Unusually Large Primary 2H/H Kinetic Isotope Effects Accompanying a syn-β-H Elimination Reaction in a σ-Alkyl−Palladium Complex

The Pd-sigma-alkyl complex 5, is stable in solution for long periods in the absence of base. On addition of simple amine bases, such as 1,8-bis(dimethylamino)naphthalene (7), complex 5 rapidly generates the triene 6 via a beta-H elimination process whose stereochemistry is demonstrated to be syn. In contrast to the low primary kinetic isotope effects (kH/kD) that normally attend syn-beta-H elimination processes from sigma-alkylmetal species, large values are observed on performing inter- and intramolecular competition experiments with 5. However, the substitution of D for H has negligible effect on the absolute rate at low temperatures (-60 degrees C). A mechanism to account for this behavior is presented, and implications for asymmetric Heck reactions are noted.