â-Heterolysis Reaction of â-Hydroxy Arylethyl Radicals
SCHEME 1
SCHEME 2
Results
reaction15 of these â-OR ketyl radicals is sometimes complicated
by difficulties associated with time-resolved detection of the
transient products. In particular, enol radical cations, if present
as intermediates, are highly acidic and typically short-lived,28-30
and R-acyl radicals28,31 often are not easily observed or
distinguished from the â-OR ketyl radical with UV spectros-
copy, a convenient spectroscopic method for monitoring fast
reactions in solution. Thus, in a manner similar to our earlier
studies into the effect of variables such as ionizing ability of
solvent and leaving group ability on the â-heterolysis of
â-substituted phenethyl radicals,11,12 we have now examined the
time-resolved formation of styrene radical cations 3 from
â-hydroxy arylethyl radicals 2 generated by photohomolysis of
substituted dibenzyl ketones 1a-c, Scheme 1, or benzyl methyl
ketones 1d,e, Scheme 2. Since these â-OH radicals are not ketyl
radicals, the radical cations generated upon loss of the â-OH
group are readily observed with UV-vis spectroscopy, and the
dynamics of the heterolysis reaction could easily be studied as
a function of substituent on the aromatic ring, solvent polarity,
and acid content with laser flash photolysis.
Generation of â-Hydroxy Arylethyl Radicals. As shown
in Figure 1a, 266 nm laser flash photolysis of the 4-Cl-
substituted precursor 1a in nitrogen-saturated acetonitrile (AcN)
at 22 °C generated a transient with an absorption maximum
near 270 nm. In nitrogen-purged AcN, this transient decayed
in a second-order manner and was relatively long-lived, Figure
1a (inset). The transient was rapidly quenched under oxygenated
conditions, Figure 1a. The location of the absorption maximum
is reminiscent of benzyl radicals that absorb in the low-UV
region, with the 4-chlorobenzyl radical possessing an absorption
maximum near 280 nm.32,33 It is also well-known that benzyl
radicals are sensitive to radical scavengers and react rapidly with
molecular oxygen.32 Thus, we assign the transient at 270 nm to
the â-hydroxy-(4-chlorophenyl)ethyl radical 2a (X ) Cl)
producedbyphotoinducedR-cleavage34-38 anddecarbonylation39-42
as shown in eq 2.
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Laser flash photolysis of compounds 1b and 1c in nitrogen-
saturated AcN resulted in the formation of transient species with
similar spectral features, Figures 1b and 1c, to those described
above. The transient absorption spectra show maxima near 280
nm, with additional sharp bands near 315 nm, that were
completely quenched when laser irradiation was carried out in
oxygenated AcN. The transients responsible for these absorption
bands possess spectral profiles that closely resemble those
reported for the benzyl radical and the 4-methylbenzyl radical,
respectively.32,33,43 The absorption maxima detected in these
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