An environmentally benign catalytic oxidation of cholesteryl acetate with molecular oxygen by using N-hydroxyphthalimide

Article abstract of DOI:10.1039/b915737a

A green and effective method is reported for the oxidation of cholesteryl acetate to 7-keto-cholesteryl acetate with O₂ in the presence of catalytic amounts of N-hydroxyphthalimide (NHPI) under mild conditions. It was found that Co(OAc)₂ could cooperate with Mn(OAc)₂ to enhance the catalytic ability of NHPI resulting in better yields. This economical and environmentally-friendly method provides a potentially new way for the synthesis of the intermediate product of vitamin D₃ in industry, which eliminates the use of large amounts of bromine in present route and overcomes the drawbacks of the known oxidation methods.

Full text of DOI:10.1039/b915737a

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www.rsc.org/greenchem | Green Chemistry
An environmentally benign catalytic oxidation of cholesteryl acetate with
molecular oxygen by using N-hydroxyphthalimide
Zhen Yao, Xingbang Hu, Jianyong Mao and Haoran Li*
Received 3rd August 2009, Accepted 7th September 2009
First published as an Advance Article on the web 12th October 2009
DOI: 10.1039/b915737a
A green and effective method is reported for the oxidation of cholesteryl acetate to
7-keto-cholesteryl acetate with O2 in the presence of catalytic amounts of N-hydroxyphthalimide
(NHPI) under mild conditions. It was found that Co(OAc)2 could cooperate with Mn(OAc)2 to
enhance the catalytic ability of NHPI resulting in better yields. This economical and
environmentally-friendly method provides a potentially new way for the synthesis of the
intermediate product of vitamin D3 in industry, which eliminates the use of large amounts of
bromine in present route and overcomes the drawbacks of the known oxidation methods.
Introduction
The allylic oxidation of cholesteryl acetate to 7-keto-cholesteryl
acetate is quite important due to its various applications in
the synthesis of pharmaceuticals and fine chemicals.1–3 One of
the most important applications is the synthesis of vitamin D3,
which is of great interest because vitamin D3 plays an essential
role in the maintenance of organic systems.4 Nowadays, some of
the vitamin D3 used worldwide is synthesized using the strategy
depicted in Scheme 1,5,6 in which, the use of large amounts
of bromine is unavoidable, thereby resulting in bromine-related
corrosion and environmental hazards. In addition, undesirable
side products (such as compounds 3 and 5, Scheme 1) cannot be
avoided in this process. Therefore, the production of vitamin
D3 by a halogen-free method is vital for the industry from
an environmental point of view and has become a focus of
scholastic interests. An innovative strategy (Scheme 2) for
the synthesis of 7-dehydrocholesteryl acetate (4), which is the
intermediate product of vitamin D3, has been developed.7,8
This method can overcome the above mentioned drawbacks, in
which, the oxidation of cholesteryl acetate to 7-keto-cholesteryl
acetate (6), is the crucial step, while the following synthesis
Scheme 2 Novel synthesis of 7-dehydrocholesteryl acetate (4).
of 4, hydrolyzation and UV irradiation are relatively mature
processes.6,8
Various methods have been developed for the oxidation of
cholesteryl acetate, which is the initial material of vitamin D3.
They can be divided into two groups: (i) reactions wherein
metal derivatives are used. The well-known classical method is
chromium(VI) oxide mediated oxidation using CrO3 or PDC10
9
11
and Cr(CO)6 in acetic acid. A great excess of the reagents
and a large volume of the solvent are required in most of these
procedures, as is complicated work-up of the environmentally
hazardous chromium residues, therefore making these methods
inconvenient. (ii) Reactions wherein peroxides are used. For
example, t-butyl hydroperoxide (TBHP) with different catalysts,
such as sodium chlorite,12,13 cuprous iodide14 and manganese(III)
acetate.15 These routes are obviously more environmentally
benign than those discussed above,9–11 however, TBHP is too
expensive for large-scale industrial application.
The methods mentioned above turned out to be unsuitable
for the oxidation from both an economic and environmental
point of view. Molecular oxygen is the ideal oxidant for
large scale processing, however, it cannot be directly uti-
lized to oxidate substrates. As a result, many high efficient
catalysts have been developed to catalyze molecular oxy-
gen for the oxidations.16–18 Recently, a new organic catalyst,
N-hydroxyphthalimide (NHPI), has been introduced as an
effective catalyst for C–H bond activation by hydrogen
abstraction,19–24 which is a cheap, non-toxic compound eas-
ily prepared by the reaction of phthalic anhydride with
hydroxylamine.25 Due to its idiosyncratic behaviour, which is
very different from that of any system proposed previously, and
Scheme 1 Traditional synthesis of vitamin D3.
Department of Chemistry, Zhejiang University, Hangzhou, 310027,
P. R. China. E-mail: lihr@zju.edu.cn; Fax: +86-571-8795-1895;
Tel: +86-571-8795-2424
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Table 1 Oxidation of cholesteryl acetate with different amounts of
to its general efficiency, NHPI has attracted increasing attention
in the past decade, from both in academia and the industries.
Herein, we present an improvement of the NHPI system,
which opens up a new and efficient access to 7-keto-cholesteryl
acetate. Molecular oxygen is introduced as oxidant, and only
catalytic amounts of NHPI is used with a better yield, which
has never been achieved before. This environmentally reliable
and practical method holds promising application potential
for chemical industries, particularly in the synthesis of vitamin
D3, for this easy method can substitute the bromination in the
traditional route (Scheme 1).
NHPIa
Entry NHPI/mmol% t/h
Conversionb (%) Isolated yield (%)
1
2
3
4
10
50
100
200
14
13
9
20
96
96
96
11
60
69c
53
3.5
a Reaction conditions: cholesteryl acetate (4.28 g, 10 mmol), acetone
(80 mL), BPO (0.016 g, 0.5 mmol) room temperature, O2 1 atm.
b Conversion rates of cholesteryl acetate were calculated using HPLC
analysis, and were unchanged after the reaction hours showed above.
c Isolated yield in ref. 26 and 27 was 73.8% under the same conditions.
Experimental
Results and discussion
Chemicals
Early research indicated that NHPI can efficiently oxidate
substrates with molecular oxygen by adding a small amount
of radical starters, such as dibenzoyl peroxide (BPO).26,27 In a
first set of experiments, the influence of the amount of NHPI
on the reaction was investigated (Table 1). After an initial
sharp increase in the conversion rate and percentage yield, an
optimal yield was obtained in the presence of about 100 mol%
NHPI, followed by a slow decrease as the amount of NHPI
was further increased. To ensure that the reaction gives the
desired yield, stoichiometric amounts of NHPI must be used
(entry 3). Therefore, the oxidation route lacked competitive
ability compared with the present bromination route from an
economical point of view, because of the large amounts of NHPI
as well as a yield of 69%. Lesser amounts of catalysts and higher
yields of products were always the ultimate goal in industrial
application. However, as shown in Table 1, when the amount
of NHPI was dropped to 10 mol%, both the reaction rate and
selectivity significantly decreased.
It is crucial for the oxidation of cholesterol using a catalytic
amount NHPI in high yield. The previously known processes
showed that the reactivity of NHPI can be enhanced by adding
different co-catalysts.28–30 We found that the yields were improved
to some extent no matter what kind of metal salt was used
(Table 2). However, the yields were lower when trivalent metal
salts (Co(III)(acac)3, Fe(III)(acac)3, Fe(III)(acac)3Cl3, entries 1–3)
were used as co-catalysts. Bivalent metal salts (Co(II)(acac)2,
All the chemicals were of AR grade. They were commercially
purchased and used without further treatment.
Synthesis of 7-keto-cholesteryl acetate
In a typical experiment, a solution of 0.163 g (1 mmol) of NHPI
in 80 mL of acetone was poured in a three-necked 100 mL round-
bottom flask, equipped with a magnetic stirrer and a reflux con-
denser. 4.28 g (10 mmol) of cholesteryl acetate and co-catalyst
used as described in the manuscript were added into the flask.
A stream of dioxygen was conducted into the reaction solution
while stirring at room temperature for 8 h. The reaction mixture
was rotary-evaporated (bath temperature 40–50 ◦C). In order to
separate the catalysts from the reaction mixture, the residue was
treated with 30 mL of hexane and heated to 40 ◦C. After the
reaction mixture was cooled to room temperature, the resulting
finely crystalline precipitate NHPI was filtered out, and ◦dried
(0.149 g, 92%). The filtrate was rotary-evaporated at 60 C to
constant mass. Then, the oily residue was treated with 5 mL
of acetic anhydride in an alkalescent system and stood at room
temperature overnight. As a dehydrating agent, acetic anhydride
can convert the primary and secondary hydroperoxides into
corresponding compounds (ketones), resulting in higher yields.
The mixture was then concentrated on a rotary evaporator. The
residue was treated with 10 mL of methanol and the resulting
suspension was cooled in an ice-bath for 1 h and then suction
filtered. Recrystallization of 7-keto-cholesteryl acetate from the
residue gave 3.78 g (86%) of the product (158–159 ◦C). [In order
to check whether the product can be used to synthesize vitamin
D3, we synthesized vitamin D3 according to known methods:6,8
0.442 g (1 mmol) 7-keto-cholesteryl acetate and 0.186 g (1 mmol)
4-methylbenzenesulfonhydrazide were added into the flask.
After stirring at room temperature for 20 h, the reaction
mixture was rotary-evaporated. The residue was treated with
25 mL dimethyl sulfoxide and stoichiometric amounts of sodium
hydride while stirring at room for 6 h. Ethyl acetate was used
for extraction in neutral solutions. 7-Dehydrocholesteryl acetate
was obtained with a yield of 60% after rotary evaporation. A
3% solution of 7-dehydrocholesterol in a methanol mixture was
irradiated with a XeBr lamp at 284 nm in a falling-film reactor.
The selectivity of vitamin D3 in the reaction mixture is 90%, with
a conversion of 50%.]
Table 2 Oxidation of cholesteryl acetate with NHPI by different co-
catalystsa
Entry Co-catalystsb/mmol t/h Conversionc (%) Isolated yield (%)
1
2
3
4
5
6
Co(acac)3/0.1
Fe(acac)3/0.1
Fe(acac)3Cl3/0.1
Co(acac)2/0.1
Mn(OAc)2/0.1
Co(OAc)2/0.1
10 99
41
28
39
50
66
59
6
8
7
91
91
91
12 91
96
8
a Reaction conditions: cholesteryl acetate (4.28 g, 10 mmol), NHPI
(0.163 g, 1 mmol), acetone (80 mL), room temperature, O2 1 atm.
b Co(acac)3: cobalt(III) acetylacetonate, Fe(acac)3: ferric acetylacetonate,
Fe(acac)3-Cl3: 3-chlorine-acetylacetonate ferric, Co(acac)2: cobalt(II)
acetylacetonate, Mn(OAc)2: manganese acetate, Co(OAc)2: cobalt(II)
acetate. c Conversion rates of cholesteryl acetate were calculated using
HPLC analysis, and were unchanged after the reaction hours showed
above.
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Table 3 Oxidation of cholesteryl acetate with different ratios of
Table 4 Oxidation of cholesteryl acetate with the optimized catalyst-
system in different conditionsa
Co(OAc)2 to Mn(OAc)2 as co-catalysta
Catalyst-system
Isolated
yield (%)
Entry
Solvent
T/◦C
Conversionb (%)
Co(OAc)2 and
Entry NHPI/mmol Mn(OAc)2/mmol t/h (%)
Conversionb Isolated
yield (%)
1
2
3
4
5
6
7
Acetone
Acetone
Acetone
Acetone
Acetone
THF
50
40
30
25
20
25
25
97
91
96
98
93
90
96
63
78
83
86
51
50
78
1
2
3
4
5
1
1
—
1
1
0.07/0.03
0.05/0.05
0.05/0.05
0.05/0.05c
0.03/0.07
8
8
95
98
14 24
64
86
14
76
69
8
8
97
94
1,4-Dioxane
a Reaction conditions: cholesteryl acetate (4.28 g, 10 mmol), ace-
tone (80 mL), room temperature, O2 1 atm. b Conversion rates of
cholesteryl acetate were calculated using HPLC analysis. c Co(acac)2
and Mn(OAc)2/mmol: 0.05/0.05.
a Reaction conditions: cholesteryl acetate (4.28 g, 10 mmol), NHPI
(0.163 g, 1 mmol), Co(OAc)2 (0.0125 g, 0.05 mmol), Mn(OAc)2 (0.0123 g,
0.05 mmol), acetone (80 mL), room temperature, O2 1 atm, 8 h.
b Conversion rates of cholesteryl acetate were calculated using HPLC
analysis.
Mn(II)(OAc)2, Co(II)(OAc)2, entries 4–6) gave better yields
under the same conditions. NHPI acts as a precursor of
the phthalimido-N-oxyl (PINO) radical, generated with the
assistance of metal salts.31 The transition metal ions worked
by forming oxo-transition metal intermediates, thereby relaying
oxygen atom from the oxidant to the cholesteryl acetate. This
requires the transition metal ions to be capable of readily losing
one electron (e.g., Co(II)–Co(III) and Mn(II)–Mn(III)), which may
explain why bivalent metal salts are more effective than trivalent
ones that are far more reluctant to lose electrons.32
We noticed that reaction rates were higher with Co(II) salts
as co-catalyst (entries 4 and 6), and the yields were higher with
Mn(II) salts (entry 5). This inspired us that a combination of
both a Co(II) salt and Mn(II) salt should give better yields and
faster reaction rates. To test this hypothesis, we ran the reaction
with different ratios of Co(OAc)2 to Mn(OAc)2 as co-catalyst.
With the total amount of co-catalyst kept invariant, the isolated
yield with the addition of Mn(II) salts reached a maximum when
the ratio of Co(OAc)2 to Mn(OAc)2 was 1 : 1, then dropped as
the amount of Mn(II) salts further increased (Table 3). The best
result (86%, entry 2) was obtained using a combination of NHPI
and mixtures of Co(OAc)2 and Mn(OAc)2 at room temperature
in acetone, with shorter reaction time but higher yield compared
with previous methods.9–15,26,27 A combination of Co(acac)2 and
Mn(OAc)2 under the same conditions gave a yield of 76% (entry
4). Blank experiments were carried out to reveal that a mixture
of Co(OAc)2 and Mn(OAc)2 or NHPI as the sole catalyst led
only to extremely low yields of products after 14 h (entry 3,
Table 3; entry 1, Table 1).
acetone to be the best solvent. These results indicated that
the reaction gave the best yields at ambient temperature and
atmospheric pressure of oxygen, both favourable for industrial
application.
Based on the above-mentioned results, NHPI was revealed
to be a key catalyst. To find the best catalytic system, al-
lylic oxidation of cholesteryl acetate as a model substrate,
was studied under the same conditions in the presence of
several other nitroxy radicals (Table 5). When the persistent
tetramethylpiperidine-N-oxyl (TEMPO) radical, being a widely
used catalyst for the oxidation of alcohols,31 was used it in
our allylic oxidation, only 9% cholesteryl acetate was converted
(entry 1). The NHPI derivatives such as N-hydroxy trimellitic
imide and its ammonium salt were also investigated, which
Table 5 Oxidation of cholesteryl acetate with different nitroxy radicalsa
Isolated
Entry Nitroxy radicals
1
t/h Conversionb (%) yield (%)
12
9
—
2c
12
23
22
3c
12
12
20
61
13
47
Further research was performed to oxidize cholesteryl acetate
with this catalyst system under different reaction conditions.
Reaction temperatures from 50 ◦C to 20 ◦C (Table 4, entries
1–5) were investigated. The reaction time and the isolated
yields increased as the temperature dropped, the isolated yield
reached a maximum at 25 ◦C and then decreased. Higher
temperature enhanced substrate solubility and shortened the
reaction time. However, they gave lower yields. At 50 ◦C, the
solvent was volatilized and the formation of by-products made
4
◦
a Reaction conditions: cholesteryl acetate (4.28 g, 10 mmol), nitroxy
radicals (1 mmol), Co(OAc)2 (0.0125 g, 0.05 mmol), Mn(OAc)2 (0.0123 g,
0.05 mmol), acetone (80 mL), room temperature, O2 1 atm. b Conversion
rates of cholesteryl acetate were calculated using HPLC analysis.
c Solvent: acetone 50 mL, acetonitrile 30 mL.
likely, resulting in low yields (entry 1). At 20 C, the solubility
of cholesteryl acetate was poor, and the yield went down to 51%
(entry 5). The optimal reaction temperature was experimentally
determined to be 25 ◦C. Different solvents were also used under
the same mild conditions (entries 6 and 7), which revealed
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Table 6 Oxidation of different hydrocarbons with the optimized catalyst systema
Entry
1
Substrate
Product
Conversionb (%)
95
Isolated yield (%)
78
1b R1 = OH, R2 = C8H17
1a R1 = OH, R2 = C8H17
2c
93
66.
3c
4d
91
61
68
14e
5d
5
4
a Reaction conditions: substrate (10 mmol), NHPI (0.163 g, 1 mmol), Co(OAc)2 (0.0125 g, 0.05 mmol), Mn(OAc)2 (0.0123 g, 0.05 mmol), acetone
(80 mL), room temperature, O2 1 atm, 8 h. b Conversion rates of cholesteryl acetate were calculated using HPLC analysis. c Solvent: 1,4-dioxane
80 mL. d Conversion rate and yield were calculated by TLC. e Yield was increased to 34% after 15 h.
resulted in relatively low conversions and yields (entries 2 and
3). A better result was achieved when the reaction was carried
out in the presence of N-hydroxysuccinimide (entry 4), however,
the yield was much lower than the best result obtained above
(Table 3, entry 2). These results indicated that using NHPI as
the key catalyst proved efficient both with respect to the final
yields and the reaction times of the referred reaction.
The optimized amounts of NHPI and co-catalysts were fur-
ther employed to catalyze oxygenations of various hydrocarbons.
Relatively good results were achieved using steroid analog as
substrates (Table 6, entries 1–3), which have similar structure
with cholesterly acetate. Cholesterol was oxygenated with high
Scheme 3 A plausible mechanism for the allylic oxidation of cholesteryl
acetate catalyzed by NHPI and transition metal salts.
conversions and converted selectively to 7-keto-cholesterol with
78% isolated yield (entry 1). The oxidation of cholesteryl
through redox reaction of transition metal salts is a key step.32 In
the next step, a hydrogen atom is abstracted from the cholesterol
to form an alkyl radical. The resultant alkyl radical is trapped
by the peroxy radicals provided by dioxygen, and eventually
converted into oxygenated products. The synergistic effects of
Co(OAc)2 and Mn(OAc)2 also suggested the presence of a radical
transformation step in the course of the oxidation.20,21
benzoate and b-sitosterol were also proceeded smoothly in
1,4-dioxane because of their poor solubility in acetone (entries 2
and 3). Essentially, the reaction of the steroid analogs belonged
to allylic oxidation. Two simple model substrates with the
possibility to be oxidized at the allylic site were investigated
(entries 4 and 5). The 61% of a-pinene was converted in 8 h,
giving a yield of 14% (entry 4). The yield was increased to 34%
after 15 h. However, it was found that a-isophorone cannot be
oxidized to ketoisophorone easily (entry 5). This may explained
by the conjugative effect of double bond and carbonyl, which
made a-isophorone difficult to be oxidized.
Conclusion
In summary, we have developed an effective method to
oxidate cholesteryl acetate to 7-keto-cholesteryl acetate by
molecular oxygen under mild conditions. This economical and
environmentally-friendly procedure led to good yields with a
A plausible reaction mechanism for the allylic oxidation of
cholesterol catalyzed by NHPI and transition metal salts is
shown in Scheme 3. The in situ generation of PINO from NHPI
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small amount of catalyst, and could eliminate the use of a large
amounts of bromine used in the present route of vitamin D3 and
overcome the drawbacks of the known oxidation methods.
Further research in cooperative catalysis with Co(OAc)2 and
Mn(OAc)2 should lead to a universal method for the effective
usage of NHPI. Our research in this field and in other aerobic
oxidations for the vitamin industry are in progress.33–36
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We thank the support of the National Natural Science Founda-
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