Asymmetric Catalysis with Ethylene. Synthesis of Functionalized Chiral Enolates.

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 ß-position. The reactions, run under ambient conditions, use trialkylsiloxy-1,3-dienes and ethylene (1 atm) 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, stereodefined trisubstituted alkene intermediates not easily accessible by current methods. Examples of Kumada, Stille, and Suzuki coupling reactions are illustrated.

Asymmetric hydrovinylation of 1-vinylcycloalkenes. Reagent control of regio- and stereoselectivity.

1-Vinylcycloalkenes undergo highly regio- and enantioselective (>98% ee) 1,4-hydrovinylation (HV) when treated with ethylene (1 atm) at room temperature in the presence of [(S,S)-2,4-bis-diphenylphosphinopentane (BDPP)]CoCl(2) (0.05 equiv) and methylaluminoxane. The minor 1,2-HV products, seen only in 1-vinylcyclohexene (~15%) and 1-vinylcycloheptene (2%), are formed as racemic mixtures. The corresponding Ni(II)-catalyzed HV reactions of these substrates give mostly the 1,2-adducts. Racemic 4-tert-butyl-1-vinylcyclohexene, when treated with Co[(S,S)-(BDPP)]Cl(2) and ethylene, undergoes a rare enantiodivergent reaction giving two diastereomers each in >98% ee.