Organometallics 2010, 29, 987–990 DOI: 10.1021/om901034w
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Unusual [3þ1] Cycloaddition of a Stable Silylene with a 2,3-Diazabuta-1,3-diene versus [4þ1] Cycloaddition toward a Buta-1,3-diene Yun Xiong, Shenglai Yao, and Matthias Driess* Institute of Chemistry: Metalorganic and Inorganic Materials, Technische Universit€ at Berlin, Sekr. C2, Strasse des 17. Juni 135, 10623 Berlin, Germany Received December 1, 2009
Cycloaddition reactions of the thermally stable N-heterocyclic silylene LSi: 1 {L=CH[(CdCH2)CMe][N(Ar)]2], Ar=2,6-iPr2C6H3} with acetone azine (1,1,4,4-tetramethyl-2,3-diazabuta-1,3-diene) and buta-1,3-diene derivatives have been probed. Unexpectedly, acetone azine undergoes a unique [3þ1] cycloaddition to give the 1-sila-2,3-diazacyclobutane 2 and its 1-sila-2,3-diazacyclobutane isomer 3. The latter rearranges further to decrease ring strain, affording the corresponding 1-sila-4, 5-diazacyclohex-3-ene 4. In contrast, reaction of 1 with isoelectronic 2,3-dimethylbuta-1,3-diene furnishes the expected [4þ1] cycloaddition product silacyclopentane 5. The new compounds 2-5 were spectroscopically characterized, including single-crystal X-ray analyses of 2, 4, and 5.
Introduction Cycloaddition reactions involving unsaturated substrates play an important role in organic and organometallic synthesis from fundamental as well as applied points of view.1 Carbenes, and their heavier homologues, are unsaturated species that are indispensable building blocks in cycloaddition reactions. They can undergo [nþ1] cycloaddition reactions (n =2, 3, 4) with n-oligoatomic π-systems, among which [2þ1] and [4þ1] cycloadditions are the most prevalent.2 In contrast, [3þ1] cycloaddition reactions are scarce. The only example involving a heavier group 14 element is the addition of GeCl2 3 dioxane to the heterocumulene RNdSdC(CF3)2 (R=1-adamantyl).3 Recently we have described the synthesis and striking reactivity of the stable ylide-like N-heterocyclic silylene LSi: 1 {L= CH[(CdCH2)CMe][N(Ar)]2], Ar=2,6-iPr2C6H3} (Scheme 1).4 As expected for a carbene homologue, silylene 1 readily undergoes [2þ1] cycloaddition with acetylene to give silacycloprop-2ene A5a and [4þ1] cycloaddition reactions with benzylideneacetone affording the corresponding silaoxacyclopent-3-ene B (Scheme 1).5b Interestingly, reaction of 1 with benzophenone *To whom correspondence should be addressed. Phone: þ49(0)30314-22265. Fax: þ49(0)30-314-29732. E-mail: matthias.driess@ tu-berlin.de. Web site: http://www.driess.tu-berlin.de. (1) (a) Huisgen, R. Angew. Chem., Int. Ed. 1968, 7, 321. (b) Xu, Y.-J.; Zhang, Y.-F.; Li, J.-Q. J. Phys. Chem. B 2006, 110, 13931. (2) See for examples: (a) Takeda, N.; Tokitoh, N. Synlett 2007, 16, 2483. (b) Gehrhus, B.; Hitchcock, P.; B. J. Organomet. Chem. 2004, 689, 1350. (c) Clendenning, S. B.; Gehrhus, B.; Hitchcock, P. B.; Moser, D. F.; Nixon, J. F.; West, R. Dalton Trans. 2002, 4, 484. (3) May, A.; Roesky, H. W.; Irmer, R. H.; Freitag, S.; Sheldrick, G. M. Organometallics 1992, 11, 15. (4) Driess, M.; Yao, S.; Brym, M.; van W€ ullen, C.; Lenz, D. J. Am. Chem. Soc. 2006, 128, 9628. (5) (a) Yao, S.; van W€ ullen, C.; Sun, X.-Y.; Driess, M. Angew. Chem., Int. Ed. 2008, 47, 3250. (b) Xiong, Y.; Yao, S.; Driess, M. Chem.;Eur. J. 2009, 15, 5545. r 2010 American Chemical Society
Scheme 1. Silylene 1 and Several [nþ1] Cycloadducts Synthesized Thereof
furnishes the initial dearomatized [4þ1] cycloaddition product C at low temperature, which isomerizes to form the rearomatized species D at ambient temperature.5b Akin to C, the dearomatized [4þ1] cycloaddition product E was reported very recently from the reaction of 1 and diphenylhydrazone (Scheme 1).6 The latter reactions prompted us to probe the reactivity of 1 toward other π-conjugated compounds containing heteroatoms. Herein we report the remarkable [3þ1] cycloaddition reaction of silylene 1 with acetone azine (1,1,4, 4-tetramethyl-2,3-diazabuta-1,3-diene), which does not proceed via the expected [4þ1] cycloaddition pathway.
Results and Discussion Treatment of an equimolar amount of 1 with acetone azine in n-hexane at 0 °C leads to the unusual zwitterionic [3þ1] cycloaddition product 2 (Scheme 2). This is in contrast to the (6) Jana, A.; Roesky, H. W.; Schulzke, C.; Samuel, P. P. Organometallics 2009, 28, 6574. Published on Web 01/19/2010
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Scheme 2. Conversion of 1 with Acetone Azine to Give 2, 3, and 4
common reactivity of acetone azine, which generally occurs by 1,2-addition,7a 1,4-addition,7b or intramolecular coupled 1,3 and 2,4 crisscross cycloaddition.7c,d To date, all known [3þ1] cycloaddition reactions involve cumulenes or related unsaturated species,3,8 and most reaction products were either only postulated as intermediates or merely characterized by 1H NMR and/or IR spectroscopy; only two products could be characterized structurally by X-ray diffraction analysis.3,8j To the best of our knowledge, there has been no report of a [3þ1] cycloaddition reaction involving a silylene and a heteroatom π-conjugated substrate. Although the mechanism for the formation of the striking product 2 is unknown, we propose that its formation occurs first by nucleophilic attack of a nitrogen atom in the acetone azine on the empty 3p orbital of the divalent Si atom in 1. The latter donor-acceptor intermediate resembles the isolable silylene-carbene adducts that have been described recently.9 Because of steric congestion, the intermediate subsequently converts into the ylide-like [3þ1] cycloaddition product 2 instead of forming the hypothetical [4þ1] cycloadduct (Scheme 2). Owing to the tendency to undergo isomerization in solution, compound 2 could be isolated only in low yield in the form of single crystals suitable for X-ray diffraction analysis. The isomerization of 2 gives the 1-sila-2,3-diazacyclobutane 3, involving a C-H activation. Because of the ring strain in the four-membered SiN2C ring in 3, the latter compound is also labile and after two days at room temperature isomerizes to generate solely the 1-sila-4,5-diazacyclohex-3-ene 4 as the final product (Scheme 2). Because of its lability, 3 could not be separated from 4, but its constitution can be unequivocally deduced from NMR spectroscopic data (see Experimental Section). However, compound 4 is (7) (a) Uhl, W.; Molter, J.; Neum€ uller, B.; Schmock, F. Z. Anorg. Allg. Chem. 2001, 627, 909. (b) Talma, A. G.; Goorhuis, J. G.; Kollogg, R. M. J. Org. Chem. 1980, 45, 2544. (c) Schantl, J. G.; Gstach, H.; Hebeisen, P.; Lanznaster, N. Tetrahedron 1985, 41, 5525. (d) Bailey, J. R.; McPherson, A. T. J. Am. Chem. Soc. 1917, 39, 1322. (8) (a) Thompson, Q. E. J. Am. Chem. Soc. 1961, 83, 845. (b) Deyrup, J. A. Tetrahedron Lett. 1971, 24, 2191. (c) Burger, K.; Fehn, J. Angew. Chem., Int. Ed. Engl. 1972, 11, 47. (d) Burger, K.; Fehn, J.; Mueller, E. Chem. Ber. 1973, 106, 1. (e) Burger, K.; Manz, F.; Braun, A. Synthesis 1975, 250. (f) Deyrup, J. A.; Kuta, G. S. Chem. Commun. 1975, 34. (g) Moderhack, D.; Lorke, M. Chem. Commun. 1977, 831. (h) Riviere, P.; Satge, J.; Castel, A.; Cazes, A. J. Organomet. Chem. 1979, 177, 171. (i) Burger, K.; Marschke, G.; Manz, F. J. Heterocycl. Chem. 1982, 19, 1315. (j) Schwarz, D. E.; Rauchfuss, T. B. Chem. Commun. 2000, 1123. (k) Choudary, B. M.; Reddy, C. R.V.; Prakash, B. V.; Kantam, M. L.; Sreedhar, B. Chem. Commun. 2003, 754. (9) (a) Xiong, Y.; Yao, S.; Driess, M. J. Am. Chem. Soc. 2009, 131, 7562. (b) Yao, S.; Xiong, Y.; Driess, M. Chem.-Eur. J. 2010, in press; DOI: 10.1002/chem.2000902467.
Figure 1. Molecular structure of 2. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Si1-N1 1.719(3), Si1-N2 1.797(4), Si1-C19 1.883(5), N2-C16 1.308(6), N3-C19 1.514(6), N2-N3 1.379(5), N1-Si1-N2 114.1(1), N2-Si1-C19 75.5(2), N1-Si1-N10 104.0(2), N1(N10 )-Si1-C19 123.2(1).
stable and can be easily isolated as colorless crystals in 71% yield. Compounds 2, 3, and 4 have been identified by 1H, 13C, and 29Si NMR spectroscopy. Each 1H NMR spectrum exhibits three characteristic singlets for the γ-CH proton and the two chemically inequivalent exocyclic methylene H atoms of the C3N2 backbone of the silylene moiety, similar to the features of 1.4 Moreover, compounds 3 and 4 exhibit additional singlets in the range of 3 to 6 ppm (three singlets for NH, NCMe(dCH2) in 3 and one singlet for NH in 4). Accordingly, in the 13C{H} NMR spectra each compound displays two characteristic resonances in the range of δ = 80 to 110 ppm for the exocyclic methylene and γ-13C nuclei of the silylene moiety. Compound 3 shows an additional signal at δ = 82.2 ppm, resulting from the NC(dCH2)Me group in the former acetone azine moiety. The compounds 2, 3, and 4 give slightly different 29Si NMR resonances at δ = -35, -44, and -25 ppm, respectively. The spectroscopically deduced structures of 2 and 4 have been confirmed by single-crystal X-ray diffraction analyses. Compound 2 crystallizes in the orthorhombic space group Pnma (Figure 1). It consists of a spirobicyclic skeleton with the two SiN2C3 and SiN2C rings perpendicular to each other. Thus the Si atom is bonded to one carbon and three nitrogen atoms in a distorted tetrahedral fashion. The short N2-C16 bond distance of 1.308(6) A˚ is consistent with a double bond, whereas the longer distance of 1.514(6) A˚ for the N3-C19 bond clearly indicates a single bond. The length of the new Si1-C19 bond (1.883(5) A˚) falls in the range of typical values for Si-C single bonds. However, the Si1-N2 bond distance of 1.797(4) A˚ is slightly longer than those in the precursor 1 (1.734(2) and 1.735(2) A˚). Compound 4 crystallizes in the triclinic space group P1 (Figure 2). Both six-membered SiN2C3 rings in 4 are strongly puckered, and the silicon atom possesses a tetrahedral coordination environment with bond angles from 101.2(1)° to 116.2(1)°. The short C31-N3 distance of 1.289(3) A˚ versus that of C16-N2 at 1.308(6) A˚ in 2 is consistent with
Article
Figure 2. Molecular structure of 4. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except for those at C1 and N4) are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Si1-N1 1.752(2), Si1-N2 1.749(2), Si1-C30 1.912(3), Si1-C33 1.892(2), N3-N4 1.415(3), N3-C31 1.289(3), N4-C33 1.498(3), N1-Si1-N2 104.1(1), C30-Si1-C33 101.2(1), N1-Si1-C30 110.4(1), N2-Si1-C33 115.7(1), N2-Si1-C30 109.3(1), N1-Si1-C33 116.2(1). Scheme 3. Reactivity of 1 toward 2,3-Dimethylbuta-1,3-diene and 1,1,4,4-Tetramethylbuta-1,3-diene
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Figure 3. Molecular structure of 5. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except for those at C1) are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Si1-N1 1.738(2), Si1-N2 1.743(2), Si1-C33 1.868(3), Si1-C30 1.871(3), C31-C32 1.334(3), C30-C31 1.506(3), C32-C33 1.519(3), N1-Si1-N2 104.1(1), N1-Si1C33 116.2(1), N2-Si1-C33 112.3(1), N1-Si1-C30 115.0(1), N2-Si1-C30 114.8(1), C33-Si1-C30 94.9(1).
CdC double bond, whereas the relatively long C32-C33 (1.519(3) A˚) and C30-C31 (1.506(3) A˚) distances represent C-C single bonds. The XSiY bond angles (X, Y = N1, N2, C30, and C33) of the SiC2N2 core are from 94.9° to 116.2° and thus significantly distorted from a regular tetrahedron.
Conclusions
a CdN double bond, whereas the N4-C33 distance of 1.498(3) A˚ is indicative of a C-N single bond. The new Si1-C30 (1.912(3) A˚) and Si1-C33 bonds (1.892(2) A˚) are slightly longer than the Si1-C19 distance in 2 (1.883(5) A˚). Interestingly, 1,1,4,4-tetramethylbuta-1,3-diene which is isoelectronic with the acetone azine, does not react with 1 even at 50 °C after several days (Scheme 3). However, 1 reacts with the less hindered 2,3-dimethylbuta-1,3-diene in hexane even below room temperature, yielding the expected [4þ1] cycloadduct 5. Compound 5 was fully characterized by elemental analysis, EI-MS, and 1H, 13C, and 29Si NMR spectroscopy. Its 29Si NMR spectrum exhibits a resonance signal at δ = 0.90 ppm, which is deshielded compared with those observed for 2, 3, and 4 (-35, -44, and -25 ppm). An X-ray single-crystal diffraction analysis revealed that 5 consists of a spirobicyclic core with a central silicon atom coordinated to two nitrogen and two carbon atoms (Figure 3). The Si-N bond distances (1.738(2) and 1.743(2) A˚) in 5 are almost identical to those in its precursor 1 (1.734(2) and 1.735(2) A˚), and the Si-C distances (1.868(3) and 1.871(3) A˚) are in the common range for Si-C single bonds. The short C32-C31 distance of 1.334(3) A˚ is indicative of a
In summary, the reaction of the stable N-heterocyclic silylene 1 with the heteroatom π-conjugated acetone azine initially affords the unusual [3þ1] cycloadduct 2, the first zwitterionic 1-sila-2,3-diazacyclobutane, which subsequently undergoes isomerization in solution to give the 1-sila-2,3diazacyclobutane 3. The latter isomerizes further in solution at room temperature by ring expansion and release of strain energy of the four-membered SiCN2 ring in 3, yielding the 1-sila-4,5-diazacyclohex-3-ene 4. In contrast, the isoelectronic 1,1,4,4-tetramethylbuta-1,3-diene does not react with 1 even at 50 °C. However, employing the less bulky 2,3dimethylbuta-1,3-diene furnishes the expected [4þ1] cycloadduct 5, which can be isolated in 90% yield.
Experimental Section General Considerations. All experiments and manipulations were carried out under dry oxygen-free nitrogen using standard Schlenk techniques or in an MBraun inert atmosphere drybox containing an atmosphere of purified nitrogen. Solvents were dried by standard methods and freshly distilled prior to use. The starting material silylene 1 was prepared according to a literature procedure.4 The NMR spectra were recorded with Bruker spectrometers ARX200 and AV400 and with residual solvent signals as internal reference (1H and 13C{H}) or with an external reference (SiMe4 for 29Si). Abbreviations: s = singlet; d = doublet; t = triplet; sept = septet; m = multiplet; br = broad. Single-Crystal X-ray Structure Determination. Crystals were each mounted on a glass capillary in perfluorinated oil and measured in a cold N2 flow. The data of 2, 4, and 5 were collected on an Oxford Diffraction Xcalibur S Sapphire at 150 K (Mo KR
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radiation, λ=0.71073 A˚). The structures were solved by direct methods and refined on F2 with the SHELX-9710 software package. The positions of the H atoms (except that on N4 in 4) were calculated and considered isotropically according to a riding model. The 2,6-iPr2C6H3 group in 2 is distorted over two orientations in a population ratio of 0.47:0.53 and was refined with distance restraints and restraints for the anisotropic displacement parameters. 2: orthorhombic, space group Pnma, a = 16.8960(9) A˚, b = 20.2468(10) A˚, c=9.5609(4) A˚, V=3270.7(3) A˚3, Z=4, Fcalc = 1.133 Mg/m3, μ(Mo KR) = 0.101 mm-1, 14 604 collected reflections, 2967 crystallographically independent reflections [Rint = 0.1133], 1659 reflections with I > 2σ(I), θmax = 25°, R(Fo) = 0.0769 (I > 2σ(I)), wR(Fo2) = 0.1654 (all data), 305 refined parameters. 4: triclinic, space group P1, a = 8.9196(4) A˚, b = 12.2620(5) A˚, c=15.7788(7) A˚, R=79.254(4)°, β=77.307(4)°, γ=85.832(4)°, V = 1653.16(12) A˚3, Z = 2, Fcalc = 1.119 Mg/m3, μ(Mo KR) = 0.100 mm-1, 12 823 collected reflections, 5816 crystallographically independent reflections [Rint=0.0344], 4150 reflections with I>2σ(I), θmax=25°, R(Fo)=0.0578 (I > 2σ(I)), wR(Fo2)=0.1426 (all data), 377 refined parameters. 5: monoclinic, space group P21/c, a = 8.9102(6) A˚, b = 20.5824(10) A˚, c =17.2568(9) A˚, β=90.286(5)°, V=3164.7(3) A˚3, Z = 4, Fcalc = 1.106 Mg/m3, μ(Mo KR) = 0.099 mm-1, 28 018 collected reflections, 5544 crystallographically independent reflections [Rint = 0.1189], 2759 reflections with I > 2σ(I), θmax = 25°, R(Fo)=0.0542 (I>2σ(I)), wR(Fo2)=0.0977 (all data), 254 refined parameters. Syntheses. 2: Acetone azine (0.16 mL, d = 0.84 g/mL, 1.19 mmol) was added to a solution of silylene 1 (0.53 g, 1.19 mmol) in hexane (10 mL) at 0 °C. The reaction mixture was then stored at 4 °C. After 24 h about 40% of 1 has been converted into 2 according to 1H NMR spectroscopy. At this stage, compound 2 could be obtained only in very low yield (ca. 10% of its amount in reaction solutions; 1 H NMR) as colorless crystals from concentrated reaction solutions at -20 °C. 1H NMR (200.13 MHz, [D6]benzene, 25 °C): δ 0.64-1.58 (m, 27 H; CHMe2), 1.20(s, 3 H, CMe2), 1.23 (s, 3 H, CMe2), 1.38 (s, 3 H; NCMe), 1.96 (m, 3 H; NCMe2), 2.11 (m, 3 H; NCMe2), 3.23 (m, 1 H; CHMe2), 3.34 (s, 1 H; NCCH2), 3.63 (m, 3 H; CHMe2), 3.93 (s, 1 H; NCCH2), 5.27 (s, 1 H; γ-H), 7.01-7.20 ppm (m, br, 6 H; 2,6-iPr2C6H3). 13C{1H} NMR (100.61 MHz, [D6]benzene, 25 °C): δ 20.0-30.1 (NCMe, CHMe2, CMe2, NC(CH2)Me), 63.5 (CMe2), 88.5 (NCCH2), 104.4 (γ-C), 124.0-147.8 ppm (NCMe, NCCH2, 2,6-iPr2C6H3, NdCMe2). 29 Si{1H} NMR (79.49 MHz, [D6]benzene, 25 °C): δ -35.5 ppm (s). EI-MS (70 eV, m/z): 556 (10) [Mþ], 541 (100) [Mþ - Me]. Anal. Calcd (%) for C35H52N4Si: C 75.49, H 9.41, N 10.06. Found: C 75.40, H 9.38, N 9.98. 3 and 4: Acetone azine (0.26 mL, d = 0.84 g/mL, 1.93 mmol) was added to a solution of silylene 1 (0.86 g, 1.93 mmol) in hexane (15 mL) at 0 °C. The reaction mixture was allowed to warm to room temperature slowly. The complete conversion of 1 can be reached within 4 days at room temperature, affording a mixture of 3 and 4 in a molar ratio of 1:5. Complete conversion of 3 to 4 can be achieved after standing of the solution for one additional day. Compound 4 can be isolated by fractional crystallization of the reaction solution at 0 °C as colorless crystals. Yield: 0.76 g (1.37 mmol, 71%). Mp: 145 °C (dec). 1H NMR (200.13 MHz, [D6]benzene, 25 °C): δ 0.36 (s, 3 H; (10) Sheldrick, G. M. SHELX-97 Program for Crystal Structure Determination; Universit€at G€ottingen: Germany, 1997.
Xiong et al. SiCMe2), 0.63 (s, 3 H; SiCMe2), 1.21-1.47 (m, 24 H; CHMe2), 1.51 (s, 3 H; SiNCMe), 1.68, 1.97 (AB system, 2 H, SiCH2), 1.91 (s, 3 H, N-NdCMe), 3.27 (m, 2 H; CHMe2), 3.31 (s, 1 H; NCCH2), 3.66 (m, 1 H; CHMe2), 3.87 (m, 1 H; CHMe2), 3.98 (s, 1 H; NCCH2), 4.13 (s, br, NH), 5.42 (s, 1 H; γ-H), 6.98-7.22 ppm (m, br, 6 H; 2,6-iPr2C6H3). 13C{1H} NMR (100.61 MHz, [D6]benzene, 25 °C): δ 20.7, 22.7, 23.2, 23.6, 24.4, 24.5, 24.8, 25.5, 25.9, 26.4, 26.6, 26.7 (NCMe, CHMe2), 28.7, 28.8, 28.9, 29.1 (CHMe2), 43.5 (SiCMe2), 87.5 (NCCH2), 107.7 (γ-C), 124.1, 124.4, 124.8, 125.6, 138.6, 139.5, 142.7, 147.7, 147.8, 148.4, 148.6, 148.8, 149.4 ppm (NCMe, NCCH2, 2,6-iPr2C6H3, NdCMe2). 29Si{1H} NMR (79.49 MHz, [D6]benzene, 25 °C): δ -25.1 ppm (s). EI-MS (70 eV, m/z): 556 (10) [Mþ], 541 (100) [Mþ - Me], 513 (45) [Mþ - iPr]. Anal. Calcd (%) for C35H52N4Si: C 75.49, H 9.41, N 10.06. Found: C 75.45, H 9.37, N 9.88. The spectroscopic data for 3 can be assigned from NMR data of reaction mixtures of 3 and 4 after subtraction of the NMR signals for 4. Spectroscopic data for 3: 1H NMR (200.13 MHz, [D6]benzene, 25 °C): δ 0.52 (s, 3 H; SiCMe2), 0.54 (s, 3 H; SiCMe2), 1.13-1.52 (m, 24 H; CHMe2), 1.39 (s, 3 H; NCMe), 2.16 (s, 3 H, NdCMe), 3.05 (s, 1 H; dCH2), 3.24 (s, 1 H; dCH2), 3.60 (m, 4 H; CHMe2), 3.88 (s, 1 H; dCH2), 3.93 (s, 1 H; dCH2), 3.94 (s, br, 1 H; NH), 5.25 (s, 1 H; γ-H), 6.98-7.22 ppm (m, br, 6 H; 2,6-iPr2C6H3). 13C{1H} NMR (100.61 MHz, [D6]benzene, 25 °C): δ 20.1, 21.8, 23.0, 23.4, 24.2, 24.5, 24.9, 25.0, 26.0, 26.3, 26.9, 27.2 (NCMe, CHMe2), 28.5, 28.6, 29.4, 29.5 (CHMe2), 57.6 (SiCMe2), 82.2 (NC(dCH2)Me), 86.8 (NCCH2), 104.2 (γ-C), 124.1, 124.5, 125.2, 125.4, 137.5, 137.8, 141.2, 141.7, 142.3, 142.7, 147.6, 148.5, 148.9, 149.1 ppm (NCMe, NCCH2, 2,6-iPr2C6H3, NdCMe2). 29Si{1H} NMR (79.49 MHz, [D6]benzene, 25 °C): δ -44.3 ppm (s). 5: 2,3-Dimethylbuta-1,3-diene (0.14 mL, d = 0.73 g/mL, 1.24 mmol) was added to a solution of silylene 1 (0.55 g, 1.24 mmol) in hexane (10 mL) at -30 °C. The reaction mixture was allowed to warm to room temperature. After 4 h stirring at that temperature the solution was concentrated to about 5 mL and cooled to -20 °C. Compound 5 was obtained as colorless crystals. Yield: 0.59 g (1.12 mmol, 90%). Mp: 163 °C (dec). 1H NMR (200.13 MHz, [D6]benzene, 25 °C): δ 1.22 (d, 3J(H,H) = 7.0 Hz, 6 H; CHMe2), 1.23 (d, 3J (H,H) = 7.0 Hz, 6 H; CHMe2), 1.31 (s, 6 H; Me), 1.32 (d, 3J(H,H) = 7.0 Hz, 6 H; CHMe2), 1.42 (d, 3J(H,H) = 7.0 Hz, 6 H; CHMe2), 1.51 (s, 3 H; NCMe), 3.32 (s, 1 H; NCCH2), 3.58 (m, 4 H; CHMe2), 3.97 (s, 1 H; NCCH2), 5.43 (s, 1 H; γ-H), 7.02-7.16 ppm (m, br, 6 H; 2,6-iPr2C6H3). 13 C{1H} NMR (100.61 MHz, [D6]benzene, 25 °C): δ 18.6, 22.2, 22.5, 24.0, 24.3, 24.8, 25.1, 26.4, 28.6, 28.7 (CHMe2, NCMe, MeCdCMe-), 86.2 (NCCH2), 106.5 (γ-C), 112.9, 124.3, 124.9, 130.4, 136.7, 138.1, 141.9, 148.4, 148.5, 148.8 ppm (NCMe, NCCH2, 2,6-iPr2C6H3, -MeCdCMe-). 29Si{1H} NMR (79.49 MHz, [D6]benzene, 25 °C): δ 0.90 ppm (s). EI-MS (70 eV, m/z): 526 (42) [Mþ], 511 (100) [Mþ - Me], 483 (60) [Mþ - iPr]. Anal. Calcd (%) for C35H50N2Si: C 79.79, H 9.56, N 5.32. Found: C 79.65, H 9.59, N 5.22.
Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie. We thank Dr. Matt Asay for critical reading of the manuscript. Supporting Information Available: CIF files giving crystallographic data for 2, 4, and 5. This material is available free of charge via the Internet at http://pubs.acs.org.
