The zwitterion 7 is probably the intermediate in this rearrangement; 7 can be considered as a (J complex of an intramolecular electrophilic substitution of 4A. Migration of the proton to the carbanionic C atom bound to the boron atom in the six-membered ring gives 8.The ' H and the 13C N M R spectra show two sharp signals for the geminal trimethylsilyl groups in 3 only below -30 "C. These signals broaden a t higher temperatures and coalesce at 30 and 20 C. respectively. The barrier for the exchange was estimated to be 14 kcal mol-'. We propose the zwitterion 9 as an intermer Dur Dur Dur ""i I 9 + 2 1 -4 L 9 10diate. In 9 the negative charge is stabilized by TI-TI delocalization in a 1,3-diboraallyl anion and the positive charge of the vinyl cation1"] is stabilized by C-Si and C-B hyperconjugation. The formation of 4 can be readily explained by the addition of 2 to 9 and the cyclization of the resulting compound 10.
Experimental Procedure3: 2 (0.46 mL. 3.3 mmol) was added dropwise to 1 (1.53 g, 3.3 mmol) in pentane (20 mL) at -78 C. The reaction mixture was slowly warmed to -3O'C and concentrated under vacuum at this temperature until crystallization began. Compound 3 crystalliied at -30 'C overnight. The crystals were separated by filtration (D3 frit) and washed with pentane. 4 : 2 (1 .O mL. 7.24 mmol) was added dropwise to a solution of 1 (1.66 g, 3.62 mmol) in pentane (20 mL) at -78 -C. After the mixture had been allowed to warm to -10 C, it was concentrated and crystals of 4 were separated by filtration (D3 frit). 8: 4 was converted quantitatively into 8 by heating for 5 min at 143 "C references therein [2] Crystal structure analyses: colorless crystals of 3 and 4 were investigated on a four-circle diffractometer (CAD4, Enraf-Nonius) at -80°C with Cu,, radiation ( A = 354.178 pm). 3: C,,H,,B,Si,, crystal dimensions 0.3 x 0.3 x 0.5 mm, monoclinic. space group P2,ic, Z = 4. u = 895.5(2). 6 = 2088.4(4), c = 1901.2(4) pm, [j = 95.01(3)'. V = 3542.0 x lo-''' m', pCalcd =1.044 gcm-3; 4954 measured retlections in the range up to 20,,, =110" (w scans), 4446 of which were independent. 3341 reflections with Fo>3u(F,) were used for the further calculations after Lorentr and polarization correction; no absorption correction (13 =13.6cm-l). The structure was solved by direct methods and refined against the 4, data with full matrix. All hydrogen atoms were located by difference Fourier syntheses and were included in the calculation riding on their bonding partners and refined groupwise with isotropic temperature factors. Anisotropic temperature factors were used for all the remaining atoms except boron. R = 0.056, W R = 0.049 ( t t = l/u2(F,)), maximal residual electron density 0.33 e k ' . 4: C,,H,,B,Si,.crystal dimensions 0.6 x 0.6 x 0.3 mm, monoclinic. space group P2,Ic. Z = 4, u = 1226.0(2), h = 1361.3(3). c = 2545.3(5) pm. /$ =102.09(3)", V = 4153.8 x m3, pcalrd =1.047 gcm-'; 5860 measured reflections in the range up to 20,., =110" (w scans), 5219 of which were independent. 3760 reflections with Fo > 3u (F,) were used...