686
Chemistry Letters 2001
New Synthetic Method of Benzaldehydes and α,β-Unsaturated Aldehydes
with I2 under Photoirradiation
Akichika Itoh,* Tomohiro Kodama, and Yukio Masaki
Gifu Pharmaceutical University, 5-6-1 Mitahora-higashi, Gifu 502-8585
(Received April 16, 2001; CL-010338)
Benzylic alcohols and allylic alcohols were found to be oxi-
dized to the corresponding benzaldehydes and α,β-unsaturated
aldehydes easily in the presence of I2 under photoirradiation.
Oxidation, as well as carbon–carbon bond formation and
reduction, is an essential and important field in synthetic chem-
istry.1 Although a variety of methods for oxidation of alcohols
have been developed by many workers over many years,2 these
methods are sometimes problematic because many reagents or
heavy metals of environmentally high-impact are required.
Benzylic alcohols and allylic alcohols, in contrast to the simple
alkanols, have proved to be effectively oxidized to the corre-
sponding aldehydes or ketones with active MnO2;3 however,
the activity of MnO2 often varies according to the way of pre-
treatment and a large amount of MnO2 is required.
Furthermore, Swern oxidation, which is a typical oxidation
using no heavy metals, has been used as a mild synthetic
method of aldehydes, while this procedure is tedious, and halo-
genated solvents of environmentally high-impact and a large
amount of several reagents such as DMSO and oxalyl chloride,
which are not suitable for a large-scale reaction, are required.2
In the course of our investigation on the application of photo-
reaction to synthetic chemistry, we have found that benzylic
alcohols and allylic alcohols provide the corresponding benz-
aldehydes and α,β-unsaturated aldehydes with iodine under
photoirradiation conditions (Scheme 1).4 In this letter, we
report our study of this new oxidation method.
Other benzylic alcohols (3, 5, 7) and allylic alcohols (11
and 13) also afforded the corresponding carbonyl compounds
(Table 2). Benzylic alcohols, generally, gave the products in
better yield than that obtained with allylic alcohols. Benzhydrol
(3) was oxidized to benzophenone (4) in high yield (97%) in the
presence of 1 equiv of I2 (entry 1). 4-tert-Butylbenzyl alcohol
(5) and 4-methoxybenzyl alcohol (7) were less reactive than
3,6,7 and 4-tert-butylbenzaldehyde (6) and 4-methoxybenzalde-
hyde (8) were obtained in 77% and 57% yield, respectively,
after 48 h (entries 2 and 3). In contrast, 4-nitrobenzyl alcohol
(9), which possess an electron-withdrawing group, resisted this
oxidation conditions, and 92% of the starting material was
recovered (entry 4). trans-2-Decenal (12) and phytenal (14)
were obtained in moderate yields, when trans-2-decen-1-ol (11)
and phytol (13), which are allylic alcohols, were used as sub-
strates (entries 5 and 6). In addition, isochroman (15), which is
a benzyl ether, was directly oxidized to isochromanone (16) in
26% yield (entry 7). Unfortunately, 4-tert-butylcyclohexyl
alcohol (17), which is a simple alkanol, was intact under the
conditions, and only the starting material was recovered quanti-
tatively even after 66 h (entry 8). Furthermore, this method is
effective for gram-scale reaction: 77% of 2 was obtained when
1 g of 1 was used in a vessel under the similar conditions. 8
Although the mechanism of this oxidation is not yet clear,
oxygen in air plays an important role to proceed this reaction
Table 1 shows the results of photooxidation of cinnamyl
alcohol (1, 50 mg) to cinnamaldehyde (2) in the presence of I2
in several solvents using 400-W high-pressure mercury lamps
at room temperature under aerobic conditions.5 Among the sol-
vents examined, i-Pr2O was found to afford the best results, and
gave 2 in 95% yield (entries 1, 8–13). This reaction was com-
pleted in 12 h, while the starting material was recovered in 6 h
(entry 2). Although a stoichiometric amount of I2 was required
to give 2 in high yield, this oxidation was, surprisingly, promot-
ed even by a catalytic amount of I2 (entries 3 and 4). On the
other hand, the result that, without either I2 or irradiation, 2 was
obtained only in low yield shows the necessity of both I2 and
irradiation for this reaction (entries 5 and 6). This reaction pro-
ceeded smoothly under aerobic condition; however, the yield of
2 was reduced under a flow of argon due to the formation of
many by-products (entry 7).
Copyright © 2001 The Chemical Society of Japan