Rate constants for ring openings of the (trans,trans-2-methoxy-3-phenylcyclopropyl)methyl radical (1M) and the (trans,trans-2-tert-butoxy-3-phenylcyclopropyl)methyl radical (1B) 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 x 10" s-' (1M) and 5 x 10" s-' (IB) 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.Paper 5/07840G
The kinetics of cyclization of the 2-methyl-6,6-diphenyl-5-hexeniminyl radical (3) were measured by laser flash photolysis methods from 0 to 50 degrees C, and the kinetics of trapping of radical 3 by thiophenol and p-chlorothiophenol were determined between 4 and 83 degrees C by indirect methods using the unimolecular rate constants as the basis values. Radical 3 cyclizes with a rate constant of 2.2 x 10(6) s(-)(1) at 25 degrees C and reacts with thiophenol and p-chlorothiophenol at this temperature with rate constants of 0.6 and 1.4 x 10(7) M(-)(1) s(-)(1), respectively. Bu(3)SnH reacted with 3 too slowly to prevent nearly complete cyclization, but a crude rate constant for the trapping reaction was estimated. The cyclization and trapping reactions of iminyl radical 3 are slower than the corresponding reactions of alkyl radicals.
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