Conjugate additions of furylcuprates to α-enones. The effect of the copper precursor and of TMSCl used as additive

Abstract

The addition of organocuprates to α,β-unsaturated substrates, namely E-cinamaldehyde and 2cyclohexen-1-one, was studied in THF. The furan moiety was chosen for the heterocuprate, because the methods for the synthesis of furan derivatives are scarce and they have a strong potentiality for tandem reactions. The performance of CuBr; CuBr.SMe 2 and CuCN as copper precursors was examined: the higher yields of the 1,4-addition product were obtained using CuCN. The reaction with 2-cyclohexen-1-one gave the best results. In some cases, a compound that arises from dehydration of the 1,2-addition product was obtained and its yield increases near 70% when the reaction is carried out with furyl-lithium in the absence of any copper salt. Addition of up to 6 equivalents of TMSCl to the reaction mixture of 2-cyclohexen-1-one with furylcuprate at-78 C, leads to a clean addition yielding more than 90% of the 1,4-addition product. The only by-product is 2-TMS-furan. The absence of silylated ether of the enolate, is a strong evidence to rule out the cyclic intermediate or transition state that was previously proposed as the mechanism whereby TMSCl (acting as a Lewis base toward the cuprate) accelerates the reaction. Promoting conversion of the initially formed π-complex to a reactive tetravalent copper species is proposed instead.