Effects of polar gamma-substituents on the structure and stability of palladacyclobutane complexes

Palladacyclobutane complexes possessing various gamma-substituents (X=H, Cl , OH) and ancillary ligands on palladium (L=NH3, Cl-, PH3) have been studie d by second order Moller-Plesset (MP2) perturbation theory. Interactions be tween the gamma-substituents and palladium have been analysed as a function of the conformation of the complexes. The gamma-substituent effects lead t o substantial structural changes in the four membered ring. The conjugative interactions increase the thermodynamic stability of the complexes, and fa cilitate the heterolytic fission of the C-X bond. The intensity of these in teractions depends on the nature of the gamma-substituent, on the conformat ion of the complex, as well as on the sigma-donor/pi-acceptor character of the ancillary ligands on palladium. Since palladacyclobutanes are key inter mediates of some important catalytic transformations, the implications for the palladium chemistry are also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.