Probing new classes of π-acceptor ligands for rhodium catalyzed hydroformylation of styrene

Three hitherto unexplored classes of strong pi-acceptor ligands for use in homogeneous catalysis-phospha-pi-aromatic compounds (class A), pyrrolyl phosphines (class B) and phosphenium cations (class C)-have been evaluatated for rhodium catalyzed hydroformylation of styrene. When testing monodentate ligands, the ortho/ortho'-disubstituted phosphabenzene derivative Ib provided a rhodium-catalyst endowed with the highest catalytic activity. Based upon these results a first series of bidentate phosphabenzene ligands have been tailored employing the concept of an electronic differentiation of the two binding sites. An oxazoline/phosphabenzene system 8 which is capable of forming an eight-membered chelation ring gave the best results. Thus, a quantitative conversion of styrene at ambient temperature afforded the desired 2-phenylpropanal in high regioselectivity (25:1). (C) 1999 Elsevier Science B.V. All rights reserved.