PICOSECOND RADICAL KINETICS - RATE CONSTANTS FOR RING OPENINGS OF (2-ALKOXY-3-PHENYLCYCLOPROPYL)METHYL RADICALS

Rate constants for ring openings of the (trans,trans-2-methoxy-3-pheny lcyclopropyl)methyl radical (1M) and the ans,trans-2-tert-butoxy-3-phe nylcyclopropyl)methyl radical (1B) have been determined between -21 an d 37 degrees 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 repo rted. Radicals 1 rearrange with rate constants of 8 x 10(11) s(-1) (1M ) and 5 x 10(11) s(-1) (1B) at 25 degrees C with predominant (160:1 an d 60:1, respectively) cleavage to give benzylic radical products. The rate constants for ring openings to the minor, alkoxy-substituted radi cal products represent the first measurements of the kinetic effects o f alkoxy substitution on cyclopropylcarbinyl radical ring openings. Pr ecursors to radicals 1 can be employed in mechanistic probe studies th at permit differentiation between radical and cationic intermediates.