Resonance driven regioselective demethylation of berberine. Microwave assisted synthesis of berberrubine and its assessment as fluorescent chemosensor for alkanes. Issue 36 (9th September 2015) - Delgado-Camón, Arantzazu
Abstract: Berberrubine has been synthesized by microwave assisted selective demethylation of berberine. The high selectivity observed in this reaction has been explained and justified by means of computational calculations using Density Functional Theory (DFT) and Natural Resonance Theory (NRT). The existence of two resonant structures of berberrubine is the driving force of regioselective demethylation. Berberrubine is a chemosensor of alkanes, and may have practical applications in petrochemical analysis as a 'mass' detector because fluorescent response of saturated hydrocarbons does not depend on hydrocarbon chain length. Berberrubine operates via dipole-induced dipole interactions. Likewise, it has two fluorescent forms in acidic and basic media, which correspond to a keto-enol tautomerism. The fluorescent signal for berberrubine and the amplification of berberrubine-alkane signals by heating can be rationalized from the predominance of enol form when berberrubine is adsorbed onto silica gel. Graphical abstract: