Rigid, Sterically Diverse N-Heterocyclic Carbene-Pyridine Chelates: Synthesis, Mild Palladation, and Palladium-Catalyzed Allylic Substitution

A series of four 1-(pyridin-2-yl)benzimidazolium salts was synthesized as precursors to rigid, bidentate N-heterocyclic carbene-pyridine ligands. With the goal of exploring the interplay between steric and electronic asymmetry in catalysis, the ligands are constructed with very small (H, Me) or very bulky (2,6-diisopropylphenyl, 2,4,6-triisopropylphenyl) groups adjacent to the nitrogen and carbon donor atoms. Cationic palladium-allyl complexes were synthesized under mild conditions, using triethylamine as base and potassium or silver hexafluorophosphate as counterion source. The structures of three complexes were verified using X-ray crystallography. All four palladium complexes were active catalysts for the allylic substitution reaction between allylic carbonates and either sodium diethyl-2-methylmalonate or N-methylbenzylamine, although significant ligand-controlled effects on regioselectivity were not observed under the conditions studied.