Uncatalyzed conjugate addition of organozinc halides to enones in DME: a combined experimental/computational study on the role of the solvent and the reaction mechanism

Both aryl and alkylzinc halides prepared by direct insertion of zinc into organic halides in the presence of LiCl underwent the conjugate addition reaction to nonenolizable unsaturated ketones in excellent yield, provided that DME was used instead of THF as the solvent. Diffusion NMR measurements highlighted that the species undergo considerable aggregation under the experimental conditions used in the synthetic procedure, but no substantial differences have been found between the two solvents. Density functional theory calculations, prompted by the experimental aggregation study, revealed an unexpected reaction mechanism, where the coordinating capabilities of DME stabilize a transition state involving two organozinc moieties, lowering the activation energy of the reaction with respect to that seen for THF, enough to explain the fast and quantitative reactions observed experimentally and the different behaviors of the two solvents.

The reaction mechanism of a new conjugate addition reaction of organozinc halides to enones is disclosed by a combined experimental/computational study.

Activation of Marginally Reactive Boron Enolates by MeLi for the Formation of Enol Phosphates and Synthesis of the Δ(9)-THC Intermediate

The addition of MeLi to boron enolates produced by the 1,4-addition of Ar2Cu(CN)Li2 to BF3·OEt2-activated enones was followed by the reaction with ClP(O)(OEt)2 to afford the corresponding enol phosphates in moderate to good yields. The scope of this method was examined with sterically hindered or electronically biased enones and/or reagents. This activation of boron enolates was successfully applied to the synthesis of the methyl ether of Δ(9)-tetrahydrocannabinol.

Alternatives to organoboron reagents in rhodium-catalyzed conjugate additions

The rhodium-catalyzed conjugate addition of organoboron reagents to alkene acceptors has emerged as an important methodology in organic synthesis. The process results in the formation of new carbon-carbon bonds, and in principle, can create new stereocenters at both the alpha and beta carbon atoms of the acceptor. Other organometallics are known to participate in the key transmetalation to rhodium and are becoming more established in stereoselective synthesis (notably arylzinc reagents). This Focus Review will describe recent developments in the use of alternative organometallic donors to organoboron reagents in rhodium-catalyzed conjugate addition reactions and their growing applications in synthesis.