Grignard reagent–acid chloride condensation in the presence of copper(I) chloride. A study of structural effects by direct and competition methods
The copper(I) chloride catalysed reaction between a Grignard reagent and an acid chloride in ether has been studied by using a competition method in which a pair of acid chlorides reacts with a single Grignard reagent, and by direct kinetic experiments in one case. The variation of relative rates produced by structural changes in the alkyl group of the acid chloride has been shown to correlate with the Taft polar parameter σ* but to be independent of the steric parameter ES(ρ*= 2·96 for EtMgBr; 6·01 for ButCH2MgCI; 12·7 for ButMgCl); this is in contrast with uncatalysed reactions of this type which have a marked sensitivity to steric effects. These results are consistent with a cyclic transition state in which chloride is displaced from acid chloride without addition to the carbonyl group, by either a mixed cuprate(I) intermediate [RCuX]–MgX+ or a loose RCu ⋯ MgX2 complex.