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 pheny l ether (1a) and 2-cyclohexenyl phenyl ether (1b) with methyl-, ethyl- , n-propyl-, and isopropylmagnesium bromide were carried out using eit her pentane-1,2-diylbis(diphenylphosphine)]-nickel(II) chloride (5a) o r henyl-2,2'-diyl)bis(diphenylphosphine)]-nickel(II) bromide (5b) as t he catalyst precursor. With both catalytic systems a maximum enantiose lectivity was observed for the ethylation reactions leading to 3-ethyl cyclopentene (up to 94% ee with 5b). Enantioselectivities are in gener al lower in the alkylation of 1b than in that of 1a. The results are d iscussed on the basis of a nickel eta3-allyl intermediate and of the p ossible change of mechanism going from a small to a bulky alkyl group.