Rate constants for ring openings of 2-phenylcyclobutylcarbinyl radicals

Kinetics of ring openings of 2-phenyl-substituted cyclobutylcarbinyl radicals were studied by laser flash photolysis methods employing N-hydroxypyridine-2-thione (PTOC) esters as radical precursors. The radicals studied were (trans-2-phenylcyclobutyl)methyl (1a), 1-(trans-2-phenylcyclobutyl)ethyl (1b), 1-methyl-1-(trans-2-phenylcyclobutyl)ethyl (1c), (cis-2-phenylcyclobutyl)methyl (2a), 1-(cis-2-phenylcyclobutyl)ethyl (2b) and (ethoxycarbonyl),(trans-2-phenylcyclobutyl)methyl (13). Arrhenius parameters for radicals 1 were determined in THF and acetonitrile and those for radicals 2 were determined in THF. Rate constants for ring openings at 20 °C in units of 107 s−1 are 1.3 (1a), 1.0 (1b), 0.8 (1c), 3.2 (2a) and 3.9 (2b). These ring openings are more than three orders of magnitude faster than those for the parent cyclobutylcarbinyl radicals lacking the phenyl groups. The family can be used in direct studies of radical substituent effects and as internal reporter groups for other radical reactions. A demonstration of the former application is the measurement of the kinetics of ring openings of radical 13 which rearranges with rate constants nearly equal to those of the analogous methyl-substituted radical 1b and, unlike radicals 1 displays a polar solvent effect in the kinetics of the fragmentation reaction.