Cationic Complexes of Palladium(II) and Platinum(II) as Lewis Acid Catalysts for the Diels−Alder Reaction

The synthesis of a series of Pd(II) and Pt(II) cationic complexes of the types [(P−P)M(solv)](Y) 2, [(P−P)M(η 2 -Y)](Y), and [(P−P)M(μ-Cl)] 2 (Y) 2 (where P−P = various diphosphines including chiral diphosphines (M = Pd, Pt; Y = TfO -, ClO 4 -, BF 4 - ) is reported. All complexes are excellent catalysts for the Diels−Alder reaction between cyclopentadiene or cyclohexadiene and a number of simple dienophiles such as acrolein, methacrolein, and methylvinyl ketone. Some of the complexes modified with chiral diphosphines can act as enantioselective catalysts in the Diels−Alder reaction under mild condition with moderate ee's. A critical point in this respect is the presence of only one vacant coordination site in the catalyst.