Bis-phosphine monoxide platinum(II) and palladium(II) cationic complexes as Lewis acid catalysts in Diels–Alder and sulfoxidation reactions

A series of bis-phosphine monoxide (BPMO) palladium(II) and platinum(II) cationic complexes of the type [M(BPMO-κ2-P,O)2][X]2 (M = Pd, Pt; BPMO = Ph2P–(CH2)n–P(O)Ph2 with n = 1 (dppmO), 2 (dppeO), 3 (dpppO); X = BF4, TfO) were prepared from the corresponding chlorides [MCl2(BPMO-κ1-P)2] upon treatment with 2 equiv. of AgX in wet acetone/CH2Cl2 or MeOH solutions. They were characterized by 1H and 31P{1H} NMR spectroscopies and, in the case of the complex [Pt(dppeO-κ2-P,O)2][BF4]2, also by X-ray crystallography. These complexes were tested as catalysts in some Diels–Alder and oxidation reactions with different substrates. In the latter reaction Pt(II) complexes showed moderate activity, while for the former one, both classes of complexes were active in the C–C coupling, in particular the Pt(II) species showed interesting high endo/exo diasteroselectivity depending on the counteranion.