Revealing phenylium, phenonium, vinylenephenonium, and benzenium ions in solution.

The 4-methoxyphenylium ion has been generated in the triplet state (3An+) by photolysis of 4-chloroanisole in polar media and detected by flash photolysis (λmax=400 nm). This is the first detection of a phenylium ion in solution by flash photolysis and the assignment is supported by time-dependent density functional theory (TD-DFT) calculations. In neat solvents, the cation was reduced to anisole, a process initiated by electron transfer from the starting compound (3An++AnCl→An.+AnCl.+, with the radical cation detected at 470 nm, then An.→AnH). Addition of π nucleophiles to the 3An+ cation offers a novel access to a number of other cationic intermediates under mild, nonacidic conditions. Two intermediates are successively formed with alkenes, a diradical cation and the phenonium ion, which are detected at 440 and 320 nm, respectively, by flash photolysis and are in accordance with calculations. Allylanisoles or β-alkoxyalkylanisoles are the end products, with a small amount of α-alkoxyalkylanisoles that arises from a Wagner–Meerwein rearrangement to form benzyl cations. Further intermediates that have been predicted and detected are the phenylvinylium ion, possibly in equilibrium with the vinylenephenonium ion, with 1-hexyne (λmax=340 nm) and the benzenium ion with benzene (λmax=380 nm). The final products were anisylhexyne and methoxybiphenyl (an analogous product and intermediate were detected with thiophene).