Catalysis of Organic..

Robitaille, Clément, Chapuzet and Lessard 283Results and DiscussionReduction of nitroaryl groupsNitrobenzene (1) was used as a model to determine the best conditions for the“chemical catalytic hydrogenation” (CCH) using the Raney Cu alloy. In this work,we used ethanol as organic co-solvent because it is greener than methanol. The bestconditions for CCH were 0.05 M KOH in ethanol-water (45:55 v/v) (pH ≈ 12.5), atroom temperature and under a nitrogen atmosphere, using an amount of alloycorresponding to 3 equation of Al. The conversion was complete after 5 h and anilinewas obtained in a quantitative yield (100% by vapor phase chromatography). TheECH of nitrobenzene at Raney Cu electrodes in alkaline aqueous methanol givesaniline in a nearly quantitative yield (99%) (7).When these CCH conditions were applied to o-iodonitrobenzene (1), o-iodoaniline (2) together with iodoaniline (4) and nitrobenzene (1) were obtained inthe yields indicated in Scheme 1 (88% mass balance), which corresponds to a 62%selectivity for the formation of o-iodoaniline (3). Under the same conditions, theECH of o-iodonitrobenzene (2) gave aniline (90% yield) as the sole product. Therewas complete hydrogenolysis of the C−I bond (no selecti-vity). Thus, in basicmedium (pH ≈ 12.5), CCH method is much more selective than ECH. However, inweakly acidic medium (pH 3, pyridine.HCl buffer), it has been reported that ECH ata RCu cathode in methanol-water 95:5 (v/v) gave o-iodoaniline in a 97% yield (1, 3).NO 2NH 2NH 2 NO 2II2 3 4 1CCH (pH 12.5) 55% 28% 5%ECH (pH 12.5) 0% 90% 0%ECH (pH 3) (1, 3) 97% n.d n.dScheme 1The CCH of p-nitroacetophenone (5) (Scheme 2) was inefficient and unselectivegiving p-aminoacetophenone (6) in about 20% yield together with some 10-12% ofunindentified compounds (30% mass balance). Attempts to recover more materialwere unsuccessful. The ECH at a RCu cathode in an alkaline (0.28 M KOH, pH ≈13.5) MeOH-H 2 O (1.5% of H 2 O) solution was reported to give exclusively p-aminoacetophenone(6) (79% yield of isolated product) (3,8).OONO 2NH 25 CCH (pH 12.5) 20% 6ECH (pH 13.5) (3,8) 79%Scheme 2