Photocatalytic oxidation of primary and secondary benzylic alcohols to carbonyl compounds catalyzed by H3PW12O 40/SiO2 under an O2 atmosphere

Article abstract of DOI:10.1016/j.tetlet.2005.10.019

An efficient, selective and green procedure for the photocatalytic oxidation of primary and secondary benzylic alcohols to the corresponding aldehydes and ketones has been achieved using silica-encapsulated H ₃PW₁₂O₄₀ as a recyclable heterogeneous photocatalyst in acetonitrile under oxygen gas as the sole reoxidant of the catalyst.

Full text of DOI:10.1016/j.tetlet.2005.10.019

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8483–8486
Photocatalytic oxidation of primary and secondary
benzylic alcohols to carbonyl compounds catalyzed by
H3PW12O40/SiO2 under an O2 atmosphere
Saeid Farhadi,* Mozhgan Afshari, Mansoureh Maleki and Zaynab Babazadeh
Department of Chemistry, University of Lorestan, Khoramabad 68135-465, Iran
Received 16 August 2005; revised 29 September 2005; accepted 6 October 2005
Available online 25 October 2005
Abstract—An efficient, selective and green procedure for the photocatalytic oxidation of primary and secondary benzylic alcohols to
the corresponding aldehydes and ketones has been achieved using silica-encapsulated H3PW12O40 as a recyclable heterogeneous
photocatalyst in acetonitrile under oxygen gas as the sole reoxidant of the catalyst.
Ó 2005 Elsevier Ltd. All rights reserved.
The catalytic oxidation of alcohols to their correspond-
ing carbonyl compounds using O2 or air is of interest
for both economic and environmental reasons.1 Many
thermal catalytic systems, for example, MII-radical
catalysts,2 CuCl.Phen,3 H5PV2Mo10O40,4 PdLn,5 M-
TEMPO,6 Ru-biomimetic-coupled systems,7 bimetallic
Os–Cu and Mo–Cu systems,8 RuO2 and perruthenate-
based catalysts,9 Pt and Pt/Bi catalysts,10 manga-
nese oxide molecular sieves,11 Ru-hydroxyapatite,12
near UV–vis irradiation of a POM solution results in
an O to M (M = W and Mo) charge-transfer excited
state which has strong oxidation ability and is responsi-
ble for the oxidation of organic substrates, photoexcited
POMs being reduced by the transfer of one or two elec-
tron(s) from the organic substrate. Reoxidation (regen-
eration) of the photoreduced POMs back to their
original oxidation state can easily be affected by oxygen
and this reaction is the basis for their photocatalytic
activity.19
13
14
RuCl2(p-cymene)2/Cs2CO3
and V2O5
have been
designed and developed for the oxidation of alcohols
by molecular oxygen. Most of these systems are partic-
ularly effective for activated (benzylic and allylic) alco-
hols. However, the search for suitable green aerobic
catalysts is still a significant challenge. In this context,
photoreactions are promising processes15 and the devel-
opment of photocatalysts is a subject that is now receiv-
ing much attention. TiO2, which catalyzes the oxidation
of various organic compounds, is one example of a prac-
tical and useful photocatalyst.16
Most POMs are highly soluble in polar solvents and suf-
fer from an inability to be recycled. Recently, progress
has been made in the preparation of desolubilized POMs
creating heterogeneous photocatalysts by encapsulating
the POM molecules into a silica matrix via a sol–gel
process.20
However, these reported studies have focused mainly on
the application of these heterogeneous systems for photo-
degradation and mineralization of organic pollutants.21
As far as we know, there is only one report on the appli-
cation of heterogenized POMs as photocatalysts for syn-
thetic purposes.22
An important class of photocatalysts that has received
more attention in recent years is the polyoxometalates
(POMs).17 Mechanistic aspects of photocatalysis by
these non-toxic compounds are well known.18 Briefly,
In this letter, we report a novel method for the oxidation
of benzylic alcohols using sol–gel silica-encapsulated
H3PW12O40 as a heterogeneous photocatalyst. The reac-
tion takes place under an O2 atmosphere, which acts as a
stoichiometric catalyst reoxidant thereby allowing recy-
cling and reuse of H3PW12O40. H3PW12O40/SiO2 was
Keywords: Benzylic alcohols; Photocatalytic; Oxidation; Polyoxometal-
ates; Silica-encapsulated; H3PW12O40
.
*
Corresponding author. Fax: +98 6614600088; e-mail: sfarhad2001@
yahoo.com
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.10.019

8484
S. Farhadi et al. / Tetrahedron Letters 46 (2005) 8483–8486
OH
taining electron-releasing substituents (e.g., entries 2–7),
O
H3PW12O40
SiO2
hv
it was found that the reaction times for the electron-rich
arenes were significantly shorter. Primary benzylic sys-
tems with electron-withdrawing groups (entries 12–14)
required longer reaction times and gave lower yields.
Most of the reactions of these substrates did not go to
completion and increasing the irradiation time did not
improve the yields.
O2
CH3CN
r. t.
1
2
Scheme 1.
prepared according to the reported method by a sol–gel
process.23
Secondary benzylic substrates generally gave very clean,
high-yielding reactions (entries 16–27). In some cases,
the rate enhancement was substantial. For example,
the oxidation of 1-(p-methylphenyl)ethanol (entry 17)
was almost complete after 1 h. The electronic effect
observed for primary benzylic systems also seemed to
be prevalent in the case of secondary benzylic systems.
1-(p-Nitrophenyl)ethanol (entry 18) required a longer
reaction time and gave a decreased yield when compared
to more electron-rich secondary benzylic substrates.
Our investigation began with 1-phenylethanol (1) as a
model substrate to check the catalytic activity of
H3PW12O40/SiO2 for the oxidation of alcohols and to
optimize the reaction conditions. We observed that
when a heterogeneous mixture of this alcohol and
H3PW12O40/SiO2 in acetonitrile was irradiated under
an O2 atmosphere, acetophenone (2) was formed as
the only product in 84% yield within 2h at room tem-
perature (Scheme 1).
Furthermore, pyridine-4-methanol and thiophene-2-
methanol were converted to the corresponding alde-
hydes in 74% and 78% yields, respectively, without
oxidation of the N or S heteroatoms on 4 h irradiation
under our reaction conditions. This reaction is also
efficient for allylic systems. For example, cinnamyl
alcohol gave cinnamaldehyde in a 78% yield after 2h
irradiation.
On the other hand, 1 was oxidized to 2 in a 75% yield
within 2.75 h when pure H3PW12O40 was used as the
homogeneous photocatalyst under the same reaction
conditions. This result clearly indicated that silica-
encapsulated H3PW12O40 is more active than the bulk
H3PW12O40. The significantly higher activity of the
H3PW12O40/SiO2 composite, as compared with that of
pure H3PW12O40, is attributed to the much higher spe-
cific surface area of H3PW12O40/SiO2.20c Using a pure
SiO2 support as a catalyst, no oxidation product 2 was
detected by GC analysis after 4 h irradiation, suggesting
that the photocatalytic activity of H3PW12O40/SiO2 is
derived from the encapsulated H3PW12O40 rather than
the SiO2 support.
In contrast to primary and secondary benzylic alcohols
and allylic alcohols which were oxidized in a highly effi-
cient way, aliphatic and aryl-substituted non-benzylic
alcohols such as 2-propanol and 3-phenyl-1-propanol
were less reactive and gave the corresponding car-
bonyl compounds in only 15% and 18% GC yields,
respectively, after very long irradiation times (10 h).
Moreover, when a mixture of 1-phenylethanol and
2-phenylethanol was irradiated in the presence of
H3PW12O40/SiO2 under O2, the former was oxidized to
acetophenone in an 85% GC yield and the latter gave
2-phenylacetaldehyde in <5% GC yield after 2 h. These
findings prompted us to oxidize 1,2-diols containing
benzylic and non-benzylic hydroxyl groups (entries 28–
32). It was observed that these diols were cleanly oxi-
dized to a-ketols without cleavage of the 1,2-diol bond.
Leaching was shown not to occur with H3PW12O40/SiO2
under the conditions employed. Thus, oxidation of 1
was allowed to proceed to 50% completion, and the
solid photocatalyst was removed by simple filtration;
the filtrate was then irradiated at rt under an oxygen
atmosphere and no further oxidation was observed by
GC even after 4 h. On the other hand, to demonstrate
the recyclability of the catalyst, the recovered catalyst
was reused several times for further reaction runs with-
out a significant decrease in yield or rate. Even after
eight runs, 80% of pure 2 was isolated. The oxidation
of 1 to 2, did not take place when light and or O2 were
eliminated from the system. Therefore, it was confirmed
that H3PW12O40/SiO2, light and O2 were essential for
this reaction.
While all the reactions were run on a 1 mmol scale, these
oxidation conditions also proved successful on a larger
scale. A 10 mmol reaction of 2,4-dimethoxybenzyl alco-
hol using the present procedure provided the corre-
sponding aldehyde in an 85% isolated yield whilst a
100 mmol reaction of benzhydrol gave benzophenone
in an 82% isolated yield.
These conditions were next applied to a variety of ring-
substituted primary and secondary benzylic alcohols.
The results are summarized in Table 1.
In conclusion, a novel and efficient procedure for the
oxidation of primary and secondary benzylic alcohols
has been developed using H3PW12O40/SiO2 as a green
and recyclable photocatalyst, under O2 gas as the sole
catalyst reoxidant. These conditions have been applied
to a wide variety of substrates producing the desired car-
bonyl compounds in moderate to excellent yields. This
reaction is highly selective for vic-diols in oxidizing only
the secondary hydroxyl group a to the benzene ring. The
The reactions of primary benzylic alcohols using this
procedure were clean and all the alcohols were con-
verted into the corresponding aldehydes in moderate
to excellent yields without over-oxidation to carboxylic
acids (entries 1–15). An interesting electronic effect
seemed to exist for these benzylic systems. Comparing
benzyl alcohol (entry 1) with the benzylic alcohols con-

S. Farhadi et al. / Tetrahedron Letters 46 (2005) 8483–8486
8485
Table 1. Photocatalytic oxidation of benzylic alcohols with O2 catalyzed by H3PW12O40/SiO2
O
OH
H3PW12O40
SiO2
hv
/
/
R
R
Ar
Ar
r. t.
O2
/
CH CN
/
3
a
1b-32b
Entry
Ar
R
Producta
Irradiation time/h
Yield/%b
1
2
3
4
5
6
7
8
C6H5
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
1b
2b
2.5
1
1
75
92
90
85
82
85
88
p-MeC6H4
o-MeC6H4
p-PriC6H4
p-ButC6H4
p-MeOC6H4
2,4-(MeO)2C6H3
o-MeOC6H4
p-ClC6H4
2,4-Cl2C6H3
p-BrC6H4
p-PhC6H4
p-CF3C6H4
p-NO2C6H4
1-C10H7
3b
4b
1
5b
6b
1.5
1.5
1
7b
8b
270
1.5
285
282
4.5
4.5
3.5
2.5
284
1
9
9b
80
10
11
12
13
14
15
16
17
18
19
2
10b
11b
12b
13b
14b
15b
16b
17b
18b
19b
20b
21b
22b
23b
24b
25b
26b
27b
28b
29b
30b
31b
32b
52
50
50
76
c
C6H5
CH3
CH3
CH3
C2H5
n-C3H7
cyclo-C3H5
C6H5
C6H5
C6H5
p-MeOC6H4
C6H5
C6H5C@O
–CH2OH
–CH2OH
–CH2OH
–CH2OH
–CH2OH
p-MeC6H4
p-NO2C6H4
C6H5
6H5
6H5
97
76
90
88
90
3
1.5
1.5
1.5
284
1.5
1.25
1
0
1
C
C
2
22
23
24
25
26
27d
2
29
30
31
32
C6H5
p-ClC6H4
p-MeOC6H4
p-MeOC6H4
p-NO2C6H4
C6H5
88
90
90
72
70
3
2.25
275
1
1
1.5
4
8
6H5
C
p-MeC6H4
p-MeOC6H4
p-ClC6H4
p-NO2C6H4
88
90
84
68
1
a Product identification was accomplished by comparison of the physical and spectral data (TLC, mp, IR, H NMR and MS) of the products with
those of the known compounds.
b Yields are for isolated and chromatographically pure products.
c 1-Naphthyl.
d Benzoin.
reaction has been shown to be effective on a 100 mmol
scale with no change in reactivity or yield. Work on
the application of this heterogenized POM and also
other heterogenized POMs for aerobic photocatalytic
oxidation of various organic substrates is now in pro-
gress in our laboratory.
fied on a silica-gel plate or by column chromatography.
Yields are shown in Table 1. All of the products were
characterized by MS, IR and 1H NMR, and by compari-
son with known compounds.24
Acknowledgements
General procedure for the oxidation of benzylic alco-
hols: A solution of the benzylic alcohol (1 mmol) in ace-
tonitrile (25 mL) was added to a Pyrex flask containing a
teflon-coated stirring bar. To this solution was added
H3PW12O40/SiO2 (0.25 g, 20 wt % of H3PW12O40). Oxy-
gen was passed through and the reaction mixture was
kept under an oxygen atmosphere (O2 balloon). The sus-
pension was vigorously stirred and irradiated with a
high pressure 400 W mercury lamp using a cut-off Pyrex
filter at room temperature. The reaction was followed by
TLC and/or GC and after an appropriate irradiation
time, as shown in Table 1, the photocatalyst was filtered
off. After removal of the solvent, the products were puri-
Support for this research by the University of Lorestan
is acknowledged.
References and notes
1. (a) Sheldon, R. A.; Arends, I. W. C. E.; Dijksman, A.
Catal. Today 2000, 57, 157–166; (b) Sheldon, R. A.;
Arends, I. W. C. E.; ten Brink, G.-J.; Dijksman, A. Acc.
Chem. Res. 2002, 35, 774–781.
2. (a) Chaudhuri, P.; Hess, M.; Muller, J.; Hildenbrand, K.;
Bill, E.; Weyhermuller, T.; Wieghardt, K. J. Am. Chem.

8486
S. Farhadi et al. / Tetrahedron Letters 46 (2005) 8483–8486
Soc. 1999, 121, 9599–9610; (b) Chaudhuri, P.; Hess, M.;
16. (a) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341–
357; (b) Hoffmann, M. R.; Martin, T. S.; Choi, W.;
Bahnemann, D. W. Chem. Rev. 1995, 95, 69–96; (c)
Fujishima, A.; Rao, T. N.; Tryk, D. A. Photochem.
Photobiol. C: Photochem. Rev. 2000, 1, 1–21.
Muller, J.; Hildenbrand, K.; Bill, E.; Weyhermuller, T.;
Wieghardt, K. Angew. Chem., Int. Ed. 1999, 38, 1095–
1098.
3. Marko, I. E.; Giles, P. R.; Tsukazaki, M.; Chelle-Regnaut,
I.; Gautier, A.; Brown, S. M.; Urch, C. J. J. Org. Chem.
1999, 64, 2433–2439.
17. Papaconstantinou, E. J. Chem. Soc., Chem. Commun.
1982, 12–13.
4. (a) Haimov, A.; Neumann, R. Chem. Commun. 2002, 876–
877; (b) Khenkin, A. M.; Neumann, R. J. Org. Chem. 2002,
67, 7075–7079; (c) Khenkin, A. M.; Shimon, L. J. W.;
Neumann, R. Eur. J. Inorg. Chem. 2001, 789–794.
5. (a) Peterson, K. P.; Larock, R. C. J. Org. Chem. 1998, 63,
3185–3189; (b) Stahl, S. S.; Thorman, J. L.; Nelson, R. C.;
Kozee, M. A. J. Am. Chem. Soc. 2001, 123, 7188–7189; (c)
Steinhoff, B. A.; Fix, S. R.; Stahl, S. S. J. Am. Chem. Soc.
2002, 124, 766–767.
6. (a) Betzemeier, B.; Cavazzini, M.; Quici, S.; Knochel, P.
Tetrahedron Lett. 2000, 41, 4343–4346; (b) Dijksman,
A.; Marino-Gonzalez, A.; Payeras, A. M. I.; Arends,
I. W. C. E.; Sheldon, R. A. J. Am. Chem. Soc. 2001, 123,
6826–6833; (c) Cecchetto, A.; Fantana, F.; Minisci, F.;
Recupero, F. Tetrahedron Lett. 2001, 42, 6651–6653; (d)
Ansari, I.; Gree, R. Org. Lett. 2002, 4, 1507–1509; (e)
Ragagnin, G.; Betmezeier, B.; Quici, S.; Knochel, P.
Tetrahedron 2002, 58, 3985–3991.
18. (a) Hill, C. L.; Bouchard, D. A. J. Am. Chem. Soc. 1985,
107, 5148–5157; (b) Nomiya, K.; Sugie, Y.; Miyazaki, T.;
Miwa, M. Polyhedron 1986, 5, 1267–1271; (c) Yamase, T.;
Usami, T. J. Chem. Soc., Dalton Trans. 1988, 183–190; (d)
Papaconstantinou, E. Chem. Soc. Rev. 1989, 18, 1–31; (e)
Hiskia, A.; Papaconstantinou, E. Inorg. Chem. 1992, 31,
163–167; (f) Maldotti, A.; Amadelli, R.; Varani, G.;
Tollari, S.; Porta, F. Inorg. Chem. 1994, 33, 2968–2973; (g)
Duncan, D. C.; Netzel, T. L.; Hill, C. L. Inorg. Chem.
1995, 34, 4640–4646; (h) Mylonas, A.; Papaconstantinou,
E. J. Photochem. Photobiol. A: Chem. 1996, 94, 77–82; (i)
Tanielian, C. Coord. Chem. Rev. 1998, 178–180, 1165–
1181.
19. (a) Yamase, T. Chem. Rev. 1998, 98, 307–325; (b) Hiskia,
A.; Mylonas, A.; Papaconstantinou, E. Chem. Soc. Rev.
2001, 30, 62–69.
20. (a) Izumi, Y.; Hisano, K.; Hida, T. Appl. Catal. A: Gen.
1998, 181, 277–282; (b) Molinari, A.; Amadelli, R.;
Carassiti, V.; Maldotti, A. Eur. J. Inorg. Chem. 2000, 6,
91–96; (c) Mizuno, N.; Misono, M. Chem. Rev. 1998, 98,
199–217.
21. (a) Guo, Y.; Li, D.; Hu, C.; Wang, E.; Wang, Y.; Zhou, Y.;
Feng, S. Appl. Catal. B: Environ. 2001, 30, 337–349;
(b) Peng, G.; Wang, Y.; Hu, C.; Wang, E.; Feng, S.; Zhou,
Y.; Ding, H.; Liu, Y. Appl. Catal. A: Gen. 2001, 218, 91–99;
(c) Ozer, R. R.; Ferry, J. L. J. Phys. Chem. B 2002, 106,
4336–4342; (d) Li, D.; Guo, Y.; Hu, C.; Mao, L.; Wang, E.
Appl. Catal. A: Gen. 2002, 235, 11–20; (e) Guo, Y.; Hu, C.;
Jiang, S.; Guo, C.; Yang, Y.; Wang, E. Appl. Catal. B:
Environ. 2002, 36, 9–17; (f) Guo, Y.; Hu, C.; Jiang, C.;
Yang, Y.; Jiang, S.; Li, X.; Wang, E. J. Catal. 2003, 217,
141–151; (g) Anandan, S.; Ryu, S. Y.; Cho, W.; Yoon, M.
J. Mol. Catal. A: Chem. 2003, 195, 201–208; (h) Jiang, C.;
Guo, Y.; Hu, C.; Wang, C.; Li, D. Mater. Res. Bull. 2004,
39, 251–261; (i) Yang, Y.; Wu, Q.; Guo, Y.; Hu, C.; Wang,
E. J. Mol. Catal. A: Chem. 2005, 225, 203–212.
7. Csjernyik, G.; Ell, A. H.; Fadini, L.; Pugin, B.; Backvall,
J.-E. J. Org. Chem. 2002, 67, 1657–1662.
8. (a) Lorber, C. Y.; Smidt, S. P.; Osborn, J. A. Eur. J. Inorg.
Chem. 2000, 655–658; (b) Muldoon, J.; Brown, S. N. Org.
Lett. 2002, 4, 1043–1045.
9. (a) Bleloch, A.; Johnson, B. F. G.; Ley, S. V.; Price, A. J.;
Shephart, D. S.; Thomas, A. W. Chem. Commun. 1999,
1907–1908; (b) Ji, H.-B.; Wang, L.-F. Chin. J. Chem. 2002,
20, 944–950; (c) Hasan, M.; Musawir, M.; Davey, P. N.;
Kozhevnikov, I. V. J. Mol. Catal. A: Chem. 2002, 180, 77–
84; (d) Ciriminna, R.; Pagliaro, M. Chem. Eur. J. 2003, 9,
5067–5073; (e) Zhan, B.-Z.; White, M. A.; Sham, T.-K.;
Pincock, J. A.; Doucet, R. J.; Rao, K. V. R.; Robertson,
K. N.; Cameron, T. S. J. Am. Chem. Soc. 2003, 125, 2195–
2199.
10. (a) Kluytmans, J. H. J.; Markusse, A. P.; Kuster, B. F. M.;
Marin, G. B.; Schouten, J. C. Catal. Today 2000, 57, 127–
141; (b) Besson, M.; Gallezot, P. Catal. Today 2000, 57,
143–155; (c) Mallat, T.; Baiker, A. Chem. Rev. 2004, 104,
3037–3058.
22. Molinari, A.; Amadelli, R.; Andreotti, L.; Maldotti, A.
J. Chem. Soc., Dalton Trans. 1999, 1203–1204.
11. Son, Y.-C.; Makwana, V. D.; Howell, A. R.; Suib, S. L.
Angew. Chem., Int. Ed. 2001, 40, 4280–4283.
23. Guo, Y.; Wang, Y.; Hu, C.; Wang, Y.; Wang, E.; Zhou,
Y.; Feng, S. Chem. Mater. 2000, 12, 3501–3508.
12. Yamaguchi, K.; Mori, K.; Mizugaki, T.; Ebitani, K.;
Kaneda, K. J. Am. Chem. Soc. 2000, 122, 7144–7145.
13. Lee, M.; Chang, S. Tetrahedron Lett. 2000, 41, 7507–7510.
14. Velusamy, S.; Punniyamuthy, T. Org. Lett. 2004, 6, 217–
219.
15. Horspool, W. M.; Song, P. S. CRC Handbook of Organic
Photochemistry and Photobiology; CRC Press: Boca
Raton, 1995.
24. (a) Moriarty, R. M.; Hu, H.; Gupta, S. C. Tetrahedron
Lett. 1981, 22, 1283–1286; (b) Sorg, G.; Mengel, A.; Jung,
G.; Rademann, J. Angew. Chem., Int. Ed. 2001, 40, 4395–
4397; (c) Mulbaier, M.; Giannis, A. Angew. Chem., Int.
Ed. 2001, 40, 4393–4394; (d) Moore, J. D.; Finney, S. N.
Org. Lett. 2002, 4, 3001–3003; (e) Reed, N. N.; Delgado,
M.; Hereford, K.; Clapham, B.; Janda, K. D. Bioorg.
Med. Chem. Lett. 2002, 12, 2047–2049.