MECHANISMS OF THE CYCLOADDITION REACTION OF METHYLENECYCLOPROPANE-PALLADIUM AND OXATRIMETHYLENEMETHANE-PALLADIUM AND AZATRIMETHYLENEMETHANE-PALLADIUM COMPLEXES WITH OLEFINS

Mechanisms of palladium-catalyzed cycloaddition reactions of methylene cyclopropane with olefins and those of oxatrimethylenemethane (OTMM) a nd azatrimethylenemethane (ATMM) complexes with olefins have been stud ied by applying the ab initio molecular orbital method. Assuming an in tramolecular coupling mechanism, we have examined some model complexes , Pd(eta(2)-methylenecyclopropane)(ethylene)(PH3) and Pd(eta(3)-OTMM o r eta(3)-ATMM)(ethylene)(PH3), to determine the transition state struc tures. The coupling of methylenecyclopropane, OTMM, and ATMM with ethy lene on the metal center takes place in two steps. In the reaction of methylenecyclopropane, the first step involves an opening of the cyclo propane ring promoted by an attack of ethylene. The methylenecycloprop ane moiety has a pi-allylic form at the first transition state which l eads to a metallacyclic intermediate. The first transition state of th e reactions of OTMM and ATMM complexes looks very similar to that of t he methylenecyclopropane complex, having pi-allylic coordinations. The two paths are separated in the second step. A [3 + 2] addition produc t is obtained by a reductive elimination from the intermediate metalla cycle, whereas a prototropic shift followed by a reductive elimination affords a [2 + 1] addition product. It is the energetics of the secon d stage starting from the metallacycles that differentiates the reacti ons of the OTMM and ATMM complexes from the reaction of the methylenec yclopropane complex. The relative stabilities of various isomeric form s of these complexes have also been studied.