Silicon−Carbon Unsaturated Compounds. 59. Stereochemistry in Addition of Carbonyl Compounds to Silenes Generated Photochemically from meso- and rac-1,2-Diethyl-1,2-dimethyldiphenyldisilane

Stereochemistry in ene-type addition of silenes produced photochemically from meso - and rac -1,2-diethyl-1,2-dimethyldiphenyldisilane ( 1a, b ) to carbonyl compounds was investigated. When the meso -isomer 1a was irradiated in the presence of acetone in hexane at −78 °C, a diastereomeric mixture of an adduct, 2-(ethylmethylphenylsilyl)[ethylmethyl(2-propoxy)silyl]benzene ( 3a, b ), in a ratio of 3a / 3b = 93/7 was obtained in 64% yield. Similar photolysis of a mixture consisting of 1a / 1b = 4/96 gave the adduct composed of 3a / 3b = 10/90 in 63% yield. The photolysis of 1a with benzophenone proceeded diastereospecifically to afford an adduct, ( R, R )- and ( S, S )-[ethylmethyl(diphenylmethoxy)silyl]-2-(ethylmethylphenylsilyl)benzene, in 83% yield. Irradiation of the disilane ( 1a / 1b = 10/90) with benzophenone, under the same conditions, produced the adducts consisting of the ( R, R )- and ( S, S )-isomers and ( R, S )- and ( S, R )-isomers in a ratio of 12/88 in 86% yield. Similar photolysis of 1a and a mixture of 1a / 1b = 6/94 in the presence of acetaldehyde proceeded also with high diastereoselectivity to afford the corresponding adduct with diastereomeric ratios of 96/4 and 9/91, respectively. Solvent effects on stereochemistry in the reaction of the silenes with acetone were investigated, and it was found that electron-donating solvents THF and acetonitrile led to decrease of diastereoselectivity of the reactions, while ether and toluene showed no influence for the stereochemistry. The ( R, S )- and ( S, R )-isomers of the adduct obtained from the photolysis of the disilane in the presence of benzophenone have been characterized by X-ray crystallography.