9,10-dihydroxyanthracene (5).6 The final product was 2-formyl-
The photoredox reaction is apparently unaffected by
dilution and was observed down to the lowest substrate
concentration in which we were able to follow the reaction
(∼10-6 M). However, the amount of side product increases
rapidly above ∼10-3 M.11 These observations suggest that
the photoredox reaction is unimolecular with respect to the
starting material but that the photoreduction side reaction is
a bimolecular process. Photolysis under oxygen gave exclu-
sively 3, suggesting that its trace yield observed under argon
arises via residual oxygenation (of either 2 or a reactive
intermediate leading to 2). Decreasing the pH to 1 or 0 gave
even higher yields of 2 consistent with acid catalysis of the
photoreaction. Photolysis of 1 in 1:1 pD 2 D2SO4-CH3CN
resulted in the formation of 2-RD (2b), indicating that the
methine proton arises from water, not from acetonitrile. This
is consistent with a water-mediated intramolecular photore-
dox reaction.12
anthraquinone (6), which arose via (thermal) air oxidation
of the initially formed 5. At that time, we also studied the
corresponding (hydroxymethyl)benzophenone derivatives 1
and 7 (in neutral aqueous solution) and found they reacted
only via hydrogen abstraction from the CH3CN cosolvent,
yielding pinacol-type photoreduction products.7 In contrast,
photolysis of 1 in aqueous solution (1:1 H2O-CH3CN, water
at pH < 3) resulted in UV-vis spectral changes with an
isosbestic point (Figure 1)8 consistent with an entirely
The observed photochemical behavior is unaffected by
relatively minor structural changes as evidenced by 3-(1′-
hydroxyethyl)benzophenone (8) and 3-benzoylbenzhydrol
(9), both of which show the same photoredox behavior. As
9 would only interchange the ketone and alcohol moieties,
resulting in the same structure upon intramolecular photo-
redox reaction, 9-RD was used instead and was shown to
exchange the deuterium for a proton at the methine position
on photolysis in acidic H2O, consistent with the water-
mediated intramolecular redox process. While similar to 9,
the photoreaction of 10 can be monitored by NMR. When
photolyzed in acidic (pH < 2) aqueous solution, 10 under-
goes the equivalent photoredox reaction (eq 3) but unlike 1
or 8, whose product are benzhydrols and less photoreactive
than the starting benzophenone, the photoproduct of 11 also
contains a benzophenone, leading to a photostationary state
after about 30% conversion. Intramolecular photoredox
chemistry was also observed in the biphenyl example 12,
strengthening the evidence that electronic transmission is
possible through a biphenyl ring.13 Secondary photochemistry
was however, evident above approximately 10% conversion,
complicating this initial study.
Figure 1. UV-vis traces for 1 with increasing photolysis time.
Samples (∼10-5 M, 1:1 H2O-CH3CN; pH 2, Ar purged) were
drawn from a semipreparative photolysis and diluted before
recording spectra traces. Each trace is 60 s apart. The resulting
spectrum is essentially identical to that of authentic 3-formyl-
benzhydrol (2).
different reaction than observed at pH 7,9 viz. eq 1. Aside
from simple photoreduction, the para isomer 7 did not
undergo photoredox chemistry and was not studied further.
The above are consistent with a meta effect that has been
observed by Wirz and co-workers for the photochemical
hydration of benzophenone.10
(3) (a) Zimmerman, H. E. J. Am. Chem. Soc. 1995, 117, 8988. (b)
Zimmerman, H. E. J. Phys. Chem. A 1998, 102, 5616.
(4) Quantum yield of reaction was estimated by comparison with the
photoredox chemistry of 2-hydroxymethylanthraquinone(Φ ∼ 0.8).6
(5) Rayonet RPR 100 photochemical reactor, 300 nm lamps, argon purged
continuously.
(6) Lukeman, M.; Xu, M.; Wan, P. J. Chem. Soc., Chem. Commun. 2002,
136.
(7) The photoreaction in these solvents was characterized by gradual loss
of the main UV-vis absorption band (255 nm) and with the formation of
new NMR signals, consistent with pinacol-type products.
(8) To maintain consistency with regards to concentration dependence
of the photolysis, the UV-vis traces followed a semipreparatory photolysis
by halting the photolysis, removing aliquots, diluting them precisely, and
taking UV-vis spectra. The photolysis was then continued and the cycle
repeated.
Laser flash photolysis14 of 1 in 1:1 H2O-CH3CN led to a
transient spectrum consisting of two peaks (λmax ) 325 nm,
(11) Semipreparative photolysis5 at pH 2 (∼10-3 M) gave a product
mixture consisting of 65% (2), 30% simple photoreduction, and 5% 3. When
the photolysis was carried out at ∼10-4 M, 2 (95%) and 3 (5%) were the
only products.
(12) Some deuteration of the aldehyde proton was observed on extended
photolysis. This is due to deuterium photoexchange of the aldehyde proton
in D2O in a manner analogous to that observed by Go¨rner and Kuhn (Go¨rner,
H.; Kuhn, H. J. J. Phys. Chem. 1986, 90, 5946-5955).
(9) At concentrations below 10-4 M, a residual photoredox pathway is
observable at pH 7 but was much less efficient compared to pH’s < 3.
Photolysis in pure CH3CN at these lower concentrations still yielded only
photoreduction products.
(10) Ramseier, M.; Senn, P.; Wirz, J. J. Phys. Chem. A 2003, 107, 3305.
3388
Org. Lett., Vol. 7, No. 15, 2005
Mitchell, Devin
