Electrophilic substitution in indoles. Part 15. The reaction between methylenedi-indoles and p-nitrobenzenediazonium fluoroborate

Abstract

p-Nitrobenzenediazonium fluoroborate (2 mol equiv.) reacts under aqueous conditions with 3,3′methylenedi-indole (9a) and N,N-dimethyl-3,3′-methylenedi-indole (9c) to give, in each instance, high yields of the corresponding 3-p-nitrophenylazoindoles (13a) and (13c). When 1 mol equiv. of diazonium salt is used, the methylenedi-indole (9a) gives < 1 mol equiv. of the 3-(p-nitrophenylazo)indole (13a) and some methylenedi-indole is recovered. In dry acetonitrile, 3,3′methylenedi-indole gives, with 1 mol equiv. of diazonium salt, a mixture of 3-(p-nitrophenylazo)indole (13a)(0.45 mol equiv.) and the unstable 3-(p-nitrophenylazo)-2,3′-methylenedi-indole (16a)(0.42 mol equiv.). The latter was also synthesized from indole-2,3-dione and chloroacetylindole via indol-2-yl, indol-3-yl, and 2,3′methylenedi-indole. Essentially similar results were obtained with N,N-dimethyl-3,3′-methylenedi-indole (9c) and the diazonium salt. The unsymmetrical compounds 3-indol-3-ylmethyl-1,2-dimethylindole (9d) and 3-indol-3-ylmethyl-2-methylindole (9e), with 2 mol equiv: of diazonium salt in aqueous solution, each give high yields of the two possible 3-p-nitrophenylazoindoles. With 1 mol equiv. of the diazonium salt only the 2-methyl-3-p-nitrophenylazo derivatives are obtained, together with 3,3′methylenedi-indole. In dry acetonitrile, with either 1 or 2 mol equiv. of diazonium salt, the unsymmetrical methylenedi-indoles each give only the 2-methyl-3-p-nitrophenylazoindoles. The displacement of an indolylmethyl residue from methylenedi-indoles by the diazonium salt, and the formation of the azo coupled rearrangement product (16a) provides firm evidence that azo coupling of 3-alkylindoles to give the 2,3-disubstituted indoles proceeds by initial attack at the 3-position followed by rearrangement, rather than by direct substitution at the 2-position.