Patel and Mishra
ammonium ions as the counterions which carry Mn(VII) oxidant
to the lipid system (organic solvents). Dash and Mishra have
substrates. This reagent is water insoluble and stable at room
temperature for more than a year when kept in sealed bottle.
Although Cr(VI) is undisputedly carcinogenic, the insolubility
in water reduces contamination of Cr(VI) in aqueous medium,
and the compound thus can be used as a green reagent. Further,
CTADC is devoid of an acidic proton and thus is relatively
milder than other Cr(VI) oxidants. In the absence of acid,
CTADC exhibits some bizarre reactions with nonconventional
products. Aromatic amines are found to yield corresponding
9
reported the product specificity of cetyltrimethylammonium
permanganate (CTAP) in a chloroform medium for olefinic
double bonds. The cis compounds are converted to the corre-
sponding diols, whereas the trans compounds lead to cleavage
of the double bond. However, CTAP is found to be a
self-destructor and may be considered as a suicidal oxidant. The
Mn(VII) oxidizes the carrier cetyltrimethylammonium ion to
hexadecanal in a mechanism akin to â-oxidation of fatty acids
by corresponding dehydrogenase.10
1
2a
diazo compounds, and arylaldoximes yielded corresponding
12c
nitriles. Recently, in an oxidation reaction of cholesterol with
A large number of Cr(VI) oxidants with onium ions have
been reported, among which pyridinium chlorochromate(PCC)
CTADC, we have observed that 7-dehydrocholesterol is ob-
tained instead of cholestenone. This dehydrogenation is a rare
1a
6
has attracted the attention of synthetic chemists the most. Its
versatility is due to the commercial availability, efficiency, and
self-stability. The anionic species in these types of oxidants is
either a chromate or a dichromate. Most of these reagents con-
tain one acidic hydrogen (pH of a 0.01 M solution of PCC,
PFC (pyridinium fluorochromate), QFC (quinolinium fluoro-
chromate), and DmpzHFC (3,5-dimethylpyrazolium fluoro-
chromate) were found to be 1.75, 2.45, 3.35, and 4.9, respec-
tively, and the corresponding pKa values were 1.4, 2.7, 4.7, and
event in Cr(VI) oxidation studies, and it is explained through a
remote functionalization mechanism. In this mechanism, the
cetyltrimethylammonium ion provides a conducive environment
for proper orientation of the oxochromium group so that the
removal of hydrogen becomes easier.
However, in the presence of acid, arylaldoximes and ketox-
imes produced carbonyl compounds with CTADC. Further, the
protonated dichromate oxidizes the secondary hydroxy group
of cholesterol to the corresponding ketone on the addition of
acid. The reaction kinetics reveals that the reaction system re-
sembles that of cholesterol oxidase, which carries FAD as the
dehydrogenating agent in the enzyme and oxidizes cholesterol
to corresponding cholestenone. In an analogy to this system,
CTADC in an organic solvent like DCM forms a reversed
micelle where the dichromate is encapsulated by the cationic
oniums and cholesterol is partitioned into the mesophase.6
To have more insight into the contribution of the hydro-
phobicity of the substrate to the reaction mechanism, a series
of alcohols have been used, in the present study, for the
oxidation reaction by CTADC. Further, attempts have been
made to provide more evidence for the hydrophobic contribution
of the solvents toward the reactivity of CTADC through reverse
micellization.
1
1
7
.8, respectively), which in some cases is sufficient to enable
the oxidative transformation and which in some cases creates a
problem for the oxidation of compounds containing acid
sensitive functionality.
In a continuation of our efforts to explore some biomimetic
oxidants to oxidize organic substrates in organic solvents,
we have reported the oxidation behavior of cetyltrimethyl-
6
,12
ammonium dichromate (CTADC)
toward various organic
(1) (a) Corey, E. J.; Suggs, W. J. Tetrahedron Lett. 1975, 2647. (b) Corey,
E. J.; Schmidt, G. Tetrahedron Lett. 1979, 399. (c) Antonioletti, R.; D’Auria,
M.; Piancatelle, G.; Scettri, A. Tetrahedron Lett. 1981, 1041. (d) Still, W.
C.; Galynker, I. J. Am. Chem. Soc. 1982, 104, 1774. (e) D’Auria, M.; Mico,
A. D.; D’Onofrio, F.; Scettri, A. Synthesis 1985, 988. (f) Cossio, F. P.;
Aizpurua, J. M.; Palomo, C. Can. J. Chem. 1986, 64, 225. (g) Banerji, K.
K. J. Chem. Soc., Perkin Trans. 2 1988, 2065. (h) Corey, E. J.; Boger, D.
L. Tetrahedron Lett. 1978, 2461. (i) Yli-Kauhaluoma, J. T.; Harwig, C.
W.; Wentworth, P., Jr.; Janda, K D. Tetrahedron Lett. 1998, 39, 2269. (j)
Maki, S.; Ishihara, J.; Nakanishi, K. J. Indian Chem. Soc. 2000, 77, 651.
Results and Discussion
(
5
2
k) Alcudia, A.; Arrayas, R. G.; Liebeskind, L. S. J. Org. Chem. 2002, 67,
773. (l) Tajbakhsh, M.; Hosseinzadeh, R.; Shakoori, A. Tetrahedron Lett.
004, 45, 2647.
The reaction mixture consisting of CTADC, alcohol, and
acetic acid in dichloromethane in almost all cases turns to green
after completion of the reaction. On chromatographic separation
of the reaction mixture, aldehydes and ketones are eluted from
the oxidation product. Further the IR characteristic bands of
(2) (a) Dey, D.; Mahanti, M. K. J. Org. Chem. 1990, 55, 5848. (b)
Chaubey, G. S.; Das, S.; Mahanti, M. K. Can. J. Chem. 2003, 81, 204. (c)
Kuotsu, B.; Tiewsoh, E.; Debroy, A.; Mahanti, M. K. J. Org. Chem. 1996,
6
1, 8875.
3) Shirini, F.; Mohammadpoor-Baltrok, I.; Hejazi, Z.; Heravi, P. Bull.
Korean Chem. Soc. 2003, 24, 517.
-1
the eluted product at 1710-1725 cm support the formation
(
of corresponding aldehydes and ketones. It is found that under
the experimental condition the carbonyl compounds are not
oxidized further.
(
(
4) Agarwal, S.; Tiwari, H. P.; Sharma, J. P. Tetrahedron 1990, 46, 1963.
5) (a) Cossio, F. P.; Lopez, M. C.; Palomo, C. Tetrahedron 1987, 43,
3
963. (b) Sekar, K. G. J. Chem. Res. 2002, 2002, 626.
From the stoichiometric analysis, it is found that 1 mole equiv
of CTADC reacts with 3 mole equiv of alcohols, and thus the
ratio of Cr(VI)/ alcohol is found to be 2:3. During the oxidation
process, Cr(VI) is reduced to Cr(IV), which disproportionates
with another Cr(VI) to Cr(V). The existence of Cr(IV) as the
reduced state in oxidation of alcohol by quinolinium bromo-
(
6) Patel, S.; Mishra, B. K. J. Org. Chem. 2006, 71, 3522.
(7) (a) Okimoto, T.; Swern, D. J. Am. Oil Chem. Soc. 1977, 54, 862A.
(
b) Sala, T.; Sargent, M. V. J. Chem. Soc., Chem. Commun. 1978, 253. (c)
Schmidt, H. J.; Schafer, H. J. Angew. Chem., Int. Ed. Engl. 1979, 18, 78.
d) Lee, D. G.; Brown, K. C. J. Am. Chem. Soc. 1982, 104, 5076. (e)
Karaman, H.; Barton, R. J.; Robertson, B. E.; Lee, D. G. J. Org. Chem.
(
1
984, 49, 4509.
8) (a) Lee, D. G.; Brown, K. C.; Karaman, H. Can. J. Chem. 1986, 64,
(
13
chromate has also been reported by Saraswat et al. Cr(V) is
1
2
054. (b) Shukla, R.; Kothari, S.; Kotai, L.; Banerji, K. K. J. Chem. Res.
001, 127. (c) Shukla, R.; Sharma, P. K.; Kotai, L.; Banerji, K. K. Proc.
Indian Acad. Sci. (Chem. Sci.) 2003, 115, 129.
further reduced to Cr(III) by the addition of two electrons. The
existence of Cr(III) in the product mixture was established from
(9) Dash, S.; Mishra, B. K. Indian J. Chem. 1997, 36A, 662.
(10) Dash, S.; Mishra, B. K. Int. J. Chem. Kinet. 1995, 27, 627.
(11) Bora, U.; Chaudhuri, M. K.; Day, D.; Kalita, D.; Kharmawphlang,
14
the absorption maximum at 580 nm.
W.; Mandal, G. C. Tetrahedron 2001, 57, 2445.
12) (a) Patel, S.; Mishra, B. K. Tetrahedron Lett. 2004, 45, 1371. (b)
Patel, S.; Kuanar, M.; Nayak, B. B.; Banichul, H.; Mishra, B. K. Synth.
Commun. 2005, 35, 1033. (c) Sahu, S.; Patel, S.; Mishra, B. K. Synth.
Commun. 2005, 35, 3123.
(13) Saraswat, S.; Sharma, V.; Banerji, K. K. Proc. Indian. Acad. Sci.-
(Chem. Sci.) 2003, 115, 75.
(14) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. In
AdVanced Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York,
1999; p 747.
(
6760 J. Org. Chem., Vol. 71, No. 18, 2006