Thermal and photochemical reactions of n-pyridinebenzopyrylium multistate of species (n = 2′,3′,4′). Exploring the synthetic potentialities from the unique reactivity of position 2′

Abstract

The kinetics and thermodynamics of the pH-dependent multistate of species generated by the trans-chalcone of n-pyridinebenzopyrylium (n = 2′, 3′) were studied by UV–vis spectroscopy, 1H NMR and HPLC-MS, and the results compared with those reported for n = 4′. Due to the slow kinetics of the multistate species interconversion, the conjugation of these techniques has shown to be a powerful tool to investigate the behaviour of these systems. The species involved in the multistate are mutatis mutandis the same observed in anthocyanins and related compounds except for the flavylium cation, which was not observed in these systems even in very acidic medium. The rates of the interconversion of the multistate species upon pH stimuli are much slower than in anthocyanins. The compound bearing the pyridine nitrogen in position 2′ gives two novel products absorbing in the visible. Formation of the new products is particularly efficient from the thermal evolution of the photochemical products obtained upon light irradiation of the protonated trans-chalcone in a mixture of methanol:acidic water (1:1). This confirms the unique capacity of the substituents in position 2′ in giving intramolecular reactions involving the benzopyrylium core. Crystal structures for the three pyridine chalcone compounds (n = 2′, 3′, 4′) were obtained and the respective structures discussed.