living lactide polymerization by dinuclear indium catalysts and its impact on isoselectivity. J. Am. Chem. Soc.2012, 134, 12758–12773. Search in Google Scholar

Zeng, C.; Yuan, D.; Zhao B.; Yao, Y. Highly enantioselective epoxidation of α,β-unsaturated ketones catalyzed by rare-earth amides [(Me3Si)2N]3RE(μ-Cl)Li·(THF)3 with phenoxy-functionalized chiral prolinols. Org. Lett.2015, 17, 2242–2245. Search in Google Scholar

Zhong, Z.; Dijkstra, P. J.; Feijen, J. [(Salen)Al]-mediated, controlled and stereoselective ring-opening polymerization of lactide in solution and without solvent: synthesis of highly isotactic polylactide stereocopolymers from racemic D,L-Lactide. Angew. Chem. Int. Ed.2002, 41, 4510–4516. Search in Google Scholar

Zhong, Z.; Dijkstra, P. J.; Feijen, J. Controlled and stereoselective polymerization of lactide: kinetics, selectivity, and microstructures. J. Am. Chem. Soc.2003, 125, 11291–11298. Search in Google Scholar

Zhou, X.; Wei, B.; Sun, X.-L.; Tang, Y.; Xie, Z. Asymmetric hydroamination catalyzed by a new chiral zirconium system: reaction scope and mechanism. Chem. Commun.2015, 51, 5751–5753. Search in Google Scholar

Zulys, A.; Dochnahl, M.; Hollmann, D.; Löhnwitz, K.; Herrmann, J.-S.; Roesky, P. W.; Blechert, S. Intramolecular hydroamination of functionalized alkenes and alkynes with a homogenous zinc catalyst. Angew. Chem. Int. Ed.2005, 44, 7794–7798. Search in Google Scholar

Received: 2016-9-15
Accepted: 2016-10-18
Published Online: 2016-11-26
Published in Print: 2016-12-1

©2016 Walter de Gruyter GmbH, Berlin/Boston

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