The development of organic reactions with metallic samarium

Abstract

The chemistry of metallic samarium (Sm) is of current interest in organic synthesis. Though samarium iodide (SmI2) is a good synthetic tool (a mild, neutral and ether-soluble one-electron reductant), its sensitivity to air makes it rather difficult to handle. We have been studying the reactivity of Sm to overcome this drawback of SmI2. Dehalogenation of vic-dihalides with Sm gave alkenes and alkynes. These reductions were accelerated with a catalytic amount of acid. Olefinic double bonds of various α, β-unsaturated carboxylic acid derivatives (ester, amide, nitrile, carboxylic acid) were also reduced to give the corresponding 1,4-reduction products. Aromatic carbonyl compounds were reduced to the corresponding pinacol coupling products with Sm-I2 or Sm-I2-Ti(OiPr)4 in methanol. Meso isomers were mainly produced in the presence of Ti(OiPr)4. Acyl- or alkoxycarbonyl-protected alcohols or amides are deprotected easily and selectively under mild and neutral conditions. Stereoselective allylation and dimerization of optically active imines with Sm and a catalytic amount of I2 have been successfully accomplished. Both Barbier-type allylation and intermolecular pinacol-like coupling reaction of imines were carried out smoothly to give allylation products and C2-2,2′-ethylenediiminodiethanol derivatives in a highly diastereoselective manner. This novel C2-dimeric amino alcohols bearing primary alcohols catalyzed enantioselective addition of diethylzinc to aldehydes.