Synthesis ofN,N′-diacylindigotins (N,N′-diacyl-2,2′-bi-indolinylidene-3,3′-diones)via an oxidative oxygen-to-nitrogen acyl shift of O,O′-diacyl-leucoindigos(3,3′-diacyloxy-2,2′-bi-indolyls)

Abstract

O,O′-Diacyl-leucoindigos (3f–j), which were readily obtained by the reaction of leucoindigo disodium salt (4) with acyl chlorides, underwent rapid oxidation by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) to generate the indigo chromophore with concomitant shift of the acyl groups intramolecularly from oxygen to nitrogen. A number of N,N′-diacylindigotins with functionalized acyl groups were prepared by this method [(2a), (2f–j)] and the direct N-acylation method [(2a–e)]. A large bathochromic shift of the visible absorption band in the cis form of N,N′-diacylindigotins with bulky acyl groups (2c), (2d), and (2j) was observed which suggested that the electronic structure of the indigo chromophore is perturbed by the steric constraint of the N-acyl groups.