Asymmetric Catalysis with Ethylene. Synthesis of Functionalized Chiral Enolates

Abstract

Trialkylsilyl enol ethers are versatile intermediates often used as enolate surrogates for the synthesis of carbonyl compounds. Yet there are no reports of broadly applicable, catalytic methods for the synthesis of chiral silyl enol ethers carrying latent functionalities useful for synthetic operations beyond the many possible reactions of the silyl enol ether moiety itself. Here we report a general procedure for highly catalytic (substrate:catalyst ratio up to 1000:1) and enantioselective (92% to 98% major enantiomer) synthesis of such compounds bearing a vinyl group at a chiral carbon at the β-position. The reactions, run under ambient conditions, use trialkylsiloxy-1,3-dienes and ethylene (1 atmosphere) as precursors, and readily available (bis-phosphine)-cobalt(II) complexes as catalysts. The silyl enolates can be readily converted into novel enantiopure vinyl triflates, a class of highly versatile cross-coupling reagents, enabling the syntheses of other enantiomerically pure, stereo-defined trisubstituted alkene intermediates not easily accessible by current methods. Examples of Kumada, Stille and Suzuki coupling reactions are illustrated.

Control of Selectivity through Synergy between Catalysts, Silanes, and Reaction Conditions in Cobalt-Catalyzed Hydrosilylation of Dienes and Terminal Alkenes

“…Even within a series of (bisphosphine)CoCl 2 complexes ease of reduction (with Zn) has been found important in other cobalt-catalyzed coupling reactions such as hydrovinylation reaction. 7a For a dramatic effect of counter ion in hydrosilylation see ref. 2c.…”

Recent developments in enantioselective cobalt-catalyzed transformations

“…Nevertheless, there are still few reports of broadly applicable catalytic methods for their synthesis. In 2015, RajanBabu and co-workers developed a general catalytic procedure for highly chemo-, regioand enantioselective synthesis of trialkylsilyl enol ethers exhibiting a vinyl-bearing chiral center at the b-position [64]. The reactions were performed at room temperature in dichloromethane in the presence of 5 mol% of cobalt catalyst ent-88 derived from (S,S)-DIOP ligand and two equivalents of methylaluminoxane.…”

Recent Advances in Enantioselective C–C Bond Formation via Organocobalt Species

“…In 2015, RajanBabu reported asymmetric 1,4-hydrovinylation of 2-siloxy-1,3-dienes 8 to afford chiral silyl enol ethers 9 bearing a vinyl group on the β-position (Scheme 2), which are challenging to access by other means. 8 The reaction was achieved by using a catalyst generated from (DIOP)-CoCl 2 or (BDPP)CoCl 2 complex and methylaluminoxane…”