Substituent effects in the intramolecular photoredox reactions of benzophenones in aqueous solution

Article abstract of DOI:10.1139/V07-081

A number of α-hydroxy-3-benzylbenzophenones 7-11 have been synthesized for the purpose of studying the effect of a phenyl substituent on the intramolecular photoredox reaction of 3-(hydroxymethyl)benzophenone (5) discovered in our laboratory. This latter compound was found to undergo a unimolecular (formal) intramolecular redox reaction upon photolysis in aqueous acid that results in clean reduction of the benzophenone ketone (to secondary alcohol) and oxidation of the alcohol to aldehyde. Three of the phenyl-substituted compounds with simple phenyl (7), p-methylphenyl (8), and p-methoxyphenyl (9) were found to undergo the acid-catalyzed intramolecular photoredox reaction with the observation that 9 also undergoes a residual photoredox reaction that is not acid-mediated and may in-volve initial photoinduced electron transfer, which is supported by LFP data. The m-methoxyphenyl (10) compound did not undergo the reaction. The trend in observed relative reactivity may be partially rationalized by examining changes in molecular orbital coefficients observed in the calculated HOMOs and LUMOs. The photoredox reaction has also been applied twice in succession in a single compound 11, demonstrating that the photoredox reaction may be useful for sequential photoredox reactions in a multifunctional compound.

Full text of DOI:10.1139/V07-081

561
Substituent effects in the intramolecular
photoredox reactions of benzophenones in
aqueous solution
Nikola Basariƒ, Devin Mitchell, and Peter Wan
Abstract: A number of α-hydroxy-3-benzylbenzophenones 7–11 have been synthesized for the purpose of studying the
effect of a phenyl substituent on the intramolecular photoredox reaction of 3-(hydroxymethyl)benzophenone (5) discov-
ered in our laboratory. This latter compound was found to undergo a unimolecular (formal) intramolecular redox reac-
tion upon photolysis in aqueous acid that results in clean reduction of the benzophenone ketone (to secondary alcohol)
and oxidation of the alcohol to aldehyde. Three of the phenyl-substituted compounds with simple phenyl (7), p-
methylphenyl (8), and p-methoxyphenyl (9) were found to undergo the acid-catalyzed intramolecular photoredox reac-
tion with the observation that 9 also undergoes a residual photoredox reaction that is not acid-mediated and may in-
volve initial photoinduced electron transfer, which is supported by LFP data. The m-methoxyphenyl (10) compound did
not undergo the reaction. The trend in observed relative reactivity may be partially rationalized by examining changes
in molecular orbital coefficients observed in the calculated HOMOs and LUMOs. The photoredox reaction has also
been applied twice in succession in a single compound 11, demonstrating that the photoredox reaction may be useful
for sequential photoredox reactions in a multifunctional compound.
Key words: intramolecular photoredox, acid catalysis, meta effect, benzophenone photochemistry.
Résumé : On a synthétisé un certain nombre de α-hydroxy-3-benzylbenzophénones (7–11) pour pouvoir étudier l’effet
d’un substituant phényle sur la réaction de photoredox intramoléculaire de la 3-(hydroxyméthyl)benzophénone (5) qui a
été découverte dans notre laboratoire. On a trouvé que lorsque ce dernier composé en solution acide aqueuse est sou-
mis à une photolyse, il subit une réaction redox intramoléculaire qui conduit à une réduction propre du groupement
carbonyle de la benzophénone en alcool secondaire avec oxydation concomitante de l’alcool en aldéhyde. On a ob-
servé que, en solution aqueuse acidulée, trois des composés substitués par un groupe phényle, soit le dérivé phényle
non substitué (7), le p-méthylphényle (8) et le p-méthoxyphényle (9), subissent la réaction photoredox intramoléculaire
qui est accompagnée dans le cas du produit 9 d’une réaction photoredox résiduelle qui n’est pas catalysée par les aci-
des et qui peut impliquer un transfert initial d’électron qui est photoinduit et cette hypothèse est supporter par des don-
nées de « LPF ». Le composé méthoxyphényle (10) ne subit pas la réaction. La tendance dans les réactivités relatives
observées peut être partiellement expliquée par un examen des changements dans les coefficients d’orbitales moléculai-
res qui sont observés dans les valeurs calculées des orbitales moléculaires HO et BV. On a aussi appliqué la réaction
photoredox deux fois en succession à un composé unique, 11; ce résultat démontre que la réaction photoredox pourrait
être utile pour des réactions photoredox séquentielles dans un composé multifonctionnel.
Mots-clés : photoredox intramoléculaire, catalyse acide, effet méta, photochimie des benzophénones.
[Traduit par la Rédaction] Basariƒ et al. 571
Introduction
the many studies on this reaction, it was not until about
60 years later that the mechanism was shown to proceed via
the triplet excited state (4, 5). Although many studies have
examined benzophenone (1) photoreduction in various or-
ganic solvents, fewer studies have examined its photochemi-
cal and photophysical behavior in aqueous solution. It is
known that the benzophenone triplet excited state is
quenched by acid. This was first reported by Ledger and
Porter (6) who noted that the phosphorescence of benzophe-
none (1) was quenched by added protons, although the de-
tailed mechanism was not understood at that time. Initially,
it was thought that the quenching was due to the formation
of the protonated triplet 2 (protonation at the carbonyl) and
Wyatt and co-workers (7, 8) assigned a pKa of 1.5 0.1 for
the protonated triplet excited state using Förster cycle analy-
sis. That is, the benzophenone triplet is more basic by sev-
The photochemistry and photophysics of benzophenone
(1) and its derivatives have been studied for over a century
(1, 2). Indeed, benzophenone photoreduction was one of the
first photoreactions described in the literature (3). Despite
Received 13 June 2007. Accepted 3 July 2007. Published on
the NRC Research Press Web site at canjchem.nrc.ca on
1 August 2007.
N. Basariƒ,1 D. Mitchell, and P. Wan.2 Department of
Chemistry, University of Victoria, Victoria, BC V8W 3V6,
Canada.
1On leave from the Ruder Boskovic Institute, Zagreb, Croatia.
2Corresponding author (e-mail: pwan@uvic.ca).
Can. J. Chem. 85: 561–571 (2007)
doi:10.1139/V07-081
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Can. J. Chem. Vol. 85, 2007
eral orders of magnitude (at the carbonyl oxygen) compared
with the ground state (pKa(So) = –5.7 (7)). In a recent study,
Wirz and co-workers (9) established that protonation (at the
carbonyl) of triplet benzophenone results in an overall
photohydration reaction, to give water-adducts 3 and 4 with
a preference for 3, i.e., hydration at the meta position
(eq. [1]). This reaction was unexpected and has probably
missed detection in the past because of the instability of the
hydration products, which readily revert back to 1. In addi-
tion, Wirz and co-workers (9) assigned a pKa of –0.4 for the
protonated triplet excited state; thus, the triplet is somewhat
less basic than originally estimated by Wyatt and co-workers
(7, 8).
[3]
tions were unreactive; the ortho isomers were not studied
because of complications that would arise from the known
competing intramolecular hydrogen abstraction pathway.
In our view, the intramolecular photoredox chemistry of
benzophenones deserves further study. Under appropriate di-
lution and at pH < 3, the reaction was found to be clean with
very high quantum yield (eq. [3], Φ = 0.6 at pH 2) (13). The
reaction is mediated by water and acid and is unimolecular
in substrate: the reaction becomes cleaner on dilution. At
high substrate concentrations (>10–5 mol/L), bimolecular
(via bimolecular hydrogen abstraction, carbonyl abstracting
the benzylic hydrogens of the benzyl alcohol) reactions com-
pete giving rise to oligomeric and polymeric products.
Photoproduct 6 does not itself undergo a photoredox reac-
tion to give 5, indicating that not all aromatic carbonyl
chromophores can mediate the chemistry and that the ques-
tion of photoredox reversibility would be an interesting issue
to address in other compounds. Moreover, the reaction itself
may be viewed as a formal transfer of electrons through the
meta positions of the benzene ring, which cannot be accom-
plished in the ground state. In this paper we report the de-
tails of a study that addresses two aspects: (i) the structural
and electronic effects on the photoredox reaction as ascer-
tained by placing a phenyl and substituted phenyl groups at
[1]
The mechanism (9) of photohydration presumably in-
volves initial protonation of the carbonyl oxygen of triplet
excited benzophenone, to generate an excited triplet state
conjugate acid that has its positive charge significantly
delocalized to the ortho and meta positions of the benzene
ring (e.g., as shown in eq. [2]), as would be anticipated
based on the Zimmerman “ortho-meta” effect for benzene
ring site activation in photochemical reactions (10, 11). At-
tack by water at these sites of positive charge would lead to
the observed hydration products, which upon deactivation to
their ground states would not be expected to be long-lived
and would readily return to benzophenone (1) by dehydra-
tion (and re-aromatization).
the benzyl alcohol carbon of
5 (i.e., α-hydroxy-3-
benzylbenzophenones 7–10) and (ii) the possibility that the
photoredox reaction can be induced twice (one after another)
in the same substrate on extended photolysis, by studying
model compound 11.
Results and discussion
[2]
Materials
Whereas α-hydroxybenzylbenzophenones 7 and 7-αD
were readily synthesized by NaBH4 (NaBD4) reduction of
commercially available 3-benzoylbenzophenone (12), 8–11
were synthesized according to the reactions outlined in
Scheme 1, all starting from 3-formylbenzophenone (13).
Compounds 8–10 were made by the reaction of 13 with the
appropriate Grignard reagent with isolated yields in the
14%–80% range. The synthesis of 11 required the intermedi-
ate aldehyde 14, which was readily made by the reaction of
13 with the Grignard reagent derived from commercially
available 3-bromobenzaldehyde diethyl acetal (Sigma-
Aldrich). This step proceeded in about 20% isolated yield.
The subsequent NaBH4 reduction gave the desired benzo-
phenone diol 11 in 50%.
Although the described acid-catalyzed photohydration of
benzophenone itself reported by Wirz and co-workers (9)
gave no isolable stable product, it seemed reasonable to as-
sume that the protonation of triplet excited benzophenone
(1) might lead to other acid-catalyzed chemistry still to be
discovered considering the known range and diversity of
chemistry for ground state carbonyl compounds that are
acid-catalyzed. Indeed, Wirz and co-workers (9) reported the
acid-catalyzed photohydrolysis of m-fluoroacetophenones
and benzophenones to give the corresponding phenols as
proceeding via a protonated triplet. We recently reported two
photochemical reactions of benzophenones that require acid-
catalysis and appear to require prior protonation (at car-
bonyl) of triplet benzophenone: (i) photochemical deuterium
exchange of the meta-substituted methyl group of 3-
methylbenzophenone and 3-methylacetophenone (12) in
deuterated aqueous acid and (ii) formal intramolecular redox
reaction of some 3-(hydroxymethyl)benzophenones (13)
(e.g., eq. [3] with the parent 3-(hydroxymethyl)benzophe-
none (5)). Notably, all the para isomers studied in these reac-
UV–vis and product studies
In view of the previous finding for the photoredox chem-
istry of 5 (13) in which higher substrate concentrations in-
creased the proportion of side products because of a
bimolecular hydrogen abstraction pathway, photolysis runs
were carried out at the most dilute substrate concentrations
deemed practical for NMR analysis. Most runs were carried
out at 10–4 mol/L (10 mg in 400 mL, 1:1 H2O–CH3CN, pH 7
and 2, Rayonet RPR 100 reactor, 300 nm lamps, argon
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Basariƒ et al.
563
Scheme 1.
Fig. 1. Product mixture observed after photolysis of 7-αD
(10–4 mol/L) in 1:1 H2O–CH3CN (pH 2) for 2 min at 300 nm.
Formation of 7 was determined by observation of a methine peak
at δ 5.9 (Ha). Relative ratios of 7 and 7-αD were calculated by
using the Hb proton at δ 7.85 common to both compounds.
and 7-αD are photochemically interconvertible (eq. [4]).
Noteworthy was the fact that photolysis of 7 or 7-αD in neat
CH3CN or in 1:1 H2O–CH3CN above pH 3 did not result in
the photoredox reaction shown in eq. [4]; only the broad sig-
nals in δ 7–7.3 were observed.
The photoredox reaction of 5 was found to be exception-
ally efficient with Φ = 0.6 at pH 2 (13). Using the reaction
of 5 as a secondary reference, we have estimated a quantum
yield of reaction for 7-αD to be about 0.3 at pH 2, which
further confirms the general high reactivity of these com-
pounds in acid.
As will be shown, all of the photoredox reactions for the
phenyl-substituted systems studied in this work (7–11), re-
gardless of their relative efficiency with respect to the
photoredox reaction itself, are accompanied by the formation
of oligomeric material as already discussed for 7 and 7-αD.
This is in contrast to what was observed for 5 in which the
reaction was exceptionally clean at 10–5 mol/L. We believe
that the competing reaction(s) are bimolecular in nature and
probably involve the triplet benzophenone-like ketone ab-
stracting hydrogen from the benzylic hydrogens of another
substrate. In the case of the phenyl-substituted compounds
7–11, the hydrogen to be abstracted is of the benzhydrol
type and hence even more prone to this reaction. A possible
reaction sequence initiated by intermolecular hydrogen ab-
straction is shown in eq. [5]. Note that in this mechanism
one would anticipate a mixture of products in which the ben-
zophenone moiety would suffer “reduction”, consistent with
the observed NMR (aromatic signals only in the δ 7–7.3 re-
gion). This offers a reasonable explanation for the observed
concentration-dependent formation of oligomeric products
for all of these compounds.
purged), and some selected runs were carried out 10–5 mol/L
(2 mg in 600 mL). Product studies were initially carried out
with compounds 7 and 7-αD. The anticipated photoredox
chemistry for these compounds (in aqueous acid solution)
would give no observable changes in their UV–vis as the re-
action would only interchange the benzophenone ketone and
benzhydrol alcohol moieties (with no overall change in mo-
lecular structure). To follow the reaction, photolyses of 7 (7-
αD) were carried out in acidic D2O (H2O) and after work-
up, NMR spectra were recorded. Photolysis (10–4 mol/L) of
7-αD in 1:1 H2O–CH3CN (pH 2, argon purged) resulted in
formation of the methine peak at δ 5.9 as would be expected
on formation of 7 (≅20% yield) (eq. [4]). The reaction
shown interconverts the ketone and alcohol carbons.
Although this was not proven in the case of 7 (or 7-αD), the
same reaction for the phenyl-substituted compounds 8 and
10 (vide infra) conclusively shows that the reaction does in-
terchange the carbons concerned. Accompanying the reac-
tion was the formation of broad aromatic signals in the δ 7–
7.3 region, indicative of oligomeric material resulting from
photoreduction of the benzophenone moiety (Fig. 1) and a
trace of the simple photooxidation product 12 (this is the
major product when photolyzed with an oxygen or air
purge). This oligomeric material proved to be intractable and
appears to be a mixture of products. Photolysis at 10–
5
mol/L gave a cleaner product mixture although the broad
peaks observed at δ 7–7.3 could not be completely elimi-
nated. In comparison, the photoredox reaction of 5 under
these conditions (i.e., at 10–5 mol/L or lower) were excep-
tionally clean (13). As expected, photolysis of 7 in D2O–
CH3CN (pD 2) (after H2O wash before NMR analysis)
showed the loss of the methine peak at δ 5.9 with essentially
no change in the aromatic region except for growth of broad
δ 7–7.3 region due to the formation of oligomers. Thus, 7
[5]
[4]
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Can. J. Chem. Vol. 85, 2007
Fig. 2. UV–vis traces observed on the photolysis of 9 in 1:1 H2O–CH3CN at (a) pH 2 and (b) pH 7, purged with N2, at 300 nm.
Traces were taken after initial irradiation of 1 min followed by increasing photolysis times of up to 20 min.
The anticipated photoredox reactions for the methoxy-
substituted derivatives 9 and 10 were studied next since
reaction would result in significant UV–vis changes.
Photolysis of the meta-isomer 10 (pH 2 and 7) resulted in
only the loss of substrate and the formation of oligmeric ma-
terial as noted above for 7, with no evidence of photoredox
reaction. On the other hand, photolysis of the para-isomer 9
resulted in the expected photoredox reaction (eq. [6]) giving
15 (major) and 16 (minor). Notably, at 10–5 mol/L substrate,
the reaction was much cleaner than 7, with reaction being
observed at pH 7 (Φ ≅ 0.007) and pH 2 (Φ ≅ 0.02), although
the quantum yields are both much lower than for 5 and 7.
UV–vis traces carried out for 9 were informative (Fig. 2):
photolysis in pH 2 resulted in the formation of a band at
290 nm that is assignable to the expected photoredox prod-
uct 15; at pH 7 a much lower yield of this product was ob-
served. Both spectral traces show competing decomposition
of the substrate, which is more pronounced at pH 7. Interest-
ingly, 15 was found to be photoinert; that is, the photoredox
reaction for 9 (eq. [6]) is not photoreversible. A possible rea-
son for this is provided later.
this assertion, we have carried out the photolysis of 8 in the
presence of added naphthalene (a known efficient triplet
quencher of benzophenone (1)) at pH 2. The presence of
10–4 mol/L naphthalene resulted in a much less efficient re-
action and longer photolysis times (up to ten fold) were
required, consistent with triplet state reactivity of the benzo-
phenone chromophore of 8. However, the photoredox reac-
tion was much cleaner and could be taken to almost
completion with formation of up to 20%–30% oliogomeric
materials as estimated by NMR. This was a significant im-
provement over runs without added naphthalene. The results
are consistent with a photoredox reaction that is unimole-
cular in substrate (although requiring water and acid media-
tion) and a competing reaction that is bimolecular in
substrate. The addition of naphthalene reduces the lifetime
of the reactive triplet state and hence all photochemical
pathways. Noteworthy is the finding that in this case, the
proportion of bimolecular pathway is retarded more than the
unimolecular photoredox pathway, although we do not com-
pletely understand why this is so.
[7]
[6]
The photoredox product 17 could not be completely sepa-
rated from substrate 8 using chromatography. The best that
could be achieved was a mixture consisting of 92% 17 and
8% 8 from photolysis in the presence of added naphthalene.
Subsequent photolysis of this mixture at pH 2 (300 nm) re-
sulted in no observable change in the ratio of these com-
pounds. Although in principle 17 could undergo the
photoredox reaction returning to 8, this result suggests that
the return is very inefficient, and that at this photolysis
wavelength the photostationary state has already been at-
tained.
Compound 11 was designed with two oxidizable benzylic
alcohol moieties. The idea was that this compound might
undergo two photoredox reactions in succession, first form-
ing 19 and then 20 as shown in eq. [8]. Initial studies by
UV–vis (Fig. 3) spectra show that photolysis of 11 in acid
The last compound studied in this series was the p-
methyl-substituted derivative 8. The intramolecular
photoredox reaction was observed at pH 2 (eq. [7]), but it
was observed that this compound gave the highest propor-
tion of oligomeric products compared with the other reactive
systems discussed so far. Even at 10–5 mol/L, low conversion
runs gave 10%–20% of the oligomeric products (estimated
by comparing the intensity of the signal corresponding to the
entire methyl proton region at δ 2.2–2.6 and the sum of the
intensities at δ 2.31 and 2.42 corresponding to 8 and 17, re-
spectively). This is consistent with the greater availability of
abstractable benzylic hydrogens (total of 4) in this molecule
and the earlier proposal (eq. [5]) that the main competing re-
action is due to bimolecular hydrogen abstraction by the
benzophenone ketone of another substrate. To further test
© 2007 NRC Canada

Basariƒ et al.
565
Fig. 3. UV–vis traces of the photolysis of 11 in 1:1 H2O–CH3CN at 300 nm at (a) pH 2 and (b) pH 7, purged with N2. Traces were
taken after 1 min of photolysis in each run, with latter runs requiring 2 min photolysis to produce noticeable spectral change.
(pH 2) resulted in significantly greater loss of the benzophe-
none chromphore absorption vs. runs in pH 7, consistent
with the acid-catalyzed photoredox reaction. Since 11 and
19 have essentially the same benzophenone chromophore, it
was not possible to follow their interconversion using UV–
vis. The UV–vis traces are consistent with the eventual
formation of 20, which lacks the benzophenone moiety.
Photolysis of 11 at pH 7 gave 14 (up to 10% yield) and
oligomeric material. Compound 14 was also observed in all
runs carried out in acid (pH 2). Its observation at pH 7 sug-
gests that it is formed via some residual oxidation pathway
because of traces of oxygen in the irradiated solution. Inter-
estingly, 14 was found to be photoinert with respect to the
photoredox reaction (eq. [9]), as was also the case for the m-
methoxyphenyl derivative 10 (vide supra). In addition to 14,
photolysis of 11 in acid (pH 2) did give both 19 and 20
shown in eq. [8], although the yield of 20 was always higher
than 19 even at the lowest conversions that we were able to
conveniently carry out. Thus 19 could not be isolated in pure
form and its presence was indicated by NMR assignment of
product mixtures that were enriched in 19 by chromatogra-
phy. It is clear that the reaction scheme shown in eq. [8] can
be achieved, in that the end product is formed. Our data sug-
gests that 19 is more reactive than 11 and preferentially un-
Nanosecond laser flash photolysis (LFP) studies
LFP studies were carried out using a Nd-Yag laser at
266 nm (pulse width ≅ 20 ns). Based on our initial LFP stud-
ies of the photoredox chemistry of 5 (13), transients assign-
able to reactive intermediates other than the reactive triplet
state were not anticipated. Indeed, LFP of 7 and 8 gave the
typical triplet state absorption expected for the benzophe-
none chromophore with maxima at 325 and 525 nm for 7
and 8 (Fig. 4) (9, 13). However, closer analysis of the spec-
tra observed for both of the methoxyphenyl-substituted com-
pounds 9 and 10 show that in addition to the triplet state
signal, bands at ≅450 and >600 nm were also observed su-
perimposed on the triplet signal. Shizuka and co-workers
(14) have shown that benzophenone radical anion (450 nm)
and methoxybenzene radical cations (630 nm) are produced
from electron transfer from methoxybenzenes to triplet ex-
cited benzophenone. That the same signals were observed
for 9 and 10 under dilute conditions and that the triplet sig-
nal itself was the dominant one indicate that intramolecular
electron transfer from the methoxybenzene to the triplet ex-
cited benzophenone is a competing channel for these two
compounds (eq. [10]).
[10]
dergoes
a photoredox reaction to give 20, although
additional studies are warranted to show this in a direct way.
With respect to the triplet absorption signal itself, the fol-
lowing general findings apply: (i) apparent lifetimes were
sensitive to oxygen and acid (pH < 3). For 9 the apparent
triplet lifetime was 5.2 µs at pH 7 and was reduced to 220 ns
at pH 2; for 10 the lifetime was 5.3 µs at pH 7 and 660 ns at
pH 2. The addition of oxygen (1 atm; 1 atm = 101.325 kPa)
to all samples reduced triplet lifetimes to <100 ns in all
cases; (ii) most decays could not be fitted to first-order de-
cays and typically required a sum of two first-order decays,
especially at pH 7 where there is no quenching of the triplet
by acid. This is consistent with competitive pseudo first-
order decays (including hydrogen abstraction) pathways. In
addition, the benzophenone ketyl absorbs at 545 nm (15)
and this overlaps with the 525 nm band of the triplet, mak-
[8]
[9]
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Can. J. Chem. Vol. 85, 2007
Fig. 4. Triplet absorption spectra of 7, 8, 9, and 10 detected by
LFP (Nd-YAG, λex = 266 nm) in 1:1 H2O–CH3CN (pH 7, N2
purged) immediately after the laser pulse (≅20 ns).
benzene ring carbon bearing the PhCH(OH) substituent
would be expected to bear most of positive charge in the (re-
distributed) electronic structure of the excited state (com-
pared with the initial ground state). This is shown in
Scheme 2 in which protonation of triplet 7 would give rise
to a conjugate acid (carbocation) that may be represented by
structures 21a and 21b. These predictions are also consistent
with the mechanism of photohydration of benzophenone re-
ported by Wirz and co-workers (9). In the case of the photo-
hydration reaction (9), water would attack the carbocation of
structure 21b. This may indeed occur, leading to a product
that would eventually return to substrate. However, another
possible pathway that is not available in the parent benzo-
phenone studied by Wirz and co-workers (9) is the depro-
tonation pathway leading to the double enol intermediate 22.
This is perhaps a pathway that initially seems unusual, but
one can quickly see that ketonization of this double enol
leads back to substrate or to a formal intramolecular redox
product that was indeed observed for most of these com-
pounds. Oxygen is expected to trap biradicals of the type 22
leading to the oxidized product 12, although other mecha-
nisms for photooxidation (photooxygenation) are not excluded.
HOMO and LUMO calculations for the p-methylphenyl
derivative 8 gave similar results. However, this was not the
case for the methoxy-substituted derivatives 9 and 10. As
shown in Fig. 6 for compound 9, examination of the HOMO
and LUMO characteristics would imply that electronic exci-
tation of this compound would transfer charge from only the
methoxybenzene ring to both of the benzene rings of the
benzophenone. This is consistent with the photoinduced sin-
gle electron transfer to form the benzophenone radical anion
and methoxyphenyl radical cation that was inferred in the
LFP results for this compound (and 10). A residual formal
photoredox pathway was observed for this compound at
pH 7 and pH 2. Indeed, initial photoinduced electron trans-
fer can lead to such a reaction (Scheme 3), and we believe
that it is the pathway taken at pH 7. At lower pH, one would
still expect protonation of the benzophenone carbonyl, and
this may lead to an overall photoredox pathway as well,
which is consistent with the higher quantum yield for this
reaction at lower pH. The question remains as to why the m-
methoxy-substituted compound 10 did not undergo
photoredox reaction at any pH. Clearly in this case the
photoinduced electron transfer appears to take place for 10
as indicated by the LFP data. We believe the explanation lies
in the electronic structure of the radical cation shown in
Scheme 3. A meta-substituted methoxy group (instead of the
para-substituted methoxy group shown) would cause the cat-
ion and radical to be more localized at benzene positions
that are ortho and para to the methoxy group (for resonance
stabilization). This will have the effect of reducing the de-
gree of positive charge that would be localized at the ben-
zene ring carbon adjacent to the Ar(HO)CH moiety. This in
term will reduce the acidity of the CH proton of the
Ar(HO)CH group and hence the overall efficiency of the for-
mal redox reaction. A similar argument may also be used to
explain why photoredox chemistry was not observed when
10 is photoprotonated at pH 2.
ing kinetic analysis even more complex. The competition of
bimolecular pathways of triplet decay can best be docu-
mented for 8 at pH 7 where dilution of the solution induced
significantly longer triplet lifetimes: the apparent lifetime at
8.8 × 10–5 mol/L was 2.2 µs whereas at 1.4 × 10–5 mol/L it
was 9.1 µs. In view of these complexities, LFP data was
used only to infer that the triplet excited state is the reactive
state and that it is subject to acid quenching, consistent with
photoprotonation at the benzophenone carbonyl (vide supra).
Mechanisms of reaction
As discussed in the Introduction section, the protonation
of the oxygen of triplet excited benzophenone in aqueous so-
lution is an important primary photochemical pathway in
acid (pH < 3). The more basic carbonyl oxygen in the triplet
excited state implies considerable charge transfer from the
benzene rings on electronic excitation that is probably en-
hanced in aqueous solution. Perhaps surprising is the fact
that the triplet excited state also retains its ability to abstract
hydrogen, as evidenced by the competing hydrogen abstrac-
tion pathway observed in this work.
We have carried out simple HOMO and LUMO (Chem
3D, MOPAC/AM1) calculations of the compounds studied
in this work to gain possible insights into their relative reac-
tivity. To our pleasant surprise, the experimental observa-
tions we have made can be rationalized by the results of
these calculations. Shown in Fig. 5 are the calculated
HOMOs and LUMOs for the unsubstituted phenyl derivative
7. The excited state (singlet or triplet) may be viewed in the
simplest approximation as attained by promoting an electron
from the HOMO to the LUMO. Thus, there would be charge
migration from two of the benzene rings (the benzophenone
benzene ring with the attached PhCH(OH) group and the
benzene ring of the attached Ph(CH)OH itself) to the benzo-
phenone carbonyl and the other benzophenone benzene ring.
One would therefore predict the increased basicity of the
carbonyl oxygen. Subsequent protonation of the ketone oxy-
gen would give rise to the conjugate acid, which one could
argue would retain the electronic distribution of its precur-
sor. A closer examination of Fig. 5 would predict that the
Finally, we have also carried out HOMO and LUMO cal-
culations for the photoredox product 15. As noted earlier,
this compound did not undergo the photoredox chemistry
© 2007 NRC Canada

Basariƒ et al.
567
Fig. 5. HOMO (left) and LUMO (right) of 7 calculated using Chem 3D (AMI).
Fig. 6. HOMO (left) and LUMO (right) of 9 calculated using Chem 3D (AM1).
Scheme 3.
Scheme 2.
on a Varian Cary 5 instrument. Preparative photolyses were
performed using a Rayonet RPR 100 photochemical reactor
equipped with 300 nm lamps.
that would have returned it back to 9. As shown in Fig. 7,
electronic excitation of this compound would result in
charge migration from the benzophenone benzene ring sub-
stituted with the methoxy group to the carbonyl and the
other adjacent benzophenone benzene ring. Although this
would make the benzophenone carbonyl more basic (hence
photoprotonated by acid), it does not predict that the
Ph(HO)CH moiety would be activated with respect to
deprotonation, as would be required for the photoredox reac-
tion to proceed and that was predicted in similar HOMO and
LUMO calculations for 7 and 8. Thus, it would appear that
simple low level HOMO and LUMO calculations offer a
useful parallel study for the intramolecular photoredox
chemistry of the benzophenones studied in this work, and
we will be applying similar theoretical studies on related
chemistry that can arise from acid–base chemistry of elec-
tronically excited states of other aromatic compounds.
Common laboratory reagents
All solvents used for synthesis (ACS grade) were pur-
chased from Sigma-Aldrich and used as received unless oth-
erwise noted. HPLC grade acetonitrile and distilled water
were used in photolyses in mixed H2O–CH3CN solutions.
D2O and CDCl3 were purchased from Cambridge Isotope
laboratory and D2SO4 was obtained from Sigma-Aldrich.
Preparative TLC was carried out on 20 cm × 20 cm silica gel
GF Uniplates (Analtech).
Materials
All readily available organic and inorganic reagents re-
quired in the synthesis reported below were purchased from
Sigma-Aldrich and used as received.
␣-D-␣-Hydroxy-3-benzylbenzophenone (7-αD)
Experimental section
3-Benzoylbenzophenone (0.30 g, 1.1 mmol) was dissolved
in 60 mL of 2-propanol and sodium borodeuteride (0.024 g,
0.56 mmol) in 25 mL of 2-propanol was added slowly with
stirring at room temperature (RT). The solution was then
refluxed for 45 min, cooled and quenched with 100 mL of
Instrumentation
1H NMR spectra were recorded on Bruker 300 or
500 MHz instruments. Mass spectra were obtained on a
Kratos Concept H instrument. UV–vis spectra were recorded
© 2007 NRC Canada

568
Can. J. Chem. Vol. 85, 2007
Fig. 7. HOMO (left) and LUMO (right) of 15 calculated using Chem 3D(AM1).
H2O. Upon extraction with 100 mL of CH2Cl2 and removal
of the organic solvent, a yellow oil was obtained. Pure 7-αD
was obtained via preparative TLC using 1:24 ether–CH2Cl2
chromatography (silica gel, 1:1 ether–hexanes), to yield 2 g
(35%) of 3-formylbenzophenone (16). H NMR (300MHz,
CDCl3) δ: 10.08 (s, 1H), 8.27 (s, 1H), 8.12–8.05 (m, 2H),
7.82–7.78 (m, 2H), 7.69–7.58 (m, 2H), 7.53–7.48 (m, 2H).
This material was used without further treatment in the syn-
thesis of 8–10 and 14.
1
1
to give 0.25 g (oil 50%) of material. H NMR (500 MHz,
CDCl3) δ: 7.84 (t, 2H, 1.8 Hz), 7.74 (dd, 2H, 8.2 Hz and
1.4 Hz), 7.65 (dt, 1H, 7.6 Hz and 1.5 Hz), 7.60 (dt, 1H,
7.7 Hz and 1.6 Hz), 7.56 (tt, 1H, 7.5 and 1.3 Hz), 7.46–7.40
(m, 3H), 7.37–7.35 (m, 2H), 7.34–7.31 (m, 2H), 7.28–7.24
(m, 1H). 13C NMR (CDCl3, 125 MHz) δ: 196.9 (s), 144.4
(s), 143.6 (s), 137.8 (s), 137.6 (s), 132.7 (s), 130.6 (s), 130.3
(s), 129.5 (s), 128.8 (s), 128.6 (s), 128.4 (s), 128.2 (s), 128.0
(s), 126.7 (s). EIMS m/z: 289 (M+, 40), 273 (40), 209 (80),
196 (35), 184 (100), 166 (40), 152 (20). HRMS calcd. for
C20H15DO2: 289.1212; found 289.1206.
General procedure for the preparation of benzophenone
derivatives 8–10 and 14
For the preparation of the Grignard reagents, 1.2 equiv. of
magnesium turnings were reacted with 1.1 equiv. of the cor-
responding bromobenzene derivative dissolved in dry THF.
The Grignard reagents were prepared in the following way.
Magnesium turnings and ≅10 mL of dry THF were placed in
a dry three-necked flask equipped with a nitrogen inlet, a
dropping funnel, and a condenser. The corresponding
bromobenzene derivative was dissolved in dry THF and
placed in the dropping funnel. Several drops were added to
the flask from the dropping funnel. The reaction was initi-
ated by the addition of a crystal of iodine followed by heat-
ing. After initiation of the reaction, the remaining solution in
the dropping funnel was added slowly to the reaction mix-
ture over 45 min, while the reaction mixture was maintained
at ≅40 °C. After the addition was completed, the reaction
mixture was heated at reflux for one additional hour. The re-
action was cooled to RT and cannulated to the dropping fun-
nel for the next step. In the next reaction, a dry three-necked
flask was equipped with a nitrogen inlet, a septum, and a
dropping funnel. In this flask was placed a solution of 1
equiv. of 3-formylbenzophenone dissolved in THF. The solu-
tion was cooled to –78 °C. The prepared Grignard reagent
was added slowly over the period of 2 h to the cooled solu-
tion, taking care that the temperature of the reaction mixture
remained at –78 °C. After the addition of the reagent, the
mixture was stirred for an additional 2 h at –78 °C and al-
lowed to slowly reach RT overnight. The next day water was
added to the reaction mixture followed by the addition of
1 mol/L HCl. The reaction mixture was extracted twice with
diethyl ether, the organic fraction washed with water and
dried over anhydr. MgSO4. The solvent was then evaporated
and the residue purified on a column of silica gel using
CH2Cl2 as the initial eluent followed with CH2Cl2–EtOAc
mixtures of increasing polarity (up to 20% of EtOAc).
␣-Hydroxy-3-benzylbenzophenone (7)
This compound was prepared the same as the preceding
except that sodium borohydride was used. 1H NMR
(500 MHz, CDCl3) δ: 7.84 (t, 2H, 1.8 Hz), 7.74 (dd, 2H,
8.2 Hz and 1.4 Hz), 7.65 (dt, 1H, 7.6 Hz and 1.5 Hz), 7.60
(dt, 1H, 7.7 Hz and 1.6 Hz), 7.56 (tt, 1H, 7.5 and 1.3 Hz),
7.46–7.40 (m, 3H), 7.37–7.35 (m, 2H), 7.34–7.31 (m, 2H),
7.28–7.24 (m, 1H). 5.90 (s, 1H), OH signal too broad for de-
tection. 13C NMR (CDCl3, 75 MHz) δ: 196.8 (s), 144.4 (s),
143.7 (s), 137.9 (s), 137.7 (s), 132.7 (s), 130.7 (s), 130.3 (s),
129.5 (s), 128.9 (s), 128.7 (s), 128.5 (s), 128.3 (s), 128.1 (s),
126.8 (s). EIMS m/z: 288 (M+, 15), 272 (10), 209 (30), 195
(10), 183 (50), 105 (100), 77 (60). HRMS calcd. for
C20H16O2: 288.1150; found 288.1152.
3-Formylbenzophenone
3-Methylbenzophenone (9.1 mL, 51 mmol) was combined
with 2.5 equiv. of N-bromosuccinimide (22.8 g, 128 mmol)
and benzoyl peroxide (0.11 g, 0.80 mmol) in 100 mL ben-
zene. The reaction mixture was refluxed overnight for 17 h
under N2. After the mixture cooled, it was washed twice
with 50 mL of distilled H2O, dried with anhydr. MgSO4, fil-
tered, and the solvent removed in vacuo. The crude 3-
(dibromomethyl)benzophenone was then combined with 5
equiv. of CaCO3 (25.8 g, 258 mmol) in 300 mL 1:1 H2O–
dioxane and refluxed overnight for 17 h. After the reaction
mixture had cooled, the solvent was removed under vacuum.
Subsequently, 200 mL of CH2Cl2 and ~400 mL of 1 mol/L
HCl was added with stirring to dissolve the solid. The or-
ganic phase was separated and the aqueous phase extracted
twice with 100 mL of CH2Cl2. The organic fractions were
combined and washed twice with 200 mL satd. NaHCO3. The
organic fraction was dried with anhydr. MgSO4, filtered, and
the solvent removed. Pure white crystals were obtained after
␣-Hydroxy-3-(4′-methylbenzyl)benzophenone (8)
Starting from 1.2 g (5.7 mmol) of 3-formylbenzophenone
and 4-bromotoluene, the reaction furnished 400 mg (23%) of
the product, a colorless oil. IR (neat from CH2Cl2 solution,
salt plates, cm–1) ν: 3434, 1656. 1H NMR (CDCl3,
© 2007 NRC Canada

Basariƒ et al.
569
300 MHz) δ: 7.83 (s, 1H), 7.74 (d, 2H, J = 7.4 Hz), 7.64 (d,
1H, J = 7.4 Hz), 7.52–7.61 (m, 2H,), 7.43 (dd, 2H, J =
7.4 Hz, J = 7.7 Hz), 7.39 (t, 1H, J = 7.7 Hz), 7.23 (d, 2H,
J = 8.0 Hz), 7.12 (d, 2H, J = 8.0 Hz), 5.82 (s, 1H), 2.65
(br s, 1H), 2.31 (s, 3H). 13C NMR (CDCl3, 75 MHz) δ:
196.9 (s), 144.6 (s), 140.8 (s), 137.7 (s), 137.7 (s), 137.6 (s),
132.6 (d), 130.6 (d), 130.2 (d), 129.5 (d), 129.3 (d), 128.5
(d), 128.4 (d), 128.1 (d), 126.7 (d), 75.8 (d), 21.3 (q). EIMS
m/z: 303 (M+, 10), 302 (M+, 45), 287 (20), 209 (30), 183
(50), 119 (50), 105 (100), 91 (30), 77 (70). HRMS calcd. for
C21H18O2: 302.1307; found 302.1306.
128.9 (d), 128.5 (d), 128.2 (d), 127.7 (d), 75.5 (d). EIMS
m/z: 317 (M+, 5), 316 (M+, 20), 270 (25), 239 (30), 212 (25),
183 (70), 165 (15), 135 (20), 122 (20), 105 (100), 77 (85).
HRMS calcd. for C21H16O3: 316.1099; found 316.1097.
␣-Hydroxy-3-[3′-(hydroxymethyl)benzyl]benzophenone (11)
The aldehyde 14 (450 mg, 1.4 mmol) was dissolved in
30 mL of MeOH and cooled under ice. To this solution was
added a solution of NaBH4 (40 mg, 1 mmol) in 10 mL of
MeOH. The reaction mixture was stirred at 0 °C for 15 min
and quenched by the addition of 50 mL of saturated ammo-
nium chloride. Extractions with CH2Cl2 were carried out and
the collected extracts were dried over anhydr. MgSO4. After
evaporation of the solvent, the residue was chromatographed
on silica using CH2Cl2 as the initial eluent followed with
CH2Cl2–EtOAc mixtures of increasing polarity (up to 30%
of EtOAc). The product was obtained as a colorless oil,
215 mg, yield 48%. IR (neat, NaCl plates, cm–1) ν: 3390,
␣-Hydroxy-3-(4′-methoxybenzyl)benzophenone (9)
Starting from 2.1 g (10 mmol) of 3-formylbenzophenone
and 4-bromoanisole, the reaction furnished 460 mg (14%) of
the product, a colorless oil. IR (neat from CH2Cl2 solution,
salt plates, cm–1) ν: 3446, 1653. 1H NMR (CDCl3,
300 MHz) δ: 7.82 (s, 1H), 7.74 (d, 2H, J = 7.5 Hz), 7.64 (dd,
1H, J = 1.4 Hz, J = 7.4 Hz), 7.52–7.61 (m, 2H), 7.44 (dd,
2H, J = 7.5 Hz, J = 8.1 Hz), 7.41 (t, 1H, J = 8.1 Hz), 7.22
(d, 2H, J = 8.8 Hz), 6.84 (d, 2H, J = 8.8 Hz), 5.83 (s, 1H),
3.76 (s, 3H), 2.50 (br s, 1H). 13C NMR (CDCl3, 75 MHz) δ:
196.9 (s), 159.4 (s), 144.6 (s), 137.8 (s), 137.7 (s), 136.0 (s),
132.7 (d), 130.5 (d), 130.3 (d), 129.4 (d), 128.5 (d), 128.5
(d), 128.1 (d), 128.1 (d), 114.2 (d), 75.6 (d), 55.5 (q). EIMS
m/z: 319 (M+, 10), 318 (M+, 60), 182 (40), 137 (50), 105
(100), 86 (75), 77 (88), 69 (100). HRMS calcd. for
C21H18O3: 318.1256; found 318.1255.
1
1651. H NMR (CDCl3, 300 MHz) δ: 7.85 (s, 1H), 7.75 (d,
2H, J = 7.4 Hz), 7.66 (d, 1H, J = 7.8 Hz), 7.53–7.63 (m,
2H), 7.45 (dd, 1H, J = 7.4 Hz, J = 7.6 Hz), 7.43 (t, 1H, J =
7.6 Hz), 7.39 (s, 1H), 7.25–7.35 (m, 3H), 5.91 (s, 1H), 4.67
(s, 2H), 2.57 (br s, 2H). 13C NMR (CDCl3, 75 MHz) δ:
197.0 (s), 144.4 (s), 144.0 (s), 141.6 (s), 137.8 (s), 137.6 (s),
132.8 (d), 130.7 (d), 130.3 (d), 129.5 (d), 129.0 (d), 128.6
(d), 128.5 (d), 128.2 (d), 126.6 (d), 126.0 (d), 125.2 (d), 75.9
(d), 65.2 (t). EIMS m/z: 319 (M+, 5), 318 (M+, 20), 209 (38),
183 (35), 135 (28), 105 (100), 77 (50). HRMS calcd. for
C21H18O3: 318.1256; found 318.1254.
␣-Hydroxy-3-(3′-methoxybenzyl)benzophenone (10)
Starting from 2.65 g (12.6 mmol) of 3-formylbenzo-
phenone and 3-bromoaniosle, the reaction furnished 3.2 g
(80%) of the product, a colorless oil. IR (neat from CH2Cl2
General photolysis procedure
A 10 mg sample of the benzophenone derivative was dis-
solved in 400 mL of 1:1 CH3CN–H2O and placed in a
600 mL quartz tube, which was continuously cooled with a
cold finger to ~15 °C. The pH of the water portion was ad-
justed by the addition of H2SO4. The solution was purged
with argon and irradiated at 300 nm in a Rayonet RPR 100
photochemical reactor using 2 or 16 lamps for different
times (1–30 min). After irradiation, the solution was neutral-
ized by the addition of NaHCO3 and extracted using
CH2Cl2. The combined extracts were dried over MgSO4 and
the solvent was evaporated. After evaporation of the solvent,
photolysis mixtures were analyzed by NMR and (or)
chromatographed on a thin layer of silica using CH2Cl2–
EtOAc as eluent to separate photoproducts.
1
solution, salt plates, cm–1) ν: 3435, 1656. H NMR (CDCl3,
300 MHz) δ: 7.83 (s, 1H), 7.74 (d, 2H, J = 8.1 Hz), 7.64 (d,
1H, J = 7.4 Hz), 7.52–7.61 (m, 2H), 7.43 (dd, 2H, J =
7.7 Hz, J = 8.1 Hz), 7.41 (t, 1H, J = 7.7 Hz), 7.23 (dd, 1H,
J = 8.1 Hz, J = 8.3 Hz), 6.90–6.95 (m, 2H), 6.78 (d, 1H, J =
8.3 Hz), 5.83 (s, 1H), 3.76 (s, 3H), 2.50 (br s, 1H). 13C NMR
(CDCl3, 75 MHz) δ: 196.8 (s), 160.0 (s), 145.3 (s), 144.3
(s), 137.8 (s), 137.6 (s), 132.7 (d), 130.6 (d), 130.3 (d),
129.9 (d), 129.5 (d), 128.6 (d), 128.5 (d), 128.2 (d), 119.0
(d), 113.4 (d), 112.3 (d), 75.9 (d), 55.4 (q). EIMS m/z: 319
(M+, 25), 318 (M+, 100), 209 (25), 183 (40), 109 (55), 105
(78), 77 (60). HRMS calcd. for C21H18O3: 318.1256; found
318.1254.
Photolysis of ␣-hydroxy-3-(4′-methylbenzyl)benzophenone
(8)
␣-Hydroxy-3-(3′-formylbenzyl)benzophenone (14)
Starting from 2.0 g (9.5 mmol) of 3-formylbenzophenone,
255 mg of Mg (10.5 mmol), and 2.71 g of 3-bromo-
benzaldehyde diethyl acetal (10.5 mmol) (Sigma-Aldrich),
the reaction furnished 560 mg (19%) of the product, a color-
less oil. IR (neat from CH2Cl2 solution, salt plates, cm–1) ν:
A solution of 8 (10 mg in 400 mL, pH 2) was photolyzed
(300 nm, 2 lamps) in a quartz tube for up to 20 min. After
work-up the residue was chromatographed on prep. TLC to
give 18 and a mixture of 8 and 17 (maximum isolated yield
of 17 was 20%). The highest content of 17 (92%) was ob-
tained on photolysis in the presence of naphthalene. IR (neat
1
3434, 1695, 1653. H NMR (CDCl3, 300 MHz) δ: 9.98 (s,
1
1H), 7.91 (s, 1H), 7.84 (s, 1H), 7.78 (d, 1H, J = 7.8 Hz),
7.74 (d, 2H, J = 8.1 Hz), 7.63–7.70 (m, 2H), 7.53–7.62 (m,
2H), 7.50 (t, 1H, J = 7.6 Hz), 7.44 (dd, 1H, J = 7.6 Hz, J =
8.1 Hz), 7.40–7.48 (m, 1H), 5.98 (s, 1H), 2.10 (br s, 1H).
13C NMR (CDCl3, 75 MHz) δ: 196.7 (s), 192.4 (d), 144.8
(s), 143.9 (s), 138.2 (s), 137.5 (s), 136.9 (s), 132.8 (d), 132.7
(d), 130.7 (d), 130.3 (d), 129.9 (d), 129.6 (d), 129.4 (d),
from CH2Cl2 solution, salt plates, cm–1) ν: 3434, 1652. H
NMR (CDCl3, 500 MHz) δ: 7.82 (dd, 1H, J = 1.7 Hz, J =
1.9 Hz), 7.66 (d, 2H, J = 8.1 Hz), 7.64 (ddd, 1H, J = 1.2 Hz,
J = 1.6 Hz, J = 7.5 Hz), 7.58 (ddd, 1H, J = 1.1 Hz, J =
1.6 Hz, J = 7.7 Hz), 7.41 (t, 1H, J= 7.7 Hz), 7.37 (d, 2H, J =
8.5 Hz), 7.33 (dd, 2H, J = 7.7 Hz, J = 8.5 Hz), 7.25–7.28
(m, 1H), 7.24 (d, 2H, J = 8.1 Hz), 5.89 (s, 1H), 2.42 (s, 3H),
© 2007 NRC Canada

570
Can. J. Chem. Vol. 85, 2007
2.35 (br s, 1H). 13C NMR (CDCl3, 125 MHz) δ: 196.5 (s),
144.3 (s), 143.7 (s), 143.5 (s), 138.3 (s), 135.0 (s), 130.5 (d),
130.4 (d), 129.4 (d), 129.2 (d), 128.9 (d), 128.6 (d), 128.1
(d), 128.1 (d), 126.8 (d), 76.2 (d), 21.9 (q). EIMS m/z: 303
(M+, 10), 302 (M+, 30), 279 (10), 223 (15), 212 (40), 197
(50), 165 (20), 149 (30), 133 (50), 119 (60), 105 (100), 91
(30), 77 (70). HRMS calcd. for C21H18O2: 302.1307; found
302.1307.
137.9 (s), 137.2 (s), 133.4 (d), 133.2 (d), 133.0 (d), 132.8
(d), 131.1 (d), 130.3 (d), 129.8 (s), 128.7 (d), 114.0 (d), 55.8
(q). EIMS m/z: 317 (M+, 5), 316 (M+, 20), 279 (30), 212
(25), 167 (40), 149 (100), 135 (95), 105 (30), 77 (35).
HRMS calcd. for C21H16O3: 316.1099; found 316.1101.
Photolysis of ␣-hydroxy-3-[4′-(hydroxymethyl)benzyl]ben-
zophenone (11)
A solution of 11 (10 mg in 400 mL, pH 2) was photolyzed
at 300 nm for up to 15 min. After work-up, the residue was
separated by prep. TLC. The main product was 20 (maxi-
mum isolated yield 5%–10%) as a mixture of diastereomers
(colorless oil). IR (neat, NaCl plates, cm–1) ν: 3402, 1694.
1H NMR (CDCl3, 500 MHz) δ: 9.96 (s, 1H), 9.95 (s, 1H),
7.86 (2d, 2H, J = 1.8 Hz), 7.76 (2dd, 2H, J = 1.4 Hz, J =
7.7 Hz), 7.62 (2dd, 2H, J = 1.8 Hz, J = 7.6 Hz), 7.47 (2dd,
2H, J = 7.6 Hz, J = 7.7 Hz), 7.42–7.45 (m, 2H), 7.12–7.34
(m, 16H), 5.87 (s, 2H), 5.81 (s, 2H). 13C NMR (CDCl3,
125 MHz) δ: 192.5 (d), 145.0 (s), 144.6 (s), 144.6 (s), 143.8
(s), 143.8 (s), 143.7 (s), 143.7 (s), 136.8 (s), 132.7 (d), 129.4
(d), 129.2 (d), 129.1 (d), 128.8 (d), 128.0 (d), 127.9 (d),
127.9 (d), 127.9 (d), 126.8 (d), 126.8 (d), 126.5 (d), 126.4
(d), 126.1 (d), 126.0 (d), 125.0 (d), 76.4 (d), 76.3 (d), 75.9
(d), 75.9 (d). EIMS m/z: 319 (M+, 7), 318 (M+, 35), 239
(30), 211 (20), 183 (20), 167 (30), 133 (20), 105 (100), 77
(45). HRMS calcd. for C21H18O3: 318.1256; found
318.1254.
Also found were 14 (identified by comparison to an au-
thentic sample made above) and a trace amount of 19 was
detected in the photomixture, which could not be purified.
1H NMR of an enriched TLC fraction assignable to 19 is as
follows: (CDCl3, 500 MHz) δ: 7.83 (s, 1H), 7.73 (s, 1H),
7.67 (d, 2H, J = 7.4 Hz), 7.62 (d, 1H, J = 7.7 Hz), 7.58 (d,
1H, J = 7.4 Hz), 7.40–7.48 (m, 2H), 7.24–7.40 (m, 5H), 5.90
(s, 1H), 4.73 (s, 2H).
The oxidized product 18 was also isolated from the pre-
parative TLC as a colorless oil. IR (neat from CH2Cl2 solu-
1
tion, salt plates, cm–1) ν: 1659. H NMR (CDCl3, 500 MHz)
δ: 8.13 (dd, 1H, J = 1.8 Hz, J = 1.7 Hz), 7.96–8.00 (m, 2H),
7.79 (dd, 2H, J = 1.7 Hz, J = 8.0 Hz), 7.70 (d, 2H, J =
8.1 Hz), 7.59 (dt, 1H, J = 1.7 Hz, J = 7.7 Hz), 7.58 (ddd, J =
1.5 Hz, J = 7.0 Hz, J = 7.5 Hz), 7.47 (dd, 2H, J = 7.7 Hz,
J = 8.0 Hz), 7.26 (d, 2H, J = 8.1 Hz), 2.41 (s, 3H). 13C NMR
(CDCl3, 125 MHz) δ: 196.1 (s), 195.8 (s), 144.0 (s), 138.4
(s), 138.0 (s), 137.3 (s), 134.5 (s), 133.6 (d), 133.5 (d), 133.1
(d), 131.3 (d), 130.5 (d), 130.3 (d), 129.4 (d), 128.7 (d), 21.9
(q). EIMS m/z: 301 (M+, 10), 300 (M+, 60), 279 (10), 223
(15), 209 (10), 167 (20), 149 (35), 119 (100), 105 (50), 91
(30), 77 (30). HRMS calcd. for C21H16O2: 300.1150; found
300.1152.
Photolysis of 8 in the presence of naphthalene
A 400 mL of the CH3CN–H2O solution (pH 2) containing
10 mg of 8 and 50 mg of naphthalene was purged with argon
and irradiated for 1 h at 300 nm (16 lamps). After irradia-
tion, extractions with CH2Cl2 were carried out. The collected
extracts were dried over anhydr. MgSO4 and the solvent
evaporated. The residue obtained from 3 photolyses was
chromatographed on a prep. TLC (silica gel; CH2Cl2–EtOAc
(2.5%)). The following fractions were isolated: ~10 mg of
18 and ~ 7 mg of the mixture containing 92% of 17 and 8%
of the starting material 8.
Nanosecond laser flash photolysis (LFP)
Photolysis of ␣-hydroxy-3-(4′-methoxybenzyl)benzophe-
none (9)
All LFP studies were conducted at the University of Vic-
toria LFP facility employing a Nd-YAG laser with a pulse
width of 20 ns and excitation wavelength of 266 nm. Flow
cells (0.7 cm) were used and solutions were purged with ni-
trogen or oxygen for 30 min prior to measurements. Optical
densities at 266 nm were ~0.1–0.7.
A solution of 9 (10 mg in 400 mL, pH 2) was photolyzed
(300 nm, 16 lamps) in a quartz tube for up to 10 min. After
work-up the residue was chromatographed on prep. TLC to
give pure 15 as a colorless oil (maximum isolated yield of
15 was 40%–50%). IR (neat, NaCl plates, cm–1) ν: 3429,
1
1646. H NMR (CDCl3, 300 MHz) δ: 7.77 (s, 1H), 7.76 (d,
2H, J = 8.8 Hz), 7.60 (d, 1H, J = 7.5 Hz), 7.57 (d, 1H, J =
8.1 Hz), 7.22–7.44 (m, 6H), 6.92 (d, 2H, J = 8.8 Hz), 5.86
(s, 1H), 3.86 (s, 3H), 2.76 (br s, 1H). 13C NMR (CDCl3,
75 MHz) δ: 195.7 (s), 163.4 (s), 144.3 (s), 143.7 (s), 138.5
(s), 132.8 (d), 130.3 (s), 130.2 (d), 129.1 (d), 128.8 (d),
128.5 (d), 128.0 (d), 126.7 (d), 113.7 (d), 76.1 (d), 55.7 (q).
EIMS m/z: 319 (M+, 10), 318 (M+, 60), 279 (15), 256 (15),
239 (15), 213 (50), 165 (20), 149 (70), 135 (100), 121 (30),
105 (100), 97 (30), 77 (87). HRMS calcd. for C21H18O3:
318.1256; found 318.1257.
Acknowledgments
This research was supported by the Natural Sciences and
Engineering Research Council (NSERC) of Canada and the
University of Victoria.
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The oxidized product 16 was also isolated as a colorless
oil. IR (neat from CH2Cl2 solution, salt plates, cm–1) ν:
1
1654. H NMR (CDCl3, 300 MHz) δ: 8.10 (s, 1H), 7.97 (d,
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(CDCl3, 75 MHz) δ: 196.2 (s), 196.1 (s), 163.8 (s), 138.7 (s),
© 2007 NRC Canada

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