Multiple transformations of 2-alkynyl-1,8-bis(dimethylamino)naphthalenes into benzo[g]indoles. Pd/Cu-dependent switching of the electrophilic and nucleophilic sites in acetylenic bond and a puzzle of porcelain catalysis.

Abstract

By means of Sonogashira reaction, a series of 2-alkynyl- and 2,7-dialkynyl derivatives of 1,8-bis(dimethylamino)naphthalene ("proton sponge") have been obtained from the corresponding iodides. It was disclosed that changing the reaction conditions and isolation protocol or conducting the model experiments with the authentic acetylenes results in several types of palladium- and copper-assisted heterocyclizations with the participation of the C≡C bond and 1-NMe2 group. These include: (i) a cyclization into isomeric 1H-benzo[g]indoles with [1,3] migration of the N-methyl group into the newly formed pyrrole ring; (ii) a similar cyclization with a loss of the methyl group; (iii) a tandem process of cyclization into benzo[g]indoles and their subsequent 3,3'-dimerization; and (iv) a copper-catalyzed oxidative transformation into 3-aroylbenzo[g]indoles. In most cases, the reactions occur in parallel, but under certain conditions, one of the above products becomes predominant or even the only one. Remarkably, in Pd-catalyzed cyclizations i-iii, the acetylenic bond behaves as an electrophile being attacked at the β-position by the amine nitrogen atom. In contrast, in transformation iv, the C≡C bond attacks by its Cα atom on the aminomethyl radical functionality N(Me)-CH2· presumably arising at copper oxidation/deprotonation of the 1-NMe2 group. Studying rearrangement i, some evidence for the porcelain catalysis was obtained.