Catalysis of Organic..

282 Chemoselective Hydrogenationthe adsorbed organic substrate and adsorbed reduction intermediates. Thestoichiometry applies to the adsorbed species only, not to the adsorption site M (1-3).ECH involves the same hydrogenation steps (steps [2] to [7]) as those of classicalcatalytic hydrogenation (CH). In both ECH and CH, the hydrogenolysis of theadsorbed dihydroxylamine, M(RNH(OH) 2 ) (step [4]), could be faster than itsdehydration to the adsorbed nitroso derivative M(RNO) (step [5]) (or alternatively,than its desorption followed by dehydration to RNO (not shown)).On Ni, Co, and Cu electrodes, two mechanisms are competing for theelectrochemical reduction of nitro groups to hydroxylamines: 1) the ECH mechanism(equations [1] to [4]); and 2) electron transfer to the nitro group (and to theintermediates formed) followed by protonation by water (electronation-protonation(EP) mechanism) (equations [8] to [11]) (3, 4). For the hydro-genolysis of thehydroxylamine to the amine, the sole mechanism operating is the ECH mechanism(equation [4]). This is because unprotonated hydoxylamines (pH > 5) are notreducible by electron transfer (EP mechanism) (5, 6). For that same reason, theelectrohydrogenation of nitro compounds at a high hydrogen overvoltage cathode(Hg, glassy carbon) in neutral (pH > 5) and basic medium stops at the hydroxylamine(5, 6).[8] RNO 2 + eRNO 2. .[9] RNO 2 + H 2 O RNO 2 H + OH.[10] RNO 2 H + e RNO + OH[11] RNO + 2e + 2H 2 O.RNHOH+ 2OHIn this paper, the selectivity of the ECH method for the reduction of nitrocompounds to the corresponding amines on RCu electrodes will be compared withthat of reduction by RCu alloy powder in alkaline aqueous ethanol. In the lattermethod (termed chemical catalytic hydrogenation (CCH)), chemisorbed hydrogen isgenerated in situ but by reduction of water by aluminium (by leaching of the alloy)(equation [12]). The reductions by in situ leaching must be carried out in a basicmedium in order to ensure the conversion of insoluble Al(OH) 3 into solublealuminate (equation [12]). The selectivity and efficiency of the electrochemicalreduction of 5-nitro-indoles, -benzofurane, and -benzothiophene at RCu electrodes inneutral and alkaline aqueous ethanol will also be compared with that of the classicalreduction with zinc in acidic medium.[12] CuAl + 3H 2 O + HO3Cu(H) + Al(OH) 4