An Exploratory Study of Regiocontrol in the Heck Type Reaction. Influence of Solvent Polarity and Bisphosphine Ligands

The regiochemistry of the addition of arylpalladium species to styrene and propene has been studied. It was found that for the reaction of P2Pd(Ph)X the counterion X, the polarity of the solvent, and the structure of the bisphosphine ligand P2 all have an influence. Using bis(diphenylphosphino)ethane as ligand, triflate as counterion, and a moderately polar solvent mixture, 98% selectivity for addition of the phenyl group to the β-carbon of styrene could be obtained. Finally, using low-temperature NMR, some of the palladium intermediates in the addition could be observed, specifically those where the palladium species is stabilized by η3-allylic interaction with a phenyl group.