Erzeugung, nachweis und stabilisierung isomerer silaethene R2Si=C(SiR3) 2 mit jeweils sechs Me-und zwei Ph-substituenten R

Article abstract of DOI:10.1016/S0022-328X(96)06428-5

Bromotrisilylmethanes Me₂SiBr-CBrRR² (2b), PhMeSiBr-CBrRR¹ (3b, two diastereomers) and Me₂SiBr-CBrR¹₂ (4b) (R = SiMe₃, R¹ = SiMe₂Ph, R² = SiMePh₂) - sources of silaethenes Me₂Si=CRR² (2), PhMeSi=CRR¹ (3, two isomers), and Me₂Si=CR²₂ (4) - are formed by bromination of Me₂SiH-CBrRR² (2a), PhMeSiH-CBrRR¹ (3a, two diastereomers; bromination takes place stereospecifically), and Me₂SiH-CBrR²₂ (4a), which themselves are formed from CBr₄ by introducing three silyl groups step by step according to: ->C-Br + R′Li + ->SiCl → ->C-Si-< + R′Br + LiCl (R′ = ⁿBu, (Me₃Si)₂CH). The structure of one of the two diastereomeric compounds, 3a, has been solved by X-ray analysis. PhLi converts 2b-4b by Br/Li-exchange to lithium organyls Me₂SiBr-CLiRR² (2c). PhMeSiBr-CLiRR¹ (3b, two diastereomers), and Me₂SiBr-CLiR¹₂ (4c), which in Et₂O at - 78°C are in equilibrium with silaethenes 2, 3 and 4 in very low concentrations. The intermediacy of the silaethenes has been established chemically by trapping 2, 3 and 4 with 2,3-dimethyl-1,3-butadiene (DMB): formation of [4 + 2] cycloadducts 2d, 3d (two diastereomers) and 4d, of ene reaction products 2e, 3e (two diastereomers), and 4e as well as of [4 + 2] cycloadducts 2f (diastereomers), 3f (diastereomers), and 4f of the formed ene reaction products. The structure of one of the two diastereomeric compounds, 3d, has been solved by X-ray analysis. By warming the etheral solutions of 2c, 3c and 4c, [2 + 2] cycloadducts 2 X 2, 2 X 4 and 4 X 4 of silaethenes 2 and 4 are found. Considering the types and yields of the products of silaethenes - including Ph₂Si=CR₂ (1) - in the absence and the presence of DMB, the following is concluded: (i) silaethenes 1, 2, 3, 4 are in equilibrium by methyl and phenyl group migrations; (ii) the relative thermodynamic silaethene stability increases in the order 1 < 3 < 4 < 4 (phenyl groups at unsaturated silicon destabilize the silaethenes as a consequence of electronic effects of phenyl; (iii) silaethenes 1-4 are kinetically more stable against [2 + 2] cycloadditions than silaethene Me₂Si=C(SiMe₃)₂ (as a consequence of steric effects of phenyl); (iv) the rate of methyl and phenyl group migration is equal; (v) substitution of methyl groups at unsaturated silicon (saturated silicons) in Me₂Si=C(SiMe₃)₂ increases (does not increase) the SiC double bond polarity.