C-N-Palladacyclic-Catalyzed Heck Reaction in EGME/Water: Rate and Regioselectivity Controlled by the Solvents Ratio

Novel as well as known C, N-palladacyclic neutral complexes were tested as precatalyst in the Heck reaction between bromobenzene and styrene under aerobic conditions. The catalytic system Pd(II) complex/K(2)CO(3)/EGME-H(2)O (EGME = ethylene glycol monomethyl ether) showed to be highly efficient. Best performances of the catalysts were achieved by controlling the amount of water: generally a H(2)O content within the 25-50% v/v range resulted in the highest conversion of the substrates into trans-stilbene. As a matter of fact, bromobenzene and styrene can be converted quantitatively using only 0.01 mol% of precatalyst with very high regioselectivity for the trans product and with a TOF of 10000 h (1). In the absence of water, all complexes were less efficient and differences in their activity were found, while such a differentiation disappeared when water was added.