Efficient and Chemoselective Reduction of Pyridines to Tetrahydropyridines and Piperidines via Rhodium-Catalyzed Transfer Hydrogenation

Promoted by iodide anion the rhodium complex dimer, [Cp*RhCl2]2, catalyzes efficiently the transfer hydrogenation of various quaternary pyridinium salts under mild conditions, affording not only piperidines but also 1,2,3,6-tetrahydropyridines in a highly chemoselective fashion, depending on the substitution pattern at the pyridinium ring. The reduction is conducted in azeotropic formic acid/triethylamine (HCOOH-Et3N) mixture at 40?degrees C, with catalyst loadings as low as 0.005 mol% being feasible.