Accessible Published by De Gruyter August 22, 2016

Synthesis and reactions of 2-azido-1,3-di(benzyloxy)imidazolium hexafluoridophosphate

Gerhard Laus, Mirco E. Kostner, Volker Kahlenberg and Herwig Schottenberger

Abstract

2-Azido-1,3-di(benzyloxy)imidazolium hexafluoridophosphate was obtained from the corresponding 2-bromo compound by reaction with sodium azide. Cycloaddition of the 2-azido compound with norbornene and norbornadiene gave the respective tricyclic aziridine and bicyclic azaoctadiene. Addition of triphenylphosphane yielded the phosphazide which upon heating eliminated dinitrogen to afford the phosphazene. The crystal structures of five compounds were determined by X-ray diffraction.

1 Introduction

Only a limited number of azidoazolium salts bearing the azido group at the carbon between two quaternary nitrogen atoms are known. They comprise 2-azido-1,3-dimethoxyimidazolium [1] and 2-azido-1,3-dimethylimidazolinium salts [2]. Recently, a 5-azido-4-(dimethylamino)-1-methyl-1,2,4-triazolium salt [3] was presented. We intended to introduce another stable 2-azidoimidazolium salt and to study its behavior in the light of the well-known versatility of organoazide chemistry [4]. A classic example is provided by the 1,3-dipolar cycloaddition [5] of strained alkenes such as bicycloheptene (norbornene) to phenyl azide giving 1,2,3-triazolines and, subsequently, mixtures of aziridines and imines [6].

Thermolytic reaction of phenyl azide with regular (non-strained) olefins at intermediate temperatures was reported to afford either aziridines or imines [7] via the corresponding 1,2,3-triazolines which were formed at lower temperatures. Photochemical decomposition of 1,2,3-triazolines was observed to yield pure aziridines [8], whereas different reaction paths were found in the thermal decomposition [9]. A kinetic investigation of the addition of aryl azides to norbornene attested to a concerted mechanism [10]. Furthermore, a concerted addition of sulfonyl azide to the double bond of norbornene with concomitant loss of dinitrogen has been suggested to afford the corresponding aziridine [11], [12]. Under identical conditions, norbornadiene yielded a 2-azabicyclo[3.2.1]octa-3,6-diene [13]. In this context, the reactivity of a cationic azidoazolium salt is expected to resemble more that of an acyl azide than that of an aryl azide. Another classic example of azide chemistry is the reaction of tertiary phosphanes with azides [14], [15], the noted Staudinger synthesis, which yields phosphazides as primary products. It is usually followed by thermal denitrogenation to afford phosphazenes (for exact nomenclature, see Experimental section) [16]. The facile hydrolysis of a phosphazene yielding an aminoazolium salt was demonstrated recently [3]. Also, alkynes are known to undergo cycloaddition with azidoazoles leading to 1,2,3-triazoles [17], [18]. To our knowledge, acyl azides gave unexpected products [19], and quaternary azidoazolium salts have not yet been employed for this so-called ‘click-reaction’ [20], [21], [22]. This versatility of azide chemistry motivated us to attempt these reactions with the new 2-azido-1,3-di(benzyloxy)imidazolium system.

2 Results and discussion

Previously, 2-azidoimidazolium salts were synthesized from the respective 2-bromo or 2-chloro compounds and sodium azide [1]. This pathway is only viable when the halogen derivatives are readily available. As we had obtained the 2-bromo-1,3-di(benzyloxy)imidazolium compound [23], the road was clear for the 2-azido-1,3-di(benzyloxy)imidazolium salt 1 (Scheme 1). The hexafluoridophosphate was chosen as an advantageous anion because these salts crystallize readily and are non-hygroscopic. Cycloaddition of the 2-azido compound with bicyclo[2.2.1]hept-2-ene and bicyclo[2.2.1]hepta-2,5-diene gave the respective 2-(3-azatricyclo[3.2.1.02,4]oct-3-yl)-1,3-di(benzyloxy)imidazolium and 2-(2-azabicyclo[3.2.1]octa-3,6-dien-2-yl)-1,3-di(benzyloxy)imidazolium salts 2 and 3 in acceptable yields. Addition of triphenylphosphane gave the yellow 1,3-di(benzyloxy)-2-(1-(triphenylphosphoranylidene)triazen-3-yl)imidazolium salt 4. The phosphazide was sufficiently stable in air at ambient temperature to be handled easily. Upon heating, it eliminated dinitrogen affording the colorless 1,3-di(benzyloxy)-2-((triphenylphosphoranylidene)amino)imidazolium salt 5. Finally, a ‘click’-cycloaddition of the azide 1 with an alkyne was attempted in order to obtain the corresponding 1,2,3-triazole. In our hands, however, the quaternary 2-azidoimidazolium salt 1 did not react at all with phenylethyne in the presence of a Cu(I) catalyst. This can be possibly explained by electron-deficiency of the cationic azide.

Scheme 1: (a) NaN3, acetone-H2O, 0 °C; (b) bicyclo[2.2.1]hept-2-ene, acetone, r. t.; (c) bicyclo[2.2.1]hepta-2,5-diene, acetone, r. t.; (d) Ph3P, acetone, 0 °C; (e) toluene, 70 °C.

Scheme 1:

(a) NaN3, acetone-H2O, 0 °C; (b) bicyclo[2.2.1]hept-2-ene, acetone, r. t.; (c) bicyclo[2.2.1]hepta-2,5-diene, acetone, r. t.; (d) Ph3P, acetone, 0 °C; (e) toluene, 70 °C.

The crystal structures of five compounds have been determined by X-ray diffraction. Conformational syn/anti isomerism in di(alkyloxy)imidazolium salts has been noticed previously [1], [23], [24], [25], [26]. In this work, the benzyloxy groups of the azide 1 and the norbornadiene adduct 3 adopt anti conformations whereas in the aziridine 2, phosphazide 4, and phosphazene 5 they adopt syn conformations. There are no directional classic hydrogen bonds in these structures. At the most, weak C–H···F interactions shorter than the sum of van der Waals radii between the imidazole H atoms and negatively polarized F atoms are worth mentioning (H2···F6, 2.38 Å in 1 and H3···F3, 2.23 Å in 3). In the crystal structure of 1, the azide group is twisted out of the ring plane and assumes a typical N3–N4–N5 angle of 169.3(3)°. In addition, an interesting anion···π interaction [27] with a F5···centroid distance of 2.913 Å was observed (Fig. 1). The diffraction data of compounds 2 and 3 confirmed the alleged molecular structures (Figs. 2 and 3) of the cycloadducts. The crystal structures of 4 and 5 are particularly valuable because they permit insights into the bonding situation in the new phosphazide and phosphazene systems. The pertinent bond lengths in 4 could be classified as a partial C1–N3 double (aromatic) bond with 1.352(3) Å, two partial double bonds N3–N4 with 1.308(2) Å and N4–N5 with 1.305(2) Å adopting (E)-geometries, and a typical single bond N5–P1 with 1.662(2) Å (Fig. 4), by comparison with accepted values [28]. For related structures of the type RC–N–N–N–PR3 in the Cambridge Structural Database, it was found that the C–N bonds were considerably longer (1.47–1.51 Å) than in 4 when the carbon substituent did not allow electron delocalization (RC = benzyl [29], [30], trityl [31], t-butyl [16], 1-adamantyl [32]) but shortened (1.39–1.42 Å) when extended resonance was possible (RC = phenyl [33], [34], naphthyl [35], pyrazolyl [36]). In contrast, the N–P bond length was only marginally affected by the nature of the substituents on P: it was comparable to that in 4 (1.64–1.65 Å) with carbon substituents (R = phenyl [33], cyclohexyl [32], isopropyl [29], [34]) and somewhat shorter (1.61–1.63 Å) with nitrogen substituents (piperidino [16], pyrrolidino [35], dimethylamino [31]). The two inner N–N bonds were rather insensitive to their surroundings. In the most closely related structures [33], [34], [35], [36], they were slightly shorter (1.27–1.31 Å) than the N3–N4 and longer (1.31–1.36 Å) than the N4–N5 bonds in compound 4 where they were of equal length. Obviously, the imidazolium cation confers additional delocalization on the phosphazide system. The crystal structure of the phosphazene 5, which was obtained as a dichloromethane solvate, revealed two typical double bonds, namely an iminium bond C1=N3+ with 1.300(3) Å and a N3=P1 bond with 1.591(2) Å, in the phosphazene unit resulting in a C–N–P angle of 137° (Fig. 5). It has been found previously that substituted 2-azaallenium cations (2-azoniaallenes) were sterically flexible around the central nitrogen atom [37]. A similar nonlinear pseudoallene with a C=N+=P fragment, C=N+ 1.31 Å and C–N–P angle 128°, has been reported [38]. Scheme 2 shows the proposed bonding situation in compounds 4 and 5 on the basis of the X-ray diffraction results. These resonance forms appear to contribute significantly to the experimentally observed structures of 4 and 5.

Fig. 1: Molecular structure of 2-azidoimidazolium salt 1 in the crystal exhibiting anion···π interactions. For clarity, H atoms have been omitted.

Fig. 1:

Molecular structure of 2-azidoimidazolium salt 1 in the crystal exhibiting anion···π interactions. For clarity, H atoms have been omitted.

Fig. 2: Molecular structure of norbornene adduct 2 in the crystal. For clarity, H atoms have been omitted. Only one of the two independent ion pairs shown.

Fig. 2:

Molecular structure of norbornene adduct 2 in the crystal. For clarity, H atoms have been omitted. Only one of the two independent ion pairs shown.

Fig. 3: Molecular structure of norbornadiene adduct 3 in the crystal. For clarity, H atoms have been omitted.

Fig. 3:

Molecular structure of norbornadiene adduct 3 in the crystal. For clarity, H atoms have been omitted.

Fig. 4: Molecular structure of phosphazide 4 in the crystal. For clarity, H atoms have been omitted.

Fig. 4:

Molecular structure of phosphazide 4 in the crystal. For clarity, H atoms have been omitted.

Fig. 5: Molecular structure of phosphazene 5 in the crystal. For clarity, H atoms have been omitted. Solvent molecule not shown.

Fig. 5:

Molecular structure of phosphazene 5 in the crystal. For clarity, H atoms have been omitted. Solvent molecule not shown.

Scheme 2: Bonding situation in the solid state structures of 4 and 5 as based on the X-ray diffraction data.

Scheme 2:

Bonding situation in the solid state structures of 4 and 5 as based on the X-ray diffraction data.

3 Experimental section

2-Bromo-1,3-di(benzyloxy)imidazolium hexafluoridophosphate [CAS-RN 1253192-86-7] was prepared as previously described [23]. NMR spectra were recorded with Bruker AC 300 and 600 spectrometers. IR spectra were obtained using a Nicolet 5700 FT spectrometer in ATR mode. High-resolution mass spectra were measured with a Finnigan MAT 95 spectrometer. Elemental analyses were conducted at the University of Vienna, Vienna, Austria.

3.1 2-Azido-1,3-di(benzyloxy)imidazolium hexafluoridophosphate (1)

Sodium azide (1.48 g, 1.06 equiv) was added to a solution of 2-bromo-1,3-di(benzyloxy)imidazolium hexafluoridophosphate (10.7 g, 21.2 mmol) in acetone-H2O (45:5 mL) at 0 °C. The mixture was stirred for 30 min, then ice-H2O (200 mL) was added. The crude product was filtered off and dissolved in CH2Cl2 (100 mL). The solution was washed with cold H2O (3 × 200 mL), dried over MgSO4 and evaporated at 0 °C. The residue was dried in vacuo at 0 °C to yield a white solid (9.45 g, 95%), which was stored at –20 °C. M.p. 94 °C (decomposition). – 1H NMR (300 MHz, [D6]acetone): δ = 5.58 (s, 4H), 7.50–7.59 (m, 10H), 7.88 (s, 2H) ppm. – 13C NMR (75 MHz, [D6]acetone): δ = 84.7 (2C), 115.8 (2C), 129.9 (4C), 131.3 (2C), 131.4 (4C), 132.6 (2C), 134.0 ppm. – IR (neat): ν = 3173 (w), 3151 (w), 2184 (w), 2165 (m), 1603 (m), 1497 (w), 1379 (w), 1270 (m), 1065 (m), 826 (s), 737 (m), 695 (m), 650 (m), 555 (s) cm–1. – C17H16F6N5O2P (467.31): calcd. C 43.69, H 3.45, N 14.99; found C 43.93, H 3.43, N 14.66.

3.2 2-(3-Azatricyclo[3.2.1.02,4]oct-3-yl)-1,3-di(benzyloxy)imidazolium hexafluoridophosphate (norbornene adduct) (2)

A solution of bicyclo[2.2.1]hept-2-ene (206 mg, 1.03 equiv) and azide 1 (983 mg, 2.10 mmol) in acetone (20 mL) was stirred for 4 h at room temperature. After removal of the solvent, the residue was crystallized by addition of MeOH (9 mL) at –20 °C. The product was collected by filtration, washed with cold MeOH (2 × 5 mL) and dried to give a white solid (460 mg, 41%). Single crystals were obtained by slow evaporation of a CH2Cl2 solution, m.p. 125–127 °C. – 1H NMR (300 MHz, [D6]acetone): δ = 0.95 (d, J = 10.1 Hz, 1H), 1.23–1.30 (m, 3H), 1.58 (d, J = 8.9 Hz, 2H), 2.67 (s, 2H), 3.43 (s, 2H), 5.40 (s, 4H), 7.51 (m, 12H) ppm. – 13C NMR (75 MHz, [D6]acetone): δ = 25.6 (2C), 28.4, 36.8 (2C), 46.0 (2C), 83.0 (2C), 113.4 (2C), 129.9 (4C), 131.1 (2C), 131.4 (4C), 133.1 (2C), 143.3 ppm. – IR (neat): ν = 3177 (w), 3150 (w), 2977 (w), 2954 (w), 2876 (w), 1599 (m), 1540 (w), 1494 (w), 1473 (w), 1457 (w), 1363 (m), 1292 (w), 1233 (w), 1217 (w), 1194 (w), 1058 (m), 995 (w), 975 (w), 943 (w), 831 (s), 759 (m), 742 (m), 702 (m), 668 (m), 650 (m), 592 (w), 555 (s) cm–1. – HRMS (ES): m/z = 388.188 (calcd. 388.202 for C24H26N3O2, [M]+).

3.3 2-(2-Azabicyclo[3.2.1]octa-3,6-dien-2-yl)-1,3-di(benzyloxy)imidazolium hexafluoridophosphate (norbornadiene adduct) (3)

A solution of bicyclo[2.2.1]hepta-2,5-diene (0.21 mL, 1.03 equiv) and azide 1 (918 mg, 1.96 mmol) in acetone (20 mL) was stirred for 3 h at room temperature. After removal of the solvent, the residue was crystallized by addition of MeOH (9 mL) at –20 °C. The product was collected by filtration, washed with cold MeOH (2 × 5 mL) and dried to give a white solid (520 mg, 50%), m.p. 120 °C (decomposition). – 1H NMR (300 MHz, [D6]acetone): δ = 1.98 (d, J = 10.7 Hz, 1H), 2.06–2.12 (m, 1H), 2.92–2.96 (m, 1H), 4.97 (s, 1H), 5.41 and 5.46 (AB, J = 9.9 Hz, 4H), 5.49 (d, J = 15.5 Hz, 2H), 5.75 (dd, J = 5.5 Hz, J = 2.3 Hz, 1H), 5.98 (d, J = 7.8 Hz, 1H), 6.48 (dd, J = 5.4 Hz, J = 2.8 Hz, 1H), 7.50–7.55 (m, 10H), 7.72 (s, 2H) ppm. – 13C NMR (75 MHz, [D6]acetone): δ = 36.7, 37.0, 65.2, 84.0 (2C), 111.5, 114.4 (2C), 122.7, 123.2, 129.8 (4C), 131.0 (2C), 131.3 (4C), 132.8 (2C), 137.2, 140.8 ppm. – IR (neat): ν = 3171 (w), 3151 (w), 1633 (w), 1604 (m), 1457 (w), 1378 (w), 1342 (w), 1229 (w), 1033 (w), 942 (w), 902 (w), 824 (s), 742 (m), 695 (m), 664 (m), 556 (s) cm–1. – HRMS (ES): m/z = 386.287 (calcd. 386.186 for C24H24N3O2, [M]+).

3.4 1,3-Di(benzyloxy)-2-(1-(triphenylphosphoranylidene)triazen-3-yl)imidazolium hexafluoridophosphate (4)

Triphenylphosphane (1.17 g, 1.03 equiv) was added to a solution of azide 1 (2.00 g, 4.28 mmol) in deoxygenated acetone (40 mL) at 0 °C. The yellow mixture was stirred for 30 min, and the solvent was evaporated. The residue was dissolved in CH2Cl2 (7 mL), and Et2O (3 mL) was added. The product was allowed to crystallize at –20 °C. It was filtered off, washed with cold Et2O and dried to yield a yellow solid (2.00 g, 64%), m.p. 114–119 °C. – 1H NMR (600 MHz, [D6]acetone): δ = 5.05 (s, 4H), 7.24 (s, 2H), 7.29 (d, J = 7.1 Hz, 4H), 7.40 (t, J = 7.2 Hz, 4H), 7.45 (t, J = 7.2 Hz, 2H), 7.73–7.78 (m, 6H), 7.84–7.95 (m, 9H) ppm. – 13C NMR (151 MHz, [D6]acetone): δ = 82.9, 114.9, 124.1 (d, J = 94.7 Hz, 3C), 129.6, 130.6, 130.7 (d, J = 11.5 Hz, 6C), 130.9, 133.4, 134.6 (d, J = 9.4 Hz, 6C), 135.3 ppm. – 31P NMR (121 MHz, [D6]acetone): δ = –142.8 (sept, J = 709 Hz), 29.8 ppm. – IR (neat): ν = 3186 (w), 3155 (w), 1568 (w), 1438 (w), 1243 (m), 1182 (m), 1112 (w), 1042 (m), 948 (m), 834 (s), 815 (s), 786 (m), 691 (m) cm–1. – HRMS (ES): m/z = 584.2227 (calcd. 584.2210 for C35H31N5O2P, [M]+). – C35H31F6N5O2P2 (729.60): calcd. C 57.62, H 4.28, N 9.60; found C 57.52, H 4.20, N 9.57.

3.5 1,3-Di(benzyloxy)-2-((triphenylphosphoranylidene)amino)imidazolium hexafluoridophosphate (5)

A suspension of the triazene 4 (183 mg, 0.25 mmol) in toluene (12 mL) was stirred at 70 °C for 40 min. After cooling, Et2O (10 mL) was added. The product was filtered off, washed with Et2O (2 × 10 mL) and dried to yield a colorless solid (122 mg, 69%). Single crystals were obtained by diffusion of Et2O into a CH2Cl2 solution, affording a CH2Cl2 solvate, m.p. 145–148 °C. – 1H NMR (300 MHz, [D6]acetone): δ = 5.17 (s, 4H), 7.18–7.23 (m, 6H), 7.36–7.48 (m, 6H), 7.60–7.66 (m, 6H), 7.76–7.88 (m, 9H). – 13C NMR (75 MHz, [D6]acetone): δ = 81.8 (2C), 111.1 (2C), 128.3 (d, J = 106 Hz, 3C), 129.6, 130.3 (d, J = 13.1 Hz, 6C), 130.5, 130.8, 133.4 (d, J = 11.2 Hz, 6C), 133.7, 134.5 (d, J = 2.1 Hz, 3C) ppm. – 31P NMR (121 MHz, [D6]acetone): δ = –142.7 (sept, J = 708 Hz), 17.9 ppm. – IR (neat): ν = 3147 (w), 1624 (m), 831 (s) cm–1. – HRMS (ES): m/z = 556.2140 (calcd. 556.2148 for C35H31N3O2P, [M]+).

3.6 X-ray structure determinations

X-ray diffraction data were collected with an Oxford Diffraction Gemini-R Ultra diffractometer by ω scans using Mo (λ = 0.7107 Å) or Cu radiation (λ = 1.5418 Å), as noted in Table 1, at 173(2) K. Absorption corrections were applied in all cases (multi-scan). The crystal structures were solved by Direct Methods and refined by full-matrix least-squares techniques [39], [40]. All non-hydrogen atoms were assigned anisotropic displacement parameters in the refinement.

Table 1:

Crystal data and structure refinement details for 15.

Compound12345
CCDC no.14711731471171147117214711741471175
FormulaC17H16N5O2·F6PC24H26N3O2·F6PC24H24N3O2·F6PC35H31N5O2P·F6PC35H31N3O2P·F6P·0.78(CH2Cl2)
Mr467.32533.45531.43729.59767.56
Crystal size, mm30.32 × 0.24 × 0.020.28 × 0.16 × 0.160.3 × 0.24 × 0.060.4 × 0.28 × 0.280.4 × 0.4 × 0.3
Crystal systemOrthorhombicOrthorhombicOrthorhombicMonoclinicMonoclinic
Space groupPca21Pna21Pca21C2/cP21/n
a, Å21.687(1)16.1404(2)28.415(1)28.501(1)11.0649(2)
b, Å8.0157(3)11.2974(1)9.5066(4)10.9937(3)17.9228(2)
c, Å11.3443(4)27.2012(3)8.8564(5)21.8971(5)18.9998(2)
β, °90909098.091(2)106.162(1)
V, Å31972.1(2)4960.0(1)2392.4(2)6792.8(3)3619.0(1)
Z48484
Dx, g cm–31.551.431.481.431.41
RadiationCuCuMoMoMo
μ, mm–12.001.640.190.200.30
F(000), e9522208109630081578
h,k,l range–25 ≤ h ≤ 23, –9 ≤ k ≤ 9, –11 ≤ l ≤ 13–19 ≤ h ≤ 19, –13 ≤ k ≤ 11, –32 ≤ l ≤ 31–34 ≤ h ≤ 31, –10 ≤ k ≤ 11, –7 ≤ l ≤ 10–34 ≤ h ≤ 32, –10 ≤ k ≤ 13, –23 ≤ l ≤ 26–12 ≤ h ≤ 13, –21 ≤ k ≤ 17, –19 ≤ l ≤ 22
Measured reflections1273637583145152211233387
Independent reflections/Rint2908/0.0338399/0.0333604/0.0576224/0.0356410/0.025
Observed reflections28027473290341355856
Parameters refined280650325451461
R1/wR2 [I > 2 σ(I)]0.035/0.0900.043/0.1160.051/0.1130.041/0.1000.055/0.153
R1/wR2 (all data)0.036/0.0910.048/0.1190.070/0.1230.065/0.1040.059/0.156
x (Flack)0.00(2)0.25(2)0.1(2)
Goodness of fit1.061.031.070.951.05
Δρmax/min, e Å–30.28/–0.250.45/–0.290.63/–0.200.44/–0.350.74/–0.81

CCDC 1471171–1471175 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

Acknowledgments

We are grateful to Dr. H. Kopacka and Dr. C. Kreutz for the NMR spectra and to Dr. T. Müller for the HR mass spectra.

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Received: 2016-5-4
Accepted: 2016-5-20
Published Online: 2016-8-22
Published in Print: 2016-9-1

©2016 Walter de Gruyter GmbH, Berlin/Boston