Catalytic Enantioselective Hydrovinylation of Trialkylsilyloxy and Acetoxy-1,3-Dienes: Cationic Co(I) Complexes for the Synthesis of Chiral Enolate Surrogates and Their Applications for Synthesis of Ketones and Cross-Coupling Reagents in High Enantiomeric Purity.

Abstract

(E)-2-Trialkylsilyloxy-1,3-dienes and the corresponding 2-acetoxy derivatives participate in cobalt-catalyzed heterodimerization reactions with ethylene, giving mostly 4,1-hydrovinylation products with addition of the vinyl group to C4 and H at C1 of the diene. The reaction, which gives highly functionalized, protected enolates, is best carried out at room temperature with the diene dissolved in methylene chloride and ethylene delivered from a balloon in the presence of a catalyst generated in situ by the reaction of (P~P)CoCl2 with methylaluminoxane (MAO). Commercially available chiral ligands, 2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis-(diphenylphosphino)butane (DIOP) and 2,4-bis-diphenylphosphinopentane (BDPP) in combination with the earth-abundant metal cobalt, gave excellent regio- and enantio-selectivities (up to 99% ee) for the chiral enolate surrogates from both silyloxy and acetoxydienes. Hydrolyses of the silyl enol ethers lead to β-vinyl ketones, thus providing a practical two-step approach to these valuable synthons starting from α,β-unsaturated ketones and ethylene. The hydrovinylated silyl enol ethers undergo typical nucleophilic reactions such as alkylation, aldol, Michael and Mannich reactions with varying degrees of diastereoselectivity (2:1-13:1). The silyl enol ethers are convenient source of lithium enolates which are readily converted into other vinyl derivatives such as vinyl acetates and vinyl triflates. The vinyl triflates are excellent partners for cross-coupling chemistry, giving potentially useful, polyolefinic chiral synthons for further applications. Chemoselective reduction and hydrosilylation of the vinyl group in the chiral β-vinyl silyl enol ether further illustrate other potential reactivities of these versatile synthons. Since isolated cationic [(P~P)Co(I)]+ [BARF]- appears to be an excellent catalyst for the heterodimerization of silyl enol ethers and ethylene giving products very similar in yield and selectivities to what is observed in the MAO-mediated reactions, we propose that a previously invoked a Co(I)/Co(III) cycle, common to other similar heterodimerization reactions, might be involved in these reactions as well.