Oxidation of organic substrates using two triethylammonium halochromates(VI), (C2H5)3NH[CrO3X], (X=F, Cl) supported on silica gel

Article abstract of DOI:10.1002/jccs.200800034

Triethylammonium fluorochromate(VI), (C₂H₅) ₃NHCrO₃F, TriEAFC and triethylammonium chlorochromate(VI), (C₂H₅)₃NHCrO₃Cl, TriEACC were prepared, supported on silica gel and used to quantitatively oxidize a number of organic substrates. These supported triethylammonium halochromates are versatile reagents ensuring effective oxidation of organic compounds, in particular of alcohols, under mild conditions. The durability, ease of work up and efficiency of TriEAFC and TriEACC are considerably increased upon their absorption on silica gel.

Full text of DOI:10.1002/jccs.200800034

Journal of the Chinese Chemical Society, 2008, 55, 229-232
229
Oxidation of Organic Substrates Using Two Triethylammonium
Halochromates(VI), (C H ) NH[CrO X], (X=F, Cl) Supported on Silica Gel
2
5 3
3
Shahriare Ghammamy* and Samaneh Dastpeyman
Department of Chemistry, Faculty of Science, Imam Khomeini International University, Qazvin, Iran
Triethylammonium fluorochromate(VI), (C
2 5 3 3
H ) NHCrO F, TriEAFC and triethylammonium chloro-
chromate(VI), (C H ) NHCrO Cl, TriEACC were prepared, supported on silica gel and used to quantita-
2
5
3
3
tively oxidize a number of organic substrates. These supported triethylammonium halochromates are ver-
satile reagents ensuring effective oxidation of organic compounds, in particular of alcohols, under mild
conditions. The durability, ease of work up and efficiency of TriEAFC and TriEACC are considerably in-
creased upon their absorption on silica gel.
Keywords: Triethylammonium fluorochromate(VI); Triethylammonium chlorochromate(VI); Het-
erogeneous oxidants; Silica gel; Oxidation; Alcohols.
INTRODUCTION
The development of oxidizing agents based upon
absorbed on silica gel, are very satisfactory and show the
new reagents to be valuable additions to the existing oxi-
dizing agents. Table 1 summarizes the products, yields and
reaction times of TriEAFC and TriEACC absorbed on sil-
ica gel, with various substrates.
higher-valent transition metal oxo derivatives has been a
subject of research in many laboratories, and a host of such
reagents derived from ruthenium, osmium, iron, manga-
nese, molybdenum, vanadium and chromium have all prov-
en to be capable of alcohol oxidation. In particular, there is
continued interest in the development of new chromium(VI)
reagents for the effective and selective oxidation of organic
substrates, in particular alcohols, under mild conditions. In
recent years, significant improvements were achieved by
Scheme I
R1
R1
R2
(
C H ) NH[CrO X] (X=F, Cl)
2 5 3 3
CH
OH
C
O
CH Cl2
2
,
rt, SiO2
R2
1
2
1
-3
the use of new oxidizing agents, such as 3-carboxypyri-
The heterogeneous reaction mixtures are thoroughly
4
dinium chlorochromate, pyridinum fluorochromate, quin-
5
stirred, at room temperature. The corresponding aldehyde
and ketone products can then be easily isolated by simple
filtration and evaporation of the solvent.
6
olinium dichromate, caffeinilium chlorochromate, quino-
7
8
linium chlorochromate, isoquinolinium fluorochromate,
9
1
0
and tetramethylammonium fluorochromate.
Oxidations may also occur using only TriEAFC and
This manuscript introduces triethylammonium fluo-
rochromate(VI) (TriEAFC) and triethylammonium chloro-
chromate(VI) (TriEACC) absorbed on silica gel as new
promising reagents with improved work-up efficiency and
durability, for the oxidation of alcohols to their correspond-
ing aldehydes and ketones, under mild conditions.
TriEACC, in the absence of SiO
provements are observed in the presence of the absorbent.
This implies that the SiO may act as a reaction medium,
2
, but considerable im-
2
providing an effective heterogeneous surface area for the
oxidation and at the same time making the work-up much
11
more convenient.
The nature of the solvent does not appear to be partic-
ularly critical. Hydrocarbons, benzene, ethers and chlori-
nated hydrocarbons are equally effective, the practical
choice being oriented by the solubility of the products and
the desired reaction temperature. The chromium(VI) con-
RESULTS AND DISCUSSION
The results obtained with triethylammonium fluoro-
chromate(VI) and triethylammonium chlorochromate(VI)
*
Corresponding author. Fax: (+98) 281-3780040; E-mail: shghamami@ikiu.ac.ir

2
30 J. Chin. Chem. Soc., Vol. 55, No. 1, 2008
Ghammamy and Dastpeyman
Table 1. Oxidations via TriEACC and TriEAFC on silica gel
TriEAFC TriEACC
470.66 MHz. All the chemical shifts are quoted in ppm us-
1
ing the high-frequency positive convention; H and
13
C
F
1
9
NMR spectra were referenced to external SiMe
NMR spectra to external CFCl
4
and
Substrate
Product
Time Yield Time Yield
min) (%) (min) (%)
3
. Chromium was estimated
(
iodometrically. In the case of the reduced product of the ox-
idant, chromium was determined after oxidizing with acidic
1
2
3
4
5
n-C H -OH
6 13
n-C H -CHO 180 90 345 95
5 11
n-C
8
H17-OH
n-C
7
H15-CHO 218 90 250 69
3-C H -OH
7 15
3-C H O
14
245 90 300 94
240 90 275 95
124 90 240 97
2 2 8
peroxodisulfate (K S O ) solution. The relative concentra-
7
2-C H -OH
8 17
2-C H O
8 16
tions of carbon, hydrogen and nitrogen were obtained from
the Microanalytical Laboratories, Department of Chemis-
try, OIRC, Tehran. Melting points were measured on an
Electrothermal 9100 melting point apparatus.
2-C11
H23-OH
22
2-C11H O
O
6
CH2OH
092 92 095 99
H
Triethylammonium Fluorochromate (TriEAFC),
7
OH
O
12h 90 380 91
2 5 3 3
(C H ) NH[CrO F]
A 1 g (10 mmol) sample of chromium(VI) oxide,
3
, and (0.9 mL, 20 mmol) of 40% hydrofluoric acid was
O
CrO
8
9
06h 65 480 95
added to 20 mL of water in a 100 mL polyethylene beaker
with stirring. After 5 min the homogeneous solution was
cooled to approx. 0-2 °C. To the resultant clear orange so-
lution, triethylamine (1.4 mL, 10 mmol) was added drop-
wise with stirring over a period of 0.5 h and stirring was
continued for 0.5 h at -4 °C. The precipitated yellow-
ish-orange solid was isolated by filtration on a polyethyl-
ene funnel, washed with petroleum ether (3 × 60 mL) and
dried in vacuum for 2 h at room temperature. Yield: (88%);
O
O
O
06h 63 480 96
1
2
tents are easily determined iodometrically.
Triethylammonium halochromates are easily pre-
pared in high yields from chromium(VI) oxide, triethyl-
amine and aqueous related acid (HCl or HF) in a conve-
nient molar ratio. The bright orange crystalline reagents
can be stored in polyethylene containers for long periods
without decomposition. The IR spectra of the reagents are
similar to that of other halochromates. Triethylammonium
mp 132 °C, C
6 3
H16CrFNO : Calc. C, 32.57; H, 7.23; N,
6.33. Found: C, 32.08; H, 7.64; N, 6.44. I.R. (KBr): 904
-1
), 648 cm n
-
cm n
1
1
(A
1
) or n(CrO
3
2 1
(A ) or n(Cr-F), 948
-
cm n
1
-1
), Electronic absorption at 22321 cm ,
4
(E) or n(CrO
3
2
1
corresponded to A
1
-1
-1
-1
® E (e = 270 M cm ); 28735 m to
1
1
-1
-1
-1
1
1
E® E (e = 845 M cm ) and 36231 cm to A
2
® E (e =
CN): d = 1.2 (t,
2
-), d = 7.5 ppm (s, 1H, -NH),
-
1
-1
1
1233 M cm ). H-NMR (500 MHZ, CD
3H, -CH ), d = 2.7 (t, 2H, -CH
C-NMR (124.44 MHZ, CD CN): d 8.10, 53.35. F-NMR
of this oxidant shows signal at 161 ppm. UV/Visible,
3
1
3
halochromates are soluble in water, dimethylformamide
3
1
3
19
acetonitrile and acetone; they are less soluble in dichloro-
methane and only sparingly soluble in benzene, carbon
tetrachloride, chloroform and hexane.
3
1
3
C
1
19
NMR, H NMR and F NMR were all consistent with the
TriEAFC structure. The above procedure can be scaled up
to larger quantities, if desired. The pH of 0.01 M solution of
TriEAFC in water was 3.45.
EXPERIMENTAL
Material and Instruments
CrO
3
(Merck, P. A.) was used without further purifi-
Triethylammonium Chlorochromate (TriEACC),
cation. Solvents were purified by standard methods. Infra-
red spectra were recorded as KBr disks on a Shimadzu
model 420 spectrophotometer. The UV/Visible measure-
ments were made on an Uvicon model 922 spectrometer.
(C
2
H
5
)
3
NH[CrO
3
Cl]
Chromium(VI) oxide (1.0 g, 10 mmol) was dissolved
3
in water in a beaker and 6 M hydrochloric acid (0.25 cm ,
15 mmol) was added under stirring at 0 °C. To the resultant
3
clear orange solution, triethylamine (0.1 cm , 10 mmol)
1
3
19
Proton, C, and F NMR (for TriEAFC) were carried out
on a Bruker AVANCE DRX 500 spectrometer at 500, 125,
was added dropwise under stirring over a period of 0.5 h

Two Silica Gel Supported Triethylammonium Halochromates(VI)
J. Chin. Chem. Soc., Vol. 55, No. 1, 2008 231
and the stirring was continued for 0.5 h at -4 °C. The pre-
cipitated clear orange solid was isolated by filtration,
3
washed with petroleum ether (3 ´ 60 cm ) and dried under
umn of silica gel to give a clear solution. The solution was
evaporated and the residual product purified by distillation,
recrystallization or column chromatography.
vacuum for 2 h at room temperature. Yield: (54%); mp 120
°
C. Calcd. For C
6
H
16ClCrNO
3
: C, 30.315; H, 6.736; N,
HAZARDS
5
.894. Found: C, 30.29; H, 6.81; N, 5.82. IR (KBr): 900
Always work in the fume hood. Organic solvents
such as toluene, acetone, hexanes, acetonitrile, dichloro-
methane, ether and petroleum ether are volatile and flam-
mable. Acidic Chromic Acid is corrosive.
-
1
-1
cm n
1
(A
(E) or n(CrO
all consistent with the TriEACC structure. Electronic ab-
1
) or n(CrO
3 2 1
), 434 cm n (A ) or n(Cr-Cl), 950
-
1
-1
1
cm n
4
3
) cm . UV/Visible and H-NMR were
-
1
1
1
sorption at 22075 cm , corresponding to A
2
® E (e = 316
-1
DANGER! Strong oxidizer contact with other materi-
als may cause a fire.
3
dm mol cm ); 28169 cm to E® E (e = 803 dm mol
-1
-1
-1
1
1
3
-
-1
1
1
® A
1
3
-1
(e = 1065 dm mol cm )
-1
2
1
CORROSIVE, Causes severe burns to every area of
contact.
1
and 41152 cm to A
3
-1
1
H-NMR (500 MHZ, CD
.9 (t, 2H, -CH -), d = 7.6 ppm (s, 1H, -NH), C-NMR
124.44 MHz, CD C
3
CN): d = 1.22 (t, 3H, -CH
3
), d =
Both it and the solid-support contain chromium, a
heavy metal, and are potentially environmentally damag-
ing so handle and dispose of it as instructed. Some of the
compounds are irritants. HARMFUL IF SWALLOWED
OR INHALED, Affects the respiratory system, liver, kid-
neys, eyes, skin and blood. May cause allergic reaction.
CANCER HAZARD. CAN CAUSE CANCER. Risk of
cancer depends on duration and level of exposure.
1
3
2
2
1
3
(
3
CN): d 8.15, 53.45. UV/Visible,
1
NMR and H NMR were all consistent with the TriEACC
structure. The above procedure can be scaled up to larger
quantities, if desired. The pH of 0.01 M solution of TriEAFC
in water was 2.4.
General procedure for supporting on silica gel
For support of the TriEAFC and TriEACC on silica
gel, in the synthesizing process silica gel (mesh 65-250)
was added before the triethylamine addition step. The sup-
ported reagents separated and were washed by cold water
and acetone. The capacity of the supported reagents were
determined by stirring overnight 0.5 g of the supported re-
agents with 10 mL of 2 N aqueous potassium hydroxide,
filtering off and titrating iodometrically the obtained chro-
mate solution. The determined average capacity of the
dried support for the above reagents is 1.6-2 mmol of
TriEAFC per 1 g of silica gel and 1.4-2.1 mmol of TriEACC
per 1 g of silica gel. The supported reagents so obtained did
not noticeably lose their activities either on storing in air at
room temperature for several weeks or on refluxing for 5 h
in benzene or hexane.
CONCLUSIONS
The new reagents triethylammonium chlorochro-
mate(VI), (TriEACC) and triethylammonium fluorochro-
mate(VI), (TriEAFC) absorbed on silica gel, are easily syn-
thesized. They prove to be low cost and readily available
oxidizing reagents for a variety of alcohols. Their advan-
tages include higher yields, shorter reaction times, lower
alcohol/oxidant molar ratios, application to pH sensitive
molecules and ease of separation of products (Table 1).
Moreover, during the reaction, the color of the oxidants
changes from orange to brown, thus providing a visual
means for ascertaining the progress of the oxidation. Many
functional groups are inert towards these oxidizing agents
enhancing the usefulness of the oxidant and the oxidation
conditions for the synthesis of highly functionalized mole-
7
-10
General procedure for oxidation of organic substrates
with TriEAFC and TriEACC
cules.
To a stirred suspension of triethylammonium halo-
chromate absorbed on silica gel in dichloromethane (gener-
ACKNOWLEDGEMENTS
3
ally 5 cm ), a solution of the substrate in the minimum
The author thanks Dr. A. R. Mahjoub and Mr. M. K.
Mohammady for valuable discussions.
amount of dichloromethane was added dropwise (Table 1).
The completion of the reaction was followed by TLC using
ether/petroleum ether (60/40) as eluant. The mixture is di-
luted with ether (1:1 v/v) and filtered through a short col-
Received November 27, 2006.

2
32 J. Chin. Chem. Soc., Vol. 55, No. 1, 2008
Ghammamy and Dastpeyman
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