Acylsilane chemistry. Synthesis of regio- and stereoisomerically defined enol silyl ethers using acylsilanes

Article abstract of DOI:10.1021/ja00170a026

The preparation of enol silyl ethers using a carbonyl addition-Brook rearrangement-elimination sequence was studied. The key intermediate α-silyl-β-X-alkoxides could be prepared in several different ways, including the addition of organolithium or hydride reagents to α-X-acylsilanes (path a, using RM with R = alkyl, aryl, vinyl, alkynyl, silyl, stannyl, phosphinyl, and cyano), the addition of α-X-lithium reagents to acylsilanes (path b, X = phenylthio, phenylsulfonyl), or the addition of silyllithium reagents to α-X-ketones (path c, X = phenylthio, alkoxy). All of the reactions gave complete regiocontrol of silyl enol ether formation, and many gave excellent (>99%) stereocontrol as well. The selectivity of the carbonyl addition, silyl rearrangement, and elimination was studied. For path a, when the R group of RM was a poor carbanion stabilizing group the elimination of the intermediate α-silyl-β-X-alkoxides was stereospecific, and there was a large difference in rate between erythro and threo (erythro > threo). When R was a carbanion stabilizing group, such as aryl or alkynyl, the elimination process became nonstereospecific in some cases, and only small differences between threo and erythro were observed. Path b was especially effective with α-sulfonyl lithium reagents, and these reactions gave predominantly E enol silyl ethers (4/1 to 20/1). The addition of organolithium reagents to β-X-acylsilanes (the homologue of path a) was also briefly explored as a synthesis of siloxy-cyclopropanes.