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Enantioselective Alkylation of Acyclic α,α‐Disubstituted Tributyltin Enolates Catalyzed by a {Cr(salen)} Complex

Catalytic a-carbonyl C C bond-forming reactions represent a proven strategy for the construction of quaternary stereocenters. Most approaches involve the addition of enolates to carbon-centered electrophiles in which both the p-facial selectivity and the enolate geometry govern the stereoselectivity of the C C bond-forming event. Unfortunately, the synthesis of stereodefined enolates from simple a,a-disubstituted carbonyl compounds that lack either tethered substituents or specific chelating functionality remains a significant challenge in organic synthesis. As a result, successful examples of asymmetric catalytic enolate addition reactions that generate a-carbonyl quaternary stereocenters are limited to b-cyano esters, b-keto esters, and cyclic ketones, and relatively little progress has been made with simple acyclic systems. Our own efforts in the area of enantioselective reactions with enolates have led to the recent discovery of a {Cr(salen)}-catalyzed asymmetric a-alkylation of cyclic tin enolates to provide 5-, 6-, and 7-memberedring ketones that contain a quaternary stereocenters (salen=N,N’-bis(salicylidene)ethylenediamine dianion; Scheme 1). While the preparation of acyclic a,a-disubstituted tin enolates leads inevitably to mixtures of E and Z isomers, tin enolates are known to undergo tautomerization between their O-stannyl and C-stannyl forms in solution. We were intrigued by the possibility that enantioselective alkylation of acyclic enolates might be achievable by a dynamic mechanism such as that outlined in Scheme 2,