Charge-transfer (CT) photoactivation of the electron donor-acceptor salts of methylviologen (MV(2+)) with carboxylate donors (RCO(2)(-)) including benzilates [Ar(2)C(OH)CO(2)(-)] and arylacetates (ArCH(2)CO(2)(-)) leads to transient [MV(*)(+), RCO(2)(*)] radical pairs. Femtosecond time-resolved spectroscopy reveals that the photogenerated acyloxy radicals (RCO(2)(*)) rapidly lose carbon dioxide by C-CO(2) bond cleavage, in competition with back-electron transfer to restore the original ion pair, [MV(2+), RCO(2)(-)]. The decarboxylation rate constants for ArCH(2)CO(2)(*) lie in the range (1-2) x 10(9) s(-)(1), in agreement with previous reports. In striking contrast, the C-CO(2) bond scission in Ar(2)C(OH)CO(2)(*) occurs within a few picoseconds (k(CC) = (2-8) x 10(11) s(-)(1)). The rate constants for decarboxylation of these donors approach those of barrier-free unimolecular reactions. Thus, real-time monitoring of the decarboxylation of benziloxy radicals represents the means for the direct observation of the transition state for C-C bond scission.
Photoinduced coupling of an acetylene with a quinone in two
wavelength regions (λDB and λCT)
can
be regioselective to yield a single quinone methide adduct when various
diarylacetylenes (DA) and
2,6-dichlorobenzoquinone (DB) are used. Thus, the
direct photoexcitation of DB at λDB = 355 nm
or the
specific activation of the 1:1 electron donor−acceptor complex
[DA,DB] at λCT = 532 nm both
lead to the
transient ion-radical pair
[DA
•+,DB
•-],
which is established by time-resolved (ps,ns) spectroscopy.
Competition
between back electron transfer (k
BET) and
ion-radical pair collapse (k
C) to the distonic
adduct DA-DB, as
described in Schemes and , limits the quantum yields for both
photochemical processes in Table . The
biradical nature of the distonic adduct in Scheme accommodates the
various facets of acetylene reactivity
and unique regioselectivity to yield the same quinone methide by both
actinic processes. In a more general
context, the electron-transfer mechanism established by the
charge-transfer excitation of [DA,DB]
provides
compelling evidence that the Paterno−Büchi coupling (by direct
excitation of DB) can proceed via the same
sequence of reactive intermediates.
We wish to report how the recent developments in timeresolved spectroscopy make it possible to observe the direct scission of a carbon-carbon bond in real time, a process of fundamental importance in organic chemistry. 1,2 We initially focus on the facile C-C bond cleavage in the decarboxylation of labile acyloxy radicals (R-CO 2 • ) since they generally have lifetimes of τ < 10 -9 s. 3 In order to generate this reactive precursor, we employ the novel method based on electrontransfer oxidation of carboxylate salts, 4 i.e.
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