Probing new classes of pi-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 pho sphines (class B) and phosphenium cations (class C)-have been evaluatated f or rhodium catalyzed hydroformylation of styrene. When testing monodentate ligands, the ortho/ortho'-disubstituted phosphabenzene derivative Ib provid ed a rhodium-catalyst endowed with the highest catalytic activity. Based up on these results a first series of bidentate phosphabenzene ligands have be en tailored employing the concept of an electronic differentiation of the t wo binding sites. An oxazoline/phosphabenzene system 8 which is capable of forming an eight-membered chelation ring gave the best results. Thus, a qua ntitative conversion of styrene at ambient temperature afforded the desired 2-phenylpropanal in high regioselectivity (25:1). (C) 1999 Elsevier Scienc e B.V. All rights reserved.