Photooxidation of arylmethyl bromides with mesoporous silica FSM-16

Article abstract of DOI:10.1021/ol0061081

(equation presented) A mesoporous silica FSM-16 was found to be a recyclable oxidizing promoter of arylmethyl bromides for the preparation the corresponding carboxylic acids, aldehydes, or ketones under photoirradiation conditions.

Full text of DOI:10.1021/ol0061081

ORGANIC
LETTERS
2000
Vol. 2, No. 16
2455-2457
Photooxidation of Arylmethyl Bromides
with Mesoporous Silica FSM-16
,†
Akichika Itoh,† Tomohiro Kodama,† Shinji Inagaki,‡ and Yukio Masaki*
Gifu Pharmaceutical UniVersity, 5-6-1 Mitahora-higashi, Gifu 502-8585, Japan, and
Toyota Central R & D labs., Inc., Aichi, 480-1192, Japan
masaki@gifu-pu.ac.jpbi
Received May 25, 2000
ABSTRACT
A mesoporous silica FSM-16 was found to be a recyclable oxidizing promoter of arylmethyl bromides for the preparation the corresponding
carboxylic acids, aldehydes, or ketones under photoirradiation conditions.
The syntheses of aryl aldehydes and aryl carboxylic acids
are essential and important processes in organic synthesis.
Although various methods for the syntheses of aryl aldehydes
from arylmethyl halides, which involve 2-nitropropane
sodium salt,1 selenium compounds,2 or DMSO,3,4 have been
developed, there have been no one-pot oxidations of aryl-
methyl halides to the corresponding aryl carboxylic acids
reported, so far. In general, this transformation has been
carried out via the corresponding alcohols or aldehydes in
at least two steps through oxidative reactions, which need
more than a stoichiometric amount of heavy metals of high
environmental impact.5 In the course of investigations on
the utilities of mesoporous silicas in organic synthesis,6 we
found that R-hydroxycarboxylic acids, phenyl acetic acid
derivatives,7 and N-protected R-amino acids8 afforded the
corresponding carbonyl compounds through an oxidative
decarboxylation reaction in the presence of a silica, FSM-
16,9 under photoirradiation. Since the substituents at the
R-position to the carboxyl group are thought to play an
important role in this reaction, we examined the reactivity
of a variety of R-substituted carboxylic acids to elucidate
the mechanism and develop its utilities. In the course of our
study, R-halocarboxylic acids were also found to afford the
corresponding carbonyl products in high yield.10 Furthermore,
4-tert-butylbenzyl bromide (1), which has no carboxyl group,
was found to give the 4-tert-butylbenzoic acid (2) with FSM-
16 under photoirradiation in acetone (Scheme 1). Now we
Scheme 1
report the utility of FSM-16 as a promoter for the one-pot
oxidation of the arylmethyl bromides to the corresponding
carboxylic acids.
† Gifu Pharmaceutical University
‡ Toyota Central R & D labs., Inc.
(1) Clanderman, B. H. J. Org. Chem. 1966, 31, 2618.
(2) Syper, L.; MLochowski, J. Synthesis 1984, 747.
(3) Epstein, W. W.; Sweat, F. W. Chem. ReV. 1967, 67, 247.
(4) Mancuso, A. J.; Swern, D. Synthesis 1981, 165.
(5) ComprehensiVe Organic Transformations: A Guide to Functional
Group Preparations; Larock, R. C., Ed.; Wiley-VCH: New York, 1989.
(6) Itoh, A.; Kodama, T.; Maeda, S.; Masaki, Y. Tetrahedron Lett. 1998,
39, 9461. Itoh, A.; Kodama, T.; Masaki, Y. Synlett 1999, 357.
(7) Itoh, A.; Kodama, T.; Inagaki, S.; Masaki, Y. Org. Lett. 2000, 2,
331.
In our previous paper on oxidative photodecarboxylation,7
the results for the oxidative reaction with FSM-16 proved
(9) Inagaki, S.; Koiwai, A.; Suzuki, N.; Fukushima, Y.; Kuroda, K. Bull.
Chem. Soc. Jpn. 1996, 69, 1449. Inagaki, S.; Fukushima, Y.; Kuroda, K. J.
Chem. Soc., Chem. Commun. 1993, 680. The unit cell dimension of FSM-
16 was 4.63 nm. The pore diameter was 2.9 nm, and the specific surface
area was 882 m2/g.
(8) Itoh, A.; Kodama, T.; Inagaki, S.; Masaki, Y. Chem. Lett. 2000, 542.
(10) Itoh, A.; Kodama, T.; Inagaki, S.; Masaki, Y. Unpublished results.
10.1021/ol0061081 CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/14/2000

to be affected by the solubility of oxygen to the solvents.11
Table 1 shows the results of the photooxidation of 4-tert-
Table 2. Mesoporous Silicas for Photooxidation of
4-tert-Butylbenzyl Bromide 1
Table 1. Solvents for Photooxidation of 4-tert-Butylbenzyl
Bromide 1 with FSM-16
entry
silica
FSM-16
MCM-41
HMS
recovery of 1 (%)
yield of 2 (%)
1
2
3
4
0
0
27
80
86
66a
58
6b
entry
solvent
recovery of 1 (%)
yield of 2 (%)
10% Ti-HMS
1
2
3
4
5
6
7
8
9
n-hexane
Et2O
80
75
0
88
0
54
88
71
28
20
0
11
5
a 4-tert-Butylbenzal bromide was obtained in 13% yield. b 4-tert-
Butylbenzaldehyde was obtained in 8% yield.
AcOEt
toluene
acetone
acetone
acetone
MeCN
MeOH
DMF
75a
trace
86
21b
0c
22
65d
0
much less reactive than the corresponding bromide 1 (entries
1 and 2). When substrates that possess an electron-withdraw-
ing group such as a nitro or cyano group were used, the yields
were somewhat lower than the the yields for the other
reactions (entries 4 and 5). The corresponding ketone 12 was
10
11
CH2Cl2
86
a 4-tert-Butylbenzal bromide was obtained in 14% yield. b The reaction
was carried out without FSM-16. c The reaction was carried out in the dark.
d Methyl 4-tert-butylbenzoate was obtained as the product.
Table 3. Photooxidation of Arylmethyl Bromides with
FSM-1612
butylbenzyl bromide (1, 50 mg) in the presence of FSM-16
(100 mg) in several solvents using 400-W high-pressure
mercury lamps at room temperature.12 The solvents, except
methylene chloride, in Table 1 are arranged in the order of
solubility of oxygen;11 however, the yield of 2 proved to
have no correlation to the order. Acetone and methylene
chloride were found to be good solvents for the reaction to
afford 2 in high yields (entries 5 and 11). Both photoirra-
diation and FSM-16 proved essential for this reaction to
proceed smoothly (entries 6 and 7).
Interestingly, 65% of methyl 4-tert-butylbenzoate was
obtained as the main product instead of 2 when methanol
was used (entry 9).
The results of the photooxidation in acetone, which is
environmentally more benign, with typical mesoporous
silicas, MCM-41,13 HMS,14 and 10% Ti-HMS,15 are shown
in Table 2. FSM-16 was found to be better as a promoter
than the others.
Table 3 shows the results for the photooxidation of several
arylmethyl bromides. 4-tert-Butylbenzyl chloride (3) was
(11) Che, Y.; Tokuda, K.; Ohsaka, T. Bull. Chem. Soc. Jpn. 1998, 71,
651.
(12) Typical procedure: In a Pyrex tube, a suspension of 4-tert-
butylbenzyl bromide (1, 50 mg) and FSM-16 (100 mg) in dry acetone (5
mL) was irradiated externally at room temperature with a 400-W high-
pressure mercury lamp for 30 h. FSM-16 was then filtered off and washed
with ethyl acetate, and the filtrate was concentrated under reduced pressure.
Pure 4-tert-butylbenzoic acid (2) (34 mg, 86%) was obtained after
purification by preparative TLC.
(13) Kresge, C. T.; Leonowicz, M. E.: Roth, W. J.; Vartuli, J. C.; Beck,
J. S. Nature 1992, 359, 710.
(14) Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865.
(15) Zhang, W.; Froba, M.; Wang, J.; Tanev, P. T.; Wong, J.; Pinnavaia,
T. J. J. Am Chem. Soc. 1996, 118, 9164. Tanev, P. T.; Chibwe, M.;
Pinnavaia, T. J. Nature 1994, 368, 321.
a A total of 57% of starting material was recovered. b Benzal bromide
was obtained in 7% yield. c A total of 26% of starting material was
recovered. d Benzoic acid was obtained in 11% yield. e A total of 91% of
starting material was recovered.
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Org. Lett., Vol. 2, No. 16, 2000

obtained from the secondary bromide 11 (entry 6). An allylic
bromide, cinnamyl bromide (13), was also oxidized to afford
5. Unfortunately, n-hexadecyl bromide (17), an aliphatic
halide, remained intact under the reaction conditions.
Furthermore, the promoter FSM-16 was found to be a
recyclable photocatalyst. Thus, the recovered FSM-16, which
was dried at rt for 2 h under reduced pressure after the
photoreaction of 1, showed no loss of activity for the reaction
after being used three times (Table 4).
Scheme 2
Table 4. Recycle of FSM-16
a A total of 72% of starting material was recovered. b A total of
15% of starting material was recovered.
To clarify the reaction mechanism, possible reaction
intermediates were examined. Bromide 1 was found to
generate 4-tert-butylbenzal bromide (18) after 12 h of
reaction in a 22% yield, which decreased with the reaction
time: 5% after 24 h and trace amounts after 30 h. Contrary
to expectation, 18 proved to afford only 23% of the product
under the same conditions (Scheme 2). Furthermore, an
alcohol, 4-tert-butylbenzyl alcohol (19), and an aldehyde,
4-tert-butylbenzaldehyde (20), are thought to be formed
through substitution of the halogen atom with oxygen at the
benzylic position. Both 19 and 20, however, showed much
lower reactivity than 1 (Scheme 2). These results suggest
that other paths would be involved in the mechanism of the
reaction. A more detailed study is necessary for elucidation
of the mechanism, since benzyl radical species, formed under
irradiation of UV,16 are thought to participate in the course
of this reaction.
In conclusion, this new method for the transformation of
arylmethyl bromides to benzoic acid derivatives is thought
to be more environmentally friendly than previous methods,
due to both the one-pot procedure and the use of a recyclable
solid catalyst instead of heavy metals. Furthermore, the
method is operationally simple from the viewpoint of the
convenience of the workup in which FSM-16 can be easily
removed only by filtration of the reaction mixture.
Supporting Information Available: Experimental de-
tails. This material is available free of charge via the Internet
at http://pubs.acs.org.
(16) Norman, I.; Porter, G. Proc. R. Soc. 1955, A230, 399.
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