DFT studies of mechanism and origin of stereoselectivity of palladium-catalyzed cyclotrimerization reactions affording syn-tris(norborneno) benzenes

Pd-catalyzed cyclotrimerization reactions of enantiopure halonorbornene derivatives furnished C₃ or C_3v symmetric syn-tris(norborneno)benzenes with high syn selectivity. To elucidate the reaction mechanism as well as the stereoselectivity of the present Pd-catalyzed cyclotrimerization, DFT calculations were carried out. The promising reaction pathway consists of (1) sequential olefin insertion followed by an HX elimination reaction of halonorbornene with the norbornenylpalladium intermediate, (2) electrocyclization of the trienylpalladium intermediate with a lower activation barrier than a triene compound, and (3) the β-elimination of HPdX of the cyclohexadienylpalladium intermediate. In addition, the stereoselectivity would be controlled by the regioselectivity in the olefin insertion process (homo and hetero positions) and the symmetry breaking in the palladacyclic intermediate.