Rate constants for ring openings of 2-phenylcyclobutylcarbinyl radicals

Kinetics of ring openings of 2-phenyl-substituted cyclobutylcarbinyl radica ls were studied by laser fash photolysis methods employing N-hydroxypyridin e-2-thione (PTOC) esters as radical precursors. The radicals studied were ( trans-2-phenylcyclobutyl)methyl (1a), 1-(trans-2-phenylcyclobutyl)ethyl (Ib ), 1-methyl-1-(trans-2-phenylcyclobutyl)ethyl (Ic), (cis-2-phenylcyclobutyl )methyl (2a), 1-(cis-2-phenylcyclobutyl)ethyl (2b) and (ethoxycarbonyl),(tr ans-2-phenylcyclobutyl)methyl (13). Arrhenius parameters for radicals 1 wer e determined in THF and acetonitrile and those for radicals 2 were determin ed in THF. Rate constants fur ring openings at 20 degreesC in units of 10(7 ) s(-1) are 1.3 (la), 1.0 (Ib), 0.8 (Ic), 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 c an 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 rare constants nearly equal to those of the analo gous methyl-substituted radical lb and, unlike radicals 1 displays a polar solvent effect in the kinetics of the fragmentation reaction. Copyright (C) 2000 John Wiley & Sons, Ltd.