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/K₂CO₃/EGME-H₂O (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 H₂O 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 10 000 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.