Reduction of Thiocarbonyl Compounds

A variety of thiocarbonyl derivatives of acids are reduced by organotin compounds by overall conjugate substitution involving addition and elimination, the reactivity depending on the formation of a strong C=0 bond at the expense of a weak C=S bond.146 

Thus secondary or tertiary alcohols ROH can be reduced to RH via their thiocarboxy-lates R C(S)OR, or dithiocarbonates [xanthates, R S(CS)OR the Barton McCombie reaction] 147-149 for example cholesteryl thiobenzoate or S-methyl dithiocarbonate gives cholest-5-ene in 90% and 78% yield respectively (equation 15-47, R = Ph or MeS). The reaction works best for secondary alcohols the 0-t-alkyl xanthates are often rather unstable at room temperature, though the reactions can be carried out at low temperature with initiation by Et3B/02. The primary alkyl xanthates undergo C-O fission only at higher temperatures, when other reactions may compete. The thiocarbonyl imidazolides provide a variant on this theme, and the cholesterol derivative (15-5, R = /V-imidazolc) reacts with tributyltin hydride to give an 18% yield of cholestene.147 

The reduction of the derivatives which carboxylic acids form with the thiohydrox-amic acids (equation 15-48) provides a valuable alternative to the Hunsdiecker reaction for decarboxylating acids RCO2H to RH.107-15°-151 

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