Revealing Phenylium, Phenonium, Vinylenephenonium, and Benzenium Ions in Solution

Abstract

The 4-methoxyphenylium ion has been generated in the triplet state ((3)An(+)) by photolysis of 4-chloroanisole in polar media and detected by flash photolysis (lambda(max)=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 ((3)An(+)+AnCl-->An(*)+AnCl(*+), with the radical cation detected at 470 nm, then An(*)-->AnH). Addition of pi nucleophiles to the (3)An(+) 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 beta-alkoxyalkylanisoles are the end products, with a small amount of alpha-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 (lambda(max)=340 nm) and the benzenium ion with benzene (lambda(max)=380 nm). The final products were anisylhexyne and methoxybiphenyl (an analogous product and intermediate were detected with thiophene).