Radical ion reactivity—I : Application of the dewar-zimmerman rules to certain reactions of radical anions and cations

Abstract

The application of the Dewar-Zimmerman rules to the interaction between radical cations, derived from 4n + 2 systems, shows that a nucleophile orbital interacting suprafacially with the ion should correspond to an antiaromatic transition state and hence to a less favored pathway relative to competing ones, e.g. electron transfer. An antarafacial interaction would on the other hand correspond to an aromatic transition state and be energetically favorable. Both types of interaction are generally possible for the same reagent, but since the suprafacial one for geometric reasons ensues and thus predominates in the early stage of the reaction, the net result should be an anomalously low reactivity of radical cations vs attack by nucleophiles. By calibration against known, qualitative reactivity data for perylene radical cation it is strongly indicated that halide ions belong to a class of reagents which do not react nucleophilically with radical cations but instead undergo electron transfer oxidation or do not react at all. This type of reaction is discussed in some detail. and several mechanisms involving radical cation/halide ion combination as a critical step can either be ruled out or considered open for reinvestigation. The same idea can be applied to the reaction between radical anions and electrophiles. Accordingly, the protonation of radical anions derived from 4n+2 aromatic systems is remarkably slow, as compared to that of analogous carbanions.