In several papers^1,3 from this laboratory, it has been shown that the formic acid reduction provides a practical method effective for selective reduction of electron-deficient carbon-carbon double bonds. The formic acid reduction of carbon-carbon double bonds conjugated with carbonyl,^3a cyano,^1,3b nitro^3b, and sulfonyl^3b group have been informed previously, but the last two only in our short communication^3b. We, therefore, wish to disclose in detail together with extensive studies.

An attractive route for synthesis of nitroalkanes involves selective reduction of conjugated nitroalkenes, the method for which previously reported includes catalytic hydrogenation⁴ and reduction with complex metal hydrides such as sodium borohydride,⁵ lithium borohydride⁶ and sodium trimethoxyborohydride.⁶ Extensive investigation of formic acid reduction of conjugated alkenes has developed a successful procedure for selective reduction of the carbon-carbon double bonds of conjugated nitroalkenes.

Conjugated nitroalkenes were efficiently reduced to the corresponding nitroalkanes by the use of the formic acid-triethylamine azeotrope (TEAF reagent⁷) in N,N-dimethylformamide (DMF) at 100-135°. The results of reduction of a number of nitrostyrenes are summarized in Table I. The failure of unsubstituted β-nitrostyrene to undergo the reduction is attributed to its polymerization. As previously known,⁸ α,β-unsaturated nitroalkenes are susceptible to the base-catalyzed polymerization. The reduction in such basic medium is accompanied by the competing polymerization which, as illustrated in Table I, apparently depends on steric influence of the olefinic substituents.

In extensive studies, 3,6-dinitrocoumarin (III) was reduced at room temperature to give 2-(2-nitroethyl)-4-nitrophenol (IV) in 37% yield. Process of this reaction may involve hydrolysis and successive decarboxylation after the reduction of the 3,4-double bond. 2,4-Dinitrostilbene did not undergo the reduction, whereas α-(p-nitrophenyl)cinnamonitrile (V) was reduced at 90-91°C to give α-(p-nitrophenyl-β-phenylpropionitrile (VI) in 73% yield.

Nature of electron-withdrawing group, with which carbon-carbon double bond is conjugated, is responsible for reactivity for the formic acid reduction. As reported previously,^3b effect of the electron-withdrawing groups is in the order; NO₂>CN>SO₂C₆H₅>COC₆H₅. Carbon-carbon double bonds conjugated with a nitro group can undergo the formic acid reduction; as indicated in the foregoing. However, groups such as cyano, sulfonyl, carbonyl and ethoxycarbonyl are not so efficient that carbon-carbon double bonds effective for the reduction have been recognized to be conjugated doubly with them at one side of the olefinic carbons.

Further experiments included an examination of the formic acid reduction of a series of alkenes conjugated with sulfonyl group. Results are summarized in Table II. β-Phenylsulfonylstyrene was inert to the reduction even at 150°C, whereas styrenes conjugated with both sulfonyl and other electron-withdrawing group underwent the reduction to give the corresponding reduction products. β,β-bis-(phenylsulfonyl)styrene was reduced to 1-phenylthio-1-phenylsulfonyl-2-phenylethane, where one of the phenylsulfonyl grouping was reduced to phenylthio grouping.