Tetrahedron Letters
Temperature dependent product distribution in photolysis
of o-alkylphenacyl benzoates
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Mi Jang, Bum Hee Lim, Hyuk Jun Ryu, Bong Ser Park
Department of Chemistry, Dongguk University (Seoul campus), Seoul 100-715, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Temperature effect on photochemical reactions of ortho-alkylphenacyl benzoates, one of the recently
developed photoremovable protecting groups (PPG), giving indanones (IN) and benzocyclobutenols
(CB) was examined. As temperature was lowered, CB was formed preferentially over IN and the amount
of IN increased as temperature was elevated. Arrhenius plot of ln(IN/CB) versus 1/T gave a straight line
from which EaIN–EaCB and AIN/ACB were obtained. The least sterically hindered 2-methylphenacyl benzo-
ate (1) gave the highest EaIN–EaCB and AIN/ACB among the ortho-alkylphenacyl benzoates tested in this
research including 2,4,6-trimethylphenacyl benzoate (2) and 2,4,6-triisopropylphenacyl benzoate (3).
Ó 2013 Elsevier Ltd. All rights reserved.
Received 12 August 2013
Revised 16 October 2013
Accepted 23 October 2013
Available online 1 November 2013
Keywords:
Temperature effect
Photochemistry
Photoremovable protecting group
Benzocyclobutenol
For the last two decades, photochemistry of ortho-alkylphenyl
ketones having a leaving group X such as chloride, carboxylates,
sulfonates, or phosphates at alpha position to carbonyl has
received much attention due to their application into photoremov-
able protecting groups (PPG) or photocages.1–3 Typically these
o-alkylphenacyl PPGs utilize an efficient release of HX together
with indanone formation upon photolysis. The reaction shares a
common feature with a well known photochemistry of ortho-alkyl-
phenyl ketones,4,5 which is a hydrogen abstraction reaction of
triplet excited state of the carbonyl, but the following step is
known to vary depending on the substituents at alpha position
to the carbonyl group and the reaction medium. When the alpha
substituent is a good leaving group such as Cl, the ground state
E-dienol formed from the excited state precursor gives the inda-
none in benzene, but the other isomer, Z-dienol, participates in
the formation of the same product in methanol.6 With a relatively
poor leaving group such as carboxylate, the E-dienol is known to be
the sole precursor of the indanone regardless of the solvent.7
Wessig had proposed the intermediacy of the 1,5-triplet biradical
formed by fast HX elimination from the initially formed 1,4-triplet
biradical in the case of the sulphonate leaving group.8 The mecha-
nistic picture known to date is shown in Scheme 1.
explained that solvation of the hydroxyl group in the E-dienol
intermediate by H-bond donor solvents and placement of the ste-
rically more demanding ortho-alkyl groups will force to twist the
enol C@C bond and make the CB formation easier and the IN
formation more difficult.
Significance of finding an optimum condition for the effective
release of protecting groups together with our continuous efforts
to understand the correlation of photochemical selectivity and
chemical structure led us to explore the temperature effect on pho-
tochemistry of ortho-alkylphenacyl benzoates.10–12 During the re-
search we found that Klan and co-workers had studied the
temperature effect of essentially the same system as our current
target molecule, but the information on the CB product was miss-
ing in their studies mainly because the exhaustive photolysis was
performed in their studies.13 Here we would like to report our re-
sults showing varying ratios of CB and IN photoproducts depending
on the reaction temperature and discuss a mechanistic reasoning
of our results.
Ketones 1–3 were prepared by a routine synthetic procedure:
a-bromination of acetophenones using CuBr2 followed by carbox-
ylation of the resulting bromoacetophenones. The ketones in de-
gassed solution of toluene-d8 (0.02 M) were irradiated in an NMR
tube using Pyrex filtered light of a 450 W Hanovia medium pres-
sure mercury arc lamp, and the progress of the reaction was mon-
itored by 1H NMR spectroscopy at regular time intervals. Photolysis
was done at various temperatures (from À30 °C to 100 °C). The de-
sired temperatures were attained by a Neslab CC-100II immersion
cooler for low temperature and by heated silicon oil bath for high
temperature. The product ratios, IN/CB, were taken using 1H NMR
integration of the corresponding peaks of each product. Table 1
Recently, we have reported that photolysis of o-alkylphenacyl
carboxylates gives not only the indanone (IN) but also the benzo-
cyclobutenol (CB).9 Product selectivity of CB over IN increases as
hydrogen bond donor ability of the solvent increases and as the
size of ortho-alkyl group becomes larger. The results were
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Corresponding author.
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