Preparation of α‐chiral crotylsilanes by retro‐[1,4]‐brook rearrangements. A stereochemical study

Abstract

AbstractTreatment of alcohols cis‐22, cis‐23, and trans‐23 with PPh3/ Ph2S2 provided three mixtures of isomers of α, γ‐disubstituted allyl phenyl sulfides. They consisted of diastereomers and ‐except starting from trans‐23–of regioisomers. Each mixture was lithiated reductively by lithium naphthalenide at −78°C in THF. α, γ‐Disubstituted allyllithium compounds resulted which underwent retro‐[1,4]‐Brook rearrangements within 30–90 min. A high degree of stereocontrol by 1,3‐asymmetric induction was observed in the rearrangement starting from a 5:1 mixture of the tBuPh2Si‐containing sulfide regioisomers cis‐25 and iso‐25 (each regioisomer consisting of more than one diastereomer): The α‐chiral crotylsilane anti, trans‐26 was formed as a racemic mixture with ds = 93:7:0:0 in preference to isomer syn,trans‐26 which possesses the opposite configuration at the silicon‐bearing stereocenter and in preference to the corresponding cis isomers (Scheme 5). The rearrangements of the two other sulfide mixtures were much less stereoselective and exhibited ds = 49:44:4:3 starting from the two tBuPh2Si‐containing diastereomers of the isomeric sulfide trans‐25 (Scheme 5) and ds = 59:41 starting from the 3.5:1 mixture of the MePh2Si‐containing sulfide regioisomers cis‐24 and iso‐24 (each regioisomer consisting of more than one diastereomer; Scheme 9). The stereostructures of the rearrangement products were determined by chemical correlations, NMR analogies, and a crystal structure analysis of 27. It was proven that the stereochemical outcome of the retro‐[1,4]‐Brook rearrangements of the MePh2Si‐containing 3.5:1 mixture of sulfides cis‐24/iso‐24 is kinetically rather than thermodynamically controlled (Scheme 10). Mechanistic aspects are discussed in Schemes 11–13.