Oxidation of Alcohols by (NH4 ) 2Cr2O7 in Solution and under Solvent-Fre Conditions
Ammonium dichromate in the presence of Mg(HSO4)2 and wet SiO2 was used as an effective oxidizing agent for
the oxidation of alcohols in solution and under solvent-free conditions.
Manuscript received: 16 August 2001.
Final Version: 2 October 2001.
Oxidation of Alcohols by (NH4 ) 2Cr2O7 in Solution and under Solvent-Fre Conditions CH01104
F. Shirini et al.
Introduction
The oxidation of alcohols to carbonyl compounds is one of
the fundamental reactions in synthetic organic chemistry.
There are many useful CrVI based oxidants for this purpose,[
1] of which Jones’ reagent,[2] CrO3/NBu4HSO4,[3] nbutylphenylphosphonium
dichromate,[4] pyridinium chlorochromate,[
5,6] pyrazinium dichromate,[7] ferric dichromate,[8]
polyvinylpyridine-supported ferric dichromate,[9] polyethyleneimine-
supported silver dichromate[10] and Dowex 1X8
(on which Cl– is replaced by dichromate and bisulfate
ions[11]) are examples.
However, the utility of CrVI reagents in the oxidative
transformation is compromised due to their power,
instability, low selectivity, long reaction time, strong protic
and aqueous conditions and tedious workup. Thus, a milder,
more selective and inexpensive reagent is still desirable.
In this paper we report a convenient and simple method
for the conversion of alcohols into their corresponding
aldehydes or ketones in solution and under solvent-free
conditions.
Results and Discussion
The oxidation of various alcohols was investigated using
(NH4)2Cr2O7 in the presence of Mg(HSO4)2 and wet SiO2
(Scheme 1), at room temperature. Yields and reaction times
are given in Table 1. Over-oxidation of the products, using
this method, was not observed.
The omission of the solvent not only eases the workup,
but reduces the reaction time. This method is not suitable for
the oxidation of allylic alcohols (Table 1, entry 13).
It should be noted that oxidation did not proceed using
any of Mg(HSO4)2, ammonium dichromate or wet SiO2
alone (Table 1, entries 14–16). These results could be
attributed to the in situ generation of H2CrO4 in low
concentration at the surface of wet SiO2 by the solid
inorganic acidic salts (Mg(HSO4)2 and (NH4)2Cr2O7).
Conclusions
The ready availability and low cost of the reagents, the
simple and clean workup, the high product yields and the
mild reaction conditions all make this method a useful
addition to the present methodologies for the oxidation of
alcohols. In addition, these properties render this method
attractive for use in large-scale operations.
Experimental
Oxidation of Benzyl Alcohol to Benzaldehyde Under Solvent-Free
Conditions: a Typical Procedure
To a mixture of Mg(HSO4)2 (0.654 g, 3 mmol), wet SiO2 (50% w/w, 0.1
g) and (NH4)2Cr2O7 (0.126 g, 0.5 mmol), was added benzyl alcohol
(0.108 g, 1 mmol). The resultant mixture was shaken at room
temperature for 3 min. The progress of the reaction was monitored by
thin-layer chromatography (TLC). The reaction mixture was triturated
with CH2Cl2 (10 mL) and then filtered. Anhydrous MgSO4 was added
to the filtrate and the mixture filtered after 10 min. Evaporation of the
solvent followed by column chromatography on silica gel gave the
benzaldehyde in 92% yield.
Oxidation of 2-Bromobenzyl Alcohol to 2-Bromobenzaldehyde in
n-Hexane: a Typical Procedure
To a suspension of Mg(HSO4)2 (0.654 g, 3 mmol), wet SiO2 (50% w/w,
0.1 g) and (NH4)2Cr2O7 (0.126 g, 0.5 mmol) in n-hexane (5 mL) was
added 2-bromobenzyl alcohol (0.187 g, 1 mmol) and the resultant
Scheme 1. (i) (NH4)2Cr2O7/Mg(HSO4)2/wet SiO2, solvent-free,
room temperature; (ii) (NH4)2Cr2O7/Mg(HSO4)2/wet SiO2, n-hexane,
room temperature.
406 F. Shirini et al.
mixture was stirred at room temperature for 10 min. The progress of the
reaction was monitored by TLC and the mixture filtered upon
completion. The residue was washed with CH2Cl2 (10 mL). Anhydrous
MgSO4 was added to the filtrate and the mixture filtered after 10 min.
Evaporation of the solvent followed by column chromatography on
silica gel gave 2-bromobenzaldehyde in 85% yield.
Acknowledgments
We are thankful to Guilan University Research Council for
the partial support of this work.
References
[1] J. Muzart, Chem. Rev. 1992, 92, 113.
[2] A. Buners, T. G. Halsall, E. R. H. Jones, A. Lemine, J. Chem.
Soc. 1953, 2548.
[3] T. Burnelet, C. Jouitteau, G. Gelbard, J. Org. Chem. 1986, 51,
4016.
[4] I. M. Baltork, M. M. Sadeghi, N. Mahmoodi, B. Kharamesh,
Indian J. Chem. 1997, 368, 438.
[5] E. J. Corey, J. W. Suggs, Tetrahedron Lett. 1975, 31, 2647.
[6] J. S. Cha, M. Kim, J. M. Kim, O. O. Kwon, J. H. Chun, S. D. Cho,
Bull. Korean Chem. Soc. 1998, 19, 724.
[7] B. Tamami, H. Alinezhad, Iran. J. Sci. Technol. 1997, 21, 375.
[8] H. Firouzabadi, B. Tamami, N. Goudarzian, M. M. Lakouraj, H.
Hatam, Synth. Commun. 1991, 21, 2077.
[9] B. Tamami, H. Firouzabadi, M. M. Lakouraj, A. Mahdavian, Iran
J. Polym. Sci. Technol. 1994, 3, 82.
[10] N. Goudarzian, P. Ghahramani, S. Hossini, Polym. Int. 1996, 39,
61.
[11] F. Shirini, H. Tajik, F. Jalili, Synth. Commun. 2001, 31, 2885.
Table 1. Oxidation of alcohols by ammonium dichromate in the presence of Mg(HSO4)2 and wet SiO2 in n-hexane
or under solvent-free conditions at room temperature
Entry Substrate ProductA Solvent-free oxidation Oxidation in solvent
Time
(min)
YieldB
(%)
Time
(min)
YieldB
(%)
1 Benzyl alcohol Benzaldehyde 5 92 5 95
2 4-Bromobenzyl alcohol 4-Bromobenzaldehyde 5 90 5 92
3 2-Bromobenzyl alcohol 2-Bromobenzaldehyde 5 90 10 85
4 2-Chlorobenzyl alcohol 2-Chlorobenzaldehyde 5 89 5 85
5 4-Methoxybenzyl alcohol 4-Methoxybenzaldehyde 5 90 15 85
6 4-Benzyloxybenzyl
alcohol
4-Benzyloxybenzaldehyde 15 90 15 80
7 2-Nitrobenzyl alcohol 2-Nitrobenzaldehyde 5 95 15 92
8 1-Phenyl ethanol Acetophenone 5 90 30 88
9 Diphenylcarbinol Benzophenone 5 92 75 90
10 Cyclohexanol Cyclohexanone 30 85 240 —C
11 1-Phenylpropan-2-ol Phenylpropan-2-one 10 90 300 —C
12 3-Phenylpropan-1-ol 3-Phenylpropanal 10 87 60 —C
13 Cinnamyl alcohol Cinnamaldehyde 5 —C 5 —C
14 Benzyl alcohol Benzaldehyde 100 5D 300 7D
15 Benzyl alcohol Benzaldehyde 100 0E 180 0E
16 Benzyl alcohol Benzaldehyde 110 0F 180 0F
A All products were characterized by infrared (IR) and 1H nuclear magnetic resonance (NMR) spectroscopy and by
comparison with authentic samples. B Isolated yield. C Mixture of products. D In the absence of Mg(HSO4)2. E In the
absence of (NH4)2Cr2O7. F In the absence of wet SiO2.
[Edited on 2-8-2005 by FriendlyFinger]praseodym - 2-8-2005 at 05:13
So what is the purpose of this and what is the question??sparkgap - 2-8-2005 at 05:43
Quote:
Originally posted by praseodym
...what is the question??
Quote:
Originally posted by FriendlyFinger
Has anybody seen this yet?
Quote:
Originally posted by praseodym
...what is the purpose of this...
Many thanks, FF.