REACTIONS OF SILENES WITH ALPHA,BETA-UNSATURATED ALDEHYDES AND KETONES

Silenes of the family (Me3Si)2Si = C(OSiMe3)R (R = Ad, t-Bu, mesity) react with unsubstituted alpha,beta-unsaturated aldehydes and ketones in a [2 + 4] manner involving both possible regiochemistries, with the 1-sila-3-oxacyclohex-4-ene isomer as the major product. In contrast, alpha,beta-unsaturated aldehydes and ketones having a beta-substituent also react in a [2 + 4] manner but with the 1-sila-2-oxacyclohex-3-ene isomer predominating. In addition, the aldehydes, but not the ketones, often formed some of the [2 + 2] siloxetane isomer. The X-ray crystal structure of 1,1-bis(trimethylsilyl)-2-(trimethylsiloxy)-2-(1-adamantyl)-4-metthyl-1-sila-3-oxacyclohex-4-ene is reported, establishing the structure of the major product of the reaction of the adamantylsilene with methyl vinyl ketone. Mechanisms are proposed to account for the observed reactions.