Cooperative Diastereoselectivity of Palladium- and Platinum-Promoted Intramolecular [4+2] Diels−Alder Cycloaddition Reactions of 3,4-Dimethyl-1-phenylphosphole

The complexes [(DMPP)2M(CH3CN)2]X2 (DMPP = 3,4-dimethyl-1-phenylphosphole; M = Pd, Pt; X = BF4-,
NO3-, ClO4-) react with 2 equiv of the dienophiles N,N-dimethylacrylamide (DMAA), 2-vinylpyridine (VyPy),
and diphenylvinylphosphine (DPVP) to form bis-[4+2] Diels−Alder cycloaddition products. The [M(DMPP)2(DMAA)2]2+ and [M(DMPP)2(VyPy)2]2+ complexes form exclusively as the cis-geometric isomers, whereas for
[M(DMPP)2(DPVP)2]2+, both cis- and trans-geometric isomers are formed. The two Diels−Alder cycloadditions
occur sequentially, and the absolute configuration of the first reaction influences the absolute configuration of the
second. In all cases, racemic mixtures of the (R,R) and (S,S) diastereomers are formed; none of the meso (R,S)
diastereomer is observed. New complexes were characterized by elemental analyses, physical properties, infrared
spectroscopy, 1H, 1H{31P}, 13C{1H}, and 31P{1H} NMR spectroscopy, and, in most cases, X-ray crystallography.