Copper-catalyzed aerobic oxidative cleavage of C-C bonds in epoxides leading to aryl ketones

Article abstract of DOI:10.1021/jo5013898

A novel copper-catalyzed aerobic synthesis of ketones from epoxides via cleavage of C-C single bonds has been discovered. This reaction constitutes a new transformation from epoxides into ketones.

Full text of DOI:10.1021/jo5013898

Note
pubs.acs.org/joc
Copper-Catalyzed Aerobic Oxidative Cleavage of C−C Bonds in
Epoxides Leading to Aryl Ketones
Lijun Gu,*,† Cheng Jin,‡ Hongtao Zhang,† and Lizhu Zhang†
†Key Laboratory of Chemistry in Ethnic Medicinal Resources, State Ethnic Affairs Commission & Ministry of Education, Yunnan
Minzu University, Kunming 650500, China
‡New United Group Company Limited, Changzhou, Jiangsu 213166, China
S
* Supporting Information
ABSTRACT: A novel copper-catalyzed aerobic synthesis of ketones from epoxides via
cleavage of C−C single bonds has been discovered. This reaction constitutes a new
transformation from epoxides into ketones.
exploration for new types of epoxide transformations is very
attractive to researchers. The classical elaborations of epoxides
are the ring-opening of epoxides with nucleophilic reagents
(Scheme 1, eq (1)).7 Recently, Zhang and co-workers reported
C−C bonds widely exist in natural and synthetic chemicals.
Undoubtedly, C−C bond activation/functionalization offers
unique opportunities to develop novel transformations, because
it allows reorganization of bond connections, leading to novel
molecular structures with high complexity.1 Although signifi-
cant progress has been achieved in the development of methods
to cleave C−C single bonds,2 the selective oxidative cleavage of
C−C single bonds still remains one of the most challenging
issues in chemistry and biology because (1) in general, the
bond dissociation energy of a C−C single bond is high, and
therefore, such bonds are thermodynamically hard to break;
and (2) the selectivity between C−C bond and C−H bond
cleavage of unstrained substrates should be controlled.
Chemists have always been pursuing new methods to solve
these challenges. For instance, the ring-opening of strained
molecules, such as three- and four-membered rings, has been
well-studied.3 Other than these studies, the cleavage of the
carbon−nitrile (C−CN) bond of nitriles and strategies that
make use of chelation assistance have been well-investigated.4,5
To cleave unstrained inert C−C bonds, harsh conditions with
stoichiometric oxidants have been required. Therefore, the
chemoselective C−C single bond cleavage, which could be used
to perform synthetic chemistry in a greener and more atom-
economical way, is highly appealing.
Scheme 1. Direct Transformations of Epoxides
a beautiful transformation from a 2,2,3-trisubstituted epoxide
into a 1,3-dioxolane via Lewis acid catalyzed C−C bond
cleavage (Scheme 1, eq (2)).8 In 2013, the group of Beller
made the remarkable observation that anilines react with 2,3-
disubstituted epoxides in the presence of Ru3(CO)12 as catalyst
to form indoles (Scheme 1, eq (3)).9 As part of our ongoing
efforts to develop transition-metal-catalyzed organic reactions,10
we herein report a novel copper-catalyzed aerobic synthesis of
ketones from 2,3-disubstituted epoxides (Scheme 1, eq (4)).11
This reaction constitutes a new transformation from 2,3-
Saturated carbonyl compounds, for example, ketones,
constitute the essential synthetic elements in organic chemistry,
which can be transformed into a large variety of functionalized
organic molecules with applications for several fields, such as
pharmaceutical chemistry and material science.6 Their
importance in synthetic and medicinal chemistry has attracted
considerable attention for the development of new synthetic
strategies for these compounds.
An epoxide is a cyclic ether with three ring atoms.
Industrially, epoxides are utilized as pharmaceutical drugs
polymer precursors, and as biologically and chemically
important building blocks.7 Because of the significance and
wide applications of such chemistry in organic synthesis, the
Received: June 22, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/jo5013898 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
a b
,
disubstituted epoxides into ketones. This protocol also provides
a practical, neutral, and mild synthetic approach to aryl ketones.
In the initial phase of this study, we investigated the reaction
of 2,3-diphenyloxirane 1a with aniline in the presence of K2CO3
(1 equiv) and CuCl2 (15 mol %) at 120 °C in dimethyl
sulfoxide (DMSO) under an O2 atmosphere. To our delight,
the desired product was isolated with 53% yield after reacting
for 15 h (Table 1, entry 1). Screening of solvents indicated that
Table 2. Scope of Epoxides
a
Table 1. Screening Optimal Conditions
b
entry
catalyst
CuCl2
solvent
yield (%)
1
2
DMSO
DCE
53
trace
trace
22
74
13
31
18
26
0
CuCl2
3
CuCl2
PhCF3
DMF
4
CuCl2
5
Cu(OAc)2
CuCl
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
a
6
Reaction conditions: 1 (0.5 mmol), catalyst (15 mol %), K2CO3 (1
equiv), aniline (0.6 mmol), DMSO (3 mL), 120 °C in O2 atmosphere
7
Cu(OTf)2
CuBr2
b
8
for 16−18 h. Isolated yield.
9
Pd(OAc)2
NiCl2
10
11
substituted aryl epoxide, a satisfactory yield was still achieved
under the same reaction conditions (Table 2, entry 2i).
Interestingly, the introduction of heterocycles into this system
made this methodology more useful for the preparation of
pharmaceuticals and functional materials (Table 2, entries 2j
and 2k). Efforts were made to apply this methodology for the
synthesis of aryl alkyl ketones. It was found that reactions with
1l and 1m proceeded sluggishly to give the aryl alkyl ketones 2l
and 2m in low yields (Table 2, entries 2l and 2m).
Unfortunately, 2,3-dialkyl-substituted substrate 1n did not
deliver the desired product (Table 2, entry 2n).
none
0
c
12
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
0
d
13
74
72
0
e
14
f
15
g
16
0
a
Reaction conditions: 1a (0.5 mmol), catalyst (15 mol %), K2CO3 (1
equiv), aniline (0.6 mmol), solvent (3 mL), 120 °C in O2 atmosphere
b
c
d
for 16 h. Isolated yield. Ar (1 atm). p-Toluidine (0.6 mmol) was
used. p-Chloroaniline (0.6 mmol) was used. n-Butylamine (0.6
mmol) was used. Without aniline.
e
f
g
Some control experiments were carried out in order to probe
the mechanism of this transformation. It has been reported that
an epoxide can be transformed into an aldehyde. Then, the
aldehyde undergoes the C−C cleavage to form a ketone in the
presence of aniline.12 To rule out this possibility, we checked
the possibilities of the formation of aldehydes by the reactions
of 2,3-diphenyloxirane 1a (0.5 mmol), in the presence of
K2CO3 (0.5 mmol) and Cu(OAc)2 (15 mol %) at 120 °C in
DMSO under an O2 atmosphere [eq (1)] (Scheme 2). No 2,2-
diphenylacetaldehyde 3 was found, which provided evidence
that the reaction does not proceed via an aldehyde
intermediate. Furthermore, in the presence of K2CO3 (0.5
mmol) and Cu(OAc)2 (15 mol %) at 120 °C in DMSO under
an O2 atmosphere, 1,2-diphenyl-2-(phenylamino)ethanol A
could give the desired product 2a in 87% yield after 15 h [eq
DMSO was optimal (Table 1, entries 1−4). DMSO is likely to
stabilize the copper catalyst and also assist in the aerobic
oxidation process. It was found that Cu(OAc)2 was superior to
other copper sources (Table 1, entries 5−8). Reactions
catalyzed by other transition metals, such as Pd(OAc)2 and
NiCl2, did not proceed or gave a poor yield (Table 1, entries 9
and 10). Both Cu(OAc)2 and O2 are essential for reactivity
(Table 1, entries 11 and 12). Replacement of aniline with p-
toluidine or p-chloroaniline did not affect the reaction efficiency
(Table 1, entries 13 and 14). However, n-butylamine did not
afford the desired product (Table 1, entry 15). Notably, the
absence of aniline resulted in no detectable amounts of
benzophenone 2a (Table 1, entry 16).
With the standard reaction conditions in hand, we next
investigated the substrate scope of the reaction by employing a
variety of epoxides 1. As summarized in Table 2, the standard
reaction conditions were found to be compatible with a wide
range of epoxides 1, including aryl and heteroaryl epoxides.
Furthermore, several substituents, such as Me, MeO, F, Cl, and
Br, on the aryl ring of epoxides were well-tolerated (Table 2,
entries 2b−2f). It is noteworthy that substituents at different
positions on the aryl ring (para, meta, and ortho positions) did
not affect the reaction efficiency (Table 2, entries 2b, 2g, 2h).
Importantly, the halogens F, Cl, and Br were tolerated under
the reaction conditions, thereby facilitating additional mod-
ifications at the halogenated positions. When using a dimethyl-
Scheme 2. Control Experiment
B
dx.doi.org/10.1021/jo5013898 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
Benzophenone (2a):14 Yield: 74%, 67 mg; H NMR (400 MHz,
CDCl3) δ: 7.83 (dd, J = 8.0 Hz, J = 1.6 Hz, 4H), 7.61−7,56 (m, 2H),
7.51−7.45 (m, 4H); 13C NMR (100 MHz, CDCl3) δ: 196.7, 137.8,
132.5, 130.2, 128.4; IR (neat cm−1): 1660 (CO); LRMS (EI 70 ev)
m/z (%): 182 (M+, 100); HRMS m/z (ESI) calcd for C13H11O (M +
H)+ 183.0804, found 183.0801.
1
(2)]. The results indicate that compound A may be involved in
this process.
On the basis of these preliminary results and previous
studies,2k,13 the catalytic cycle of this transformation was
hypothesized as shown in Scheme 3. Ring opening of 2,3-
Phenyl(p-tolyl)methanone (2b):14 Yield: 71%, 70 mg; H NMR
1
Scheme 3. Possible Mechanism
(400 MHz, CDCl3) δ: 7.79 (d, J = 7.2 Hz, 2H), 7.73 (d, J = 8.0 Hz,
2H), 7.59 (t, J = 7.4 Hz, 1H), 7.48 (t, J = 7.6 Hz, 2H), 7.28 (d, J = 8.0
Hz, 2H), 2.43 (s, 3H); 13C NMR (100 MHz, CDCl3) δ: 196.4, 143.2,
137.9, 134.8, 132.1, 130.2, 129.8, 128.9, 128.1, 21.6; IR (neat cm−1):
1658 (CO); LRMS (EI 70 ev) m/z (%): 196 (M+, 100); HRMS m/
z (ESI) calcd for C14H13O (M + H)+ 197.0960, found 197.0963.
(4-Methoxyphenyl)(phenyl)methanone (2c):14 Yield: 64%, 68 mg;
1H NMR (400 MHz, CDCl3) δ: 7.80 (d, J = 8.4 Hz, 2H), 7.73 (d, J =
8.0 Hz, 2H), 7.51−7.45 (m, 3H), 6.96 (d, J = 8.4 Hz, 2H), 3.91 (s,
3H); 13C NMR (100 MHz, CDCl3) δ: 197.1, 163.2, 138.2, 132.4,
131.7, 130.0, 129.5, 128.2, 113.6, 55.8; IR (neat cm−1): 1652 (CO);
LRMS (EI 70 ev) m/z (%): 212 (M+, 100); HRMS m/z (ESI) calcd
for C14H13O2 (M + H)+ 213.0909, found 213.0913.
(4-Florophenyl)(phenyl)methanone (2d):14 Yield: 70%, 70 mg; 1H
NMR (400 MHz, CDCl3) δ: 7.86−7.83 (m, 2H), 7.78 (d, J = 4.2 Hz,
2H), 7.62 (dd, J = 7.2 Hz, J = 1.2 Hz, 1H), 7.51 (t, J = 7.6 Hz, 2H),
7.18 (t, J = 8.6 Hz, 2H); 13C NMR (100 MHz, CDCl3) δ: 195.5, 165.5
(JC−F = 252.9 Hz), 137.6, 132.7 (JC−F = 2.5 Hz), 132.6 (JC−F = 14.8
Hz), 132.0, 129.8, 128.3, 115.5 (JC−F = 21.8 Hz); IR (neat cm−1): 1661
(CO); LRMS (EI 70 ev) m/z (%): 200 (M+, 100); HRMS m/z
(ESI) calcd for C13H10FO (M + H)+ 201.0710, found 201.0719.
(4-Chlorophenyl)(phenyl)methanone (2e):14 Yield: 61%, 66 mg;
1H NMR (400 MHz, CDCl3) δ: 7.78 (t, J = 7.2 Hz, 4H), 7.62 (t, J =
7.4 Hz, 1H), 7.50 (dd, J = 7.6 Hz, J = 8.4 Hz, 4H); 13C NMR (100
MHz, CDCl3) δ: 195.4, 138.8, 137.2, 135.8, 132.6, 131.4, 129.9, 128.6,
128.3; IR (neat cm−1): 1664 (CO); LRMS (EI 70 ev) m/z (%): 218
(41), 216 (M+, 100); HRMS m/z (ESI) calcd for C13H10ClO (M +
H)+ 217.0415, found 217.0410.
diphenyloxirane 1a with aniline gives intermediate A. There-
after, intermediate A is oxidized to produce intermediate B
and/or C. With the aid of Cu and O2, intermediate B and/or C
is readily transformed into α-imine ketone D, which quickly
converts into hydrated species E by picking up one molecule of
water. Following a benzylic acid rearrangement, F affords
intermediate G, which decomposes to the desired product
benzophenone 2a.
In summary, the chemoselective oxidative cleavage of the C−C
single bond of 2,3-disubstituted epoxides that yields ketones
has been described for the first time. The application of
selective C−C bond cleavage in organic synthesis presents one
of the most attractive and challenging projects. This chemistry
provides a new means of C−C bond cleavage. A wide range of
2,3-disubstituted epoxides can be subjected to this copper-
catalyzed method under an oxygen atmosphere; the oxidation
terminates at the ketone stage. Mechanistic, scope, and
limitation studies of the reaction are in progress in our
laboratory.
(4-Bromophenyl)(phenyl)methanone (2f):14 Yield: 57%, 74 mg;
1H NMR (400 MHz, CDCl3) δ: 7.78 (t, J = 4.2 Hz, 2H), 7.69 (dd, J =
2.0 Hz, J = 2.0 Hz, 2H), 7.64−7.58 (m, 3H), 7.51 (t, J = 7.6 Hz, 2H);
13C NMR (100 MHz, CDCl3) δ: 195.6, 137.1, 136.3, 132.6, 131.6,
131.5, 129.9, 128.4, 127.5; IR (neat cm−1): 1659 (CO); LRMS (EI
70 ev) m/z (%): 260 (M+, 100), 258 (81); HRMS m/z (ESI) calcd for
C13H10BrO (M + H)+ 260.9909, found 260.9913.
Phenyl(o-tolyl)methanone (2g):14 Yield: 66%, 65 mg; H NMR
1
(400 MHz, CDCl3) δ: 7.74 (d, J = 7.2 Hz, 2H), 7.54−7.50 (m, 1H),
7.43−7.36 (m, 2H), 7.33−7.26 (m, 1H), 7.25−7.20 (m, 3H); 2.33 (s,
3H); 13C NMR (100 MHz, CDCl3) δ: 198.5, 138.8, 138.1, 137.0,
133.5, 131.7, 130.6, 130.3, 129.0, 128.8, 125.4, 20.4; IR (neat cm−1):
1647 (CO); LRMS (EI 70 ev) m/z (%): 196 (M+, 100); HRMS m/
z (ESI) calcd for C14H13O (M + H)+ 197.0960, found 197.0961.
EXPERIMENTAL SECTION
Instrumentation and Chemicals. Reagents were obtained
commercially and used as received. Solvents were purified and dried
■
Phenyl(m-tolyl)methanone (2h):15 Yield: 74%, 72 mg; H NMR
1
1
(400 MHz, CDCl3) δ: 7.81 (dd, J = 1.2 Hz, J = 8.4 Hz, 2H), 7.62−7.57
(m, 3H), 7.46−7.40 (m, 2H), 7.38 (dd, J = 4.4 Hz, J = 4.4 Hz, 2H),
2.41 (s, 3H); 13C NMR (100 MHz, CDCl3) δ: 196.8, 138.1, 137.4,
137.1, 133.0, 132.1, 130.6, 130.1, 128.4, 128.0, 127.2, 21.3; IR (neat
cm−1): 1663 (CO); LRMS (EI 70 ev) m/z (%): 196 (M+, 100);
HRMS m/z (ESI) calcd for C14H13O (M + H)+ 197.0960, found
197.0954.
by standard methods. All products were characterized by IR, MS, H
NMR, 13C NMR, and high-resolution mass spectrometry (HRMS). 1H
NMR spectra were recorded on 400 MHz in CDCl3, and 13C NMR
spectra were recorded on 100 MHz in CDCl3 using TMS as an
internal standard. Chemical shift values (δ) are given in parts per
million (ppm). Coupling constants (J) were measured in Hz. HRMS
spectra were recorded on a micrOTOF-Q II analyzer.
(3,4-Dimethylphenyl)(phenyl)methanone (2i):16 Yield: 65%, 68
mg; 1H NMR (400 MHz, CDCl3) δ: 7.79 (t, J = 4.2 Hz, 2H), 7.61 (s,
1H), 7.59−7.52 (m, 2H), 7.49 (t, J = 7.6 Hz, 2H), 7.23 (d, J = 7.6 Hz,
1H), 2.35 (s, 3H), 2.32 (s, 3H); 13C NMR (100 MHz, CDCl3) δ:
198.3, 141.9, 138.0, 136.7, 135.3, 132.0, 131.1, 129.9, 129.4, 128.1,
128.0, 20.0, 19.7; IR (neat cm−1): 1661 (CO); LRMS (EI 70 ev) m/
z (%): 210 (M+, 100); HRMS m/z (ESI) calcd for C15H15O (M + H)+
211.1116, found 211.1111.
Typical Experimental Procedure for Synthesis of Ketones 2.
An oven-dried Schlenk tube was charged with a magnetic stir bar, 2,3-
disubstituted epoxides 1 (0.5 mmol), aniline (0.6 mmol), K2CO3 (0.5
mmol), Cu(OAc)2 (0.075 mmol), and DMSO (3 mL). The tube was
sealed, and oxygen was purged through a syringe. The reaction was
stirred at 120 °C for 16−18 h. After the reaction was finished, the
reaction mixture was diluted in 30 mL of ethyl acetate and filtered on a
Celite pad. The organic portion was washed with a saturated solution
of brine (8 mL), saturated NH4Cl (8 mL), and a saturated solution of
brine (8 mL), dried (Na2SO4), and concentrated in vacuum, and the
resulting residue was purified by silica gel column chromatography
(hexane/ethyl acetate) to afford the desired products 2.
Phenyl(thiophen-2-yl)methanone (2j):17 Yield: 58%, 54 mg; H
1
NMR (400 MHz, CDCl3) δ: 7.87 (d, J = 7.6 Hz, 2H), 7.73 (d, J = 4.8
Hz, 1H), 7.65 (d, J = 3.6 Hz, 1H), 7.61 (t, J = 7.4 Hz, 1H), 7.51 (t, J =
7.8 Hz, 2H), 7.17 (t, J = 4.4 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ:
C
dx.doi.org/10.1021/jo5013898 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
188.2, 143.6, 138.1, 134.8, 134.1, 132.2, 129.1, 128.3, 127.9; IR (neat
cm−1): 1638 (CO); LRMS (EI 70 ev) m/z (%): 188 (M+, 100);
HRMS m/z (ESI) calcd for C11H9OS (M + H)+ 189.0368, found
189.0361.
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Dithiophen-2-ylmethanone (2k):2k Yield: 54%, 52 mg; H NMR
1
(400 MHz, CDCl3) δ: 8.08 (dd, J = 4.0 Hz, J = 1.2 Hz, 2H), 7.86 (dd,
J = 4.8 Hz, J = 1.2 Hz, 2H), 7.22−7.17 (m, 2H); 13C NMR (100 MHz,
CDCl3) δ: 182.8, 138.5, 137.3, 137.0, 128.6; IR (neat cm−1): 1631
(CO); LRMS (EI 70 ev) m/z (%): 194 (M+, 100); HRMS m/z
(ESI) calcd for C9H7OS2 (M + H)+ 194.9932, found 194.9936.
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Propiophenone (2l):14 Yield: 31%, 21 mg; H NMR (400 MHz,
1
CDCl3) δ: 7.81 (dd, J = 1.6 Hz, J = 0.8 Hz, 2H), 7.50 (dd, J = 6.4 Hz, J
= 1.6 Hz, 1H), 7.40−7.35 (m, 2H); 2.92−2.89 (m, 2H), 1.19−1.14
(m, 3H); 13C NMR (100 MHz, CDCl3) δ: 200.0, 136.4, 132.3, 128.0,
127.4, 31.2, 7.6; IR (neat cm−1): 1681 (CO); LRMS (EI 70 ev) m/z
(%): 134 (M+, 100); HRMS m/z (ESI) calcd for C9H11O (M + H)+
135.0805, found 135.0813.
2-Methyl-1-phenylpropan-1-one (2m):14 Yield: 33%, 24 mg; H
1
NMR (400 MHz, CDCl3) δ: 7.96−7.92 (m, 2H), 7.58−7.52 (m, 1H),
7.48−7.42 (m, 2H); 3.69−3.54 (m, 1H), 1.26 (d, J = 6.8 Hz, 6H); 13C
NMR (100 MHz, CDCl3) δ: 204.1, 136.0, 132.4, 128.6, 128.1, 35.0,
18.9; IR (neat cm−1): 1677 (CO); LRMS (EI 70 ev) m/z (%): 148
(M+, 100); HRMS m/z (ESI) calcd for C10H13O (M + H)+ 149.0961,
found 149.0967.
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ASSOCIATED CONTENT
■
S
* Supporting Information
1
Copies of H and 13C NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: gulijun2005@126.com (L.G.).
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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We are grateful for the financial support from The Educational
Bureau of Yunnan Province (2010Y431) and the State Ethnic
Affairs Commission (12YNZ05).
REFERENCES
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