ENANTIOSELECTIVE NICKEL-CATALYZED GRIGNARD CROSS-COUPLING OF ALLYL ELECTROPHILES - THE INFLUENCE OF THE ALKYL GROUP OF THE GRIGNARD-REAGENT

The enantioselective cross coupling reactions of 2-cyclopentenyl phenyl ether (1a) and 2-cyclohexenyl phenyl ether (1b) with methyl-, ethyl-, n-propyl-, and isopropylmagnesium bromide were carried out using either [(RR)-trans-cyclopentane-1,2-diylbis(diphenylphosphine)]-nickel(II) chloride (5a) or [(R)-(6,6'-dimethylbiphenyl-2,2'-diyl)bis(diphenylphosphine)]-nickel(II) bromide (5b) as the catalyst precursor. With both catalytic systems a maximum enantioselectivity was observed for the ethylation reactions leading to 3-ethylcyclopentene (up to 94% ee with 5b). Enantioselectivities are in general lower in the alkylation of 1b than in that of 1a. The results are discussed on the basis of a nickel eta3-allyl intermediate and of the possible change of mechanism going from a small to a bulky alkyl group.