ABSTRACT
Simon Robitaille, Geneviève Clément, Jean Marc Chapuzet and Jean Lessard
Laboratoire de Chimie et Electrochimie Organiques, Département de Chimie, Université de Sherbrooke, Sherbrooke, Québec, J1K 2R1 Canada
Jean.Lessard@USherbrooke.ca
Abstract The selectivity of the electrocatalytic hydrogenation (ECH) method for the reduction of nitro compounds to the corresponding amines is compared with that of reduction by Raney copper (RCu) alloy powder in alkaline aqueous ethanol. In the former method, chemisorbed hydrogen is generated in situ by electrochemical reduction of water. In the latter method (termed “chemical catalytic hydrogenation” (CCH)), chemisorbed hydrogen is also generated in situ but by reduction of water by aluminium (by leaching of the alloy). Finally, the selectivity and efficiency of the electrochemical reduction of 5-nitro-indoles, -benzofurane, and -benzothiophene at RCu electrodes in neutral and alkaline aqueous ethanol is compared with that of the classical reduction with zinc in acidic medium. Introduction The electrocatalytic hydrogenation (ECH) of a nitro group to the corresponding amine in neutral or basic aqueous or mixed aqueous-organic media is described by equations [1] to [7] where M represents an adsorption site of the catalyst and M(H) chemisorbed hydrogen, and M(RNO2), M(RNH(OH)2), and M(RNHOH) represent
the adsorbed organic substrate and adsorbed reduction intermediates. The stoichiometry 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 classical catalytic hydrogenation (CH). In both ECH and CH, the hydrogenolysis of the adsorbed dihydroxylamine, M(RNH(OH)2) (step [4]), could be faster than its dehydration to the adsorbed nitroso derivative M(RNO) (step [5]) (or alternatively, than its desorption followed by dehydration to RNO (not shown)).
