Isotope effects and the mechanism of palladium-catalyzed allylic alkylation

The palladillm-catalyzed allylic alkylation reaction of 1,1-diniethylatlyl acetate with dimethyl malonate is studied by a combination of isotope effects and theoretical calculations. A large C-13 isotope effect of approximate to1.037 is observed at the tertiary carbon, while small isotope effects are observed at the olefinic carbons. These results support rate-limiting ionization of a eta(2)-Pd complex. The observed isotope effects are compared with predictions from calculational models employing either solvent models or ionization of an amidinium ion. The calculated transition structures are notably eta(2) in character, and the implications of this observation are discussed. (C) 2005 Elsevier Ltd. All rights reserved.