PICOSECOND RADICAL KINETICS - FAST RING OPENINGS OF CONSTRAINED, ARYL-SUBSTITUTED CYCLOPROPYLCARBINYL RADICALS

The kinetics of ring openings of the exo- and endo-(2,3-benzobicyclo [3.1.0]hex-2-en-6-yl)methyl radicals (4a and 5a), the trans-(spiro[cyclopropane-1,1'-indan]-2-yl)methyl radical (6a), and the (spiro[cyclopropane-1,9'-fluorene]-2-yl)methyl radical (7a) have been studied by competition kinetics employing benzeneselenol trapping. Arrhenius functions for ring openings were determined for reactions conducted between -80 and 50 degrees C. Each radical rearranges rapidly, with rate constants at 25 degrees C of 3 x 10(11) (4a), 2 x 10(11) (5a), 6 x 10(11) (6a), and 6 x 10(12) s(-1) (7a). Precursors to these radicals represent hypersensitive mechanistic probes with unambiguous rate constants for rearrangement. The results confirm the utility of a previously employed semiquantitative method for estimating rate constants for aryl-substituted cyclopropylcarbinyl rearrangements based on Marcus theory. However, they also show that severe dihedral angles between the aromatic pi-systems and the breaking cyclopropyl C-C bonds in the energy-minimized structures cannot be used to predict kinetic effects in the rearrangement reactions. The ramifications of the kinetic results for mechanistic studies of enzyme-catalyzed oxidations of hydrocarbons that employed hypersensitive probes are discussed.