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

The intramolecular exo-cycloaddition reaction between N,N-dimethylthioacryl amide and 3,4-dimethyl-1-phenylphosphole in the presence of a perchloratopa lladium template containing ortho-metallated (S)-(1-(dimethylamino)ethyl)na phthalene as the chiral auxiliary gave the corresponding thioamide-substitu ted P-chiral phosphanorbornene stereospecifically in 6 d. The exo-cycloaddu ct coordinated to the palladium template as a bidentate chelate via its pho sphorus and thioamide-sulfur donor atoms. The corresponding intermolecular endo-cycloaddition reaction using the analogous chloropalladium template co ntaining the same ortho-metallated naphthylamine auxiliary produced a pair of separable diastereomeric endo-cycloadducts in 60 d. Both the endo-cycloa dducts coordinated to the palladium template as monodentates via their phos phorus donor atoms and their thioamide functions were not involved in the m etal complexation. The faster rate observed in the exo-cycloaddition reacti on 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 prod uct complexes by treatment with aqueous cyanide. For comparison purposes, t he novel ligand Ph2PC(S)NMe2 was prepared. This short-chain ligand displace d acetonitrile and monodentate phosphine ligands on Pd-II to form stable 4- membered P-S chelates. The C=S bond in these sterically hindered chelates a re unreactive toward cycloaddition reaction with the phosphole cyclic diene . The thioamido (S)C-N bonds in this series of P-S palladium(II) chelates w ere found to be significantly shorter than those reported for organic thioa mides and their non-chelating counterparts in the endo-cycloadducts, thus i ndicating that nitrogen in the thioamide function contributed electronicall y to the stability of the Pd -->S bonds.