Remarkable differences in photo and thermal (acid-catalyzed) reactivities between ortho-and para-acylcyclohexadienones as essential factors determining the overall efficiency of the photo-fries rearrangement

Article abstract of DOI:10.1246/cl.2004.256

Successful isolation of ortho and para - acylcyclohexadienones allowed us to comparatively study their ground- and excited-state behavior under a variety of conditions. In neutral solutions, the two isomeric cyclohexdienones showed completely different reactivities for photochemical and thermal reactions, while in acidic methanol both quantitatively afforded the corresponding transesterification product and naphthol. These studies help us understand the detailed photo-Fries rearrangement mechanism, which involves several crucial photochemical and thermal steps.

Full text of DOI:10.1246/cl.2004.256

256
Chemistry Letters Vol.33, No.3 (2004)
Remarkable Differences in Photo and Thermal (Acid-catalyzed) Reactivities between ortho-
and para-Acylcyclohexadienones as Essential Factors Determining the Overall Efficiency
of the Photo-Fries Rearrangement
Tadashi Mori,ꢀ Makoto Takamoto, Hideaki Saito, Takahiro Furo, Takehiko Wada, and Yoshihisa Inoueꢀy
Department of Molecular Chemistry, Osaka University, 2-1 Yamada-oka, Suita 565-0871
yEntropy Control Project, ICORP, JST, 4-6-3 Kamishinden, Toyonaka 560-0085
(Received December 9, 2003; CL-031208)
Successful isolation of ‘‘ortho’’ and ‘‘para’’-acylcyclohexa-
dienones allowed us to comparatively study their ground- and
excited-state behavior under a variety of conditions. In neutral
solutions, the two isomeric cyclohexdienones showed complete-
ly different reactivities for photochemical and thermal reactions,
while in acidic methanol both quantitatively afforded the corre-
sponding transesterification product and naphthol. These studies
help us understand the detailed photo-Fries rearrangement
mechanism, which involves several crucial photochemical and
thermal steps.
absorption at 330 nm; these spectral changes are assignable to
the initial formation of 6-acetylcyclohexa-2,4-dienone and the
subsequent ring-opening to the corresponding dienic ketene. In-
terestingly, the formation of isomeric 4-acylcyclohexadienone
was not detected.
In this study, we comparatively investigated the photoreac-
tivities of ‘‘para’’ and ‘‘ortho’’-acylcyclohexadienones 1 and 2
for a more comprehensive understanding of the process. An ace-
tonitrile solution of 1 (0.5 mM) was irradiated under argon at
25 ꢁC in a Pyrex tube at 313 nm with a high-pressure mercury
lamp fitted with an aqueous K2CrO4 filter, with the photolyzate
analyzed by HPLC. Upon short irradiation (5 min), ester 3, or-
tho-cyclohexadienone 4 and phenol 5 were obtained in 7, 24,
and 9% yields, respectively. Although the photolysis of aryl es-
ters is thought to occur in the singlet state, 1 does not fluoresce,
suggesting fast intersystem crossing to the triplet. The low ma-
terial balance may also be explained by a greater contribution
of the cage-escape process of a triplet radical pair. These results
evoke possible involvement of the secondary photoreaction of
acylcyclohexadienones during the photo-Fries reaction of the ar-
yl esters, especially when the conversion is high for synthetic
purposes.
The photo-Fries rearrangement has frequently been em-
ployed as a bench-mark reaction for assessing the confinement
effects of zeolites,1,2 cyclodextrins,3 micelles,4 and polyethylene
films2,5 on the fate of geminate organic radical pairs. Neverthe-
less, the complete photorearrangement mechanism has not yet
been fully established. Furthermore, precise reaction control is
often difficult to achieve and the reproducibility of product yield
and distribution is not good in general, probably owing to the in-
tervention of several consecutive processes, i.e. the initial photo-
cleavage to a geminate radical pair and the subsequent migration
of the resulting acyl fragment to the o- and p-positions to form
cyclohexadienone intermediates, which eventually undergo ther-
mal (acid- or base-catalyzed) rearrangements to give the corre-
sponding acylphenols.6 For instance, recent studies7,8 revealed
puzzling solvent effects that by the use of different batch or pu-
rification procedures lead to significantly different product distri-
butions. These are rationalized mostly by the side reactions pos-
sibly arising from long-lived radical species, but secondary
photochemical, as well as thermal, reactions of the intermedi-
ate(s) may also be responsible at least in part. Thus, for a com-
prehensive understanding and precise control of the photo-Fries
rearrangement process, it is indispensable to elucidate the photo-
chemical and thermal reactivities of acylcyclohexadienone inter-
mediates. In the preceding paper,9 we showed that acylcyclohex-
adienones 1 and 2, produced as photo-Fries rearrangement
products, are reasonably stable and isolable under ambient con-
ditions. This enabled us to closely examine the photochemical
and acid-catalyzed reactions of 1 and 2.
hν (313 nm)
+
+
O
O
Mes
Mes =
Mes
O
O
O
OH
O
Mes
3
4
5
MeOH, H+
1
MesCO2Me (6)
5
+
(quantitative)
hν (313 nm)
O
No reaction
O
Mes
7 (not formed)
O
O
MeOH, H+
Mes
+
6
2
(quantitative)
OH
8
Scheme 1.
In sharp contrast to 1, 2 was extremely photostable, being re-
covered totally under the same conditions and in 98% even after
30 min irradiation, where ortho-analogue 4 was completely con-
sumed and 62% of ester 7 was converted.13 The dienone 2 was
also recovered exclusively when the photolysis was performed
in methanol.
Nucleophilic reactions of water and alcohol to 4-benzoylcy-
clohexadienone have been known,11 but no similar reaction is
known for ortho-acylcyclohexadienone. We therefore compared
4-Benzoyl-4-methyl-2,5-cyclohexadienone has been syn-
thesized,10 and photolyzed to give benzoic acid and p-cresol as
major products, although no further details have been reported.11
The photobehavior of transient 2,4-cyclohexadienone species
obtained upon flash photolysis of phenyl acetate has been inves-
tigated.12 In this two-laser, two-color excitation experiment, the
first pulse at 266 nm produces a transient species, which is fur-
ther photolyzed by the second pulse at 308 nm to give a transient
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.3 (2004)
257
(a)
(b)
(c)
0.8
0.4
0
3
2
1
0
260 nm (x 1/4)
1
2
2
1
0
330 nm
5000 10000 15000
Time / s
0
250
300
350
250
300
350
Wavelength / nm
Wavelength / nm
Figure 1. UV spectral changes for the reactions of cyclohexadienones 1 and 2 in methanol in the presence of methansulfonic acid at
25 ꢁC. (a) ½1ꢄ0 ¼ 0:26 mM, [MeSO3H] = 0.20 mM. (b) ½2ꢄ0 ¼ 0:28 mM, [MeSO3H] = 0.17 mM. (c) Typical curve fitting analyses of
the absorption changes at 260 and 330 nm for the transesterification of 1 in the presence of acid, with an fitting equation: ꢀA ¼
c1 ꢅ expðꢃkobstÞ þ c2 ꢅ ½1 ꢃ expðꢃkobstÞꢄ.
cal and thermal (acid-catalysis) activations. More detailed and
quantitative studies are currently in progress for revealing the
origin of differences in the photochemical and thermal reactivi-
ties, with the aim of eventually understanding the overall photo-
Fries rearrangement mechanism.
Table 1. Observed rate constants for the acid-catalyzed meth-
anolyses of 1 and 2 in methanol at 25 ꢁC
[MeSO3H]/mM kobs/sꢃ1
½1ꢄ0 ¼ 0:26 mM
[MeSO3H]/mM kobs/sꢃ1
½2ꢄ0 ¼ 0:28 mM
0.097
0.146
0.182
0.195
0.292
0.389
0.478
0.955
1.91
<10ꢃ7
<10ꢃ7
0.051
0.072
2:2 ꢅ 10ꢃ6
2:3 ꢅ 10ꢃ6
4:3 ꢅ 10ꢃ6
7:4 ꢅ 10ꢃ6
8:0 ꢅ 10ꢃ6
1:0 ꢅ 10ꢃ3
3:7 ꢅ 10ꢃ3
8:5 ꢅ 10ꢃ3
1:1 ꢅ 10ꢃ2
1:9 ꢅ 10ꢃ2
Financial support of this work by the 21st Century COE
Program for Integrated EcoChemistry (to TM) is gratefully
acknowledged.
1:29 ꢅ 10ꢃ4 0.102
1:98 ꢅ 10ꢃ4 0.129
9:87 ꢅ 10ꢃ4 0.144
1:67 ꢅ 10ꢃ3 0.170
2:44 ꢅ 10ꢃ3 0.174
5:96 ꢅ 10ꢃ3 0.183
1:23 ꢅ 10ꢃ2 0.190
0.200
References and Notes
1
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ble in methanol at room temperature for more than one week.
Such lower reactivities may be attributed to the steric hindrance
caused by the two methyls on the mesityl moiety, which protects
the carbonyl group from nucleophilic attack by methanol. In the
presence of acid (1 mM MeSO3H added to a 10 mM solution of 1
or 2), however, the transesterification immediately occurred to
give the methyl ester 6 and the corresponding naphthol (5 or
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the apparent rate constants (kobs) (Table 1).
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For both substrates, the effect of acid was discontinuous, ex-
hibiting a sudden increase in kobs at [MeSO3H] = 0.15–0.17 mM.
However, even at low [MeSO3H], 2 suffers transesterification in
appreciable rates, giving a good linear correlation with [MeSO3-
H], whilst 1 does not react in a comparable time scale. At [MeS-
10 L. B. Jackson and A. J. Waring, J. Chem. Soc., Chem. Commun.,
1985, 857; J. Chem. Soc., Perkin Trans. 1, 1988, 1791.
11 L. B. Jackson and A. J. Waring, J. Chem. Soc., Perkin Trans. 2,
1990, 907; J. Chem. Soc., Perkin Trans. 2, 1990, 1893.
O3H] >0.17 mM, the rate constants are well described by kobs
=
´
12 M. C. Jimenez, M. A. Miranda, J. C. Scaiano, and R. Tormos, J.
Chem. Soc., Chem. Commun., 1997, 1487.
ka[MeSO3H] + k0, where ka ¼ 7:1 ꢂ 0:1 and 590 ꢂ 40 Mꢃ1 sꢃ1
for 1 and 2, respectively. The much larger ka for 2 may be attrib-
uted to the 1,3-diketonic structure.
13 Note that absorption coefficient of 10 is much higher than that of
9 at 313 nm, being 7900 and 720 Mꢃ1 cmꢃ1, respectively.
14 Note that the absorption coefficient of 10 is always larger than
that of 12, thus not showing an isosbestic point. However, the
reaction does not involve any side reactions, as judged from the
results of curve-fitting analysis.
We have demonstrated that the ortho- and para-acylcyclo-
hexadienones, sharing an apparently similar dienone structure,
are totally different in stability and reactivity upon photochemi-
Published on the web (Advance View) February 2, 2004; DOI 10.1246/cl.2004.256