Picosecond radical kinetics. Rate constants for ring openings of (2-alkoxy-3-phenylcyclopropyl)methyl radicals

Abstract

Rate constants for ring openings of the (trans,trans-2-methoxy-3-phenylcyclopropyl)methyl radical (IM) and the (trans,trans-2-tert-butoxy-3-phenylcyclopropyl)methyl radical (IB) have been determined between –21 and 37 °C by indirect kinetics employing benzeneselenol trapping as the competition reaction. Radicals 1 were formed in chain reactions of the appropriate ‘PTOC esters’, 2-thioxopyridine-N-oxy derivatives of the corresponding carboxylic acids, the syntheses of which are reported. Radicals 1 rearrange with rate constants of 8 × 10¹¹ s^–1(1M) and 5 × 10¹¹ s^–1(1B) at 25 °C with predominant (160:1 and 60:1, respectively) cleavage to give benzylic radical products. The rate constants for ring openings to the minor, alkoxy-substituted radical products represent the first measurements of the kinetic effects of alkoxy substitution on cyclopropylcarbinyl radical ring openings. Precursors to radicals 1 can be employed in mechanistic probe studies that permit differentiation between radical and cationic intermediates.