Lewis acid-catalyzed formation of triply S-confused hexaphyrin (1.0.1.0.1.0) and its structure-property correlations

Abstract

A simple rigid precursor termed as a “diheterole” bearing thiophene linked to [Formula: see text]-pyrrole at 4-position was prepared using a three-step synthetic strategy. This functionalized diheterole was allowed to undergo acid-catalyzed condensation in the presence of Lewis acid (BF3 [Formula: see text] Et2O) to produce the [2+2+2] cyclotrimer 1 as a major product with trace formation of 2and higher homologues.Various 1D and 2D NMR analyses along with theoretical investigations indicate that the molecule 1 exhibits nonaromaticity due to its non-conjugated annulenic structure. The optical properties and electronic structures were analyzed using UV-vis absorption spectroscopy and time-dependent density functional theory calculations. Detailed structural analyses revealed that the conformationally rigid triply S-confused hexaphyrin 1 adopts nearly planar geometry with three thiophene rings inverted. Upon protonation, the individual rings are tilted largely due to the steric congestion by both [Formula: see text]-CH’s and pyrrolic NH’s inside the core. The calculated NICS(0) values of +1.73 ppm for the protonated species, 1 [Formula: see text] 3H[Formula: see text] indicates the distinct non-aromatic feature as observed in 1 (NICS value of +2.34 ppm).