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Publicly Available Published by De Gruyter January 10, 2019

π-Excess-aromatic and non-aromatic 1,3-azaphospholes – impact of annulation and multiple reactivity

  • Joachim W. Heinicke ORCID logo EMAIL logo

Abstract

Syntheses, properties, structure aspects and reactivity of non- and aromatically carbo- and heterocyclic annulated 1H- and 3H-1,3-azaphospholes are compared to illuminate the impact of annulation, substituent effects, aromatic stabilization and π-excess at phosphorus of the 1H-isomers, to demonstrate the current knowledge and open questions in this field of research.

Introduction

For many years compounds of phosphorus with (p–p)π bond were unknown and believed to be unstable. Thus, the first report by Märkl on a stable heterocycle with dicoordinated phosphorus, triphenylphosphabenzene [1], was a landmark that opened a new area in the research field of phosphorus chemistry. This discovery was soon followed by the synthesis of the first P,N heterocyle, a diazaphosphole from Ignatova et al. [2], and later a large number of further compounds with dicoordinated (σ2) phosphorus with (p–p)π bonds [3], [4], [5], [6], [7], [8] were developed. My first investigations in this field began in 1976 after initial studies of C=N-unsaturated arsenic heterocycles and comprised arsenic and later phosphorus heterocycles with E=C–N and E=C–O fragments [7], [8]. Whereas research on compounds with neutral dicoordinate phosphorus focused in the first 2–3 decades on the syntheses and characterization of new compound types [9], [10], [11], [12], more recent investigations in this field are directed mainly towards reactivity and potential applications, involving new synthetic methodology [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. The aim of this contribution is to highlight different types of chemistry of the aromatic 1H-1,3-azaphospholes and their non-aromatic 3H-isomers, the impact of annulation, substituent effects and electronic structure on the reactivity, and to show some first examples for potential applications.

General synthetic routes to 1,3-azaphospholes

The principal synthetic strategies for 1,3-azaphospholes differ for non-annulated 1H-isomers 1 (3H-isomers still unknown), C–C bridgehead annulated 1H- and 3H-1,3-azaphospholes 2–5 and 3H-2, 3H-3 (Scheme 1a–c) and for a larger variety of N-heterocyclic C–N bridgehead annulated 1,3-azaphospholes [4], [11]. The latter have been reviewed recently in much detail by Bansal et al. [20], [21], [22], [23] and are not included in this contribution. Non- or 2-monosubstituted compounds of type 1 were prepared by flash-vacuum pyrolysis of C=N unsaturated 1,3-azaphospholines, multiple substituted derivatives by O→P replacement of oxazolium salts with P(SiMe3)3 or cycloadditions of phosphaalkynes with suitable azadiene precursors [3], [8], [10]. Benzo- and pyrido[b]-annulated 3H-1,3-azaphospholes 3H-2 and 3H-3 were synthesized by organometallic routes whereas the 1H-isomers 2–4 [10], [18] as well as naphtho- and tetracyclic systems 5–7 (see also Fig. 1) [24], [25], [26], including the skeleton of 2, were synthesized by cyclocondensation of o-aminoarylphosphanes with suitable carboxylic acid derivatives or aldehydes. NH-Derivatives of 2 are also directly accessible by reductive cyclization of N-secondary o-phosphonoanilides of alkyl- or arylcarboxylic acids. This circumvents the preparation of highly air-sensitive primary o-anilinophosphanes by reduction of the respective o-anilinophosphonates or -phosphonites but is applicable only for N-secondary amides that were reduced slower by LiAlH4 than the PO3Et2 group [18], [27], [28], [29], [30]. The above mentioned cyclocondensation with RCHO leads primarily to 2,3-dihydro compounds, which on heating in the presence of p-toluene sulfonic acid undergo aromaticity-driven redox processes leading in part to reductive P–C bond cleavage, RCH2OH and, for primary o-anilinophosphanes with excess RCHO, to N-CH2R-substituted 1H-1,3-benzazaphospholes. For o-diformylarenes this opens a high-yield route to 6 and related systems with extended planar π-system, confirmed by crystal structure analysis and spectroscopic data [25].

Scheme 1: (a–c) General synthetic routes to 1H- and 3H-1,3-azaphospholes (R2CH(X)Y: X=NH+Cl−, Y=OMe; X=NTol or O, Y=Cl; X=(OMe)2, Y=NMe2).
Scheme 1:

(a–c) General synthetic routes to 1H- and 3H-1,3-azaphospholes (R2CH(X)Y: X=NH+Cl, Y=OMe; X=NTol or O, Y=Cl; X=(OMe)2, Y=NMe2).

Molecular and electronic structure

The general structure elucidation of the azaphospholes was based on multinuclear NMR data, particularly characteristic 13C chemical shifts and diagnostic P-13C and P-H coupling constants, which clearly indicate 1H-1,3-azaphospholes and the 1H- or 3H-benzo- or pyridoazaphosphole skeleton. In addition, several of these compounds and complexes thereof were characterized by crystal structure analyses, confirming high double bond character within the -P=CR2-NR1-↔-P-CR2=N+R1-substructure and lower double bond character of the P–C and N–C bonds to the annulated ring. The increased π-electron density at phosphorus, represented by the polar resonance structure (and strengthened by the – I-effect of N in β-position to σ2P), is indicated by the phosphorus chemical shifts. Like 13C chemical shifts of unsaturated carbon compounds, these are usually found in the downfield region but strongly upfield shifted by increasing π-density compared to σ2P compounds lacking a π-donor atom in conjugation to phosphorus. δ31P values of selected 1H-1,3-azaphosphole-type compounds (Fig. 1) show by general upfield shifts a different electronic nature compared to phosphabenzenes, by more or less significant up- or downfield shifts the impact of annulation and the influence of inductive and mesomeric effects of substituents and of N instead of CH within the ring. A comparison with 2-phosphaindolizines reveals the effect of the position of σ2P relative to the annulated six-membered ring and indicates higher π- and total electron density in the α-rather than in the β-position, which is confirmed by early MNDO calculations [41]. More detailed information was gathered by combined photoelectron spectroscopic and quantum chemical studies on 1H-1,3-azaphospholes, 1H- and 3H-1,3-benzazaphospholes and the analogous N,N′- and As,N-heterocycles (Fig. 2). Whereas the π-orbitals of benzannulated 3H-phospha- and -arsazoles are similar and indicate heterocyclic iminoaryl-phosphanes and -arsanes, formally related to phosphols and arsols, the HOMO π-orbitals of the 1H-isomers show a strong increase in the order N, P and As. This is similar to what was found for the heterobenzenes with dicoordinate N, P and As. The higher HOMO energies of the heteroazoles, i.e. smaller IPs, indicate stronger nucleophilicity for the dicoordinated heteroatoms than in the heterobenzenes, as is typical for π-excess systems. Furthermore, calculations on isodesmic bond separation reactions show that 1H-1,3-azaphospholes are more stable than the aromatic pyrroles while the gain in energy for 3H-1,3-azaphospholes is much lower and similar as in the electronically related phospholes [47], [48]. It should be noted that suitably annulated 1H-1,3-benzazaphospholes (7) also exhibit fluorescence [26].

Fig. 1: Comparison of δ31P values of selected non-, benzo- and pyrido-annulated 1H-1,3-azaphospholes and phosphabenzenes.
Fig. 1:

Comparison of δ31P values of selected non-, benzo- and pyrido-annulated 1H-1,3-azaphospholes and phosphabenzenes.

Fig. 2: Photoelectron spectroscopically determined ionization potentials of 1,3-azaphospholes and analogous N and As heterocycles compared with that of phosphabenzene and its congeners (orbital assignments by quantum chemical calculations).
Fig. 2:

Photoelectron spectroscopically determined ionization potentials of 1,3-azaphospholes and analogous N and As heterocycles compared with that of phosphabenzene and its congeners (orbital assignments by quantum chemical calculations).

Reactivity of 1H- and 3H-type 1,3-azaphospholes

Both the high π-basicity at phosphorus and the aromatic stabilization determine the reactivity of the 1H-1,3-azaphosphole-type heterocycles, which differs from that of σ2P-aromatic phosphabenzenes and that of the 3H-isomeric heterocyclic iminoarylphosphanes. While 3H-benzazaphospholes are sensitive to aqueous acids and decomposed even during column chromatography on silica gel, most 1H-1,3-benzazaphospholes are stable to addition of nucleophiles like water, aqueous acid and bases, alcohols, amines or thiols. 1,2-Dimethyl-1,3-benzazaphosphole proved quite stable to even undergo attack by tBuLi, possibly disfavored by sterical hindrance and the high electron-density at phosphorus. Therefore, we studied NH and CH-lithiations with tBuLi, NH-lithiation also with LDA. NH-Functional 1,3-azaphospholes and benzazaphospholes are lithiated to ambident azaphospholides 11. These are alkylated at phosphorus to 3H-isomers, unstable in the case of 3-methyl-1,3-azaphospholes [31] and partly oligomerizing in the case of 2-unsubstituted 3-methyl-1,3-benzazaphosphole [8]. In contrast, silylation or phosphanylation may occur at both, N- and P atom (Scheme 2). N-Substitution is preferred for small C2-substitutents and not too bulky electrophiles, P-substitution is favored in the case of stronger steric hindrance. DFT calculations revealed that the energetic differences of 2,5-dimethyl-1H- and 3H-benzazaphosphole model compounds are large for N- or P-PtBu2 groups (ca. 16–20 kJ/mol for the more stable conformer) and small for N- or P-PCy2 or PiPr2 (<4 kJ/mol). The observed isomers and preferred conformers were always those with the lowest energy, indicating that the reactivity was controlled by steric effects and highlighting their influence on the stability of the products [49]. A temperature dependent equilibrium between the isomers, as observed for PhAs=C(tBu)N(SiMe3)Tol and PhAs(SiMe3)-C(tBu)=NTol (preferred at high T) [50], was not detected. η1P- and μ2P-coordinated organo-transition-metal benzazaphospholides have been obtained from the reaction of 11 with the respective organometallic halides or also through direct heating NH-benzazaphospholes with nickelocene. Attempts to synthesize N-coordinated transition metal benzazaphospholide complexes failed because of extreme moisture sensitivity [17] (Scheme 3).

Scheme 2: N-Lithiation of non- or benzannulated 1H-1,3-azaphospholes and ambident reactivity of the azaphospholides 11 (with selected solid-state structures 11a–13, representing a NLi(THF)3 solvate, N- and P-substitution).
Scheme 2:

N-Lithiation of non- or benzannulated 1H-1,3-azaphospholes and ambident reactivity of the azaphospholides 11 (with selected solid-state structures 11a–13, representing a NLi(THF)3 solvate, N- and P-substitution).

Scheme 3: Organometallic 3H-1,3-benzazaphospholides.
Scheme 3:

Organometallic 3H-1,3-benzazaphospholides.

Reaction of tBuLi with 2-unsubstituted N-alkyl- or N-aryl-1,3-benzazaphospholes led in polar media, for more bulky N-substituents in the presence of KOtBu, to lithiation at C2. Isomerization of the 2-lithio compounds 14 to P,C-coordinated dimer lithium phosphidocarbene complexes seems not to take place, although the 13C solution NMR data (for R=Me d13C2=247 ppm) do not exclude this possibility. The crystal structure of the THF solvate 14a (R1=Me) displays a clear μ-C2-bridged dimer lithium compound. It reacts with various electrophiles in the typical way of aryllithium species with substitution at C2 and allows introduction of a variety of functional groups including OH or COOH [18] (Scheme 4). It should be noted here that for pyrido[b]-annulated 1H-1,3-azaphospholes addition of tBuLi at the P=C bond is preferred under the same conditions [29].

Scheme 4: C2-lithiation of N-substituted 1H-1,3-benzazaphospholes and introduction of functional groups.
Scheme 4:

C2-lithiation of N-substituted 1H-1,3-benzazaphospholes and introduction of functional groups.

Without prior lithiation, the reactivity of 1H-1,3-benzazaphospholes towards the electrophilic alkylhalides is very low. For 3H-1,3-benzazaphospholes P-alkylation was not yet studied, but for the homologous As-heterocycles methylation by methyl iodide was shown to give a 3,3-dimethylbenzazarsolium iodide in 50% yield [51]. Alkylation of 1H-1,3-benzazaphospholes was achieved only by prolonged heating with the highly reactive triethyloxonium tetrafluoroborate (Meerwein salt), yielding P-ethylated benzazaphospholium salts 15 (Scheme 5). In contrast to homologous benzimidazolium salts, which with silver oxide in the presence of molecular sieves form silver complexes of N-heterocyclic carbenes [52], [53], [54], the P-ethyl-benzazaphospholium salts did not form a defined complex, possibly due to the high affinity to moisture or to low stability and high tendency of cyclic phosphinoaminocarbenes for rearrangements [55]. In the presence of transition metal catalysts, 1H-1,3-benzazaphospholes react with alkyl halides and even arylbromides at σ2P, possibly favored by the high π-density at this site, similar to electrophilic o-or p-alkylation of aromatic compounds facilitated by +M-substituents. Because of the extreme reactivity of the primary addition products towards moisture, even with carefully dried reagents and solvents, only the dihydrobenzazaphosphole-P-oxides 16 could be isolated [56].

Scheme 5: P-alkylation and catalytic P-arylation of 1H-1,3-benzazaphospholes.
Scheme 5:

P-alkylation and catalytic P-arylation of 1H-1,3-benzazaphospholes.

Transition metal electrophiles react with 3H-1,3-benzazaphospholes [57] and with pyrido[b]-annulated 3H-1,3-azaphospholes [58] at the phosphane-type σ3P-atom. Alternative coordination at the imino-N atom has not yet been observed. For pyrido-3H-1,3-azaphospholes additional N-coordination is favored at the pyridine nitrogen, affording Pd and Ag dimer complexes, e.g. 17 (Scheme 6). The reactivity of 1H-1,3-benzazaphospholes towards transition metal salts or complexes is completely different. Reactions with d8-transition metal salts or complexes did not lead to isolable complexes with these ligands, possibly due to interactions of the π-HOMO of the ligand with the empty d-orbital of the metal and loss of aromatic stabilization. This may be the reason for consecutive reactions such as (i) addition of moisture, observed in the formation of the Rh(COD) complex 18, (ii) oxidation of phosphorus by working up in DMSO, e.g. in the case of PdCl2 complex 19, and (iii) reductive C2-dimerization or oxidative chlorination of 1-neopentyl-1H-1,3-benzazaphosphole in reactions with Fe3(CO)12 and AuCl, respectively [17].

Scheme 6: Examples of Rh(I)- and Pd(II)-complexes, formed with 3H-1,3-pyrido[b]- and 1H-1,3-benzazaphospholes.
Scheme 6:

Examples of Rh(I)- and Pd(II)-complexes, formed with 3H-1,3-pyrido[b]- and 1H-1,3-benzazaphospholes.

Only complexes with Cu(I) and Ag(I) salts or HgCl2 with d10 electron configuration of the metal (Scheme 7) or with metal(0)carbonyl complexes, studied mainly for group 6 metals (Scheme 8), provided isolable complexes. In contrast to complexes of d10 cations with phosphane ligands or σ-coordinated phosphabenzenes (without π-donor substituents), the 1H-1,3-benzazaphosphole ligands prefer μ2-bridging or tilted η1-coordination, which led to dimer or cluster structures, as elucidated by single crystal XRD analysis. In solution, they are all labile and in the NMR show spectra with in part averaged ligand signals if mixed with free ligand [59], [60]. Dimer μ2-bridging coordinated copper halide complexes were recently reported also for phosphabenzene ligands with π-donor substituents in o-position [61], [62]. The o-phosphinophenyl-benzazaphosphole P,P′-chelate complex 20 with HgCl2 shows coordination of Hg(II) at the ring P- atom almost perpendicular to the benzazaphosphole ring plane (angle of P–Hg vector to ring plane 80.3°). Quantum chemical calculations confirm considerable contributions of π-bonding in this case and smaller π-bond contributions also in the aforementioned complexes [59], [60].

Scheme 7: CuX, AgX and HgCl21H-1,3-benzazaphosphole complexes with μ2-bridging and/or tilted η1-coordination.
Scheme 7:

CuX, AgX and HgCl21H-1,3-benzazaphosphole complexes with μ2-bridging and/or tilted η1-coordination.

Scheme 8: Metal(0) carbonyl complexes of annulated 1H- and 3H-1,3-azaphospholes and activation of AgSbF6 for ROP of cyclic ethers (without light).
Scheme 8:

Metal(0) carbonyl complexes of annulated 1H- and 3H-1,3-azaphospholes and activation of AgSbF6 for ROP of cyclic ethers (without light).

In the metal (0) carbonyl complexes of 1H-1,3-benzazaphospholes 21, the metal is only slightly tilted out of the ring plane (ca. 7°). For the chelate 22 this tilting is somewhat more (14°), but in complex 23 with a less π-rich 2-phosphaindolizine ligand the metal lies strictly within the ring plane, similarly as in phosphabenzene-M(CO)5 complexes with M=Cr, Mo or W [6]. Upfield coordination chemical shifts of the phosphorus NMR signals for the tungsten and molybdenum and only weak downfield shifts for the chromium pentacarbonyl complexes hint at π-back bonding contributions. In contrast, the (3H-pyridoazaphosphole)M(CO)5 complexes 24 display generally downfield coordination chemical shifts, as is typical for phosphane ligands. This coordination is stronger than that at σ2P as shown by preferred coordination of the W(CO)5 fragment at the phosphanyl group of the 2-phosphanyl-benzazaphosphole ligand 25 [63]. When attempts were made to synthesize mixed metal carbonyl silver hexafluoroantimonate complexes starting from 21 (R1=neopentyl, R and R2=H), we observed rapid ring-opening polymerization of the solvent THF. This reaction occurred also in the dark and at −30°C, whereas THF polymerization only with AgSbF6 required irradiation and stopped if irradiation was switched off. Therefore and because of decomposition of 21 and formation of colloidal black silver, we suppose that labile coordination of silver at the benzazaphosphole facilitates the oxidation of the ether oxygen to radical cations, which then start the ring-opening polymerization (ROP). The weakly coordinating SbF6 anion is crucial for this reaction. The ROP of oxetane or epoxides with these initiators requires dilution by toluene to avoid a violent reaction [64].

Although N- and C2-substituted 1H-1,3-benzazaphospholes are quite stable towards addition of nucleophiles at the P=C double bond, such reactions are possible for t-BuLi and 2-unsubstituted 1H-1,3-benzazaphospholes in non- or less polar media like hexane, hexane/Et2O or toluene (Scheme 9). They are also favored in the presence of ClSiMe3 as trapping reagent, which reacts very slowly with tBuLi itself but fast with the addition products. Reaction of t-BuLi with 1H-1,3-pyrido- or with N- and C2-unsubstituted 1H-1,3-benzazaphospholes so far gave only addition products. Bulky N-substituents allow for only electronegativity-controlled “normal” addition with t-Bu at P and Li at C2 whereas for smaller N-substituents “inverse” addition with t-Bu at C2 and Li at P is also possible. In special cases, the primarily formed 2-lithiated product adds inverse to a second molecule benzazaphosphole to give a C2–C2-bonded bis(dihydrobenzazaphospholide). The benzo and pyrido-annulated 2-lithio-azaphospholines are highly reactive and deprotonate THF, if present in the solvent mixture, whereas in hexane/Et2O or toluene as solvent trapping by ClSiMe3 or introduction of functional groups is possible [18]. The reaction with carbon dioxide was studied preferably because this opened a route to new types of α-phosphino amino acids, another research field of my group [65]. In addition, the P-tert-butyl-substituted dihydrobenzazaphospholes may be of interest as ligands for transition-metal catalysts. Bulkiness and high basicity at P, increased here by the t-Bu and the intrinsic o-amino group with +M-effect, should facilitate reductive elimination and oxidative addition, respectively, the rate-controlling steps in many catalytic reactions. Preliminary tests in a Pd-catalyzed C–N cross-coupling with the 2-unsubstituted ligand 26 gave moderate yields, which might be improved by use of more bulky 2-SiMe3-substituted ligands. Use of the 2-COOH substituted tert-butyl-benzazaphospholine 27 in the in-situ generation of nickel catalysts led to rapid and almost quantitative conversion of ethylene to oligomers (TON>3500). The good selectivity for liquid and waxy linear α-olefins (ca. 90%) [66], similar to that for phosphanylacetate-based Ni-catalysts in the oligomerization step of the Shell Higher Olefin Process [67], [68], may be attributed to the common P–C–C(O)O catalyst backbone and an analogous mechanism.

Scheme 9: Addition reactions of tBuLi at the P=C bond of 1H-benzazaphospholes, C2-functionalization and use in catalysis.
Scheme 9:

Addition reactions of tBuLi at the P=C bond of 1H-benzazaphospholes, C2-functionalization and use in catalysis.

Conclusions

1H-1,3-Azaphospholes and benzo- or pyrido[b]-annulated systems thereof with NH, N-alkyl or N-aryl substituents are stable aromatic compounds, except for pyrido-annulation not basic and stable to water, alcohols and similar protic compounds. In contrast, 3H-1,3-azaphospholes are not aromatic, possess a basic nitrogen like Schiff-bases and are nucleophilic at P like phosphines. Moreover, they need stabilization by bulky substituents at C2 and/or P and annulation at the backside. Acylation, silylation and phosphinylation of lithium-benzazaphospholides lead in the absence of steric stress to N-substituted 1H-1,3-benzazaphospholes and in the case of bulky 2-substituents or electrophiles to the sterical more stabilized 3H-isomers. The π-excess at P of 1H-benzazaphospholes allows catalytic P-alkylation or P-arylation, leads to μ2- or tilted η1P-coordination in d10 metal complexes and hinders addition of THF-, particular of KOtBu-complexed t-BuLi with a polarized C-Li bond at the P=C double bond. Thus, NH- or 2-CH-lithiation and substitution by electrophiles at N, P or C2 is enabled. In non- or less polar media, however, addition of t-BuLi at the P=C double bond is preferred, opening access to P-tert-butyl-dihydrobenzazaphospholes, which may be useful as ligands in transition metal catalyzed reactions.


Article note

A collection of invited papers based on presentations at the 22nd International Conference on Phosphorous Chemistry (ICPC-22) held in Budapest, Hungary, 8–13 July 2018.


Acknowledgements

I thank my earlier coworkers involved in these investigations for their engaged work, particularly Dr. Mohammed Ghalib, Dr. Basit Niaz, Dr. Bhaskar R. Aluri, Dr. Mohamed S. S. Adam, Dr. Normen Peulecke, Prof. Dr. Neelima Gupta, Dr. Nidhi Gupta, Dr. Shreeyukta Singh, Dr. Anushka Surana, Kinga Steinhauser, Dr. Markus K. Kindermann, Brigitte Witt, Gabriele Thede, Waltraut Heiden and Mike Steinich. Furthermore, I am grateful to the Professors Tamas Veszprémi, Laszlo Nyulászi, Raj K. Bansal, Peter G. Jones, Konstantin Karaghiosoff and Carola Schulzke for the successful cooperation and their valuable contributions to these results and to DFG, DAAD and the University Greifswald for the financial support of this research.

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Published Online: 2019-01-10
Published in Print: 2019-05-27

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