Erzeugung, nachweis und stabilisierung isomerer silaethene R 2Si [dbnd]C(SiR 3) 2 mit jeweils sechs Me- und zwei Ph-substituenten R

Abstract

Bromotrisilylmethanes Me 2SiBr CBrRR 2 ( 2b), PhMeSiBr CBrRR 1 ( 3b, two diastereomers) and Me 2SiBr CBrR 2 1 ( 4b) (R SiMe 3, R 1 SiMe 2Ph, R 2 SiMePh 2) - sources of silaethenes Me 2Si CRR 2 ( 2), PhMeSi CRR 1 ( 3, two isomers), and Me 2Si CR 2 2 ( 4) - are formed by bromination of Me 2SiH CBrRR 2 ( 2a), PhMeSiH CBrRR 1 ( 3a, two diastereomers; bromination takes place stereospecifically), and Me 2SiH CBrR 2 2 ( 4a), which themselves are formed from CBr 4 by introducing three silyl groups step by step according to: ≤Cz.sbnd;Br+R′Li+≤SiCl→≤C Si≤+R′Br+LiCl(R′ = ″Bu, (Me 3Si) 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 Me 2SiBr CLiRR 2 ( 2c). PhMeSiBr CLiRR 1 ( 3b, two diastereomers), and Me 2SiBr CLiR 2 1 ( 4c), which in Et 2O 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 × 2, 2 × 4 and 4 × 4 of silaethenes 2 and 4 are found. Considering the types and yields of the products of silaethenes - including Ph 2Si CR 2 ( 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 Me 2Si C(SiMe 3) 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 Me 2Si C(SiMe 3) 2 increases (does not increase) the SiC double bond polarity.