Regio- and enantioselective allylic alkylation of an unsymmetrical substrate: A working model

The evolution of a model for understanding asymmetric allylic alkylations catalyzed by palladium with the use of ligands derived from chiral diamines and 2-diphenylphosphinobenzoic acid provides a basis for attacking the problem of regio- and enantioselective alkylations proceeding through the intermediacy of l-monosubstituted allyl complexes. The model predicted that in the kinetic ionization of an achiral precursor the major enantiomer of the product resulting from attack at the more substituted terminus would be the mirror image of that obtained under Curtin-Hammett conditions. Experimentally, the ee was rationally varied from 66% of one enantiomer to 83% of the mirror image using the same ligand. Nonpolar solvents and the absence of counterions that coordinate to palladium favor the kinetic product. More polar solvents and counterions that coordinate well to palladium favor Curtin-Hammett conditions, For maximum regio- and enantioselectivity, the chiral racemic 3-substituted-1-alkene is the preferred substrate.