REGIOCONTROLLED ANODIC CYANATION OF NITROGEN-HETEROCYCLES - PYRROLES AND INDOLES

A series of 1-substituted pyrroles and indoles was potentiostatically oxidized at platinum sheet. The anolyte was methanol-sodium cyanide and the reference electrode an SCE. In all instances substitution with cyanide ion was achieved. Annular replacement at a free 2 (or 5) position was favored for pyrroles lacking the substituent on either 2 or 5 (or both) position(s), whereas lateral substitution was favored for compounds with the methyl group on both 2 and 5 positions. Methyl displacement by the cyano group occurred slightly in some cases. The methyl or phenyl group at the 1 position was not attacked. Cyanation of indoles occurred exclusively on the pyrrole moiety of the molecule. In this case side-chain substitution was not observed. A scheme involving initial electron loss from the organic substrates, followed by a fast chemical reaction, can well elucidate products, coulometric data, and voltammetric results. A comparison with the photosensitized electron transfer cyanation in the same solvent system indicates that the above anodic system involves initial cation radical formation. Pyrrolecar-bonitriles, annular substitution products, come from direct nucleophilic attack by cyanide ion on methylpyrrole cation radicals prior to competitive deprotonation from a methyl substituent. An MO calculation for the cation radicals by the ω technique supports the observed positional reactivity. © 1979, American Chemical Society. All rights reserved.