Zr-catalyzed olefin alkylations and allylic substitution reactions with electrophiles

Disubstituted aryl olefins undergo efficient alkylations in the presence of 5 mol % Cp2ZrCl2, n-BuMgCl, and alkyl tosylates or alkyl bromides. In one class of reactions (Table 1), the resulting alkyl zirconocene (initial alkylation product) undergoes P-hydride abstraction with a hydrogen atom from within the substrate to afford allylic alkylation products (Scheme 5). In another category of reactions, where cyclic allylic ethers (chromenes) are used (Table 2), Zr-alkoxide elimination occurs after the C-C bond formation to effect a net allylic substitution. It is proposed that these reactions involve the nucleophilic attack of substrate-derived zirconates or zirconocene-Grignard reagents on various tosylates and bromides. There is little or no competitive electrophile alkylation by the n-alkyl Grignard reagent. Several mechanistic and synthetic aspects of these Zr-catalyzed C-C bond forming reactions are examined and discussed.