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 ty pes [(P-P)M(solv)](Y)(2), [(P-P)M(eta (2)-Y)](Y), and [(P-P)M(mu -Cl)](2)(Y )(2) (where P-P = various diphosphines including chiral diphosphines (M = P d, Pt; Y = TfO-, ClO4-, BF4-) is reported. All complexes are excellent cata lysts for the Diels-Alder reaction between cyclopentadiene or cyclohexadien e and a number of simple dienophiles such as acrolein, methacrolein, and me thylvinyl ketone. Some of the complexes modified with chiral diphosphines c an act as enantioselective catalysts in the Diels-Alder reaction under mild condition with moderate ee's. A critical point in this respect is the pres ence of only one vacant coordination site in the catalyst.