Aa. Martinesker et al., PICOSECOND RADICAL KINETICS - FAST RING OPENINGS OF CONSTRAINED, ARYL-SUBSTITUTED CYCLOPROPYLCARBINYL RADICALS, Journal of the American Chemical Society, 116(20), 1994, pp. 9174-9181
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[cycl
opropane-1,1'-indan]-2-yl)methyl radical (6a), and the (spiro[cyclopro
pane-1,9'-fluorene]-2-yl)methyl radical (7a) have been studied by comp
etition kinetics employing benzeneselenol trapping. Arrhenius function
s for ring openings were determined for reactions conducted between -8
0 and 50 degrees C. Each radical rearranges rapidly, with rate constan
ts 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 h
ypersensitive mechanistic probes with unambiguous rate constants for r
earrangement. The results confirm the utility of a previously employed
semiquantitative method for estimating rate constants for aryl-substi
tuted cyclopropylcarbinyl rearrangements based on Marcus theory. Howev
er, they also show that severe dihedral angles between the aromatic pi
-systems and the breaking cyclopropyl C-C bonds in the energy-minimize
d structures cannot be used to predict kinetic effects in the rearrang
ement reactions. The ramifications of the kinetic results for mechanis
tic studies of enzyme-catalyzed oxidations of hydrocarbons that employ
ed hypersensitive probes are discussed.