N-methyl piperidinium chlorochromate adsorbed on alumina: A new and selective reagent for the oxidation of benzylic alcohols to their corresponding carbonyl compounds

Article abstract of DOI:10.1007/s007060070030

N-Methyl piperidinium chlorochromate adsorbed on alumina oxidizes a wide variety of benzylic alcohols to the corresponding carbonyl compounds.

Full text of DOI:10.1007/s007060070030

Monatshefte fuÈr Chemie 131, 1213±1216 (2000)
N-Methyl Piperidinium Chlorochromate
Adsorbed on Alumina: A New and Selective
Reagent for the Oxidation of Benzylic Alcohols
to their Corresponding Carbonyl Compounds
Mahmood Tajbakhsh1, Moosa Ghaemi1, Siavosh Sarabi1,
Mitra Ghassemzadeh2, and Majid M. Heravi3;Ã
1
Department of Chemistry, School of Sciences, University of Mazandaran, Babolsar, Iran
2
Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran
3
Department of Chemistry, School of Sciences, Azzahra University, Tehran, Iran
Summary. N-Methyl piperidinium chlorochromate adsorbed on alumina oxidizes a wide variety of
benzylic alcohols to the corresponding carbonyl compounds.
Keywords. N-Methyl piperidinium chlorochromate; Oxidation; Benzylic alcohols; Carbonyl com-
pounds.
Introduction
Oxidation of alcohols to carbonyl compounds is a fundamental type of reaction
encountered at all levels and aspects of organic synthesis [1]. Oxidation of primary
and secondary alcohols to the corresponding aldehydes and ketones is often
accomplished using chromium IV [2], manganese reagents [3], and oxalyl chloride/
DMSO [4]. A large number of catalytic systems employing a metal complex
associated with a peroxo species have been described for the oxidation of alcohols
to the corresponding carbonyl compounds [5], but these reagents are in most cases
relatively expensive and rarely commercially available. Pyridinium chlorochromate
in dichloromethane [6] is a versatile oxidizing reagent of high ef®ciency; however,
work-up of the reaction mixture is tedious [7].
Results and Discussion
Reagents adsorbed on mineral supports have gained popularity in organic synthesis
due to their selectivity and their simplicity of manipulation [8]. In continuation of
our program to develop methodologies using solid supports [9], we report a very
facile method of oxidation of primary and secondary alcohols employing N-methyl
piperidinium chlorochromate (MPPC) adsorbed on alumina.
Ã
Corresponding author

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M. Tajbakhsh et al.
The reagent is inexpensive and easily prepared by reaction of N-methyl
piperidine with chromium trioxide in 6 N hydrochloric acid. MPPC is soluble in
polar solvents such as dioxane and acetone and insoluble in chloroform, benzene,
and hexane. The oxidizing reagent can be stored at least for three months without
losing its activity. The pH of a 0:01 M aqueous solution of MPPC is 1.85, which is
quite close to that reported for pyridinium chlorochromate (1.75) and lower than
that reported for zinc chlorochromate monohydrate (2.3) [10].
Preliminary investigations demonstrated that some primary and secondary
alcohols (1 mmol) can be oxidized to the corresponding aldehydes and ketones with
MPPC (2 mmol). We observed conversions up to 65%; additional oxidizing agent
failed to drive the reaction to completion. The moderate yields can presumably be
attributed to dif®culties with the work-up of the reaction mixture.
The concept of utilizing reagents adsorbed on inert inorganic supports for
oragnic reactions is well documented [8, 9] and has been applied especially to
chromium compounds [9a±f,j]. MPPC adsorbed on alumina provides a particular
environment capable of enhancing the reactivity of several reactants. Perhaps
neutral alumina modi®es the mildly acidic character of piperidinium chlorochro-
mate. Among the various solid supports examined such as montmorillonite K-10,
silica gel, and alumina, the latter turned out to work best. This reagent is easily
prepared by adding alumina to an aqueous solution of MPPC followed by
evaporation to dryness on a rotary evaporator. The yellow-orange solid can be stored
at room temperature and in light at least for three months. The reaction is simply
performed by stirring an excess of oxidant with alcohol in e.g. dioxane. The
products can be easily isolated by mere ®ltration and removal of solvent.
Interestingly, this adsorbed reagent is effective only in oxidizing primary and
Scheme
Table 1. Oxidation of benzylic alcohols with MPPC adsorbed on alumina in dioxane
Reaction time
min
Yielda
R1
R2
%
C6H5
H
100
119
75
93
89
91
92
96
80
81
89
75
83
m-ClC6H4
p-ClC6H4
p-BrC6H4
m-OHC6H4
C6H5
H
H
H
60
H
80
C6H5
C6H5
CH3
C6H5
H
120
125
110
720
240
p-ClC6H4
C6H5
C6H5CO
o-NH2C6H4
a
Yields refer to isolation of DNPH derivatives

Oxidation of Benzyl Alcohol with MPPC/Alumina
1215
secondary benzylic alcohols (Table 1) and therefore can be used for selective
oxidation of benzylic alcohols in the presence of other hydroxy groups.
In conclusion, a safe oxidant on solid support, which is a good alternative to
pyridinium chlorochromate, is introduced. It has all the advantages of pyridinium
chlorochromate and, in addition, is selective for benzylic alcohols.
Experimental
All oxidation products are known and were identi®ed by comparison of their physical and
spectroscopic data with those of authentic samples. Yields refer to isolation of the 2,4-
dinitrophenylhydrazones (DNPH). N-Methylpiperidine was obtained commercially.
Preparation of N-methyl piperidinium chlorochromate adsorbed on alumina
To a solution of 20 g CrO3 (0.2 mol) in 36.8 g 6 N HCl (0.22 mol) at 40ꢀC, 19.8 g N-methylpiperidine
(0.2 mol) were added within 15 min. The resulting solution was stirred at 10ꢀC for 2 h by which time
a yellow-orange solid precipitated. Isolation of this material and drying gave MPPC (found: C 30.10,
H 5.70, N 5.60, Cr 21.9%; calcd.: C 30.64, H 6.00, N 5.96, Cr 22.1%). Reheating of the above MPPC
suspension to 40ꢀC resulted in a homogeneous solution. Alumina (100 g, 100±200 mesh) was then
added while stirring at 40ꢀC. After evaporation under reduced pressure, the yellow-orange solid was
dried in vacuum. MPPC and MPPC/alumina can be preserved for at least 3 months in air and at room
temperature without losing their activity.
Oxidation of benzylalcohol
1 g of the above reagent (2 mmol) was added to a solution of 0.147 g benzylalcohol (1 mmol) in
10 cm3 of dioxane. The reaction mixture was stirred for 100 min at room temperature. After
completion of the reaction (monitored by TLC) the solid was ®ltered off and washed with 10 cm3 of
CHCl3. The combined ®ltrates were evaporated and vacuum distilled to give 83% benzaldehyde.
References
[1] a) Hudlicky M (1990) Oxidation in Organic Chemistry. ACS Monograph, p 186; b) Procter G
(1991) In: Ley SV (ed) Comprehensive Organic Synthesis. vol. 7. Oxford, Pergamon, p 305; c)
Heravi MM, Ajami D, Aghapoor K, Ghassemzadeh M (1999) J Chem Soc Chem Commun 833
[2] Cainelli G, Cardolli G (1984) Chromium Oxidations in Organic Chemistry. Springer, New York
[3] a) Fatiadi AJ (1976) Synthesis 56: 133; b) Pizey JS (1974) Synthetic Reagents, vol 2. Halstead
Press, New York, p 143
[4] a) Fatiadi AJ (1987) Synthesis 85; b) Epstein WW, Swead FW (1967) Chem Rev 67: 247; c)
Mancuso AJ, Swern D (1981) Synthesis 165
[5] Grif®th WP, Ley SV (1990) Aldrichimica Acta 23: 13
[6] Corey EJ, Suggs W (1975) Tetrahedron Lett 2647
[7] Shia Cheng Y, Liang Liu W, Chen S (1980) Synthesis 223
[8] a) McKillop A, Young DW (1979) Synthesis 401 and 481; b) Balogh M, Laszlo P (1993) Organic
Chemistry Using Clays. Springer, Berlin
[9] a) Heravi MM, Ajami D, Tabar-Hydar K (1998) Monatsh Chem 130: 1305; b) Heravi MM, Ajami
D, Tabar-Hydar K (1999) Synth Commun 29: 163; c) Heravi MM, Ajami D, Tabar-Hydar K
(1999) Synth Commun 29: 1009; d) Heravi MM, Ajami D, Tabar-Hydar K, Mojtahedi MM
(1999) J Chem Res 620; e) Heravi MM, Kiakoojori R, Tabar-Hydar K (1999) Monatsh Chem

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M. Tajbakhsh et al.: Oxidation of Benzyl Alcohol with MPPC/Alumina
130: 581; f) Heravi MM, Kiakoojori R, Tabar-Hydar K (1998) J Chem Res 656; g) Heravi MM,
Ajami D, Ghassemzadeh M (1999) Synth Commun 29: 1013; h) Heravi MM, Ajami D, Tabar-
Hydar K, Ghassemzadeh M (1999) J Chem Res 334; i) Heravi MM, Ajami D, Mojtahedi MM;
Ghassemzadeh M (1999) Tetrahedron Lett 40: 561; j) Heravi MM, Ajami D, Ghassemzadeh M
(1999) Synthesis 393
[10] Firouzabadi H, Shari® A (1999) Synthesis 999
Received May 18, 2000. Accepted (revised) July 4, 2000