Quinoxalinium fluorochromate (QxFC): A new and efficient reagent for the oxidation of alcohols in solution, under solvent-free conditions and microwave irradiation

Article abstract of DOI:10.1080/00207450500272248

The new mild oxidizing agent, quinoxalinium fluorochromate (QxFC), has been easily prepared by reacting quinoxaline with an aqueous solution of CrO ₃ and HF. This reagent is suitable for oxidizing various primary and secondary alcohols to their corresponding carbonyl compounds and anthracene to antraquinone. The reactions were carried out in solution, under solvent-free conditions and microwave irradiation. The results show that the rates of the reactions and the yields are usually highest under microwave irradiation. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00207450500272248

This article was downloaded by: [University of Stellenbosch]
On: 04 October 2014, At: 12:47
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Phosphorus, Sulfur, and Silicon
and the Related Elements
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gpss20
Quinoxalinium Fluorochromate
(QxFC): A New and Efficient
Reagent for the Oxidation of
Alcohols in Solution, Under
Solvent-Free Conditions and
Microwave Irradiation
a
a
Kıvılcim Şendil & Beytiye Özgün
a
Gazi Üniversitesi, Fen-Edebiyat Fakültesi, Kimya
Bölümü , Teknikokullar, Ankara, Türkiye
Published online: 15 Aug 2006.
To cite this article: Kıvılcim Şendil & Beytiye Özgün (2006) Quinoxalinium
Fluorochromate (QxFC): A New and Efficient Reagent for the Oxidation of
Alcohols in Solution, Under Solvent-Free Conditions and Microwave Irradiation,
Phosphorus, Sulfur, and Silicon and the Related Elements, 181:4, 959-964, DOI:
10.1080/00207450500272248
To link to this article: http://dx.doi.org/10.1080/00207450500272248
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with

primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Phosphorus, Sulfur, and Silicon, 181:959–964, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/00207450500272248
Quinoxalinium Fluorochromate (QxFC): A New
and Efficient Reagent for the Oxidation of Alcohols
in Solution, Under Solvent-Free Conditions
and Microwave Irradiation
Kıvılcım S¸ endil
¨
Beytiye Ozg u¨ n
¨
Gazi Universitesi, Fen-Edebiyat Fak u¨ ltesi, Kimya B o¨ l u¨ m u¨ ,
Teknikokullar, Ankara, T u¨ rkiye
The new mild oxidizing agent, quinoxalinium fluorochromate (QxFC), has been
easily prepared by reacting quinoxaline with an aqueous solution of CrO3 and
HF. This reagent is suitable for oxidizing various primary and secondary alcohols
to their corresponding carbonyl compounds and anthracene to antraquinone. The
reactions were carried out in solution, under solvent-free conditions and microwave
irradiation. The results show that the rates of the reactions and the yields are
usually highest under microwave irradiation.
Keywords Carbonyl compounds; fluorochromate; microwave irradiation; oxidation; sol-
vent free
INTRODUCTION
The oxidation of primary and secondary alcohols to the corresponding
carbonyl compounds is most frequently accomplished by chromium (VI)
reagents. Extensive work has led to the development of a good number
1
of these oxidants such as the Collins reagent, pyridinium chlorochro-
2
3
ꢀ
mate (PCC), pyridinium dichromate (PDC), 2,2 -bipyridinium
4
5,6
chlorochromate (BIPCC), pyridinium fluorochromate (PFC), quino-
linium fluorochromate,7 3,5-dimethylpyrazolium fluorochromate,
8
9,16
2
,6-dicarboxypyridinium chlorochromate,
chlorochromate, and tetramethylammonium fluorochromate.
N-methylpiperidinium
11
12
Received March 23, 2005; accepted April 12, 2005.
The authors are grateful to the Research Foundation of Gazi University, Ankava,
T u¨ rkiye, for supporting this study.
¨
¨
Address correspondence to Beytiye Ozg u¨ n, Gazi Universitesi, Fen-Edebiyat Fak u¨ ltesi,
Kimya B o¨ l u¨ m u¨ , Teknikokullar, Ankara 06500 T u¨ rkiye. E-mail:beytiye@gazi.edu.tr
959

¨
K. S¸ endil and B. Ozg u¨ n
9
60
However, these reagents have some limitations such as long reaction
times, low yields of the products, the use of expensive reagents, and a
tedious work-up procedure.
In continuation of our studies on the development of new halochro-
mates and dichromates1
3,14
with heterocyclic bases, we now report
the preparation, characterization, and synthetic utility of quinox-
alinium fluorochromate with a view to minimizing the existing dif-
ficulties encountered in the oxidation of organic compounds with
Cr(VI) based oxidants. The oxidation reactions of alcohols were car-
ried out in solution, under solvent-free conditions and microwave
irradiation.
RESULTS AND DISCUSSION
Quinoxalinium fluorochromate can be easily prepared in an excel-
lent yield (95%) from quinoxaline, 40% aqueous hydrofluoric acid and
chromium (VI) oxide in a molar ratio of 1:1:1. The orange crystalline
reagent can be stored in a sealed polythene bag for long periods with-
out decomposition. QxFC is also a moisture-insensitive reagent. The
structure of the product was confirmed by elemental analysis and IR
1
13
(
KBr), H NMR, and C NMR spectra. The infrared absorption frequen-
−
1
cies for the fluorochromate group 948, 870, and 470 cm in QxFC are
attributable to υasym (Cr O), υsym(Cr O), and υ(Cr F); these assign-
1
5
ments are in accordance with those found for KCrO3F. The NMR data
are consistent with the OxFC structure. The reagent QxFC is soluble
in dimethylformamide, dimethylsulfoxide, and water; sparingly solu-
ble in dichloromethane, acetone, acetonitrile, ethylacetate, and chlo-
roform; and insoluble in benzene, ether, and toluene. These results
are indicative of the ionic nature of QxFC. The reagent melts sharply
◦
2
−1
at 117–118 C. It is a 1:1 electrolyte (ꢀM = 124 mho·cm mol , ace-
tonitrile) showing a pH of 2.76 in a 0.01 M aqueous solution. The
pH values of 0.01 M aqueous solutions of PCC, PFC, and QFC were
2
,6,16
found to be 1.75, 2.45, and 2.65, respectively.
Thus, the acidity of
QxFC is less pronounced than that of PCC but comparable to PFC and
QFC.
In this work, at first the oxidation of various substrates was per-
formed in a solution. The effect of the solvent in the oxidation rate
was evaluated by carrying out the oxidation reaction of benzyl alco-
hol with QxFC in a 1:1.5 molar ratio in n-hexane, carbon tetrachloride,
dichloromethane, acetone, and dimethylformamide. The results are pre-
sented in Table I. The use of more polar solvents such as DMF and
acetone resulted in moderate yields. The other solvents gave relatively
good yields. Dichloromethane was chosen as a reaction medium.

Quinoxalinium Fluorochromate (QxFC)
961
TABLE I Oxidation of Benzyl Alcohol in Different Solvents
using QxFC
Substrate/oxidation
Solvents
(molar ratio)
Reaction period/h
Yielda
%
n-hexane
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
2
2
2
2
2
89
85
82
74
70
Carbon tetrachloride
Dichloromethane
Acetone
DMF
aYields refer to the isolated 2,4-DNP derivative.
In order to find out how the stability and activity was changed with
time, we have used QxFC for the oxidation of benzyl alcohol, which
was kept for different periods of storage. The percentage yield of ben-
zaldehyde obtained in each case showed that QxFC is a quite stable
compound (Table II).
The oxidation of various alcohols with 1.5 molar equivalents of QxFC
was investigated in CH2Cl2 at r.t. As shown in Table III, substrates re-
acted with the reagent to afford the corresponding carbonyl compounds
in 57–85% yields within 1–4 h.
The oxidation of the alcohols listed in Table III also was performed
under solvent-free conditions with 1.5 molar equivalents of the reagent.
Under these conditions, the carbonyl compounds were obtained in 65–
9
7% yields within 3–120 min. It is important to note that grinding
is necessary for the reactions to proceed smoothly. Finally, the trans-
formation of alcohols to corresponding carbonyl compounds was inves-
tigated under microwave irradiation. The oxidation was carried out
in the presence of 1.5 molar equivalents of QxFC with reaction pe-
riods ranging between 10 sec–3 min, and the corresponding carbonyl
TABLE II Oxidation of Benzyl Alcohol Using QxFC
Stored for Different Periods
Storage
Oxidant period/week
Substrate/oxidation Reaction
(molar ratio)
period/h
Yielda
%
QxFC
1
2
3
4
6
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
2
2
2
2
2
82
81
81
79
78
aYields refer to the isolated 2,4-DNP derivative.

¨
9
62
K. S¸ endil and B. Ozg u¨ n
a
TABLE III Oxidation of Alcohols by QxFC in Solution, Under
Solvent-Free Conditions, and Under Microwave Irridiation
Solvent-free
Solution
condition
MW
Time
Time
Yieldb
(%)
Time
Yieldb
(%)
Yieldb
(%)
Substrate
(h)
(min)
(s)
1
-octanol
2
3
2
2
3
3
1
4
66
82
85
70
70
80
61
60
60
120
45
20
25
10
45
36
60
3
97
95
85
88
78
95
72
72
360
108
180
180
180
240
480
30
300
10
20
15
180
90
98
98
96
93
82
97
87
93
Benzyl alcohol
4
4
4
4
-methoxybenzyl alcohol
-cholorobenzyl alcohol
-nitrobenzyl alcohol
-methylbenzyl alcohol
Cyclohexanol
-tert-butylcyclohexanol
4
c
c
c
Benzoin
2
83
84
93
Cinnamyl alcohol
Citronellol
Menthol
Anthracene
Benzhydrol
3.5
3.5
4
1.5
4
64
57
65
70
73
68
87
82
84
88
94
90
4
30
25
c
c
c
80
84
75
86
aOxidations were carried out in solution and under a solvent-free condition with a
substrate-oxidant ratio. 1:1.5 (1:3 for an thracene) at r.t.
b
Yields refer to the isolation of 2,4-DNP derivatives identified by melting points.
c
Yields refer to isolated benzil and antraquinone whose melting points were taken
directly and confirmed by comparison with authentic samples (IR, TLC).
products were obtained in 82–98% yields. The results show that the rate
of the reactions and the yields are usually higher under microwave ir-
radiation compared to solution and solvent-free reactions (Scheme 1)
(
Table III).
SCHEME 1
No overoxidation to carboxylic acids was observed in the case of
primary alcohols. Cinnamyl alcohol was oxidized to the corresponding
α,β-unsaturated aldehyde in this reaction, no trace benzaldehyde could
be detected. Obviously, the oxidant can not cleave the double bond of

Quinoxalinium Fluorochromate (QxFC)
963
cinnamyl alcohol. That QxFC is a relatively mild oxidation agent was
illustrated by the reaction with benzoin forming benzil with no sign of
bond cleavage products. During the reactions, the colour of the oxidant
changes from orange to brown, providing visiual means for ascertaining
the progress of the oxidation.
In conclusion, the easily preparable new reagent QxFC has a number
of advantages over its companion reagents like PCC, PFC and QFC as
evident from the consideration of the amount of the reagent required for
the oxidation, solubility in different solvents, controlled acidity, shorter
reaction time and high to very high yields of products. The reduced
chromium species can be trapped on a silica gel column for safe disposal.
Moreover, it is stable and can be stored for long periods without much
loss in its activity and hence turns out to be a useful reagent in synthetic
organic chemistry.
EXPERIMENTAL
Preparation of Quinoxalinium Fluorochromate
Chromium (VI) oxide (20 g, 0.2 mol) was dissolved in water (25 mL) in
a polythene beaker, and 40% hydrofluoric acid (11.3 mL, 0.23 mol) was
added to it while stirring at r.t. Within 5 min, a clear solution resulted.
To this solution, quinoxaline (26.4 g, 0.2 mol) was added slowly with
stirring. The mixture was heated on a steam bath for half an hour, then
cooled to r.t. and allowed to stand for 1 h. The bright orange crystalline
quinoxalinium fluorochromate was isolated by filtration and dried in
◦
vacuo for 1 h. M.p.: 117–118 C, yield 95%; calcd. for C8N2H7CrO3F: C,
3
1
8.40; N, 11.20; H, 2.80; Cr 12.02. Found: C, 38.31; N, 11.19; H, 2.91; C,
2.12%.
General Procedure for Oxidation in Solution
The oxidations were all conducted in a dry apparatus and under ef-
ficient stirring. To the thoroughly stirred suspension of QxFC (1.5–3
3
mmol) in 10 cm of CH2Cl2, l mmol of the substrate dissolved in a
small amount of the solvent was added. The mixture was stirred at
r.t. for the period indicated in Table III. The progress of the reaction
was followed by TLC (n-hexane:ethylacetate = 2:1). After the comple-
3
tion of the reaction, the solid was filtered and washed with 10 cm
CH2Cl2, and the filtrate was evaporated on a rotary evaporator to fur-
nish the product, which was isolated as the 2,4-dinitrophenyl hydrazone
(
2,4-DNP).

¨
K. S¸ endil and B. Ozg u¨ n
9
64
General Procedure for Oxidation Under
Solvent-Free Conditions
QxFC (1.5–3 mmol) was added to the substrate (1 mmol) in a mortar.
Starting materials were instantly mixed and then stored for the appro-
priate period at r.t. or in an oven without any further agitation (Table
III). The progress of the reaction was monitored by dissolving a sam-
ple in CH2Cl2 and using TLC on silica gel (n-hexane:ethylacetate =
2
:1). Upon completion of the reaction, HCl (20%, 30 mL) was added and
extracted with Et2O (3 × 25 mL). Evaporation of the solvent gave the
corresponding carbonyl compounds in 67–97% yields.
General Procedure for Oxidation Under Microwave Irradiation
The alcohol compound (1 mmol) and 1.5 mmol of QxFC were mixed,
3
and then 0.5 cm CH2Cl2 was added. The mixture was subjected to
microwave irradiation at 800 W. Upon completion of the reaction, HCl
(
20%, 30 mL) was added, and the mixture was extracted with Et2O
3 × 25 mL). Evaporation of the solvent gave the corresponding carbonyl
(
compounds in 82–98% yields.
REFERENCES
[
[
[
[
[
[
1] J. C. Collins and F. J. Frank, Tetrahedron Lett., 30, 3363 (1968).
2] E. J. Corey and J. W. Suggs, Tetrahedron Lett., 16, 2647 (1975).
3] E. T. Corey and G. Schmidt, Tetrahedron Lett., 20, 399 (1979).
4] F. S. Guziec and F. A. Luzzio, Synthesis, 9, 691 (1980).
5] M. N. Bhattacharjee and M. K. Chaudhuri, Tetrahedron, 43, 5389 (1987).
6] M. N. Bhattacharjee, M. K. Chaudhuri, H. S. Dasgupta, N. Roy, and D. T. Khathing,
Synthesis, 7, 588 (1982).
[7] V. Murugesan and A. Pandurangan, Indian J. Chem. Sec. B, 31B, 377 (1992).
[8] U. Bora and M. K. Chaudhuri, Tetrahedron, 57, 2445 (2001).
[9] M. Tajbakhsh, R Hosseinzadeh, and M. Y. Niaki, J. Chem. Research (S), 10, 508
(
2002).
10] R. Hosseinzadeh, M. Tajbakhsh, and M. Yazdani-Niaki, Tetrahedron Lett., 43, 9416
2002).
[
(
[
[
[
[
[
[
11] M. Tajbakhsh and M. M. Hevari, Monatsh. Chem., 132, 1229 (2001).
12] M. K. Kassaee, A. R. Mahjob, and S. Ghammami, Tetrahedron Lett., 44, 4555 (2003).
¨
¨
13] N. De gˇ irmenba s¸ ı and B. Ozg u¨ n, Monatsh. Chem., 135, 407 (2004).
14] N. De gˇ irmenba s¸ ı and B. Ozg u¨ n, Monatsh. Chem., 133, 1417 (2002).
15] M. K. Chaudhuri, H. S. Dasgupta, and N. Roy, Indian J. Chem., 20A, 1227 (1981).
16] V. Murugesan and A. Pandurangan, Indian J. Chem., 31B, 377 (1992).