Polyenyl cations and radical cations – synthesis, spectroscopic properties and reactions

Abstract

In the present work, polyene precursors for the formation of charged polyenyl species were synthesised. Their conversion to charge delocalised radical cations and diamagnetic mono- and dications was carried out by treatment with Brønsted and Lewis acids. Reaction conditions were optimised to achieve sufficient stability of the charged polyenes for characterisation by modern spectroscopic methods; in particular nearinfrared absorption spectroscopy (NIR), two-dimensional nuclear magnetic resonance (2D NMR), electron paramagnetic resonance (EPR) and dynamic light scattering.

The full assignment of 13C NMR chemical shifts of mono- and dications was used as basis for charge distribution estimates in charge delocalised polyenes. Coulombic repulsions are dominant features in dicationic systems, whereas monocations show a more even distribution of charge throughout the polyene system. The size limit of charge delocalisation regions, suggested by current theories, has been investigated using an empirical approach. Treatment of the cationic polyenes with nucleophiles gave conversion to neutral compounds, which were characterised by mass spectrometry (MS) and ultraviolet/ visible absorption spectroscopy (UV/Vis), in addition to NMR. In most cases, theidentified neutral compounds supported the structural characterisation of their charged precursors.

The work is of relevance to the classical blue colour reactions of polyenes developed in the early 20th century. The chemistry behind these reactions has previously not been elucidated. Only during the last 15-20 years has the instrumental methods necessary for the study of this chemistry been available.

In current studies on the colouration mechanism of the carotenoproteins present in external tissues of some marine organisms, a polarisation of the polyene chain appear to be of importance. Moreover, radical cations of polyenes are unstable intermediates in Nature, where they are shown to be of importance in photosynthetic processes in green plants and bacteria.

The delocalised polyenylic cations studied here, representing a new direction of structural studies in the carotenoid/retinoid field, are of interest in a wider context for carbocation structure and stability in basic organic chemistry and the various applied aspects outlined above.