Efficient and Chemoselective Reduction of Pyridines to Tetrahydropyridines and PiperidinesviaRhodium-Catalyzed Transfer Hydrogenation

Abstract Promoted by iodide anion the rhodium complex dimer, [Cp*RhCl 2 ] 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‐Et 3 N) mixture at 40 °C, with catalyst loadings as low as 0.005 mol% being feasible.