Catalysis of Organic..

286 Chemoselective HydrogenationCNNO 2NH 2OHCNCNCNCNCN21 22 23 24 25 26CCH (pH 12.5) 84% 14% 5%ECH (pH 12.5) 26% 36% 17% 8%ECH (pH 7) (1, 3) 91% -- - -Scheme 7tron transfer to 21 (equation [8]) followed by fast cleavage of the resulting radicalanion (11) (Scheme 8) and the electron transfer does compete with reaction withchemisorbed hydrogen (equations [2] and [3]) as previously shown (12).n.dCNn.d18%NO 2NO 2..CNeeCNk c( NO 2 )fastCN21Couplinge , H1/2 NCCN26CN25Scheme 8ECH of 5-nitroindoles, 5-nitrobenzofurane, and 5-nitrobenzothiopheneAs shown in Scheme 9, the ECH of the 5-nitro derivatives 26 of the indole family inEtOH-H 2 O 50:50 (v/v) gives higher yields of 5-amino derivatives 27 (range overthree experiments) in basic medium (0.15 M KOH, pH ≈ 13) than in neutral medium(acetate buffer, pH ≈ 6) (13, 14). By comparison, reduction under the classical acidicconditions (Zn/HCl in the same solvent) gave a mixture of 5-amino derivative 27 inthe yields indicated in Scheme 9 and of 4-chloro-5 amino derivatives 28 (see inset ofScheme 9) in 15%, 17%, 8% and 23% yield for X = NH, NCH 3 , O, and Srespectively (13, 14).