| Abstract
| - A model for glycol radicals was employed in laser flash photolysis kinetic studies of catalysis of thefragmentation of a methoxy group adjacent to an α-hydroxy radical center. Photolysis of aphenylselenylmethylcyclopropane precursor gave a cyclopropylcarbinyl radical that rapidly ringopened to the target α-hydroxy-β-methoxy radical (3). Heterolysis of the methoxy group in 3 gavean enolyl radical (4a) or an enol ether radical cation (4b), depending upon pH. Radicals 4 containa 2,2-diphenylcyclopropane reporter group, and they rapidly opened to give UV-observablediphenylalkyl radicals as the final products. No heterolysis was observed for radical 3 under neutralconditions. In basic aqueous acetonitrile solutions, specific base catalysis of the heterolysis wasobserved; the pKa of radical 3 was determined to be 12.5 from kinetic titration plots, and the ketylradical formed by deprotonation of 3 eliminated methoxide with a rate constant of 5 × 107 s-1. Inthe presence of carboxylic acids in acetonitrile solutions, radical 3 eliminated methanol in a generalacid-catalyzed reaction, and rate constants for protonation of the methoxy group in 3 by severalacids were measured. Radical 3 also reacted by fragmentation of methoxide in Lewis-acid-catalyzedheterolysis reactions; ZnBr2, Sc(OTf)3, and BF3 were found to be efficient catalysts. Catalytic rateconstants for the heterolysis reactions were in the range of 3 × 104 to 2 × 106 s-1. The Lewis-acid-catalyzed heterolysis reactions are fast enough for kinetic competence in coenzyme B12 dependentenzyme-catalyzed reactions of glycols, and Lewis-acid-catalyzed cleavages of β-ethers in radicalsmight be applied in synthetic reactions.
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