Coordination chemistry, reactivities, and stereoelectronic properties of chelating phosphine ligands containing thioamide substituents

Abstract

The intramolecular exo-cycloaddition reaction between N,N-dimethylthioacrylamide and 3,4-dimethyl-1-phenylphosphole in the presence of a perchloratopalladium template containing ortho-metallated (S)-(1-(dimethylamino)ethyl)naphthalene as the chiral auxiliary gave the corresponding thioamide-substituted P-chiral phosphanorbornene stereospecifically in 6 d. The exo-cycloadduct coordinated to the palladium template as a bidentate chelate via its phosphorus and thioamide-sulfur donor atoms. The corresponding intermolecular endo-cycloaddition reaction using the analogous chloropalladium template containing the same ortho-metallated naphthylamine auxiliary produced a pair of separable diastereomeric endo-cycloadducts in 60 d. Both the endo-cycloadducts coordinated to the palladium template as monodentates via their phosphorus donor atoms and their thioamide functions were not involved in the metal complexation. The faster rate observed in the exo-cycloaddition reaction is attributed to the electronic polarization and hence the activation of N,N-dimethylthioacrylamide via thioamide-S coordination. Optically active thioamide-substituted phosphanorbornenes could be liberated from these product complexes by treatment with aqueous cyanide. For comparison purposes, the novel ligand Ph₂PC(S)NMe₂ was prepared. This short-chain ligand displaced acetonitrile and monodentate phosphine ligands on Pd^II to form stable 4-membered P–S chelates. The CS bond in these sterically hindered chelates are unreactive toward cycloaddition reaction with the phosphole cyclic diene. The thioamido (S)C–N bonds in this series of P–S palladium(II) chelates were found to be significantly shorter than those reported for organic thioamides and their non-chelating counterparts in the endo-cycloadducts, thus indicating that nitrogen in the thioamide function contributed electronically to the stability of the Pd→S bonds.