Highly Specific Reciprocal Methyl/Hydrogen Transfer Reactions Preceding Some Unimolecular Dissociations of Crowded Enol Cation Radicals in the Gas Phase as Examples for Conformationally Controlled Processes

The molecular ions of α-(o-methyl)-substituted triarylethenols contain in their metastable ion (MI) mass spectra signals for radical losses that are derived from highly site specific reciprocal CH₃/H migrations. The study of analogues, including some deuterium-labeled isotopomers, reveals the following unusual features: The ortho position of the α-aryl ring is the migration origin, and the ipso position of the β-ring (cis to it) is the terminus of the formal [1,5]-CH₃ migration. The hydrogen transferred back to the a-ring stems exclusively from the migrating CH₃ group. Other methyl rearrangements, from the β-to the a-ring or between the β-and the β′-rings, resulting in the eventual loss of a radical were not observed. No evidence was obtained for the operation of hydrogen scrambling processes. For the overall reaction, i.e., CH₃/H transfer and radical loss, a kinetic isotope effect is not operative. The rearrangement is facilitated by the proximity of the migration origin and terminus in the propeller conformation of the crowded enols. The stereochemical and electronic properties of the C_β-aryl groups exert a directing influence on the direction of the CH₃ transfer. Steric and electronic effects in the products generated apparently determine the detailed course of the reaction in that the elimination of the β-aryl ring together with the transferred “CH₂” unit as a benzyl-type radical results exclusively in the generation of an α-aryl-substituted vinyl cation 20, whereas a similar elimination of the α-aryl ring will generate the less stable isomeric ion 24. When the cation resulting from the rearrangement is unstable, e.g., that obtained from the substituted indene 32, the reciprocal CH₃/H migration followed by radical loss is of minor importance.