Site Selectivity in the Gas‐Phase Ionic Phenylation of Alcohols and Alkyl Chlorides

Abstract

AbstractFree, unsolvated phenylium ions formed by the spontaneous β decay of [1,4‐3H2]benzene have been allowed to react with gaseous alcohols (ROH: R = Et, CF3CH2, Pr, or i‐Pr; partial pressure: 3–56 Torr) and alkyl chlorides (R′Cl: R′ = Pr, i‐Pr, or Bu; partial pressure: 20–450 Torr), in the presence of a thermal radical scavenger (O2: 4 Torr). Phenylium ion confirms its considerable site selectivity, demonstrated by the distinct preference toward the n‐centre of the substrate (46–100%), although significant insertion into the alkyl group of alcohols is observed as well. Phenylium ion displays significant positional selectivity even between different n‐type sites in a bidentate molecule such as CF3CH2OH. An affinity F < O < Cl trend is observed, which indicates a direct relationship between the polarizability of the n‐centre of the molecule and its orienting properties toward phenylium ion. The stability features of the ionic intermediates from addition of phenylium ion with ROH or R′Cl have been evaluated, as well as their fragmentation and isomerization mechanisms. The behaviour of phenylium ion toward the selected substrates in the gas phase is discussed and compared with previous mechanistic hypotheses from related nuclear‐decay studies.