Tm. Bockman et al., DIRECT OBSERVATION OF ULTRAFAST DECARBOXYLATION OF ACYLOXY RADICALS VIA PHOTOINDUCED ELECTRON-TRANSFER IN CARBOXYLATE ION-PAIRS, Journal of organic chemistry, 62(7), 1997, pp. 2210-2221
Charge-transfer (CT) photoactivation of the electron donor-acceptor sa
lts of methylviologen (MV(2+)) with carboxylate donors (RCO(2)(-)) inc
luding benzilates [Ar2C(OH)CO2-] and arylacetates (ArCH2CO2-) leads to
transient [MV(.+), RCO(2)(.)] radical pairs. Femtosecond time-resolve
d spectroscopy reveals that the photogenerated acyloxy radicals (RCO(2
)(.)) rapidly lose carbon dioxide by C-CO2 bond cleavage, in competiti
on with back-electron transfer to restore the original ion pair, [MV(2
+), RCO(2)(-)]. The decarboxylation rate constants for ArCH2CO2. lie i
n the range (1-2) x 10(9) s(-1), in agreement, with previous reports.
In striking contrast, the C-CO2 bond scission in Ar2C(OH)CO2. occurs w
ithin a few picoseconds (k(CC) = (2-8) x 10(11) s(-1)). The rate const
ants for decarboxylation of these donors approach those of barrier-fre
e unimolecular reactions. Thus, real-time monitoring of the decarboxyl
ation of benziloxy radicals represents the means for the direct observ
ation of the transition state for C-C bond scission.