The photochemistry of carbenium ions and related species

113Remarkable strides have been made in recent years in the study of carbenium ions.^{1, 2} While a variety of reasons lie behind this expansion of knowledge, it has been particularly helped by the development of non-nucleophilic super acid media which has allowed for the widespread preparation and characterization of carbocations as long lived species.³ Indeed, techniques for the isolation of carbenium ions have developed sufficiently that many of these species are now readily available, some even being sold commercially. Wide ranging studies, including structure determinations by x-ray crystallography⁴, have been conducted. The stage has now been reached that carbenium ions can no longer be considered as fleeting intermediates or, even if prepared as stable entities, as chemical curiosities, but rather as one of the standard forms in which carbon compounds exist. Perhaps no better indication of the “arrival” of carbenium ions is the recognition that they occur as natural products. The iminium salts of retinal which form the basis of the visual pigment chromophores are prime examples of “natural” carbenium ions.⁵ Other important examples are the anthocyanin pigments which, among other things, are responsible for the color of red wine.².