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 toX-4+in aqueous solution. The existing relationships between carbocation lifetime and nucleophilic selectivity, and the known lifetime ofca140 sec forspiro[2.5]oct-4,7-diene-6-one provide rough estimates of the lifetimes for4-Me-X-4+and4-MeO-X-4+in aqueous solution. Evidence is presented that nucleophile addition toX-4+proceeds through an "exploded" transition state, with relatively weak bonding the nucleophile and leaving group, and the development of significant positive charge at the reacting primary cyclopropyl carbon.
Department of Biochemistry and Applied Chemistry, Kurume National College of Technology, Komorinomachi, Kurume 830-8555, Japan.
Yutaka
Y
Tsuji
Department of Chemistry, University at Buffalo, SUNY, Buffalo, NY 14260, USA.
John P
JP
Richard