Litera´k et al.
well in the order of kH measured by Falvey. The second part of
the dependence in Figure 1 at higher IPA concentrations must
reflect a complex interplay among the rate constants of
photoinduced and ground-state reduction of the ester.
OH group: a proton-coupled electron transfer. It was estimated
that the reducing potential of the ketyl radical can increase by
∼0.8 V if the electron transfer is coupled with proton transfer
to a base such as lutidine.49 Since the oxidation potentials of
•EH and •IPA species will change correspondingly, there must
be an increasing probability of the electron-transfer participation
in the mechanism. In addition, a hydrogen-bonded complex
between •EH and pyridine could facilitate elimination of benzoic
acid, thus enhancing the rate constant k2 at the expense of the
Tanner and co-workers showed that one-electron reduction
of 10 causes rapid elimination of BA from [PhCOCH2OCOPh]•-
with the rate constant in the order of 108 s-1 47
likewise,
;
•
elimination of BA from EH (k2) could be rapid. The rate
constant k3 of the H-atom transfer is expected to be high. The
H-atom exchange rate constants (k4) are known to be in the
order of 103 to 104 L mol-1 s-1, and the equilibrium constants
(k4/k-4) of the exchange process were reported to be on the order
of 10-100.41 If only recombination production of BA occurs,
100% is still an upper limit of the quantum yield (eq 1). If k2
becomes dominant (eq 2), the subsequent reactions trigger the
chain propagation (eqs 3 and 4). Such an event will accompany
formation of three chain carriers in the first cycle (two ketyl
•
termination coupling with IPA. The fact that such an effect
was observed in reactions in which a chain propagation
dominated advocates the latter concept. We should also note
that esters 9a-c contain a pyridine moiety with the same basic
site as pyridine.
There is a significant difference in the quantum yields of the
photoreduction of esters 9b and 10 in the presence of H-atom
donors, exhibiting signs of a chain propagation, compared to
that of 9a and 9c (Table 1). The nitrogen atom in the ortho or
para positions, in contrast to that of meta, is in a direct
conjugation with the carbonyl group. It was shown that the
radicals IPA• and one ArC(dO)CH2 radical) (âγR), and each
•
of them is subsequently responsible for further BA production.
The partitioning between the initial processes then depends
principally on the diffusion efficiency of the two radicals, EH•
and IPA•, from the solvent cage. Any increase in the IPA
concentration will of course affect the efficiency of the H-atom
exchange (â). An analysis of eq 7 indicates that the total
quantum yield ΦBA is dependent on the term φTτTkH[IPA]P and
on its multiples by the expressions containing R, â, and γ. The
R efficiency then plays the most important role in the sum (7)
and defines principally the extent of the process.
•
attack of a ketyl radical, such as IPA, to the phenyl ring of a
ketyl radical formed from 3-benzoylpyridine is most efficient
to the positions ortho and para in respect to the nitrogen atom.26
Various substituted phenacyl esters were used to study sub-
stituent effects on both the redox potential of the phenacyl group
and the rate of carboxylate anion elimination.50,51 It was
concluded that there was a tradeoff between the ease of reduction
and the efficiency of the elimination. The reduction of a ground-
state aromatic ketone by aliphatic 1-hydroxyradical is a favorable
reaction. The reactivity of substituted aromatic ketones to
1-hydroxyradicals produced from secondary alcohol correlates
with the σ constants, giving F ) +1.59.52 The higher stability
of radicals possessing electron-withdrawing substituents can be
explained by captodative stabilization of the ketyl radical site.
Additionally, excited 2-acyl-substituted ketones can abstract a
hydrogen atom either by oxygen or by nitrogen, and a
1-hydropyridinyl radical can rearrange to a ketyl radical.53-57
The emissive CIDEP spectrum from the photolysis of 4-acetylpy-
ridine in IPA proved that the hydrogen atom is abstracted by
the carbonyl group in the T1 (mainly n,π*) state.33 1-Hydro-
pyridyl radical was also observed, and it was likely produced
by secondary reduction of ground-state 4-acetylpyridine by the
2-hydroxy-2-propyl radical. Thus, it is obvious that the position
of the nitrogen atom in 9a-c can directly affect the rate constant
of BA elimination from •EH (k2) as well as that of the reduction
of the ground-state ester (k4). While a direct conjugation of the
nitrogen atom with the carbonyl group will decrease the
efficiency of BA release in the former case, a stabilization of a
The quantum yield variations, when three different aliphatic
alcohols were used, correlated generally with their hydrogen
atom donating ability (Table 1). The Φ values increased in the
order of MeOH < EtOH < IPA as the bond dissociation
energies decreased from 96 to 91 kcal mol-1, apparently
affecting the efficiency of an endothermic hydrogen transfer
from the excited ester (ET values of acetophenone and the 2-,
3-, and 4-acetylpyridines are 74, 70, 71, and 69 kcal mol-1
,
respectively48). The correlation, however, is not straightforward
since diffusion as well as abilities of the corresponding radical
involved in a radical coupling with EH• are different. Despite
the fact that the radical formed from methanol will be more
electron deficient than that formed from IPA, the radical
coupling must still be a diffusion-limited step.
Although this investigation suggested that the reduction of
all esters basically involved hydrogen transfer steps, it can be
argued that an electron-transfer mechanism may participate. An
enhancement of the ester photolysis quantum yields in the
presence of pyridine as a basic additive was shown in Table 1
and Figure 4. The Φ value increased by a factor of ∼4 in the
case of 10 in IPA, whereas it increased by a factor of 2.5 in the
case of 9b in ethanol. Interestingly, this effect was insignificant
for other systems. Bases such as pyridine can interact with the
OH group of ketyl radicals via a hydrogen bond. There are two
types of ketyl radicals in our system (those of the reduced ketone
and those formed by the hydrogen abstraction from alcohols),
and the reactivity of both should be affected similarly. A
reducing ability of a 1-hydroxy radical can be enhanced when
oxidation of the radical is accompanied by deprotonation of the
•
ketyl radical site of EH will enhance the ester reduction.
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