Platinum complexes of 2-diphenylphosphinobenzaldehyde-derived P-alkene ligands and their application in the hydroformylation of styrene

Neutral complexes of the formula PtCl2L2 (where L = diethyl 2-diphenylphosphino-benzylidene-malonate (1), diisopropyl 2-diphenylphosphino-benzylidene-malonate (2), di-tert-butyl 2-diphenylphosphino-benzylidene-malonate (3), methyl E-2-(2′-diphenylphosphinophenyl)-acrylate (4), tert-butyl E-2-(2′-diphenylphosphinophenyl)acrylate (5)) were prepared. These complexes proved to be excellent precursors to active catalysts for the hydroformylation of styrene. The platinum-containing catalytic systems prepared from malonate-based ligands 1 and 2 provided the highest activity. Chemoselectivities of up to 87% were obtained, while the two aldehyde regioisomers were formed in almost equimolar ratio. The in situ studies by using lower ligand to Pt ratios resulted in slight decrease in both regio- and chemoselectivities. 31P NMR studies on the PtCl2L2 complexes revealed that the formation of trans isomers is highly preferred in the case of benzylidene malonate-type ligands with two ester functionalities (1–3) probably due to steric hindrance. A mixture of cis/trans geometrical isomers (on the Pt) with a predominance of the trans isomer was formed when acrylate-type ligands with one ester functionality (4 and 5) were used.