Porphyrins fused to N-heterocyclic carbene palladium complexes as tunable precatalysts in Mizoroki–Heck reactions: How the porphyrin can modulate the apparent catalytic activity?

AbstractPorphyrins fused to N-heterocyclic carbenes (NHCs) 1M, with inner metal M=Ni, Cu, Zn and H2, were used to synthesize palladium(II) complexes. These complexes behaved as precatalysts for high-temperature Mizoroki–Heck reactions between iodobenzene and styrene. Despite similar electronic properties between 1M ligands, catalytic activity was found to significantly depend on the presence, or not, of a central metal incorporated in the porphyrin core: a conversion of iodobenzene between 80–90% was observed with metalloporphyrins (M=Ni, Cu and Zn), whereas only 30% was observed with free-base porphyrins (M=H2) after 10 h. Palladium leaching and subsequent trapping of free palladium in the inner core of the free-base porphyrin modified the concentration of Pd(0) active species and may explain the lower catalytic activity observed