Formation and mechanism for reactions of ring-substituted phenonium ions in aqueous solution

The results of studies on the structure and reactivity of spiro[5.2]oct-5,7-diene-4yl carbocation [phenonium ion] have had a significant impact on the course of discussion about the distinction between classical and nonclassical carbocations. This minireview will present a brief overview of the structure, bonding, and reactivity of ring-substituted phenonium ions (X-4⁺), with an emphasis on work completed since 2004. The discussion will focus on the development of new experimental protocol for determination of the selectivity for addition of nucleophilic anions to X-4⁺ in aqueous solution. The existing relationships between carbocation lifetime and nucleophilic selectivity, and the known lifetime of ca 140 s for spiro[2.5]oct-4,7-diene-6-one provide rough estimates of the lifetimes for 4-Me-4⁺ and 4-MeO-4⁺ in aqueous solution. Evidence is presented that nucleophile addition to X-4⁺ proceeds through an “exploded” transition state, with relatively weak bonding of the nucleophile and leaving group, and the development of significant positive charge at the reacting primary cyclopropyl carbon.