Stable Tetrakis(trialkylsilyl)disilenes; Synthesis, X‐Ray Structures, and UV/VIS Spectra

Abstract

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...

The Chemistry of Disilyne with a Genuine Si–Si Triple Bond: Synthesis, Structure, and Reactivity

“…The last signal at 89.9 ppm is assignable to triply-bonded silicon atoms. The resonance of the sp-hybridized silicon atoms is apparently shifted to a higher field when compared with that of silyl-substituted disilenes (142.1 to 154.5 ppm), 24 as was observed in the case of the 13 C NMR chemical shifts of silyl-substituted alkenes (188 to 197 ppm) and alkynes (112 to 114 ppm). 25 The mass spectrum shows a clear parent ion peak at m=z ¼ 834, and reasonable fragmentation peaks for disilyne.…”

Structure and Reactions of an Isolable Ge=Si Doubly Bonded Compound, Tetra(t-butyldimethylsilyl)germasilene

“…The bond dissociation energy and the stability of a double bond of heaver group-14 elements may increase by introducing trialkylsilyl substituents as first predicted theoretically by Apeloig et al [39] and elucidated experimentally by us [40][41][42]. We wish herein to report the comparison of structure among a series of acyclic E=E'-type doubly bonded compounds 8a-8c (8a, E, E' = Si; 8b, E = Si, E' = Ge; 8c, E, E' = Ge), which bear the same t-butyldimethylsilyl substituents at the unsaturated atoms (Chart 2).…”

“…We wish herein to report the comparison of structure among a series of acyclic E=E'-type doubly bonded compounds 8a-8c (8a, E, E' = Si; 8b, E = Si, E' = Ge; 8c, E, E' = Ge), which bear the same t-butyldimethylsilyl substituents at the unsaturated atoms (Chart 2). For this purpose, a novel germasilene 8b was synthesized using a germa-metallation method [43] and X-ray structures of 8b and 8c were analyzed; the synthesis of 8a [40] and 8c [41] and the X-ray analysis of 8a [40] had been previously reported. Several fundamental reactions of germasilene 8b are discussed in comparison to those of known germasilenes.…”

Fused Tricyclic Disilenes with Highly Strained SiSi Double Bonds: Addition of a SiSi Single Bond to a SiSi Double Bond

“…A similar tendency is observed between highly distorted (tBu 2 MeSi) 2 Si = Si(SiMetBu 2 ) 2 (7; t = 54.58, l max = 612 nm, e = 1300 mol À1 L cm À1 ) [10] and less distorted (tBuMe 2 Si) 2 Si = Si(SiMe 2 tBu) 2 (t = 8.98, l max = 420 nm, e = 2800 mol À1 L cm À1 ). [12] The NMR resonances for the unsaturated 29 Si nuclei, d(Si u ), of these disilenes (d = 135.9 ppm (trans-2), 128.1 ppm (cis-2)) are considerably low-field-shifted compared with that for 8 (d = 87.4 ppm). [13] Because d(Si u ) values in disilenes are known to be proportional to the reciprocal of the energy difference between the s and p* orbitals of the Si = Si bond, [13] it is reasonable that the order of d(Si u ), trans-2 > cis-2 > 8, is the same as that of l max of the p-p* transition bands among these disilenes.…”