29955-10-0Relevant articles and documents
Erzeugung, nachweis und stabilisierung isomerer silaethene R2Si=C(SiR3) 2 mit jeweils sechs Me-und zwei Ph-substituenten R
Wiberg, Nils,Joo,Polborn
, p. 147 - 161 (2007/10/03)
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-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 2Si=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.
