Cetyltrimethylammonium bromochromate: A new and efficient oxidant for organic substrates

Article abstract of DOI:10.1080/00397910601055164

Cetyltrimethylammonium bromochromate is a versatile reagent for the effective and selective oxidation of organic substrates, particularly for alcohols, under mild conditions. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910601055164

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Cetyltrimethylammonium Bromochromate:
A New and Efficient Oxidant for Organic
Substrates
Shahriar Ghammamy a , Hossein Eimanieh a & Mohammad Kazem
Mohammady a
a Department of Chemistry, Faculty of Science, Imam Khomeini International
University, Qazvin, Iran
Version of record first published: 06 Mar 2007
To cite this article: Shahriar Ghammamy, Hossein Eimanieh & Mohammad Kazem Mohammady (2007):
Cetyltrimethylammonium Bromochromate: A New and Efficient Oxidant for Organic Substrates, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 37:4,
599-605
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Synthetic Communicationsw, 37: 599–605, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910601055164
Cetyltrimethylammonium Bromochromate:
A New and Efficient Oxidant
for Organic Substrates
Shahriar Ghammamy, Hossein Eimanieh,
and Mohammad Kazem Mohammady
Department of Chemistry, Faculty of Science, Imam Khomeini
International University, Qazvin, Iran
Abstract: Cetyltrimethylammonium bromochromate is a versatile reagent for the
effective and selective oxidation of organic substrates, particularly for alcohols,
under mild conditions.
Keywords: cetyltrimethylammonium bromochromate, oxidation, organic substrates,
efficient
There is continued interest in the development of new chromium(VI) reagents
for the effective and selective oxidation of organic substrates, particularly
alcohols, under mild conditions. In recent years, significant improvements
were achieved by the use of new oxidizing agents,[1–3] such as pyridinium
chlorochromate,[4] pyridinium fluorochromate,[5] quinolinium dichromate,[6]
2,20-bipyridinium chlorochromate,[7] quinolinium chlorochromate,[8] and iso-
quinolinium fluorochromate.[9] We synthesized tetramethylammonium fluoro-
chromate with the belief that this reagent may have oxidative properties and
could be used for oxidation of organic substrates. This may be because of the
phase-transfer catalytic property of its cation.[10] The cetyltrimethylammonium
ion is better phase-transfer catalyst than the tetramethylammonium ion.[11]
We have now investigated the synthetic potential of cetyltrimethylammo-
nium bromochromate, C19H42NCrO3Br (CTMABC), and we have found that
Received in the U.K. January 17, 2006
Address correspondence to Mohammad Kazem Mohammady, Department of
Chemistry, Faculty of Science, Imam Khomeini International University, Qazvin,
Iran. E-mail: m_k_mohammadi@yahoo.com
599

600
S. Ghammamy, H. Eimanieh, M. K. Mohammady
this reagent has certain advantages over similar oxidizing agents in terms of
amounts of oxidant and solvent required, easier workup, and higher yields.
Further, CTMABC does not react with acetonitrile, which is a suitable
medium for studing kinetics and mechanism. The results obtained with
CTMABC are very satisfactory and show the new reagent as a valuable
addition to the existing oxidizing agents.
CTMABC in dichloromethane oxidizes primary (1a–g) and secondary
alcohols (3a–f) to the corresponding aldehydes (2a–g) or ketones (4a–f) in
high yields.
CTMABC
R1R2 ꢀ CH ꢀ OHꢀꢀꢀꢀ! R1R2 ꢀ C55O
CH2Cl2;r:t
1
2
CTMABC in dichloromethan also oxidizes anthracene (5a) and phenan-
threne (5b) to anthraquinone (6a) and phenanthraquinone (6b) in 98% and
93% yields, respectively. These yields are higher than those obtained by
other oxidizing agents under mild conditions. This reagent works as efficiently
as activated manganese dioxide or Collins reagent.[12]
CTMABC has also been used for oxidation of carbohydrates such as
1,2:5,6-di-O-isopropylidine-a-D-glucofuranose (7a) to its relative ketosugare
(7b), like pyridinium chlorochromate,[13] but in 97% yield (mp 105–1078C;
lit.[14] 1068C) in 105 min.
CTMABC is easily prepared in 98% yield from chromium(VI) oxide and
cetyltrimethylammonium bromide in a molar ratio of 1:1 in acetonitrile. The
bright orange crystalline reagent can be stored in polyethylene containers for
long periods whitout decomposition. The chromium(VI) content may be easily
determined iodometrically. The IR spectrum is similar to that of other bromo-
chromates.[15] CTMABC is soluble in water, dimethylforamide, acetonitrile,
and acetone; it is less soluble in dichloromethane and only sparingly soluble
in benzen, carbontetetrachloride, chloroform, and hexane.

Cetyltrimethylammonium Bromochromate
601
CETYLTRIMETHYLAMMONIUM BROMOCHROMATE (VI),
C19H42NCRO3Br, CTMABC
Inside a glove box purged by argon, a measured amount of chromium(VI)
oxide (CrO3: 1.07 g, 10.7 mmol) is dissolved in dry acetonitrile (25 ml) in
a polyethylene beaker and a stoichiometric amount of cetyltrimethyl-
ammonium bromide, CH3(CH2)15(CH3)3 Nþ Br2 (3.89 g, 10.7 mmol) is
added with stirring at room temperature. Within 5 min, a clear orange
solution results. Upon refrigerating, solid CTMABC forms, which is
isolated by filtration, washed with dry isopropanol and diethyl ether,
pressed between the folds of a filter paper, and dried in vacuum for 1 h.
Slow evaporation of the concentrated solution of CH3(CH2)15 Nþ(CH3)3
[CrO3Br]2 in acetonitrile gives suitable crystals after 1 week. Yield:
4.71 g (95%); mp 1048C. C19H42BrCrNO3: calc. C, 49.14; H, 9.05; N,
1
3.01. Found: C, 48.76; H, 9.19; N, 2.95. H NMR (CDCl3): d 3.31 (s, 9
H), 3.25 (t, 2H), 1.73 (t, 2 H), 1.30 (s, 26 H), 95 (t, 3 H). IR (KBr):
904 cm21 n1(A1) or n(CrO3), 394 cm21 n2(A1) or n(Cr-Br), 940 cm21
n4(E) or n(CrO3) cm21. UV (CH3CN): l ¼ 350, 1 ¼ .85, l ¼ 220,
1 ¼ 1.8.
This procedure can be performed on a 250-g scale without any
difficulty.
GENERAL PROCEDURE FOR OXIDATION OF ORGANIC
SUBSTRATES WITH CETYLTRIMETHYLAMMONIUM
BROMOCHROMATE
To a stirred suspension of CTMABC (2.5 g) in dichloromethane (generally
5 ml), a solution of the substrate in the minimum amount of dichloro-
methane is added dropwise, the molar ratio of substare to the oxidant
being 1:1 in the case of alcohols (1, 3) and 1:2 in the case of arenes (5)
(see Table 1). The mixture is refluxed for the time indicated in the table.
[The completion of the reaction is followed by UV/vis and thin-layer
chromatography (TLC) using ether/petroleum ether (60/40) as eluant.]
The mixture is diluted with ether (1:1 vol/vol) and filtered through a
short column of silica gel to give a clear solution. The solution is evapor-
ated, and the residual product is purified by distillation, recrystallization, or
column chromatography (Table 1). All boiling points of oxidized
compounds are according to the literature.[5,10,16] The progress of the
reactions was monitored by UV/vis spectrophotometry. The amount of
the oxidant during the reaction was measured spectrophotometrically at
350 nm (Fig. 1). Oxidation reaction progress is illustrated in Fig. 1 and
shows that absorption of CTMABC decreases during the reaction
progress. Spectrophotometric measurements were performed on a UV/visi
Uvicon 922 spectrophotometer using thermostated matched 10-mm

Table 1. Oxidation of alcohols and polycyclic arenes with CTMABC
Substrate/
oxidant ratio
Time
(min)
Yield
(%)
Substrate
Product
Mp (8C)
Bp (8C)
1a n-C3H7-OH
1b n-C4H9-OH
1/1
1/1
120
95
2a n-C2H5-CHO
2b n-C3H7-CHO
92
90
—
—
48–50
74–75
1c n-C5H11-OH
1/1
1/1
80
2c n-C4H9-CHO
92
90
—
—
102
1d
135
2d
63–64
1e n-C8H17-OH
1f n-C11H23-OH
1/1
1/1
60
50
2e n-C7H15-CHO
2f n-C10H21-CHO
93
92
—
—
170–172
110–113
1g
1/1
35
2g
90
—
177–179
3a 3-C7H15-OH
3b 2-C11H23-OH
1/1
1/1
75
65
4a n-C6H13-CHO
4b 2-C10H21-CHO
90
90
—
—
146–148
230–234
3c
1/1
30
4c
90
—
179–181

1/1
400
89
47–50
—
3d
3e
4d
4e
1/1
1/1
40
90
95
50–61
—
3f
130
4f
—
162–163
5a
5b
1/2
1/2
60
72
6a
6b
92
98
284–285
208–210
—
—

604
S. Ghammamy, H. Eimanieh, M. K. Mohammady
Figure 1. Monitoring of oxidation reaction progress with UV/vis spectrophotometer
at 350 nm.
quartz cell. A very small magnetic stirrer was designed at the cell
compartment in the bottom of sample cell in the spectrophotometer to
stir up the solution in cell. The reaction mixtures remain homogenous in
the solvent system used.
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