, 2003, 13(6), 265–266
chromium(VI) reagents have been reported, such as n-butyl-
phosphonium dichromate,2 pyridinium chlorochromate,3,4 pyra-
zinium dichromate,5 ferric dichromate,6 Dowex 1-X8 (on which
Cl– is replaced by dichromate and bisulfate ions),7 1,1,3,3-tetra-
methylguadinium dichromate,8 γ-picolinium chlorochromate9
and CrO3/NBu4HSO4.10
Table 2 Comparison of the results obtained by oxidation with ammonium
dichromate in the presence of Al(HSO4)3 and wet SiO2 (1), with published
data for 1,1,3,3-tetramethylguanidium dichromate (2)8 and γ-picolinium
chlorochromate (3).9
Oxidant/substrate, time/h, yield (%)
Entry Substrate
However, the utility of CrVI reagents in the oxidative trans-
formation is compromised due to their power, instability, low
selectivity, long reaction time, strong protic and aqueous con-
ditions and tedious work-up. Thus, a milder, more selective and
inexpensive reagent is still desirable.
(1)
(2)
(3)
1
2
Benzyl alcohol
Diphenylcarbinol 0.3, 0.25, 95 2, 8, 98
0.3, 0.5, 92
2, 4, 93
1, 0.3, 75
1, 0.3, 92
Note that oxidation did not proceed with ammonium di-
Here we report a mild, simple, efficient and convenient
method for the oxidation of alcohols into carbonyl compounds
in solution and under solvent-free conditions.†
The oxidation of alcohols in n-hexane as a solvent, using
ammonium dichromate in the presence of Al(HSO4)3 and wet
SiO2 (Scheme 1) at room temperature was studied. Yields and
reaction times are given in Table 1. Over-oxidation of the
products, using this method, was not observed.
chromate, Al(HSO4)3 or wet SiO2 alone, even after prolonged
heating. These results can be explained by the probable in situ
generation of H2CrO4 in a low concentration at the surface of
wet SiO2 by Al(HSO4)3 and ammonium dichromate.
To illustrate the efficiency of the proposed method, Table 2
compares our results with relevant published data.8,9
In conclusion, the ready availability and low cost of the
reagents, the simple and clean work-up, the high product yields
and the mild reaction conditions all make this method a useful
addition to the present methodologies for the oxidation of alco-
hols. In addition, these properties render this method attractive
for use in large-scale operations.
In order to compare the results with those obtained in the
absence of a solvent, we studied the oxidation reaction under
solvent free conditions (Table 1).
i or ii
R1R2CHOH
R1R2CO
This work was supported in part by the Guilan University
Research Council.
Scheme 1 Reagents and conditions: i, (NH4)2Cr2O7/Al(HSO4)3/wet SiO2,
solvent free, room temperature; ii, (NH4)2Cr2O7/Al(HSO4)3/wet SiO2,
n-hexane, room temperature.
References
Although, in most cases, by omitting the solvent, the reaction
time increased, but the work-up procedure became easier and
the need for solvent was avoided. This method is unsuitable for
the oxidation of allylic alcohols (Table 1, entry 13).
1
J. C. Collins, W. W. Hess and F. J. Frank, Tetrahedron Lett., 1968, 24,
3363.
2
I. Mohammadpoor-Baltork, M. M. Sadeghi, N. Mahmoodi and
B. Kharamesh, Indian J. Chem., 1997, 368, 438.
J. S. Cha, M. Kim, J. M. Kim, O. O. Kwon, J. H. Chun and S. D. Cho,
Bull. Korean Chem. Soc., 1998, 19, 724.
3
4
†
Commercial chemicals (Fluka, Merck and Aldrich) and silica gel 60
(0.063–0.2 mm, 70–230 mesh) from Merck were used. Products were
separated and purified by chromatography and identified by comparison
of melting points, IR and NMR spectra and refractive indices with those
reported for authentic samples. All the yields refer to isolated products.
The purity determination and reaction monitoring were performed by
TLC on silica gel Polygram SILG/UV 254 plates. Column chromato-
graphy was carried out on Merck Kisselgel 60H.
5
6
B. Tamami and H. Alinezhad, Iran J. Sci. Tech., 1997, 21, 375.
H. Firouzabadi, B. Tamami, N. Goudarzian, M. M. Lakouraj and
H. Hatam, Synth. Commun., 1991, 21, 2077.
7
8
S. Kim, D. C. Lhim and P. H. Lee, Bull. Korean Chem. Soc., 1986,
7, 86.
Preparation of Al(HSO4)3. A 500 ml suction flask was equipped with
a constant-pressure dropping funnel. A gas outlet was connected to a
vacuum system through an absorbing solution (water) and an alkali trap.
Anhydrous aluminium chloride (66.7 g, 0.5 mol) was charged into the
flask, and concentrated sulfuric acid (147.1 g, 1.5 mol) was added drop-
wise for 40 min at room temperature. HCl gas was evolved immediately.
After completion of the addition of H2SO4, the mixture was shaken for
30 min, meanwhile, the residual HCl was exhausted by suction. A white
solid material was thus obtained (158.5 g). Al(HSO4)3 was characterised
by the determination of its H+ content by titration with NaOH (calc.
0.95%, found 0.96%), and SO42– was determined by precipitation with
Ba2+ (calc. 90.57%, found 90.48%). Al3+ was determined by conversion
to Al2O3 (calc. 8.48%, found 8.31%) and spectrophotometry using com-
plex formation with aluminon (calc. 8.48%, found 8.38%).11
9
M. M. Khodaie, P. Salehi and M. Goodarzi, Synth. Commun., 2001, 31,
1253.
10 T. Burnelet, C. Jouitteau and G. Gelbard, J. Org. Chem., 1986, 51,
4016.
11 A. I. Vogel, Quantitative Inorganic Analysis, Longman, London, 1972,
pp. 462, 472, 792.
Oxidation of 2-bromobenzyl alcohol to 2-bromobenzaldehyde in
n-hexane. A typical procedure. 2-Bromobenzyl alcohol (0.187 g, 1 mmol)
was added to a mixture of Al(HSO4)3 (0.316 g, 1 mmol), wet SiO2
(50 wt%, 0.1 g) and (NH4)2Cr2O7 (0.076 g, 0.3 mmol) in n-hexane (5 ml).
The resulting mixture was stirred at room temperature for 14 min. The
progress of the reaction was monitored by TLC and the mixture was
filtered upon completion. The residue was washed with CH2Cl2 (10 ml).
For the removal of trace water, which may be transfered from wet SiO2
to the filtrate, anhydrous MgSO4 was added to the filtrate and the mix-
ture was filtered after 10 min. Evaporation of the solvent followed by
column chromatography gave 2-bromobenzaldehyde in 90% yield.
Oxidation of 4-chlorobenzyl alcohol to 4-chlorobenzaldehyde under
solvent-free conditions. A typical procedure. 4-Chlorobenzyl alcohol
(0.142 g, 1 mmol) was added to a mixture of Al(HSO4)3 (0.316, 1 mmol),
wet SiO2 (50 wt%, 0.1 g) and (NH4)2Cr2O7 (0.076 g, 0.3 mmol). The
resulting mixture was shaken at room temperature for 10 min. The progress
of the reaction was monitored by TLC. The reaction mixture was treated
with 10 ml of CH2Cl2 and then filtered. For the removal of trace water,
which may be transfered from wet SiO2 to the filtrate, anhydrous MgSO4
was added to the filtrate and the mixture was filtered after 10 min.
Evaporation of the solvent followed by column chromatography gave
4-chlorobenzaldehyde in 95% yield.
Received: 20th August 2003; Com. 03/2173
– 266 –
Shirini, Farhad
