Oxidation of alcohols with 4-aminopyridinium chlorochromate supported on silica gel

Article abstract of DOI:10.1007/s00706-003-0129-7

4-Aminopyridinium chlorochromate supported on silica gel was found to be an efficient reagent for the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds. Springer-Verlag 2004.

Full text of DOI:10.1007/s00706-003-0129-7

Monatshefte f€ur Chemie 135, 793–797 (2004)
DOI 10.1007/s00706-003-0129-7
Oxidation of Alcohols
with 4-Aminopyridinium
Chlorochromate Supported on Silica Gel
Mohammed M. Hashemi1;ꢀ, Dadkhoda Ghazanfari2,
and Maryam Akhbari1
1
Department of Chemistry, Sharif University of Technology,
P.O. Box 11365-9516 Tehran, Iran
2
Department of Chemistry, Science and Research Branch, Islamic Azad University,
P.O. Box 14515-775 Tehran, Iran
Received August 15, 2003; accepted (revised) October 16, 2003
Published online February 16, 2004 # Springer-Verlag 2004
Summary. 4-Aminopyridinium chlorochromate supported on silica gel was found to be an efficient
reagent for the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds.
Keywords. Alcohol; Oxidation; 4-Aminopyridinium chlorochromate; Silica gel.
Introduction
Solid phase organic synthesis for preparation of various compounds has provided
an attractive option for organic synthesis [1–3]. Micro porous solids such as silica
gel offer a wide range of active sites for the supported reagents. Silica gel as an
inorganic support has been used in various chemical reactions, however, there are
few examples using supported organic molecules on silica gel as oxidizing agents
[4–7].
Oxidation of alcohols to the carbonyl compounds is an important transforma-
tion in organic synthesis and because of its significance and in spite of the avail-
ability of plethora of methods to accomplish this objectives [8–10], a practically
simple procedure is yet to be desired.
Pyridinium chlorochromate is used for oxidation of organic compounds con-
sistently [11–13]. Chromium oxidants, such as pyridinium chlorochromate adsor-
bed on alumina [14, 15], on silica [16], chromic acid on silica [17], ammonium
chlorochromate on silica [18] and chromyl chloride on silica–alumina [19] have
ꢀ
Corresponding author. E-mail: mhashemi@sharif.edu

794
M. M. Hashemi et al.
Scheme 1
been reported to give better yields under milder conditions as compared to the
unsupported oxidants. During the course of our systematic study on the develop-
ment of supported reagents and catalysts for the oxidation of organic compounds
[20–30], we are reporting a novel heterogeneous oxidant based on 4-aminopyri-
dinium chlorochromate supported on silica gel as an efficient and convenient
method for the oxidation of alcohols to the corresponding carbonyl compounds
in high yields (Scheme 1).
Results and Discussions
Silica gel supported 4-aminopyridinium chlorochromate is a stable solid. It can be
prepared from the reaction of 4-aminopyridine with activated silica gel (Si–NH
pyridine bond formation), which is then reacted with a solution of chromium
trioxide in hydrochloric acid.
The oxidation of alcohols is performed in dichloromethane as solvent and work
up is simply by separation of the heterogeneous supported oxidant by filtration and
removal of the solvent followed by purification of the residue using column chro-
matography to give aldehydes and ketones in high yields. The results are summa-
rized in the Table 1.
Table 1. Oxidation of primary and secondary alcohols with silica gel supported 4-aminopyridinium
chlorochromate
Entry Substrate
Time Producta
h
Yieldb Mp or Bp (ꢁC)
Found Reported [31]
%
1c
2c
3c
4c
5
C6H5CH2OH
2
C6H5CHO
97
97
180
202
248
106
202
48
179
204
247
106
200
48
p-CH3C6H4CH2OH
p-CH3OC6H4CH2OH
p-O2NC6H4CH2OH
C6H5CH(OH)CH3
2
p-CH3C6H4CHO
1.5
4
p-CH3OC6H4CHO 100
p-O2NC6H4CHO
C6H5COCH3
93
92
95
97
85
3
6
C6H5CH(OH)C6H5
C6H5CH(OH)COC6H5
3
C6H5COC6H5
C6H5COCOC6H5
7
3
95
95
8
6
135
135
9c
10c
CH3(CH2)4CH2OH
CH3(CH2)6CH2OH
6
6
CH3(CH2)4CHO
CH3(CH2)6CHO
85
83
128
172
128
171
a
All compounds were fully characterized by their mp or bp, IR and 1H NMR spectra; b yields refer to
c
pure isolated products; no indications of over-oxidation could be observed in the case of primary
alcohols

4-Aminopyridinium Chlorochromate Supported on Silica Gel
795
ꢀ
Table 2. Reusability of silica gel supported 4-aminopyridinium chlorochromate
Experimental trail
Substrate
Product
Yield (%)
1st
C6H5CH2OH
C6H5CH2OH
C6H5CH2OH
C6H5CH2OH
C6H5CHO
C6H5CHO
C6H5CHO
C6H5CHO
97
94
92
92
2nd
3rd
4th
ꢀ
All reactions were carried out according to the typical procedure
Reusability of Silica Gel Supported 4-Aminopyridinium Chlorochromate
Silica gel supported 4-aminopyridinium chlorochromate after using as oxidant for
the oxidation of alcohols can be re-converted to corresponding chlorochromate salt
by treatment by a solution of chromium trioxide in hydrochloric acid. In a typical
procedure silica gel supported 4-aminopyridinium chlorochromate after the first
use for the oxidation of benzyl alcohol to benzaldehyde was re-converted to chloro-
chromate salt by a solution of chromium trioxide in hydrochloric acid according
to the mentioned procedure for the preparation of silica gel 4-aminopyridinium
chlorochromate and then it was re-used for the oxidation of benzyl alcohol to benz-
aldehyde. The second, third, and fourth run according to this procedure showed no
considerable decrease in the reaction yields (Table 2).
The silica gel supported 4-aminopyridine was recovered quantitatively after
each experiment. The IR spectrum of the recovered reagent exhibited signals at
ꢀꢀ ¼ 890 and 1563 cmꢂ1 corresponding to Si–N stretching.
In conclusion, silica gel supported 4-aminopyridinium chlorochromate can be
used as an efficient and recoverable oxidant for controlled oxidation of primary
and secondary alcohols to their corresponding carbonyl compounds. Mild reaction
conditions, high yields, easy work up, and reusability of the supported reagent are
the most significant aspects of this method.
Experimental
Chemicals were purchased from Merck, Aldrich, and Riedel-de Haen AG and were used without
further purification. IR spectra were recorded on a FT-IR Unicam Mattson 1000 spectrophotometer. 1H
NMR spectra were recorded on a Bruker AC-80 (80 MHz) spectrometer in CDCl3. All products are
1
known compounds and they were identified by their melting and boiling points, IR and H NMR
spectra [31]. All yields refer to pure isolated products.
Activation of Silica Gel
Chromatography grade silica gel (10 g, 70–230 mesh) was heated in an electric furnace at 700ꢁC for
3 h. Then it was cooled to room temperature to obtain activated silica gel (9.3g).
Supporting of 4-Aminopyridine on Activated Silica Gel
To a stirred solution of 4-aminopyridine (3g, 30mmol) in 70ml of ethyl acetate activated silica gel
(9.3g) was added and the mixture was refluxed for 9 h. Then it was cooled to room temperature and

796
M. M. Hashemi et al.
filtered. The solid matter was continuously extracted with ethyl acetate in a Soxhlet apparatus for 12h
to remove unreacted 4-aminopyridine. It was filtered and dried in an oven at 70ꢁC for 3 h to yield silica
gel supported 4-aminopyridine (11.2 g), which contained 1.9 g (20 mmol) of 4-aminopyridine on
activated silica gel (9.3g).
Preparation of Silica Gel Supported 4-Aminopyridinium Chlorochromate
To a fresh solution of 2 g of chromium trioxide (20 mmol) in 4 cm3 of 6 N hydrochloric acid, silica gel
supported 4-aminopyridine (11.2 g, contain 20mmol of 4-aminopyridine) was added within 3 min at
7–12ꢁC, and the mixture was stirred at ambient temperature for 30min until a lemon solid was formed.
After evaporation of the solvent, the solid was dried at 50ꢁC for 2 h. The supported reagent can be kept
for weeks in the dark without losing its activity.
Typical Procedure for the Oxidation of Benzyl Alcohol to Benzaldehyde
Benzyl alcohol (1.1g, 10 mmol) was dissolved in 50 cm3 of dichloromethane. Then silica gel supported
4-aminopyridinium chlorochromate (8.4g, containing 15mmol of 4-aminopyridinium chlorochromate)
was added and the mixture was stirred at ambient temperature for 2 h, during which the progress of the
reaction was monitored by TLC. The reaction mixture was filtered and the residue was washed with
two 10 ml portions of dichloromethane. The combined filtrates were evaporated, and the residue was
passed through a short silica gel pad (eluent: petroleum ether:ethyl acetate ¼ 4:1) to obtain 1.05g
(97%) of pure benzaldehyde.
References
[1] Laszlo P (1987) Preparative Chemistry Using Supported Reagents. Academic Press,
San Diego
[2] Smith K (1992) In: Harwood E (ed) Solid Supports and Catalyst in Organic Synthesis.
Ellis Harwood, Chichester
[3] Terrett NK (1998) Combinatorial Chemistry. Oxford University Press, New York
[4] a) McKillop A, Young DW (1979) Synthesis 401; b) McKillop A, Young DW (1979) Synthesis
481
[5] Hashmat A, Bohnert M, Gary J (1998) Synthesis 1238
[6] Borkar S, Khadilkab D, Bhushan M (1999) Synth Commun 29: 4295
[7] Bolm C (1999) Chem Commun 18: 1795
[8] Marko IE, Giles PR, Tsukazaki MS, Brown M, Urch CJ (1996) Science 274: 2044
[9] Trost BM (1997) Comprehensive Organic Synthesis, vol 7. Pergamon, New York
[10] Lee RA, Donald DS (1999) Tetrahedron Lett 40: 21
[11] Ramesh V, Sumathy M (2001) Oxid Commun 24: 241
[12] Saras W, Sonu S, Vinita B, Kalyan K (2001) Indian J Chem (A) 40: 583
[13] Vyas S, Sharma PK (2001) Oxid Commun 24: 248
[14] a) Bhar S, Kumar SC (2003) Tetrahedron 59: 3493; b) Sengh RP, Subbaro HN, Dev S (1979)
Tetrahedron Lett 20: 1789
[15] Heravi MM, Kiakoojori R, Aghayan MM, Hydar KT, Bolourtchian M (1999) Monatsh Chem
130: 481
[16] Fitch FA, Moore WJ, Mudd KJ (1999) J Chem Edu 76: 974
[17] Sanfillippo JR (1997) J Org Chem 42: 2182
[18] Heravi MM, Kiakoojori R, Mojtahedi MM (2001) Indian J Chem (B) 40: 329
[19] Sanfillippo J, Chern CI (1977) J Org Chem 42: 2182
[20] Hashemi MM, Ghazanfari D (1995) Iran J Chem & Chem Eng 13: 77

4-Aminopyridinium Chlorochromate Supported on Silica Gel
797
[21] Hashemi MM, Ahmadibeni Y (1998) J Sci I R Iran 9: 237
[22] Hashemi MM, Ahmadibeni Y (1998) J Chem Res(S) 138
[23] Hashemi MM, Ahmadibeni Y (1999) J Chem Res(S) 434
[24] Hashemi MM, Manuchehri N (1997) Iran J Chem & Chem Eng 16: 82
[25] Hashemi MM, Kalantari F (2000) Synth Commun 30: 1857
[26] Hashemi MM, Ahmadibeni Y (2000) J Chem Res(S) 196
[27] Hashemi MM, Ahmadideni Y (2001) Synth Commun 31: 295
[28] Hashemi MM, Ahmadideni Y (1999) J Chem Res(S) 672
[29] Hashemi MM, Ahmadibeni Y, Ghafuri H (2003) Monatsh Chem 134: 107
[30] Hashemi MM, Ahmadibeni Y (2000) J Chem Res(S) 224
[31] a) Rappoport Z (1967) CRC Handbook of Tables for Organic Compound Identification, The
Chemical Rubber Co, Ohio; b) Hashemi MM, Ahmadibeni Y (2002) Iran J Chem & Chem Eng
21: 71