Sulfonyl-stabilized 4-alkylidene dihydropyridines as C-donor ligands in organotransition metal catalysis: Synthesis of palladium complexes

Three palladium(II) complexes supported by new neutral η¹-C-donor ligands derived from alkylidene dihydropyridine derivatives are reported. The ligand frameworks are easily constructed from simple 4-alkylpyridines. The ligands feature peripheral aryl sulfonyl groups that increase acidity of picolyl C–H bonds, facilitate subsequent generation of relatively stable alkylidene dihydropyridines, and provide sites for attachment of additional metal ligating groups. Sulfonyl-stabilized alkylidene dihydropyridines readily coordinate PdCl₂ through the picolyl carbon in monodentate fashion or as C,N-chelating ligands when 2-pyridylsulfonyl groups are present. All complexes are air-stable and X-ray crystal structures of two complexes are reported. One Pd(II) complex was screened for catalytic activity and found to be moderately effective in mediating Suzuki cross-coupling reactions.