Oxidation of cholesterol by a biomimetic oxidant, cetyltrimethylammonium dichromate

Article abstract of DOI:10.1021/jo060128k

The oxidation of cholesterol by cetyltrimethylammonium dichromate (CTADC) in dichloromethane (DCM) yielded 7-dehydrocholesterol, while with addition of acetic acid in DCM the product was found to be 5-cholesten-3-one. The kinetics of oxidation of cholesterol by CTADC in DCM, in the presence of acid, was investigated with change in [acid], [cholesterol], [CTADC], [surfactant], temperature, and solvents. The reaction was found to be first order with acetic acid and fractional order with CTADC and cholesterol. Michaelis-Menten-type kinetics was observed with respect to cholesterol. The solvent isotope effect was found to be k(D₂O)/k(H₂O) = 0.72. The observed experimental data suggest that the reaction occurs in reversed micellar system, akin to an enzymatic environment, and the reaction path involves the intermediate formation of an ester complex, which undergoes decomposition to give the product.

Full text of DOI:10.1021/jo060128k

Oxidation of Cholesterol by a Biomimetic Oxidant,
Cetyltrimethylammonium Dichromate
Sabita Patel and B. K. Mishra*
Center of Studies in Surface Science and Technology, Department of Chemistry,
Sambalpur UniVersity, Jyoti Vihar-768019, Orissa, India
bijaym@hotmail.com
ReceiVed January 20, 2006
The oxidation of cholesterol by cetyltrimethylammonium dichromate (CTADC) in dichloromethane (DCM)
yielded 7-dehydrocholesterol, while with addition of acetic acid in DCM the product was found to be
5-cholesten-3-one. The kinetics of oxidation of cholesterol by CTADC in DCM, in the presence of acid,
was investigated with change in [acid], [cholesterol], [CTADC], [surfactant], temperature, and solvents.
The reaction was found to be first order with acetic acid and fractional order with CTADC and cholesterol.
Michaelis-Menten-type kinetics was observed with respect to cholesterol. The solvent isotope effect
was found to be k(D O)/k(H O) ) 0.72. The observed experimental data suggest that the reaction occurs
2 2
in reversed micellar system, akin to an enzymatic environment, and the reaction path involves the
intermediate formation of an ester complex, which undergoes decomposition to give the product.
Introduction
to affect the molecular order of cell membrane changing its
permeability leading to vascular cell injury. The products also
4
As an unsaturated lipid, cholesterol is found in all mammalian
cells and lipoproteins. Most of the cellular cholesterols located
in the plasma membrane are found to be oriented in such a way
that the alcoholic -OH of a cholesterol is exposed to the
aqueous system and the other hydrophobic unit is buried in the
membrane. Cholesterol is susceptible to spontaneous oxidation,
and it yields a variety of oxidized products such as epoxide,
lead to change in internalization pathway of endothelin receptor
type A. However, most of the oxidized products are due to
increase in oxygen functionality. Further, the oxidized products
of cholesterol are precursors of many pharmaceutically important
products. Cholestenone is a precursor of androsta-1,4-diene-
5
3
,17-dione, which can be chemically modified to manufacture
oral contraceptives.
1
peroxides, diols, ketones, dienes, acids, etc. The preponderance
In our efforts in exploring some biomimetic oxidants to
oxidize organic substrates in organic solvents, we have reported
the oxidation behavior of cetyltrimethylammonium permanga-
of cholesterol in the plasma membrane allows preferential
2
probing of oxidative damage in the concerned compartment.
Most of the products are found to be cytotoxic and mutagenic.
One of the products, 7-dehydrocholesterol, is reported to be
responsible for mental retardation and various congenital
6
7
8-10
nate (CTAP), cerate (CTACN), and dichromate (CTADC)
(3) Waterham, H. R.; Wanders, R. J. A. Biochim. Biophys. Acta 2000,
1529, 340.
4) Verhagen, J. C.; Braake, P. ter; Teunissen, J.; van Ginkes, G.;
3
abnormalities. The oxidized products of cholesterol are found
(
Sevanian, A. J. Lipid Res. 1996, 37, 1488.
(5) Okamoto, Y.; Ninomiya, H.; Miya, S.; Masaki, T. J. Biol. Chem.
2000, 275, 6439.
(6) Dash, S.; Mishra, B. K. Int. J. Chem. Kinet. 1995, 27, 627.
(7) Mishra, B. K.; Kuanar, M.; Sharma, A.; Nayak, B. B. Indian J. Chem.
2001, 40B, 724.
(
1) (a) Cainelli, G.; Cardillo, G. In Chromium Oxidation in Organic
Chemistry; Spinger Verlag: New York, 1984. (b) Zulak, I. M.; Maerker,
G. J. Am. Oil Chem. Soc. 1989, 66, 1499 and references therein.
(2) Korytowski, W.; Wrona, M.; Girotti, A. W. Anal. Biochem. 1999,
2
70, 123.
10.1021/jo060128k CCC: $33.50 © 2006 American Chemical Society
3
522
J. Org. Chem. 2006, 71, 3522-3526
Published on Web 03/29/2006

Oxidation of Cholesterol by a Biomimetic Oxidant
toward various organic substrates. These are inorganic oxidants
SCHEME 1
with an amphipathic organic carrier, cetyltrimethylammonium
+
(
CTA ) ion, to carry the oxidants into the organic (lipid) phase.
However, these oxidants are hydrophobic and thus support the
existence of a tight ion pair of the cationic carrier and the anionic
oxidant counterion in nonpolar medium. In organic solvents,
+
CTAP oxidizes its carrier, CTA , in a manner similar to
6
â-oxidation of fatty acids. Other aforesaid oxidants are found
to be inert toward their carrier.
Chromium oxidants with lipopathic carriers are well estab-
lished. Quaternary ammonium ions, e.g., pyridinium and quino-
linium ions, with halochromate as the counterions are reported
to be mild oxidants for organic substrates. Pyridinium chloro-
1
1,12
12
chromate,
pyridinium dichromate, and pyridinium fluoro-
chromate13 have been used in the allylic oxidation of cholesterol.
We have used CTAP for oxidation of cholesterol to yield a diol
at the double bond.14 Recently, we have synthesized CTADC
from cetyltrimethylammonium bromide and potassium dichro-
mate and have investigated its oxidation behavior of the oxidant
on various organic substrates.8
-10
CTADC is highly soluble in
a benzophenone moiety to position 3R of a steroid molecule
for dehydrogenation at different positions under various ex-
perimental conditions. A m-dichloroiodobenzoyl group has been
organic solvents and does not dissociate in the medium. It does
not contain any acidic proton, and without acid it shows bizarre
behavior. Aromatic amines and thiols give rise to coupled
products, and aldoximes produce nitriles. However, in the
presence of acid, it shows normal oxidation behavior of
Cr(VI).
16
used by Breslow et al. for dehydrogenation at C9-C11 of
8
10
cholestanol. In the present case, the reaction process may be
initiated with an association of the 3-OH group with the
chromate ion of CTADC, and subsequent reaction takes place
at an equidistant site of the active center of the reagent at the
cholesterol nucleus (Scheme 1). The secondary overlap of
π-orbitals of cholesterol at the C5-C6 position with that of
CrdO may assist the system in achieving proper orientation.
This type of noncovalent bond interaction for remote function-
alization has also been proposed for double-bond insertion in
the sterol system by benzophenone derivatives, where electro-
static interactions or hydrogen bonding between the two
substrates are sufficient to bring the two molecules close enough
Herein, we have made an attempt to investigate the biomi-
metic oxidation activities of CTADC toward cholesterol. To
achieve the objectives, the oxidation products were character-
ized, and kinetics were run in media with varied polarities and
also in microheterogeneous systems generated from the presence
of cationic surfactant, CTAB (cetyltrimethylammonium bro-
mide), anionic surfactant, SDS (sodium dodecyl sulfate), and
nonionic surfactant, Triton X-100 (isooctylphenoxypolyoxyet-
anol), at different concentrations. The kinetic model that mimics
the biological system was used to analyze the resultant data.
By varying [substrate], [acid], and [CTADC] in the reaction
process and from the solvent isotope effect, a suitable mecha-
nism for the reaction was proposed. The thermodynamics of
the reaction was also analyzed by running the kinetics at various
temperature.
15
together for bond insertion and hydrogen abstraction. Dehy-
drogenation of organic molecules due to Cr(VI) oxidation is
not new. Zhu and Okamura have reviewed the synthesis of
calciferol (vitamin D), wherein there is a report on formation
of a dehydrogenated product from cholesteryl acetate derivative
17
by using Cr(VI). In a four-step reaction process a double bond
is introduced at the C-7 position of the B ring of the cholesteryl
unit. The oxidative couplings of amines and thiols by CTADC
to yield corresponding diazo and disulfide derivatives have also
been proposed to be a dehydrogenation coupling process. The
dehydrogenation, in the present case, may occur through a
seven-membered cyclic transition state involving a change of
oxidation state of Cr(VI) to Cr(IV) (Scheme 1).
Results and Discussion
8
Reaction in the Absence of Acid. When cholesterol was
refluxed with CTADC in DCM for 6 h, a product separated
through chromatotron was isolated and was identified as
13
1
7-dehydrocholesterol (1) from its C NMR, H NMR, and FAB-
MS spectral characteristics. The introduction of the double bond
by a dehydrogenation process at C7-C8, thus, is carried out by
CTADC. Earlier dehydrogenation of cholestanol has been
achieved by remote functionalization, wherein the free radical
is involved for abstraction of hydrogen. Breslow15 has attached
The three-dimensional structure of cholesterol oxidase (ChOX)
from BreVibacterium sterolicum has been solved, and it is
proposed that the enzyme’s catalytic site is formed by a
hydrophobic cavity where the sterol binds and interacts with
18
the flavin of FAD. Flavin adenosine dinucleotide (FAD) is
known to be a dehydrogenating agent and is responsible for
(
8) Patel, S.; Mishra, B. K. Tetrahedron Lett. 2004, 45, 1371.
dehydrogenation at vicinal sites during â-oxidation of fatty
(9) Patel, S.; Kuanar, M.; Nayak, B. B.; Banichul, H.; Mishra, B. K.
19
+
acids. In analogy to ChOX, the CTA has an affinity to the
Synth. Commun. 2005, 35, 1033.
(
(
10) Sahu, S.; Patel, S.; Mishra, B. K. Synth. Commun. 2005, 35, 3123.
11) Parish, E. J.; Chitrakron, S.; Wui, T. Y. Synth. Commun. 1986, 16,
(16) Breslow, R.; Corcoran, R. J.; Snider, B. B. J. Am. Chem. Soc. 1974,
96, 6791.
1
371.
(
(
(
12) Parish, E. J.; Wui, T. Y. Synth. Commun. 1987, 17, 1227.
13) Parish, E. J.; Sun, H.; Kizitu, S. A. J. Chem. Res. 1996, 544.
14) Mishra, B. K.; Dash, S.; Nayak, B. B. Indian J. Chem. 2001, 40A,
(17) Zhu, G. D.; Okamura, W. H. Chem. ReV. 1995, 95, 1877.
(18) Gadda, G.; Wels, G.; Pollegioni, L.; Zucchelli, S.; Ambrosius, D.;
Pilone, M. S.; Ghisla, S. Eur. J. Biochem. 1997, 250, 369.
(19) Berg, J. M.; Tymoczko, J. L.; Stryer, L. In Biochemistry, 5th ed.;
W. H. Freeman and Co.: New York, 2002; p 607.
1
59.
15) Breslow, R. Chem. Soc. ReV. 1972, 92, 553.
(
J. Org. Chem, Vol. 71, No. 9, 2006 3523

Patel and Mishra
TABLE 1. Effect of [Chol], [CTADC], and [Acetic Acid] on the
Oxidation of Cholesterol by CTADC at 300 K
CTADC] × 10 (M) [Chol] (M) [acetic acid] (M) kobs × 10 (s-1)
4
3
[
0.47
0.94
1.89
2.83
4.70
1.89
1.89
1.89
1.89
1.89
1.89
1.89
1.89
0.05
0.05
0.05
0.05
0.05
0.002
0.01
0.02
0.08
0.05
0.05
0.05
0.05
3.24
3.24
3.24
3.24
3.24
3.24
3.24
3.24
3.24
0.81
1.62
6.48
8.1
3.934
3.255
2.28a
1.973
1.457
0.326
0.844
1.274
2.959
0.557
1.179
4.479
5.197
FIGURE 2. Plot of 103kobs vs [Chol] in the oxidation reaction of
CTADC with cholesterol at 300 K.
a
03kobs at 298, 303, and 308 K were found to be 2.073, 2.43, and 3.197
1
s-1 respectively.
yielding 1 mol of 7-dehydrocholesterol and 1 mol of Cr(IV).
The existence of Cr(IV) as the reduced state in oxidation of
benzyl alcohol by quinolinium fluorochromate has also been
2
0
reported by Dave et al. Cr(IV) further changes to Cr(III) by a
disproportionation reaction in a sequential manner. The existence
of Cr(III) in the product mixture was established from the
2
1
absorption maximum at 580 nm. However, the change in the
intensity at 580 nm was not reliable to study the rate of
formation of Cr(III).
Cr(IV) + Cr(VI) f 2Cr(V)
Cr(V) f Cr(III) + 2e
The reaction rate was found to increase with an increase in
the concentration of cholesterol achieving a plateau at higher
concentration. Thus, the oxidation of cholesterol exhibited
FIGURE 1. Plot of 103kobs vs [acetic acid] in the oxidation reaction
of CTADC with cholesterol at 300 K.
normal Michaelis-Menten kinetics with a maximum value of
electron-rich site of cholesterol, i.e., 3-hydroxy and ∆5,6
π-electron clouds, and as a consequence, the tightly bound
dichromate to the quaternary ammonium ion remains close to
the reaction site, i.e., C7-C8, and thus is responsible for the
dehydrogenation process.
-3 -1
kobs at 3.3 × 10
s
(Figure 2). The steady-state dissociation
constant of the oxidant-substrate complex known as the
Michaelis-Menten constant, Km () (k-1 + k2)/k+1), was
-3
obtained as 24.0 × 10 M. While investigating the effect of
water pool in a reversed micelle on oxidation of cholesterol by
Reaction in the Presence of Acid. Under reflux conditions,
the solution of CTADC and cholesterol in 20% acetic acid of
DCM became green. The completion of the reaction was
ascertained by monitoring TLC of the reaction mixture. The
product was obtained from the green reaction mixture as a white
crystal, which was characterized from its IR and NMR (H and
cholesterol oxidase in a CTAB-octane/hexanol system, Gupte
-3
22
et al. obtained a Km value of 2.408 × 10 M. By using a
Lineweaver-Burk-type double-reciprocal equation (eq 5), the
binding constant K () k+1/k-1) and k2 are obtained as 63.3 L
-1
-3 -1
mol and 2.893 × 10 s , respectively The corresponding
double-reciprocal curve is shown in the inset of Figure 2. From
K (63.3 L mol ) and Km and k2 values the k+1 and k-1 were
calculated to be 358.2 × 10 and 5.597 × 10
1
3
C) spectral data and the melting point to be 5-cholesten-3-
-1
one (2). The results were compared with the data of authentic
samples. The formation of keto functionality may be explained
by assuming the formation of an acid dichromate. In presence
of acetic acid, the dichromate ion becomes free from the grasp
of the quaternary onium ion due to the change in polarity of
the medium and also the probable substitution of onium ion by
proton of acetic acid. It becomes easier to abstract the R-proton
of cholesterol yielding cholestenone. To obtain further support
for this, the reaction kinetics of the oxidation reaction was
monitored in the presence of acid, and the kinetic data are
tabulated in Table 1.
Without acid, the reaction became too slow to measure. The
linear plot of [acid] vs rate constant (Figure 1) was found to
pass through the origin, substantiating almost no reaction with
acid. The rate of reaction was found to be first order with respect
to acid.
-3
-3 -1
s
respectively.
k
+1
cholesterol + CTADC y z [complex]
(1)
(2)
k-1
k2
[complex] 98 products
Thus, by applying the steady-state approximation23
d[Complex] Kk
[cholesterol][CTADC]
2
rate ) -
)
(3)
dt
1 + K[cholesterol]
d[complex]
1
Kk [cholesterol]
2
-
×
) kobs
)
dt
[CTADC]
1 + K[cholesterol]
(4)
The stoichiometry of the reaction was found to be 2:3 for
Cr(VI) and cholesterol. However, in the earlier case, i.e., without
acetic acid, 1 mol of cholesterol reacts with 1 mol of CTADC
1
kobs
1
1
k2
)
+
(5)
k2K[cholesterol]
3524 J. Org. Chem., Vol. 71, No. 9, 2006

Oxidation of Cholesterol by a Biomimetic Oxidant
TABLE 2. Observed Rate Constants for the Oxidation Reaction of
Cholesterol in Various Organic Solvents at 300 Ka
3
3
kobs × 10
kobs × 10
-1
-
1
solvent
benzene
toluene
chloroform
dichloromethane
(s
)
solvent
ethyl acetate
acetone
tetrahydrofuran
N,N-dimethylformamide
(s )
3.792
3.036
2.441
2.28
0.672
0.315
0.234
0.0844
a
Chol] ) 0.05 M, [CTADC] ) 1.89 × 10-4 M, and [acetic acid] )
[
3
.24 M.
FIGURE 3. Effect of surfactant in the oxidation reaction of cholesterol
with CTADC.
With increasing concentration of CTADC, the rate constant
was found to decrease nonlinearly in a concave fashion (Table
1). The decrease in rate constant with increase in [CTADC]
may be ascribed to the formation of reversed micelle wherein
the dichromate ion is enveloped by CTA+ and cholesterol is
more partitioned to bulk DCM. Thus, the effective concentration
of cholesterol at proximity of dichromate decreases. CTAB
forms reversed micelles in DCM24 and catalyzes various
chemical reactions by partitioning the reagent and the substrate.
When CTAB was added to the reaction mixture, the rate
constant was found to decrease sharply with increasing [CTAB].
FIGURE 4. Plots of 5 + log k vs surface tension of the solvents for
the oxidation of cholesterol with CTADC.
between two classes of solvents was experienced, each class
contributing differently to the reaction system. Benzene, toluene,
DCM, and chloroform formed a group, while the rest of the
solvents (tetrahydrofuran, dimethylformamide, ethyl acetate, and
acetone) formed another group. In the former group, the change
in the polarity of the solvent did not have a significant effect
on the rate, while with other solvents a decreasing trend in
reactivity with increasing polarity scale was observed. When
-3
Above a concentration of 5 × 10 M of CTAB, the rate constant
-
4
-1
almost levels up around 1 × 10
s
(Figure 3). This
observation also corroborates the earlier argument on partition
of the oxidant and substrate in two different subphases.
Assuming the rate decrease is due only to the partition effect,
-
4
for complete entrapment of a dichromate (2 × 10 M) by
+
-3
CTA (5 × 10 M) it requires a composition of 1:25 of
2
5
CTADC/CTAB. From the logarithmic plot of rate constant and
log k was plotted against â (hydrogen bond acceptor basicity)
27
[CTADC], the order of reaction is found to be 0.432, indicating
and log P (partition coefficient) values separately, straight lines
with negative and positive slopes were observed. The resulting
regression equation considering both the parameters is presented
in eq 6.
a complex mechanism being operated during the reaction
process.
In presence of SDS, the dichromate ions become free from
the engulfment of the reversed micelle due to the anionic
amphiphile and thus is more available to cholesterol for
oxidation. With increasing [SDS], the rate constant experiences
a plateau above a concentration of 0.002 M. Addition of TX-
log k ) 0.215((0.093) log p - 0.98((0.528)â -
2
.899((0.231) R ) 0.9798, F ) 48.13 (6)
1
00 did not exhibit any significant change in the rate constant.
When the reaction was monitored in various solvents with
This result suggests the existence of a relatively less polar
transition state than the reacting species. Thus, a mechanism of
the oxidation process can be proposed wherein the anionic
dichromate forms a neutral ester as the intermediate which,
during R-hydrogen abstraction, forms a more nonpolar cyclic
different polarity, the rate was found to change significantly
Table 2). Interestingly, both bulk parameters and polarity
(
parameters of the solvents were found to exhibit specific trends
in the reactivity.
28
transition state. On the contrary, Banerji have observed a rate
When the log k values were plotted against polarity param-
eters such as dielectric constant (ꢀ), dipole moment (µ), solvent
polarity (π*),25 anion-solvating power of the solvent (A), and
acceleration in Cr(VI) oxidation of organic sulfides using
pyridinium fluorochromate due to an increase in polarity of the
medium, and have assigned the results to the increased polarity
in the transition state.
2
6
cation-solvating power of the solvent (B), an ordination
Further, the correlation of the reactivity with the bulk
parameters such as surface tension (γ) and viscosity (η) of the
solvents were analyzed. In this case also, the plots demarcated
the two classes of solvents with two opposite trends (Figure 4).
In the nonpolar solvents, there was an increase in the rate
constant with an increase in viscosity or surface tension, while
(
20) Dave, I.; Sharma, V.; Banerji, K. K. Indian J. Chem. 2002, 41A,
93.
21) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. In
4
(
AdVanced Inorganic Chemistry, 6th ed.; John Wiley and Sons: New York,
1
999; p 747.
22) Gupte, A.; Nagarajan, R.; Kilara, A. Ind. Eng. Chem. Res. 1995,
4, 2910.
23) Laidler, K. J. In Chemical Kinetics, 2nd ed.; McGraw-Hill: New
York, 1968; p 327.
24) Rosso, F. D.; Bartoletti, A.; Profio, D. P.; Germani, R.; Savelli, G.;
Blasko, A.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1995, 673.
25) Taft, R. W.; Abboud, J. L. M.; Kamlet, M. J. J. Org. Chem. 1984,
9, 2001.
(
3
(
(26) Swain, C. G.; Swain, M. S.; Powel, A. L.; Alunni, S. J. Am. Chem.
Soc. 1983, 105, 502.
(27) Katritzky, A. R.; Fara, D. C.; Kuanar, M.; Hur, E.; Karelson, M. J.
Phys. Chem. A 2005, 109, 10323.
(
(
4
(28) Banerji, K. K. J. Chem. Soc., Perkin Trans. 2 1988, 2065.
J. Org. Chem, Vol. 71, No. 9, 2006 3525

Patel and Mishra
SCHEME 2
the variation in the bulk properties for relatively polar solvents
experienced a decrease in the rate constants.
The changes in entropy, enthalpy, and the free energy of
activation were calculated for the oxidation of cholesterol
and sodium dodecyl sulfate (SDS) were purified by recrystallization
from methanol solution, and their purity was checked from physical
constants.
Kinetic Measurements. The oxidation of cholesterol by CTADC
in the presence of acetic acid was studied in different solvents and
surfactant systems. The temperature in the reaction cell was
controlled by circulating water by using an INSREF thermostat
within a temperature fluctuation of (0.1 K. The reactions were
performed under pseudo-first-order conditions by keeping a large
excess (100 times or more) of the cholesterol with respect to
CTADC. The rate of disappearance of the Cr(VI) species was
followed spectrophotometrically by monitoring the absorption band
at 350 nm from which the first-order rate constant, kobs, was obtained
from the linear (r ) 0.99) plot of log[CTADC] against time for up
to 75% completion of the reaction. The values reported are the
average of at least duplicate runs and were reproducible within (4%
error. Some of the kinetics were run under nitrogen atmosphere,
and the data were found to be almost the same (within an error of
(
1
[Cholesterol] ) 0.05 M) with CTADC ([CTADC] ) 1.89 ×
-
4
0 ) in the presence of 3.24 M acetic acid by using the Eyring
-
1
-1
equation and are found to be -208.1 J K mol , 26.368 kJ
mol , and 89.392 kJ mol , respectively. A high negative value
in ∆S supports the proposal of involvement of a cyclic transition
-1
-1
q
state during H-abstraction from the substrate.
From the above findings a tentative mechanism has been
proposed (Scheme 2) wherein the CTADC equilibrates with
acetic acid to form the protonated dichromate, which subse-
quently reacts with cholesterol giving rise to a dichromate ester.
The proximal oxygen anion of the dichromate abstracts the
R-hydrogen leading to formation of cholestenone.
To have more insight into the reaction mechanism, the solvent
isotope effect was investigated using 1:1 mixtures of H2O/acetic
acid and D2O/acetic acid. Acetic acid was equilibrated with H2O
and D2O separately for 3 h before its use in the reaction process.
The solvent isotope effect k(H2O)/k(D2O) was found to be 1.35
2%) as those under normal atmospheric conditions. Hence, most
of the reactions were carried out without a nitrogen environment.
Product Analysis. After keeping the reaction mixture of
CTADC, cholesterol, and acetic acid in proper composition for 12
h in DCM, the volume of the reaction mixture was reduced to 30%
under vacuum. The mixture was kept at 5 °C for 12 h. A white
crystal separated out, which was recrystallized from its alcohol
solution. The IR and NMR spectral analyses of the product agreed
well with those of 3-cholestenone.
-
4
-4
×
10 /1.87 × 10 ) 0.72. When a preequilibrium protonation
is involved, the acid-catalyzed rate is faster in D2O than in
H2O.29 The trend, k(D2O) > k(H2O), indicates that the hydroxyl
group is not involved in either preequilibrium or in the rate-
determining step. Further, significant changes due to change in
solvent on the rates preclude the possibility of breaking an O-H
bond in the rate-determining step and thus support the formation
of the dichromate ester in the reaction process.
Stoichiometry. The stoichiometry of the reaction was determined
by performing the experiment at 298 K, under the conditions of
[
CTADC] ≈ [cholesterol] at varying cholesterol concentrations. The
disappearance of Cr(VI) was followed until the absorbance values
became constant. The [CTADC] was estimated after 48 h. A
stoichiometry ratio, ∆[CTADC]/∆[cholesterol] ≈ 0.35, was ob-
served, which confirmed a 1:3 CTADC/cholesterol relationship.
Experimental Section
Materials. Cetyltrimethylammonium dichromate (CTADC) was
8,9
Acknowledgment. We thank the University Grants Com-
mission (UGC) and the Department of Science and Technology
DST), New Delhi, for financial support through DRS and FIST
programs, respectively. S.P. thanks the Council of Scientific
and Industrial Research (CSIR), New Delhi, for a Senior
Research Fellowship.
prepared by the method reported earlier, and its purity was
checked by estimating Cr(VI) iodometrically.30 Cholesterol was
(
purified by recrystallization from alcohol. Glacial acetic acid was
used as a source of hydrogen ion and was used without further
purification. The organic solvents were purified by standard
methods.31 The surfactants cetyltrimethylammonium bromide (CTAB)
JO060128K
(
29) Kuotsu, B.; Tiewsoh, E.; Debroy, A.; Mahanti, M. K. J. Org. Chem.
996, 61, 8875.
30) Vogel, A. I. In A Textbook of QuantitatiVe Inorganic Analysis, 3rd
ed.; ELBS and Longmans: Essex, 1961.
1
(
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