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Organometallics 2009, 28, 378–381
Synthesis and Reactivity of Three New N-Heterocyclic Silylenes Adam C. Tomasik, Amitabha Mitra, and Robert West* Organosilicon Research Center, Department of Chemistry, UniVersity of Wisconsin-Madison, 1101 UniVersity AVenue, Madison, Wisconsin 53706 ReceiVed June 30, 2008 Summary: Three new stable silylenes, rac-1,3,4-tri-tert-butyl1,3-diaza-2-silacyclopentane-2-ylide (5), 1,3-di-tert-butyl-4,4dimethyl-1,3-diaza-2-silacyclopentane-2-ylide (6), and rac-1,3di-tert-butyl-4-methyl-1,3-diaza-2-silacyclopentane-2-ylide (7), haVe been synthesized by the reaction of their corresponding dibromides with KC8. Unlike the analogous silylene 2, which lacks any backbone substitution and tetramerizes in concentrated solution or as a solid, silylenes 5, 6, and 7 show no tendency to oligomerize. The reactions of 5 with tert-butanol and chloroalkanes giVe only 1:1 O-H or C-Cl insertion products; with adamantyl azide 5 yields the spirosilatetrazoline 8, while with mesityl azide it giVes the azadisilacyclopropane 9. Silylenes, divalent, dicoordinate silicon species, have long been known to be key intermediates in numerous thermal and photochemical reactions.1 Until relatively recently, however, silylenes have been observable only at low temperatures. The N-heterocyclic silylenes 1-3, silicon analogues of the Arduengo N-heterocyclic carbenes,2-5 were first reported during the mid1990s along with a few other examples.6,7 These molecules are stable at room temperature under anaerobic conditions and benefit from both electronic stabilization by the amino groups and steric protection by the bulky alkyl groups. Silylene 2, however, is stable only in dilute solution.
In the pure solid state or in a concentrated solution, 2 reversibly tetramerizes into a diaminodisilyldisilene (a dark red solid) (Chart 1).4,5 Recently, silylene 4 was added to the group * To whom correspondence should be addressed. E-mail: west@ chem.wisc.edu. (1) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165–4183. (2) Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Belyakov, A. V.; Verne, H. P.; Haaland, A.; Wagner, M.; Metzler, N. J. Am. Chem. Soc. 1994, 116, 2691–2692. (3) Gehrhus, B.; Lappert, M. F.; Heinicke, J.; Boose, R.; Blaser, D. J. Chem. Soc., Chem. Commun. 1995, 1931–1932. (4) Haaf, M.; Schmedake, T. A.; Paradise, B. J.; West, R. Can. J. Chem. 2000, 78, 1526–1533. (5) West, R.; Denk, M. Pure Appl. Chem. 1996, 68, 785–788. (6) Heinicke, J.; Oprea, A.; Kindermann, M. K.; Karpati, T.; Nyulaszi, L.; Veszpremi, T. Chem.-Eur. J. 1998, 4, 541–545. (7) Kira, M.; Ishida, S.; Iwamoto, T.; Kabuto, C. J. Am. Chem. Soc. 1999, 121, 9722–9723.
Chart 1. Tetramerization of 2 (R ) tBu)
of known thermally stable silylenes; it shows no tendency to undergo oligomerization.8 Several studies have examined the behavior of silylenes 1 and 2 toward a wide range of organic, inorganic, and organometallic substrates.1,9 Among these, reactions with organic azides, alcohols, and alkyl halides have been reported. The reactions of 2 with PhN3, p-tolN3, Ph3CN3, and Ph3SiN3 all yield the silatetrazolines A-D (Figure 1).10 The reaction of 1 with adamantyl azide (AdN3) gives a similar product, E, while the reaction of silylene 3 with AdN3 produces the azadisilacyclopropane F, although the latter species was not crystallographically authenticated.11 Typically, silylenes will react with alcohols and alkyl halides via oxidative addition. For silylenes 1 and 2 most alcohols give a 1:1 adduct, while reactions with alkyl halides can give either the 1:1 adduct or the 1:2 disilane radical adduct depending on the substrate (Scheme 1).12,13 We herein report the synthesis of three new saturated N-heterocyclic silylenes, 5 (as a racemic mixture), 6, and 7 (racemic), and the reactions of 5 toward a variety of substrates. The silylenes were characterized by 1H, 13C, and 29Si NMR, high-resolution mass spectrometry, and (for 5 and 7) elemental analysis. The 29Si resonance of 5 in C6D6 falls at δ +140.6 ppm, slightly more deshielded than the resonances of 2 (δ +119 ppm) and 4 (δ +123.4 ppm). The UV-vis spectrum of 5 in n-heptane (ca. 2.6 × 10-4 M) shows a band maximum at 279 mm (ε ) 1700 M-1 cm-1) and shoulders at 300 and 333 nm. Silylene 2 has similar band maxima at 268 and 292 nm, while silylene 4 shows maxima at 270 and 295 nm.
Silylene 6, substituted with two methyl groups on the same backbone carbon, is a pale yellow liquid that exists solely as the monomeric species. The 29Si resonance for 6 in C6D6 falls (8) Li, W.; Hill, N. J.; Tomasik, A. C.; Bikzhanova, G.; West, R. Organometallics 2006, 25, 3802–3805.
10.1021/om8006147 CCC: $40.75 2009 American Chemical Society Publication on Web 11/25/2008
Notes
Organometallics, Vol. 28, No. 1, 2009 379
Figure 1. Products of silylenes 1, 2, and 3 with organic azides. Scheme 1. Reactions of Silylene 2 with Alcohols and Alkyl Halides
at δ +130.5 ppm, in the same range as the resonances of 2, 4, and 5. Silylene 7 has one methyl group in the backbone and can be viewed as the smallest structural perturbation of silylene 2. It is a colorless liquid that exists exclusively as the monomeric species. The 29Si NMR shift appears at 121.4 ppm, which is in between those of 2 and 4. The observed trend in the 29Si NMR shifts, 2 < 7 < 4 is what would be expected as the silylene structure becomes more deshielded upon the sequential addition of methyl groups. The reason 4, 5, 6, and 7 remain as monomers rather than tetramerizing like 2 is still under active investigation. The methyl or tert-butyl substituents on the ring carbon atoms contribute some steric effect, but inspection of models does not suggest that these groups should interfere with the oligomerization. Another possibility is that the alkyl substituent groups make the nitrogen atoms slightly better π-electron donors, stabilizing the monomeric silylenes. In either argument, it is clear that silylene 2 is a unique species. Silylenes 5, 6, and 7 were prepared by the reduction of their corresponding dibromosilanes 5a, 6a, and 7a with potassium graphite (KC8) in dimethoxyethane (DME) at room temperature, as shown in Scheme 2. (9) Haaf, M.; Schmedake, T. A.; West, R. Acc. Chem. Res. 2000, 33, 704–714. (10) Hill, N. J.; Moser, D. F.; Guzei, I. A.; West, R. Organometallics 2005, 24, 3346–3349. (11) Gehrhus, B.; Hitchcock, P. B.; Lappert, M. F. Z. Anorg. Allg. Chem. 2001, 627, 1048. (12) Moser, D. F.; Bosse, T.; Olson, J.; Moser, J. L.; Guzei, I. A.; West, R. J. Am. Chem. Soc. 2002, 124, 4186–4187. (13) Moser, D. F.; Naka, A.; Guzei, I. A.; Mu¨ller, T.; West, R. J. Am. Chem. Soc. 2005, 127, 14730–14738. (14) Gardiner, M. G.; Lawrence, S. M.; Raston, C. L. Inorg. Chem. 1996, 35, 1349–1354.
Scheme 2. Synthesis of Silylenes 5, 6, and 7
Diamines 5b, 6b, and 7b were synthesized via methods derived from literature procedures.14-16 The diamines were converted to the dibromosilanes 5a, 6a, and 7a by reaction with silicon tetrabromide in the presence of triethylamine.17,18 When (15) Lai, J. T. Tetrahedron Lett. 1982, 23, 595–598. (16) D’Angeli, F.; Marchetti, P.; Cavicchioni, G.; Bertolasi, V.; Maran, F. Tetrahedron: Asymmetry 1991, 2, 1111–1121. (17) Synthesis of rac-2,2-dibromo-1,3,4-tri-tert-butyl-1,3,2-diazasilolidine (5a): A solution of the diamine (5b) (6.28 g, 27.5 mmol), triethylamine (12 mL), and heptane (125 mL) was stirred at 0 °C. SiBr4 (11.0 g, 31.6 mmol) in 25 mL of heptane was added dropwise to the cooled solution. The reaction mixture was then fitted with a reflux condenser and refluxed for 2 d, after which it was cooled and filtered, and the solvent removed. The crude yellow oil was distilled in a Kugelrohr oven (110 °C, 1 Torr) to give a colorless oil. Yield: 8.60 g, 75.5%. 1H NMR (C6D6): δ 1.00 (s, 9H); 1.28 (s, 9H); 1.33 (s, 9H); 2.77 (d, 1H, J ) 5.8 Hz); 2.83 (d, 1H, J ) 9.2); 2.89 (dd, 1H, J ) 9.2, 5.8 Hz). 13C NMR (C6D6): δ 29.17; 30.07; 31.89; 36.48; 46.95; 53.33; 55.12; 62.64. 29Si{1H} NMR (C6D6): δ-50.06. Highresolution mass spectrometry: calcd for [C14H30N2SiBr2-CH3]+ 397.0305, found 397.0310. Anal. Calcd for C14H30N2SiBr2: C, 40.59; H, 7.30; N, 6.76. Found: C, 40.38; H, 7.27; N, 6.67. (18) 2,2-Dibromo-1,3-di-tert-butyl-4,4-dimethyl-1,3,2-diazasilolidine (6a): Yield: 8.29 g, 69%. 1H NMR (C6D6): δ 1.17 (s, 6H); 1.28 (s, 9H); 1.44 (s, 9H); 2.56 (s, 2H). 13C NMR (C6D6): δ 29.42; 29.46; 33.62; 52.63; 54.85; 59.25; 60.14. 29Si{1H} NMR (C6D6): δ-60.96. Highresolution mass spectrometry: calcd for [C12H26N2SiBr2]+ 384.0227, found 384.0221. Anal. Calcd for C12H26N2SiBr2: C, 37.32; H, 6.78; N, 7.25. Found: C, 37.60; H, 6.55; N, 7.41. rac-2,2-Dibromo-1,3-di-tertbutyl-4-methyl-1,3,2-diazasilolidine (7a): Yield: 7.90 g, 76.5%. 1H NMR (C6D6): δ 1.12 (d, 3H, J ) 6.7 Hz); 1.26 (s, 9H); 1.27 (s, 9H); 2.29 (d, 1H, J ) 8.6 Hz); 2.98 (dd, 1H, J ) 8.3, 5.8 Hz); 3.03 (dq, 1H, J ) 11.8, 6.0 Hz). 13C NMR (C6D6): δ 23.93; 29.57; 31.18; 50.19; 51.27; 52.58; 53.58. 29Si{1H} NMR (C6D6): δ-60.64. High-resolution mass spectrometry: calcd for [C11H24N2SiBr2]+ 370.0070, found 370.0065. Anal. Calcd for C11H24N2SiBr2: C, 35.49; H, 6.50; N, 7.53. Found: C, 35.34; H, 6.58; N, 7.70.
380 Organometallics, Vol. 28, No. 1, 2009
Notes
Scheme 3. Reaction of 5 with AdN3 and MesN3
the dibromosilanes were reduced with KC8 in dimethoxyethane, the silylenes 5, 6, and 7 were obtained.19,20 The silylenes were purified by careful distillation at reduced pressure. Silylene 5 shows reactivity similar to that of silylenes 1-4. Treatment of 2 equiv of adamantyl azide with 5 in hexane at -78 °C was accompanied by the evolution of N2 and yielded the white solid 8. A similar reaction between 2 equiv of 5 and 1 equiv of mesityl azide gives the azadisilacyclopropane derivative 9 quantitatively by NMR (Scheme 3). Formation of 8 and 9 is accompanied by a large upfield shift in the 29Si NMR spectrum relative to the free silylene, from δ +140 ppm for 5 to ca. δ -52 ppm for 8 and δ -33 ppm for 9.
Figure 2. Thermal ellipsoid (40%) plot of 8 with H atoms omitted for clarity. Selected bond distances (Å): Si-N(1) 1.7288(15), Si-N(2) 1.6934(17), Si-N(3) 1.7380(15), Si-N(6) 1.7384(16), N(3)-N(4) 1.383(2), N(4)-N(5) 1.267(2), N(5)-N(6) 1.383(2). Selected bond angles (deg): N(1)-Si-N(2) 96.59(8), N(3)-SiN(6) 85.99(7). Selected torsion angles (deg): N(1)-C(5)-C(10)N(2) 20.9(2). Planarity of C(5) (sum of angles) (deg): 343.34.
Single-crystal X-ray diffraction studies confirmed 8 to be a spirocyclic silatetrazoline, consisting of a central silicon atom coordinated to four separate nitrogen atoms (Figure 2). The SiN4 core is comprised of two planar, orthogonal rings fused at the central silicon atom. The azadisilacyclopropane product 9 (Figure 3) provided the first crystallographic evidence for a compound of its type. The two silylene units in 9 are of the same enantiomer (the R-silylene), possibly indicating spontaneous resolution of the racemic mixture of 5, although inspection of models does not indicate a strong steric reason for this preference. In addition, the 1H NMR of the mother liquor does not show the detectable presence of the “meso” isomer.21 (19) Synthesis of rac-1,3,4-tri-tert-butyl-1,3-diaza-2-silacyclopentane2-ylide (5): In a 250 mL Schlenk flask was placed potassium graphite (8.41 g, 62.3 mmol). To this flask was added 150 mL of DME. Then, rac-2,2dibromo-1,3,4-tri-tert-butyl-1,3,2-diazasilolidine (5a) (8.60 g, 20.76 mmol) dissolved in 50 mL of DME was added dropwise. The reaction was monitored by 1H NMR until all the starting material disappeared (∼3 h). The reaction mixture was filtered and the solvent removed, resulting in a pale yellow oil. The yellow oil was vacuum distilled (95 °C, 1 Torr) to give a very pale yellow liquid. Yield: 2.78 g, 52.7%. 1H NMR (C6D6): δ 0.90 (s, 9H); 1.27 (s, 9H); 1.38 (s, 9H); 3.00 (d, 1H, J ) 10.5 Hz); 3.14 (d, 1H, J ) 7.2 Hz); 3.22 (dd, 1H, J ) 10.5, 7.2 Hz). 13C NMR (C6D6): δ 29.04; 32.01; 35.21; 35.48; 49.74; 53.54; 54.99; 67.12. 29Si{1H} NMR (C6D6): δ +140.59. High-resolution mass spectrometry: calcd for [C14H30N2Si]+ 254.2173, found 254.2170. Anal. Calcd for C14H30N2Si: C, 66.07; H, 11.88; N, 11.01. Found: C, 66.12; H, 12.09; N, 10.80. (20) 1,3-Di-tert-butyl-4,4-dimethyl-1,3-diaza-2-silacyclopentane-2ylide (6): Yield: 3.05 g, 62.8%. 1H NMR (C6D6): δ 1.26 (s, 9H); 1.27 (s, 6H); 1.43 (s, 9H); 2.96 (s, 2H). 13C NMR (C6D6): δ 30.77; 31.69; 36.35; 52.56; 55.09; 63.10; 65.09. 29Si{1H} NMR (C6D6): δ +130.47. Highresolution mass spectrometry: calcd for [C12H26N2Si]+ 226.1860, found 226.1860. rac-1,3-Di-tert-butyl-4-methyl-1,3-diaza-2-silacyclopentane-2ylide (7): Yield: 1.93 g, 52.0%. 1H NMR (C6D6): δ 1.10 (d, 3H, J ) 6.8 Hz); 1.26 (s, 9H); 1.31 (s, 9H); 2.63 (d, 1H, J ) 8.9 Hz); 3.34-3.44 (m, 2H). 13C NMR (C6D6): δ 26.16; 31.93; 34.14; 52.68; 53.55; 53.95; 55.43. 29 Si{1H} NMR (C6D6): δ +121.43. High-resolution mass spectrometry: calcd for [C11H24N2Si]+ 212.1704, found 212.1697. Anal. Calcd for C11H24N2Si: C, 62.20; H, 11.39; N, 13.19. Found: C, 62.11; H, 11.54; N, 13.47.
Figure 3. Thermal ellipsoid (40%) plot of 9 with H atoms omitted for clarity. Selected bond distances (Å): Si(1)-Si(2) 2.2286(6), Si(1)-N(3) 1.7757(12), Si(2)-N(3) 1.7913(12). Selected bond angles (deg): N(3)-Si(1)-Si(2) 51.65(4), N(3)-Si(2)-Si(1) 51.02(4), Si(1)-N(3)-Si(2) 77.33(5). Selected dihedral angles (deg): N(1)-Si(1)-Si(2)-N(5) 24.79(9), N(2)-Si(1)-Si(2)-N(4) 19.86(9).
The Si-N(silylene) bond distances in 8 are slightly longer than in the structural analogue 2 (Si-N 1.719(3) vs Si-N(1) 1.7288(15), Si-N(2) 1.6934(17) Å, respectively),10 while the Si-N(azo) distances are very similar to each other (Si-N(3) 1.7380(15) and Si-N(6) 1.7384(16) Å). The C2N2Si ring (21) Compound 9 has four stereogenic centers, which lead to 16 possible enantiomers (six are duplicates). Of the 10 sterographically unique possibilities, seven of them are sterically infeasible, as they contain tBu groups, which conflict with the mesityl moiety. The three remaining enantiomers are RSSR, SRRS, and RSRS (the meso compound). The crystal of 9 represents the RSSR isomer. Presumably it is mixed with SRRS, as they are mirror images.
Notes
Organometallics, Vol. 28, No. 1, 2009 381 Scheme 4. Reaction of 5 with tBuOH, tBuCl, CHCl3, and PhCH2Cl
Figure 4. Thermal ellipsoid (40%) plot of 10 with H atoms omitted for clarity. Selected bond distances (Å): Si(1)-N(1) 1.7237(13), Si(1)-N(2) 1.7017(15), Si(1)-N(3) 1.7472(16), Si(1)-N(6) 1.7461(15), N(3)-N(4) 1.3869(16), N(4)-N(5) 1.2668(17), N(5)-N(6) 1.3813(17). Selected bond angles (deg): N(1)-Si(1)-N(2) 95.57(6), N(3)-Si(1)-N(6) 85.98(7), C(6)C(5)-C(7) 108.52(11).
becomes somewhat planar due to the steric repulsion of the adamantyl groups, with the sum of angles around C(5) (343.34°) being a good indicator of this planarity. The N(3)-N(4) and N(5)-N(6) distances (1.383(2) Å) are typical of N-N single bonds, while the N(4)-N(5) distance is much shorter (1.267(2) Å), indicative of a NdN bond. The N-Si-N bond angles of the SiN4 core are in the range 86.0-121.2° and thus significantly distorted from a regular tetrahedron. Silylene 6 also reacts with adamantyl azide to yield the silatetrazoline 10 (Figure 4), which is analogous to compound 8 (Scheme 3). Crystals suitable for crystallographic studies were grown from a concentrated solution in tetrahydrofuran. Silylene 5 reacts with tert-butanol in hexane at -78 °C via an expected insertion into the O-H bond to afford the corresponding tert-butoxyhydrosilane 11 in 78% isolated yield (Scheme 4). The 29Si NMR shows the characteristic upfield shift from divalent silicon (δ +140 ppm) to tetravalent silicon (δ -43 ppm, JSi-H ) 240 Hz). Similar reactions with chlorocarbons are also observed. The reaction of 5 with tBuCl forms the 1:1 adduct 12 (29Si δ ) +2.24 ppm), as expected with a probable preference for the “trans” isomer. The reactions of 1 and 2 with CHCl3 and PhCH2Cl give the 1:2 disilane adducts preferentially. However, the reactions of 5 with CHCl3 and PhCH2Cl give the 1:1 adducts 13 and 14 exclusively, which were obtained in 97% and 98% yield (by NMR), respectively (Scheme 4). This preference may be due to the increased steric bulk of 5, which does not allow the insertion of a second silylene molecule to take place.
Although silylene 2 reacts with 2,3-dimethylbutadiene to afford a 1-silacyclopent-3-ene compound, the new silylene 5 catalyzes the polymerization of 2,3-dimethylbutadiene, as does the silylene 4. This polymerization takes place even at very low concentrations of silylene.
Conclusion Three new N-heterocyclic saturated silylenes, 5, 6, and 7, which are monomeric in their pure state, have been synthesized. Reactions of 5 are similar to those for the unsubstituted analogue 2, but unlike 2, 5 does not undergo dimerizing additions to alkyl chlorides. Further investigations regarding the reactivity of 5, 6, and 7 as well as theoretical calculations may shed light on the differences between 2 and its substituted analogues 5, 6, and 7.
Acknowledgment. The authors thank Dr. Ilia Guzei for assistance with X-ray crystallographic studies and the National Science Foundation for support of this research. Supporting Information Available: Text giving general experimental procedures, details of the preparation of 5-14 and their synthetic precursors, and spectroscopic information and CIF files giving X-ray structural information on 8, 9, and 10. This material is available free of charge via the Internet at http://pubs.acs.org. OM8006147
