Arylation of cyclic ethers by gaseous phenylium ions. Formation and behavior of phenoxenium ions in the gas phase

Abstract

Free, unsolvated phenylium ions formed by the spontaneous decay of 1,4-ditritiobenzene have been allowed to react with gaseous cyclic ethers (oxirane, propene oxide, and oxetane) and acetaldehyde in the pressure range 30–250 Torr and in the presence of a thermal radical scavenger (O2, 4 Torr). The effects of a gaseous base (NMe3, 20 Torr) and of an energy moderator (He, 630–720 Torr) were also investigated. Phenylium ion confirms its considerable site selectivity, demonstrated by the distinct preference for the n-type center of the substrate, although appreciable insertion into the carbocyclic structure of propene oxide and oxetane is observed as well. The stability features of the ionic intermediates from addition of phenylium ion to selected substrates have been evaluated as well as their fragmentation and isomerization mechanisms. The behavior of gaseous phenylium ion toward cyclic ethers, in particular its ability to formally abstract an oxygen atom from the ether to give the phenoxenium ion, a reaction first observed in the present study, is discussed and compared with previous mechanistic investigations carried out in the gas phase and in solution.