[Rapid reduction of a variety of organic functionalities including a new selective reduction with samarium diiodide]

This review deals with the rapid reduction of a variety of organic functionalities, including a new selective reduction of carboxylic acid, with samarium diiodide in the presence of additives under mild conditions. The single electron donor ability of samarium diiodide can be enhanced with ligands around Sm2+ when sufficient electrons are supplied from the ligands. Mechanistic consideration of this enhancement led to our finding that carboxylic acid, ester, amide, nitrile, pyridine and other functionalities were rapidly reduced with samarium diiodide in the presence of a base, acid, water and methanol as ligands at room temperature in good yields. Particularly, these systems directly reduced carboxylic acid into alcohol and a samarium diiodide--85% phosphoric acid system immediately reduced an aromatic primary amide to aldehyde in good yield. Pyridines were reduced to piperidines with a samarium diiodide-base or water system in excellent yields; also, the aromatic nucleus of phenol derivatives was rapidly reduced with a samarium diiodide-base system. Furthermore, these organic functionalities were rapidly reduced with an Sm or Yb metal-hydrochloric acid system. Aliphatic and aromatic carboxylic acids with a coexisting aldehyde or that bearing a formyl group were selectively reduced to the corresponding alcohols with a new samarium diiodide-samarium triflate-methanol-base or water system within a few minutes at room temperature in good to almost quantitative yield.