Isolation of cyclohexadienone intermediates in the photo-fries rearrangement of 2,4-dimethylnaphth-1-yl and 1,4-dimethylnaphth-2-yl 2,4,6-trimethylbenzoates

Article abstract of DOI:10.1246/cl.2004.254

Labile cyclohexadienones were isolated for the first time in good yields in the photo-Fries rearrangement of partially blocked naphthyl esters. Upon direct excitation at 313 nm, 1,4-dimethylnaphth-2-yl and 2,4-dimethylnaphth-1-yl 2,4,6-trimethylbenzoates afforded 1-acyl-2-naphthalenone and 2- and 4-acyl-1-naphthalenones, respectively. This isolation is of particular importance as a direct mechanistic proof and also as a convenient route to these thermally less-accessible compounds.

Full text of DOI:10.1246/cl.2004.254

254
Chemistry Letters Vol.33, No.3 (2004)
Isolation of Cyclohexadienone Intermediates in the Photo-Fries Rearrangement
of 2,4-Dimethylnaphth-1-yl and 1,4-Dimethylnaphth-2-yl 2,4,6-Trimethylbenzoates
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-031207)
Labile cyclohexadienones were isolated for the first time in
Here, we report the first successful isolation of such cyclohexa-
dienoid intermediates obtained in the photolysis of blocked
naphthyl esters, which unambiguously reveals its intervention
in the photo-Fries rearrangement.
good yields in the photo-Fries rearrangement of partially
blocked naphthyl esters. Upon direct excitation at 313 nm, 1,4-
dimethylnaphth-2-yl and 2,4-dimethylnaphth-1-yl 2,4,6-trimeth-
ylbenzoates afforded 1-acyl-2-naphthalenone and 2- and 4-acyl-
1-naphthalenones, respectively. This isolation is of particular
importance as a direct mechanistic proof and also as a conven-
ient route to these thermally less-accessible compounds.
Since unblocked 1-naphthyl arenecarboxylates are known to
undergo photo-Fries rearrangements to give the corresponding
2- and 4-aroylnaphthols,5 we first chose 2,4-dimethylnaphth-1-
yl 2,4,6-trimethylbenzoate (1) as substrate, which was synthe-
sized from 2,4-dimethynaphth-1-ol.6 A hexane solution of 1
(2.5 mM), placed in a donut-shaped Pyrex vessel, was irradiated
at ꢁ40 ꢂC under Ar with a 300-W medium-pressure Hg lamp
(ꢀ > 280 nm). After irradiation for 2 h, the photolyzate was con-
centrated, and the residue was purified by column chromatogra-
phy over alumina with hexane eluent to yield 4-cyclohexadie-
none (2)7 in 22% isolated yield (Scheme 2). Cage-escape
product, 2,4-dimethylnaphthol (4), was also obtained, along with
trace amounts of radical escape products 5–8. In contrast, no or-
tho-isomer 3 or decarboxylation product was isolated by the
above photolysis/work-up procedures. In a similar photolysis
in acetonitrile, 2 was isolated in 15% yield. The lack of 3 is a lit-
tle puzzling, since the spin density of 2,4-dimethylnaphthoxy
radical (calculated at the B3LYP/EPRII level) is similarly large
at the 2- and 4-positions (Figure 1), suggesting the facile forma-
tion of both isomers. However, the formation of a small amount
of 3 was detected at low conversions by HPLC analysis, but 3
was photochemically labile under the irradiation conditions par-
ticularly upon prolonged irradiations (ꢀ > 280 nm), probably
owing to the longer ꢀmax of 3 (ꢃ320 nm) than that of 2
(ꢃ260 nm).
The photo-Fries rearrangement of aryl esters has been stud-
ied extensively and reviewed frequently with an emphasis on its
synthetic advantage of the unique positional selectivity of the re-
arrangement.1 The mechanism of the photo-Fries rearrange-
ment has also been investigated in considerable detail, in con-
trast to the thermal Fries-rearrangement, in which ionic inter-
mediates are involved. CIDNP experiments revealed that the
photoreaction occurs in the singlet manifold to give the acyl–ar-
yloxy radical pair;2 this radical pair recombines to cyclohexadi-
enone intermediate(s), which eventually tautomerize to the cor-
responding acylphenol derivatives, as shown in Scheme 1. A
recent flash photolysis study on the photo-Fries rearrangement
of phenyl acetate revealed the intervention of two different tran-
sient species, the decay profiles of which are nicely correlated
with the formations of o- and p-acetylphenols, thus supporting
the assignment of these intermediates to the ortho- and para-cy-
clohexadienones (Scheme 1).3
OH
OH
O
Photo-Fries Rearrangement
+
O
O
O
O
Mes
h
ν (313 nm)
h
ν
+
+
O
O
Mes
O
H
1
*
O
OH
O
O
O
O
Mes =
O
O
Mes
+
1
2
3
4
O
O
H
O
O
O
O
Scheme 1.
Mes
+
+
+
+
Mes
Mes
C6H13
Mes
H
O
OC6H13
We have recently investigated the photochemistry of partial-
ly and fully blocked aryl esters.4 Irradiation of these aryl esters in
neutral solutions predominantly affords the photodecarboxyla-
tion and photo-Fries rearrangement products, while the addition
of alcohol and a catalytic amount of acid to the solution switched
the photoreactivity to transesterification, which is well under-
stood in the framework of the photo-Fries reaction mechanism,
involving the initial CO–O bond cleavage and subsequent rear-
rangement to the ortho and para positions to give the corre-
sponding cyclohexadienones. However, all attempts to isolate
or detect such intermediates were unsuccessful at least with me-
sityl and related substituted phenyl esters, probably owing to the
thermal and/or photochemical instability of the intermediates.
5
6
7
8
Scheme 2.
In order to minimize the secondary photodecomposition of 3
(and presumably of 2 as well), we irradiated the sample solution
at 313 nm with a medium-pressure Hg lamp fitted with an aque-
ous K2CrO4 filter. Upon irradiation at 313 nm for 1 h, ester 1
gave the isomeric cyclohexadienones (2 and 3)8 and naphthol
4 in 26, 22, and 15% yields based on consumption of substrate,
respectively, at 15% conversion as determined by the HPLC
analysis. The ortho-rearranged cyclohexadienone 3, which elut-
ed slightly later than 2, was isolated by preparative HPLC on a
Kanto ODS column (150 mm ꢄ 4.6 mm, ꢀ = 5 mm) with aque-
Copyright ꢀ 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.3 (2004)
255
0.10
(+19.5)
449 (1974).
0.40 (≡ 0)
0.18 (+10.7)
4
5
T. Mori, M. Takamoto, T. Wada, and Y. Inoue, Photochem. Pho-
tobiol. Sci., 2, 1187 (2003); T. Mori, H. Saito, and Y. Inoue, Chem.
Commun., 2003, 2302; T. Mori, T. Wada, and Y. Inoue, Org. Lett.,
2, 3401 (2000).
0.07 (+15.7)
0.38 (+0.9)
0.09 (+13.1)
0.33
O
0.17 (+15.4)
0.50
(≡ 0)
O
0.27
H. S. Banu, K. Pitchumani, and C. Srinivasan, Tetrahedron, 55,
9601 (1999); N. P. Gritsan, Y. P. Tsentalovich, A. V.
Yurkovskaya, and R. Z. Sagdeev, J. Phys. Chem., 100, 4448
(1996); K. Pitchumani, M. Warrier, C. Cui, R. G. Weiss, and V.
Ramamurthy, Tetrahedron Lett., 37, 6251 (1996); R. Nakagaki,
M. Hiramatsu, T. Watanabe, Y. Tanimoto, and S. Nagakura, J.
Phys. Chem., 89, 3222 (1985); Y. Ohto, H. Shizuka, S. Sekiguchi,
and K. Matsui, Bull. Chem. Soc. Jpn., 47, 1209 (1974).
Y. Terao, T. Satoh, M. Miura, and M. Nomura, Tetrahedron, 56,
1315 (2000).
Figure 1. Calculated spin densities of relevant radical species.
Vaules in the parentheses are the heats of formation (in kcal/
mol) of relevant 2,4,6-trimethylbenzoyl dimethylbeozocyclo-
hexadienones derived from each radical species.9
ous acetonitrile as eluent. Prolonged irradiations led to increased
yields of 2 and the complete loss of 3.
6
7
We further performed the photolysis of 1,4-dimethylnaphth-
2-yl 2,4,6-trimethylbenzoate (9).10 Irradiation (ꢀ > 280 nm; 2 h)
at ꢁ40 ꢂC gave 1-acylcyclohexadienone 1011 in 22% isolated
yield, along with 6-acylnaphthol 11 in 13% and naphthol 13 in
13% (Scheme 3). A small amount of 12 was also detected.
The absence of 8-acylnaphthol may be due to the larger heat
of formation for this rearrangement (Figure 1).
2,4-Dimethyl-4-(2,4,6-trimethylbenzoyl)-4H-naphthalen-1-one (2).
mp 75–77 ꢂC; IR (KBr) ꢁmax 3067, 2997, 2970, 2926, 1701,
1659 cmꢁ1; H NMR (400 MHz, CD3CN) ꢂ 1.35 (br s, 3H), 1.80
1
(d, 3H, J ¼ 1:5 Hz), 1.85 (s, 3H), 2.10 (br s, 3H), 2.18 (s, 3H),
6.70 (br s, 1H), 6.79 (br s, 1H), 6.92 (q, 1H, J ¼ 1:5 Hz), 7.45–
7.51 (m, 1H), 7.58–7.64 (m, 1H), 7.82 (m, 1H), 8.07 (m, 1H);
13C NMR (100 MHz, CD3CN) ꢂ 16.29, 19.32, 20.46, 20.99,
27.75, 55.96, 127.23, 128.94, 129.13, 129.38, 132.56, 133.35,
135.43, 139.42, 139.66, 143.50, 146.82, 185.21, 209.88; HRMS:
calcd for C22H22O2 318.1620, found 318.1617; Anal. Calcd for
O
h
ν
O
Mes
+
O
O
Mes
OH
O
C
22H22O2: C, 82.99; H, 6.96%. Found: C, 82.52; H, 6.90%. Rear-
Mes
10
rangement position was confirmed by difference NOE spectrum
with presaturation at 1.85 ppm (2-Me) and 1.80 ppm (4-Me). The
former showed only H3 signal, while the latter showed both H3
and H5 enhancements.
Dimethyl-1-(2,4,6-trimethylbenzoyl)-2H-naphthalen-1-one (3).
1H NMR (400 MHz, CD3CN) ꢂ 1.62 (s, 3H), 1.96 (s, 3H), 2.13
(d, 3H, J ¼ 1:5 Hz), 2.16 (s, 3H), 6.06 (q, 1H, J ¼ 1:5 Hz), 6.70
(s, 2H), 7.35–7.40 (m, J ¼ 1:1 Hz, 7.7 Hz, 1H), 7.45 (d, J ¼
7:7 Hz, 1H), 7.63–7.68 (m, J ¼ 1:1 Hz, 7.7 Hz, 1H), 7.80 (dd,
J ¼ 1:1 Hz, 7.7 Hz, 1H). The spectrum was obtained by subtrac-
tion from the spectrum of isomeric mixture (2 and 3).
9
11
O
Mes
O
+
+
OH
O
Mes
8
9
12
13
Scheme 3.
4-Benzoyl-4-methyl-2,5-cyclohexadienone and its deriva-
tives have already been prepared but requires wearisome mul-
ti-step syntheses.12 To the best of our knowledge, no 2-acylcy-
clohexadienone derivative has been isolated so far. In contrast,
the present photorearrangement allows us to conveniently pre-
pare and isolate such benzocyclohexadienones in appreciable
yields as ortho- and para-rearranged products.
In summary, we have shown for the first time that acylcyclo-
headienone intermediates are involved in the photo-Fries rear-
rangement, by employing naphthyl esters that carry methyl
groups at the photo-Fries rearrangement-susceptible positions.13
Studies on thermal and photochemical behavior of these chemi-
cally intriguing species are in progress.
B3LYP/6-311+g(2d,p)//B3LYP/6-31g(d) calculated heats of
formation of possible cyclohexadienoid intermediates (2, 3, 10,
and 14–18).
O
Mes
H
H
Mes
O H
Mes
O
H
O
O
Mes
O
O
H
O
O
O
Mes
18
14
15
16
17
10 C. W. Jefford, J. C. Rossier, S. Kohmoto, and J. Boukouvalas,
Helv. Chim. Acta, 68, 1804 (1985); Note that not 4 but 13 was ob-
tained by the reported procedure: C. T. Wigal, J. D. McKinley, J.
Coyle, D. J. Porter, and D. E. Lehman, J. Org. Chem., 60, 8421
(1995).
11 Spectral data: 1,4-dimethyl-1-(2,4,6-trimethylbenzoyl)-1H-naph-
thalen-2-one (10). mp 83–84 ꢂC; IR (KBr) ꢁmax 3064, 2978,
Financial support by the 21st Century COE Program for In-
tegrated EcoChemistry (to TM) is gratefully acknowledged. We
thank Dr. Guy A. Hembury for editorial assistance.
2921, 2861, 1700, 1654 cmꢁ1 1H NMR (400 MHz, CD3CN) ꢂ
;
1.75 (s, 6H), 1.85 (s, 3H), 2.19 (s, 3H), 2.30 (d, 3H, Jallyl
1:1 Hz), 5.91 (q, 1H, Jallyl ¼ 1:1 Hz), 6.72 (s, 2H), 7.39–7.54
¼
(AA0XX0, 2H), 7.63–7.67 (AA0XX0, 1H, J5{6 ¼ 7:7 Hz, J5{7
¼
References and Notes
1:5 Hz), 7.72–7.76 (AA0XX0, 1H, J7{8 ¼ 7:9 Hz, J6{8 ¼ 1:3 Hz);
13C NMR (100 MHz, CD3CN) ꢂ 20.03, 20.50, 20.92, 26.50,
66.74, 125.53, 126.98, 128.84, 128.97, 129.08, 131.08, 132.21,
134.06, 139.32, 140.00, 141.26, 154.22, 198.66, 207.38; EI-MS
m=z (relative intensity) 318 (Mþ, 14), 172 (8), 147 (100), 119
(15), 91 (4), 57 (4); Anal. Calcd for C22H22O2: C, 82.99; H,
6.96%. Found: C, 82.76; H, 7.02%. Rearranged position was con-
firmed by difference NOE spectrum with presaturation at 2.34 ppm
(4-Me), showing both H3 and H5 enhancements.
1
M. A. Miranda and F. Galindo, in ‘‘CRC Handbook of Organic
Photochemistry and Photobiology,’’ 2nd ed., ed. by W. M.
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2
3
W. Adam, Chem. Commun., 1974, 289; W. Adam and J. Arce de
Sanabia, J. Org. Chem., 38, 2571 (1973); N. P. Gritsan, Y. P.
Tsentalovich, A. V. Yurkovskaya, and R. Z. Sagdeev, J. Phys.
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1985, 857; J. Chem. Soc., Perkin Trans. 1, 1988, 1791.
13 Related intermediate has been isolated in the photo-Claisen rear-
rangement of naphthyl ester. Y. Yoshimi, A. Sugimoto, H. Maeda,
and K. Mizuno, Tetrahedron Lett., 39, 4683 (1998).
Published on the web (Advance View) February 2, 2004; DOI 10.1246/cl.2004.254