Tetramethylammonium fluorochromate(VI): A new and efficient oxidant for organic substrates

Article abstract of DOI:10.1016/S0040-4039(03)00989-4

Tetramethylammonium fluorochromate(VI), (CH₃)₄N⁺CrO₃F^- (TMAFC), was prepared and used for quantitative oxidation of several organic substrates. This new compound is more efficient and has certain advantages over similar oxidizing agents in terms of the amount of oxidant and solvent required, short reaction times and high yields.

Full text of DOI:10.1016/S0040-4039(03)00989-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 4555–4557
Tetramethylammonium fluorochromate(VI): a new and efficient
oxidant for organic substrates
Ali Reza Mahjoub,a,* Shahriare Ghammamib and Mohammad Zaman Kassaeea
aDepartment of Chemistry, Tarbiat Modarres University, PO Box 14155-111, Tehran, Iran
bDepartment of Chemistry, Faculty of Science, Imam Khomeini International University, Ghazvin, Iran
Received 17 July 2002; revised 10 April 2003; accepted 17 April 2003
Abstract—Tetramethylammonium fluorochromate(VI), (CH3)4N+CrO3F− (TMAFC), was prepared and used for quantitative
oxidation of several organic substrates. This new compound is more efficient and has certain advantages over similar oxidizing
agents in terms of the amount of oxidant and solvent required, short reaction times and high yields. © 2003 Published by Elsevier
Science Ltd.
The oxidation of organic substrates in aprotic sol-
vents, under mild and neutral conditions, is important
in modern organic synthesis. Therefore, the search for
new oxidizing agents is of interest to synthetic organic
chemists. Many such reagents have been developed in
recent years with some success.1 In particular, there is
continued interest in the development of new chromi-
um(VI) reagents for the effective and selective oxida-
tion of alcohols, under mild conditions. Of the large
number of ‘mild’ chromium based oxidizing agents
available, many prove impractical when the reactions
are performed on a large (mol) scale,2 although in
recent years, significant improvements have been
achieved by the use of new oxidizing agents.3–5 Exam-
ples are: pyridinium chlorochromate,6 pyridinium
dichromate,7 pyridinium fluorochromate8 and 2,2%-
bipyridinium chlorochromate.9 Tetramethylammonium
fluorochromate (TMAFC) was synthesized with the
belief that this reagent could be used for the oxidation
of organic substrates. There were two primary incen-
tives for selection of (CH3)4N+ as the counter ion.
Firstly, quaternary ammonium ions are often used as
phase transfer catalysts. This could make TMAFC a
more efficient and stronger oxidizing agent.10,11 Sec-
ondly, quaternary ammonium ions are used as crystal
growing agents. Hence using this counter ion,
improves the quality of the TMAFC crystals.12
TMAFC was prepared inside a glove box purged with
argon. Chromium(VI) oxide (1.07 g, 10.7 mmol) was
dissolved in dry acetonitrile (25 ml) in a polyethylene
beaker and
a
stoichiometric amount of tetra-
methylammonium fluoride (1 g, 10.7 mmol) was added
with stirring at room temperature. Within 5 min a
clear orange solution formed which upon refrigerating,
gave solid TMAFC, which was isolated by filtration.
The solid was washed with dry isopropanol and
diethyl ether, and dried under vacuum for 1 h. The
solid was dissolved in acetonitrile and slow evapora-
tion (1 week) furnished suitable crystals of TMAFC.
Satisfactory elemental analysis was obtained.
(CH3)4N[CrO3F]: calcd: C, 24.85; H, 6.21; N, 7.25,
found: C, 24.52; H, 6.42; N, 7.21%. IR, UV/vis, 19F,
1H NMR and single-crystal X-ray diffraction analysis
were all consistent with the TMAFC structure.
This work provides a useful new oxidant for organic
chemists. The chemistry of TMAFC has been explored
as an oxidizing agent (Scheme 1, Table 1).
Scheme 1.
Keywords: oxidant; chromium(VI) reagents; tetramethylammonium fluorochromate(VI); oxidation; organic substrates.
* Corresponding author.
0040-4039/03/$ - see front matter © 2003 Published by Elsevier Science Ltd.
doi:10.1016/S0040-4039(03)00989-4

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A. R. Mahjoub et al. / Tetrahedron Letters 44 (2003) 4555–4557
Table 1. Oxidation of alcohols 1 and polycyclic arenes 3 with TMAFC, PFCa and IQFCb
the basis of a number of observations made by their group
In order to examine the oxidative ability of TMAFC the
following typical procedure was employed: In a small-
scale experiment TMAFC (1 mmol) was suspended in
methylene chloride (ca. 2 ml) and an alcohol (l mmol in
0.5–1.5 ml of CH2Cl2) was rapidly added at room
temperature and the resulting mixture stirred vigorously
for the appropriate time (Table 1). The molar ratio of
substrate to oxidant was 1:1 in the case of alcohols 1 and
1:2 in the case of polycyclic arenes 3. The solution became
homogeneous briefly before the black-brown reduced
reagent precipitated. The progress of the reactions were
monitored by TLC and UV/visible spectrophotometry (at
352 nm). Analysis of the reaction mixture for the
corresponding carbonyl compound was accomplished by
the procedure reported in earlier papers.8,13 The above
procedure may be carried out on 1–100 g scales without
any problem.
and others.16 Granier et al.17 have already linked higher
reactivities with the extent of symmetry about the Cr for
MCrO3F (M=K, Rb, Cs and NH4). They concluded that
the inequality between the CrꢀO and the CrꢀF bonds is
responsible for higher reactivity. Interestingly, our X-ray
data clearly demonstrate such inequality. The reason for
this inequality is due to the CH···F hydrogen bond that
forms between the methyl hydrogen of the cation and the
fluoride atom of the anion. This type of hydrogen bonding
in tetramethylammonium salts has been studied by
Harmon.18 TMAFC appears to be a stronger reagent than
others; this could be due to its lower symmetry.
It has also been found that this reagent has certain
advantages over similar oxidizing agents in terms of
amounts of oxidant and solvent required, and especially
in the short reaction times required and in higher product
yields8,19,20 (Table 1).
Table 1 summarizes the products, yields and times of
reaction of TMAFC with various substrates. The oxida-
tive effect of TMAFC on polycyclic rings appears more
pronounced than alcohols (Table 1). This could be due
to the interaction of p electrons with the quaternary cation
as suggested by Felder14 and Pullman.15 The probable
importance of such interactions in biological recognition
processes has been emphasized by Dougherty et al. on
Because of the stability and solubility of TMAFC,
reactions can be performed at room temperature and the
separation of the products is facile. During the reactions,
the color of the oxidant changes from orange to brown,
providing visual means for ascertaining the progress of
the oxidation. The mechanism for the present oxidation
is still unclear. However, we assume that the mechanism

A. R. Mahjoub et al. / Tetrahedron Letters 44 (2003) 4555–4557
4557
of oxidation is similar to that of other fluorochromates.
In addition this oxidant and the oxidation conditions
can be used for the synthesis of highly functionalized
molecules.
9. Guziec, F. S.; Luzzio, F. A. Synthesis 1980, 691.
10. Deakyne, C. A.; Meot-Ner, M. J. Am. Chem. Soc. 1985,
107, 474.
11. March, J. Advanced Organic Chemistry; John Wiley &
Sons: New York, 1985; p. 320.
12. Hurle, D. T. J. Handbook of Crystal Growth Part B. 1994,
Elsevier Science, pp. 1219–1269.
13. Piancatelli, G.; Scettri, A.; D’Auria, M. Synthesis 1982,
245.
14. Felder, C.; Jiang, H.-L.; Zhu, W.-L.; Chen, K.-X.; Sil-
man, I.; Botti, S. A.; Sussman, J. L. J. Chem. Phys. 2001,
A105, 1326.
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