Selective copper(II)-catalyzed aerobic oxidative cleavage of aromatic gem-disubstituted alkenes to carbonyl compounds under neutral and mild conditions

Article abstract of DOI:10.1016/j.tet.2013.11.075

Three copper(II) catalytic systems, CuCl₂·2H ₂O, CuCl₂·2H₂O+phenanthroline, and [Cu(μ-Cl)Cl(phen)]₂ were used to cleave alkenes to their corresponding carbonyl compounds under aerobic and neutral conditions. [Cu(μ-Cl)Cl(phen)]₂ shows enhanced selectivity over the other two catalytic systems. The oxidative cleavage reactions were carried out in mixed H₂O/THF solvent system under oxygen (4 atm) at 60 C. The real oxidant is 2-hydroperoxytetrahydrofuran, which is generated in situ in the process through the reaction between THF and oxygen catalyzed by copper(II). The cleavage reactions are selective for aromatic gem-disubstituted alkenes. Aromatic internal alkenes are slow to be oxidized, and both aliphatic terminal and internal alkenes are inert to oxidative cleavage. Free radical scavenger 2,2,6,6, tetramethylpiperidinyl-1-oxyl (TEMPO) deactivates the reaction indicating the involvement of free radical path in the reaction mechanism.

Full text of DOI:10.1016/j.tet.2013.11.075

Tetrahedron 70 (2014) 251e255
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Selective copper(II)-catalyzed aerobic oxidative cleavage of aromatic
gem-disubstituted alkenes to carbonyl compounds under neutral
and mild conditions
Md. Munkir Hossain, Shin-Guang Shyu *
Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Three copper(II) catalytic systems, CuCl $2H O, CuCl $2H Oþphenanthroline, and [Cu(
m-Cl)Cl(phen)]2
2
2
2
2
Received 23 September 2013
Received in revised form 15 November 2013
Accepted 22 November 2013
Available online 27 November 2013
were used to cleave alkenes to their corresponding carbonyl compounds under aerobic and neutral
2
conditions. [Cu(m-Cl)Cl(phen)] shows enhanced selectivity over the other two catalytic systems. The
oxidative cleavage reactions were carried out in mixed H
2
O/THF solvent system under oxygen (4 atm) at
ꢀ
60 C. The real oxidant is 2-hydroperoxytetrahydrofuran, which is generated in situ in the process through
the reaction between THF and oxygen catalyzed by copper(II). The cleavage reactions are selective for
aromatic gem-disubstituted alkenes. Aromatic internal alkenes are slow to be oxidized, and both ali-
phatic terminal and internal alkenes are inert to oxidative cleavage. Free radical scavenger 2,2,6,6, tet-
ramethylpiperidinyl-1-oxyl (TEMPO) deactivates the reaction indicating the involvement of free radical
path in the reaction mechanism.
Keywords:
Alkene
Aerobic
Oxidation
Carbonyl
Copper
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
The use of copper salts and complexes as the catalysts has
gained much prominence recently because of their economic via-
Alkene oxidation reactions are of diverse utility with immense
impact on the development of synthetic organic chemistry. In
particular, oxidative cleavage of olefins to the corresponding car-
bility, less hazardous, good functional group tolerance, and scal-
ability in large-scale synthetic procedures. Copper is involved in the
cleavage of different types of CeC single bonds, C]C double
1
15
1
6
17
bonyl compounds is pivotal for the preparation of valuable in-
bonds in ketenimines and aromatic enol ethers, C^C triple
2
18
termediates and fine chemicals. There are numerous diverse
bonds in O-propargyl oximes, and aromatic ring cleavage in cat-
echol. Copper containing enzyme laccase is also used to cleave the
alkene double bond. Use of CuCl
of styrene in the presence of oxygen is also reported. Although
biological and electrochemical cleavage of the C]C double
bond involving copper have been reported, aerobic oxidative
cleavage of alkene to carbonyl compounds using copper salts or
19
methods to accomplish this oxidation. Among those, ozonolysis is
a classical one, which involves in direct oxidative cleavage of the
2
0
2
in the electrochemical oxidation
3
21
C]C double bond in alkenes. However, its utility is often limited
4
5
6
7
20
21
due to safety concerns. Transition metals, such as Ru, Os, Mn,
8
9
10
11
12
W, Re, Pd, Fe, and Au were used to catalyze the cleavage
reaction without using ozone, and transition metal free aryl-l -
iodane-based methods have also been reported. In spite of great
progress in the field of oxidative cleavage of alkenes, processes that
involve inexpensive and green catalysts are scarce.
3
13
22
complexes is rare. Herein, we report selective Cu(II) catalyzed
aerobic oxidative cleavage of the C]C double bonds of gem-di-
substituted alkenes to the corresponding ketones. Possible reaction
paths are also proposed.
Regarding oxidant, for simplicity and easy availability, molecular
oxygen is one of the ultimate goals in oxidation chemistry. How-
ever, transition metal catalyzed C]C double bond cleavage re-
actions by oxygen are quiet limited, and in most cases reactions are
not selective.14 Thus, the development of a safe, simple, and green
protocols for selective alkene aerobic oxidation is highly desirable.
2. Results and discussion
Alkene 1,1-diphenylethylene was used as the starting substrate.
The oxidation reaction is excellent in yield using the recently re-
2
3
ported oxidation catalyst [Cu(
control reaction, i.e., oxidation using CuCl
m
-Cl)Cl(phen)]
2
(1). However, the
O alone, produced
2
$2H
2
similar results under similar conditions. The reaction conditions
were then optimized in different solvents with different catalytic
*
Corresponding author. Tel.: þ886 2 27898592; fax: þ886 2 2783 123; e-mail
addresses: sgshyu@chem.sinica.edu.tw, sgshyu@gate.sinica.edu.tw (S.-G. Shyu).
0
040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.11.075

252
Md.M. Hossain, S.-G. Shyu / Tetrahedron 70 (2014) 251e255
systems: CuCl
complex 1 (C). The results are summarized in Table 1.
2
$2H
2
O (A), CuCl
2
$2H
2
Oþphenanthroline (B), and
Table 2
Aerobic oxidation of alkenes into aldehydes and ketones
5
mol% Cu(II) Catalyst
Alkenes
Products
Table 1
0
THF + H O (9:1), 60 C, O (4 atm)
2
2
Optimization of the reaction conditions for aerobic oxidation of 1,1-
diphenylethylene in different solvents and Cu(II) catalytic systems
Yield %a/b
Entry
Alkenes
Products
Ac
Bd
Ce
O
31/19 (10)f 29 (5)
1
2
3
4
Trace (15)
Entry
Solvents
O
2
Catalyst used
Yield %a
O
28/17 (10)
21/16 (20)
91 (5)
24 (15) Trace (15)
11 (20) 18 (30)
Ab
Bc
Cd
e
1
2
3
4
5
6
7
8
9
Toluene
n-Pentane
Yes
Yes
Yes
Yes
Yes
Yes
Yesf
No
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No
d
d
d
d
d
d
CH
CH
3
CN
Cl
d
d
d
O
2
2
d
d
d
89 (5)
85 (7)
83 (7)
98/93 (5)
98/91 (15)
96/89 (15)
THF
Trace
91
d
Trace
89
d
Trace
98
53
d
d
d
d
THFþH
THFþH
THFþH
THFþH
THFþH
2
2
2
2
2
O (9:1)
O (9:1)
O (9:1)
O (9:1)
O (1:1)
g
O
d
d
5
85 (5)
81 (7)
Yes
Yes
Yes
d
d
1
1
0
1
Yes
Yes
d
d
O
H
2
O
d
d
6
a
b
c
d
e
f
Yields were determined by GC based on 100% conversion.
F
F
CuCl
CuCl
Cu(
No reaction.
2
$2H
2
O.
$2H
2
Oþphenanthroline.
O
O
2
[
m
-Cl)Cl(phen)]
2
.
7
80 (7)
83 (7)
97/90 (15)
Cl
Cl
2
atm.
g
67% conversion.
15g (10)
11h
i
8
9
(10) 5 (15)
j
O
d (10)
d (10) d (15)
d (10) d (15)
Oxidation reactions for all the three catalytic systems (A, B, and
C) were carried out in polar (CH CN, CH Cl , THF, and H O), non-
polar (toluene and n-pentane), and mixed solvent systems
O) (Table 1). Reactions in polar and nonpolar solvents were
3
2
2
2
1
0
d (10)
d (10)
O
(
THFþH
2
11
d (10) d (15)
unsuccessful (entries 1e5 and 11). Using mixed solvent of THF and
water with THF to water ratio 9:1 (v/v) achieved good yield (entry
). However, water and THF alone were found to be unsuitable
entries 5 and 11). It is also clear that both oxygen and Cu(II) are
O
1k (5)
1 (5)
l
4m (5)
73 (5)
1
6
10 (5)
6
(
1
2
63 (5)
O
needed (entries 8 and 9). All the three catalytic systems can cleave
the alkene to form the corresponding ketone (entry 6). Under 2 atm
of oxygen, lower yield (53%) was obtained (entry 7). Instead of
phenanthroline, bulky ligands, such as neocuproine and bath-
ocuproine afforded similar yields (83% and 85% with 100% con-
version, respectively) and selectivity under identical reaction
conditions.
Catalytic systems (A, B, and C) were applied to various alkenes in
2
THF/H O and the results are summarized in Table 2. Aromatic gem-
disubstituted alkenes were successfully oxidized to their corre-
sponding ketones with high yields and selectivity (entries 4e7).
Among these Cu(II) catalytic systems, C afforded the highest se-
lectivity. The aromatic mono-substituted alkenes are less selective
a
Yields were determined by GC based on 100% conversion except internal alkene
in the entries 8 and 12.
b
Isolated yields.
c
CuCl
2
$2H
$2H
2
-Cl)Cl(phen)] .
2
O.
d
e
f
CuCl
Cu(
2
2
Oþphenanthroline.
[
m
Figures in parenthesis are reaction time in hours.
Conversion 23%.
Conversion 19%.
Conversion 12%.
No reaction.
Conversion 19%.
Conversion 17%.
Conversion 8%.
g
h
i
j
k
l
m
(entries 1e3) irrespective of electronically neutral (entry 1), rich
(entry 2), or poor (entry 3) alkenes. Aliphatic mono-substituted
(entry 9), gem-disubstituted (entry 10), and internal alkene (entry
alkene to the corresponding ketone was observed when additional
alkene (e.g., 1,1-diphenylethylene) was added to the reaction
mixture.
11) are inert. Furthermore, the internal aromatic alkene is much
less reactive than the terminal aromatic alkenes (entry 8). The se-
lectivity is demonstrated by the reaction of equimolar amounts of
mixed terminal and internal alkenes, which leads to the products
with much higher yield in the terminal alkene (entry 12), and C is
more effective for the selectivity among the three catalytic systems.
Although regioselective oxidation of alkenes to epoxides is well
Oxidation of alkenes to carbonyl compounds exclusively in the
THF solvent (although it is trace in pure THF, Table 1, entry 5) in-
dicates a vital role of the solvent in the process. In this study, the
ideal solvent system is the mixed solvent of THF and water (9:1 v/v
ratio; Table 1, entry 6), however, excess water is not effective (Table
1, entry 10). These observations indicate that only a small portion of
water in the mixed solvent is required for the oxidation and excess
of water inhibits the process.
24
known, examples of the cleavage of alkene double bonds are
2
5
rare. The observed selectivity in Table 2 may be useful in synthetic
organic chemistry when there are different types of double bonds
in the same substrate. At the end of the oxidation reaction, the
Cu(II) catalyst was still active because further conversion of the
Furthermore, THF or water alone is also unsuitable (Table 1,
entries 5 and 11). It has been reported that Rh catalyzed formation

Md.M. Hossain, S.-G. Shyu / Tetrahedron 70 (2014) 251e255
253
of 2-hydroperoxytetrahydrofuran (2) from THF requires small
amount of water, and larger amount of water inhibits the re-
action.26 Detection of
-butyrolactone (3), the decomposition
product of 2, in the reaction further supports the formation of 2
Table 3
Control reactions of alkenes and epoxides
g
5 mol% Cu(II) Catalyst
Substrates
Products
2
7
0
THF + H O (9:1), 60 C, O (4 atm)
during catalytic reactions (Figs. S15eS29 in Supplementary data).
2
2
In addition, blank reactions reveal that the formation of 2 requires
the presence of both catalyst (Cu(II)) and alkene since absence of
either of them makes the cleavage reaction or formation of 3 in-
effective. Therefore, the role of copper is to catalyze the formation
Entry Alkene & epoxide
Products
Catalyst TEMPOb GC yielda
%
Ac
Bd
Ce
1g
O
Yes
Yes
Yes
No
Yes
Yes
No
No
No
d
d
24
d
53
f
d
d
2 2
of 2 from THF in the presence of alkene, O , and H O and may also
take part in the decomposition of 2 to 3 (Scheme 1). Based on
these observations, 2 is proposed to be the real oxidant in the al-
kene cleavage reactions because besides of THF, all the other sol-
O
2g
d
d
vents (CH
carry out the reaction under identical conditions (Table 1, entries
e4, 11).
2 2 3
Cl , CH CN, toluene, pentane and water) are unable to
O
3g
O
O
14 17
1
O
4g
d
d
O
O
O
5h
Yes
41 49
O
6h
No
No
d
d
d
a
Yields were determined by GC based on 100% conversion.
TEMPO (46.8 mg, 0.3 mmol).
b
c
CuCl
2
$2H
$2H
-Cl)Cl(phen)]
No reaction.
2
O.
d
e
f
CuCl
Cu(
2
2
Oþphenanthroline.
[
m
2
.
g
h
Reaction time 15 h.
Reaction time 5 h.
Table 4
Control reactions for aerobic oxidation of 1,1-diphenylethylene in different solvents
and oxidant 2
Scheme 1. Proposed reaction paths for the alkene oxidative cleavage.
Oxidative cleavage of alkenes by hydroperoxides is generally
Aa
Conv./yield%b
Entry
Solvents (9:1)
2
8
proceeded through free radical pathways. For metal ion catalyzed
hydroperoxy reactions, the most important function of the catalyst
is the decomposition of relatively stable hydroperoxides into radi-
c
1
2
3
4
5
6
THF/H
THF/H
2
O
O
No
Yes
No
Yes
Yes
Yes
45/34
100/81
47/37
100/83
65/17
71/21
c
2
c
TolueneþH
2
O
O
2
8
c
TolueneþH
2
cal. The cleavage reactions of alkenes in our case were completely
inhibited in the presence of radical scavenger TEMPO (Table 3,
entries 1 and 2), which is in support of the free radical pathways.
However, it is still not clear whether it is a direct peroxide free
radical oxidation or copper-catalyzed oxidation of alkenes by per-
oxide 2.
d
2
THF/H O
d
TolueneþH
2
O
a
2 2
CuCl $2H O.
b
Yields were determined by GC (Figs. S26eS29 in Supplementary data).
c
2
Reaction under O (4 atm).
Reaction under 1 atm nitrogen.
d
To clarify the above, 2 was prepared,29 and alkene oxidations
using 2 with and without Cu(II) were carried out under identical
conditions (Table 4). Cleavage reaction was incomplete (Table 4,
entry 1) in the absence of A, however, the reaction was completed
when A was used as a catalyst (Table 4 entry 2). This indicates the
involvement of Cu(II) as a catalyst in the cleavage reactions. The
reactions with 2 were also carried out in toluene to verify the in-
fluence of extra 2 possibly formed in situ during the catalytic pro-
The alkene cleavage may go through the epoxide as an in-
termediate, and alkylperoxy radical may involve in the alkene ox-
idation to epoxide. It might proceed in two steps: initially via
epoxidation of alkene and its subsequent oxidation to the carbonyl
3
0
compound (path a in Scheme 1). In addition, trace amount of
epoxides (styrene oxide and 1,1-diphenylethylene oxide) were
observed in the final reaction mixture of the corresponding re-
actions (Figs. S15eS17 and S26eS28 in Supplementary data).
The oxidation of styrene epoxide and 1,1-diphenylethylene ox-
ide afforded the respective carbonyl compounds (Table 3, entries 3
and 5) indicating that epoxides could be the intermediates formed
in the reactions. The reactions in the absence of Cu(II) were un-
successful (Table 3, entries 4 and 6) indicating Cu(II) is essential in
the carbon elimination of epoxide to form the carbonyl compound.
2
cess in THF/H O solvent system. The results of the cleavage
reactions with and without catalyst A under identical conditions
in toluene are comparable to those in THF (Table 4, entry 3 and 4)
indicating the extra 2 formed in situ is minor as compared to
the pure 2 added. The reaction between alkene and 2 under inert
atmosphere (1 atm nitrogen) producing the corresponding car-
bonyl compound further supports the argument (Table 4, entries 5
and 6).

254
Md.M. Hossain, S.-G. Shyu / Tetrahedron 70 (2014) 251e255
Alkene cleavage via dioxetane without going through epoxide
identical conditions. The products were also analyzed by GC with
the same procedure stated above.
31
however cannot be totally eliminated (path b in Scheme 1).
. Conclusion
In conclusion, the Cu(II) catalyst systems (A, B, and C) can se-
3
4.3.1. Benzaldehyde. TLC (hexane/ethyl acetate¼1:0.5) gave benz-
1
aldehyde as a colorless liquid; yield: 4 mg (19%). H NMR (400 MHz,
13
CDCl
CDCl
3
)
)
d
ppm 9.98 (s, 1H), 7.85e7.47 (m, 5H); C NMR (400 MHz,
ppm 192.1, 136.2, 134.2, 129.5, 128.8.
lectively catalyze the aerobic oxidative cleavage of terminal C]C
double bonds in aromatic gem-disubstituted alkene with high yield
at low temperature. The catalytic systems involve the coupling of
two oxidation reactions, i.e., the oxidation of THF to 2 and the ox-
idative cleavage of alkene, in one-pot.
3
d
4.3.2. m-Tolualdehyde. TLC (hexane/ethyl acetate¼1:0.5) gave m-
1
tolualdehyde as a colorless liquid; yield: 4 mg (17%). H NMR
(400 MHz, CDCl
m, 2H), 2.39 (s, 3H); C NMR (400 MHz, CDCl
136.7, 135.5, 130.2, 129.1, 127.4, 21.4.
3
) d ppm 9.95 (s, 1H), 7.65e7.64 (m, 2H), 7.42e7.26
13
(
3
) d ppm 192.7, 139.1,
4
4
. Experimental section
.1. General
4.3.3. 3-Nitrobenzaldehyde. TLC (hexane/dichloromethane¼1:1)
1
gave 3-nitrobenzaldehyde as a colorless solid; yield: 5 mg (16%). H
All chemicals were obtained from commercial sources and used
3
NMR (400 MHz, CDCl ) d ppm 10.11 (s, 1H), 8.67e8.66 (m, 1H),
13
without further purification. Phenanthroline, neocuproine, 2,2,6,6,
tetramethylpiperidinyl-1-oxyl (TEMPO), -methylstyrene, styrene,
styrene oxide, 4-chloro- -methylstyrene, cis-stilbene, 1,1-diphenyl
8.46e8.44 (m, 1H), 8.23e8.21 (m, 1H), 7.76 (t, J¼7.9 Hz, 1H);
C
a
NMR (400 MHz, CDCl
124.4.
3
) d ppm 189.9, 148.9, 137.5, 134.9, 130.5, 128.7,
a
ethylene, 3-nitrostyrene, 3-nitrobenzaldehyde, 1,1-diphenyethy
lene oxide, valeraldehyde, benzaldehyde, 4-chloroacetophenone,
4.3.4. Benzophenone. TLC (hexane/ethyl acetate¼1:0.5) gave ben-
1
acetophenone, 4-fluoroacetophenone, heptaldehyde, 1-octene,
g-
zophenone as a colorless solid; yield: 34 mg (93%). H NMR
butyrolactone, 2,3-dihydrofuran, ammonium chloride, and cupric
chloride dihydrate were purchased from Across. trans-5-Decene, 3-
(400 MHz, CDCl
3
)
d
ppm 7.80 (d, J¼7.8 Hz, 4H), 7.57 (t, J¼7.6 Hz, 1H),
13
7.47 (t, J¼7.6, 4H), C NMR (400 MHz, CDCl
3
) d ppm 196.9, 137.8,
methylstyrene, 4-fluoro-
a-methylstyrene, benzophenone, and m-
132.6, 130.2, 128.5.
tolualdehyde were purchased from Alfa Aesar. Bathocuproine, 2-
methyl-1-1-heptene and 34.5e36.5 wt. % aqueous solution of
4.3.5. Acetophenone. TLC (hexane/ethyl acetate¼1:0.5) gave ace-
1
H
2
O
2
were purchased from Aldrich. [(phen)Cu(
m
-Cl)(Cl)]
2
and 2-
tophenone as a colorless liquid; yield: 22 mg (91%). H NMR
hydroperoxytetrahydrofuran were synthesized according to liter-
(400 MHz, CDCl
3
)
d
ppm 7.87 (d, J¼7.6 Hz, 2H), 7.50e7.46 (m, 1H),
13
2
3,29
ature.
Agilent 6890 instrument with an FID detector and an Agilent
0 mꢁ0.53 mmꢁ3.0 m HP-1 capillary column. Products isolation
was carried out by TLC (Merck, TLC silica gel 60 F254 25 Aluminum
Gas chromatographic analyses were performed on an
3
7.40e736 (m, 2H), 2.51 (s, 3H); C NMR (400 MHz, CDCl ) d ppm
198.1, 137.1, 133.1, 128.6, 128.3, 26.6.
3
m
4.3.6. 4-Fluoroacetophenone. TLC (hexane/ethyl acetate¼1:0.5)
sheets 20ꢁ20 cm). NMR spectra were recorded in CDCl
3
on a Bruker
gave 4-fluoroacetophenone as a colorless liquid; yield: 24 mg (89%).
1
AV 400 MHz.
3
H NMR (400 MHz, CDCl ) d ppm 7.95e7.91 (m, 2H), 7.09e7.05 (m,
13
3
2H), 7.40e736 (m, 2H), 2.53 (s, 3H); C NMR (400 MHz, CDCl )
4
.2. Typical procedure for alkene oxidation
d
ppm 196.6, 167.2, 164.7, 133.8, 131.2, 131.1, 115.9, 115.7, 26.7.
A stock solution of CuCl
pared. To a Pyrex tube with a Teflon screw cap and side arm inlet,
catalyst A (100 L of the stock solution, 0.01 mmol of CuCl ), or B
100 L of a stock solution, 0.01 mmol of CuCl , 2.0 mg, 0.01 mmol
of phenanthroline), or C (6.3 mg, 0.01 mmolþ100 L water) was
added. Then 900 L of THF and 0.2 mmol of alkene were added in
2
$2H
2
O in water (0.0171 g/cc) was pre-
4.3.7. 4-Chloroacetophenone. TLC (hexane/ethyl acetate¼1:0.5)
gave 4-chloroacetophenone as a colorless liquid; yield: 28 mg
1
m
2
(90%). H NMR (400 MHz, CDCl
3
)
d
ppm 7.85e7.83 (m, 2H),
13
(
m
2
3
7.38e7.26 (m, 2H), 2.54 (s, 3H); C NMR (400 MHz, CDCl ) d ppm
m
196.9, 139.7, 135.6, 129.9, 129.0, 26.7.
m
each case. The tube was filled with oxygen (4 atm) and heated to
Acknowledgements
ꢀ
6
0 C with stirring till to its reaction time specified in Table 2. The
reaction mixture was cooled and diluted with ethyl acetate. In-
ternal standard 1,4-di-tert-butylbenzene (11.9 mg, 0.06 mmol) was
then added, and the products were analyzed by GC. Similar pro-
cedure was followed for each alkene and reactions of alkene and
epoxide with TEMPO (46.8 mg, 0.3 mmol).
We are grateful to the National Science Council, Republic of
China, and Academia Sinica for financial support of this work.
Supplementary data
Supplementary data ( H NMR, 13C, and important GC spectra)
related to this article can be found at http://dx.doi.org/10.1016/
j.tet.2013.11.075.
1
4
.3. Typical oxidation of 1,1-diphenylethylene by 2-hydrope
roxytetrahydrofuran (2)
To a Pyrex tube with a Teflon screw cap and side arm inlet, THF
(
2
900 mL), H O (100 mL), 1,1-diphenylethylene (0.2 mmol, 35.7 mL),
References and notes
and 2 (1.0 mmol, 104.0 mg) were added. The tube was filled with
oxygen (4 atm) and heated to 60 C with stirring for 5 h. The re-
ꢀ
1. (a) Hudlic, M. Oxidations in Organic Chemistry; American Chemical society:
Washington, DC, 1990; (b) Larock, R. C. Comprehensive Organic Transformations;
Wiley: New York, NY, 1999.
action mixture was cooled and diluted with ethyl acetate. Internal
standard 1,4-di-tert-butylbenzene (11.9 mg, 0.06 mmol) was then
added, and the products were analyzed by GC. Similar procedure
was followed for the reaction in toluene (900
with catalyst A (100 L of the stock solution, 0.01 mmol of CuCl
and 900 L of solvent THF or toluene were used for each case under
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