An insight into copper catalyzed allylation of alkyl zinc halides. Comparison of reactivity profiles for catalytic and stoichiometric alkylzinc–copper reagents

The γ-selective allylation of catalytic and stoichiometric alkylzinc–cuprates have been kinetically studied. The reactivity profiles generated by allylation reactions of n-butylzinc chloride catalyzed by CuX compounds (X = I, Br, Cl, CN, SCN) and also catalyzed by n-butylzinc–copper reagents and di n-butylzinc–copper reagents were evaluated. Reactivity profiles for allylation of stoichiometric n-butylzinc–copper reagents and di n-butylzinc–copper reagents were also prepared. All CuX compounds have been screened for the preparation of Grignard reagent derived n-butylzinc–copper reagents and di n-butylzinc–copper reagents.The evaluation of the profiles indicates that the active catalyst might be RCu(X)ZnCl and also to some degree, R2CuZnCl · ZnClX, which both could favor formation of γ-product. All data supports the reductive elimination of σ-allyl Cu (III) complex formed at vinylic terminal to give γ-allylated product with a quite slow isomerization to σ-allyl Cu (III) complex formed at allylic terminal to give α-allylated product. In the allylation mechanism of zinc cuprates, the role of counter ion, ZnCl+ has been discussed.

Graphichal abstractEvaluation of the reactivity profiles generated by γ-selective allylation of catalytic and stoichiometric alkylzinc–cuprates supports the formation of both RCu(X)ZnCl and R2CuZnCl as active catalysts and also reductive elimination of σ-allyl-Cu(III) complex to give γ-allylated product.Figure optionsDownload as PowerPoint slide