Copper catalyzed C-O bond formation via oxidative cross-coupling reaction of aldehydes and ethers

Article abstract of DOI:10.1039/c4ob00862f

A practical and efficient construction of C-O bonds via oxidative cross-coupling reaction of aldehydes and ethers has been realized under open air. When 2 mol% copper was used as the catalyst, various α-acyloxy ethers were obtained with up to 93% isolated

Full text of DOI:10.1039/c4ob00862f

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Copper catalyzed CꢀO bond formation via oxidative
crossꢀcoupling reaction of aldehydes and ethers
Cite this: DOI: 10.1039/x0xx00000x
Quan Wang,a Hao Zheng,a Wen Chai,a Dianyu Chen,a Xiaojun Zeng,a Renzhong
Fu,*a and Rongxin Yuan*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
HO
A practical and efficient construction of CꢀO bonds via
oxidative crossꢀcoupling reaction of aldehydes and ethers
has been realized under open air. When 2 mol % copper
was used as the catalyst, various αꢀacyloxy ethers were
obtained with up to 93% isolated yield.
O
HO
O
H
HO
O
O
O
O
O
O
O
HO
O
O
H
HO
HO
H
O
OH
HO
O
O
OH
O
HO
OH
(a) Artemisinin5a (b) Heterocyclic cage compound5b
(c) Sanguiin Hꢀ55c
Scheme 1 Selected examples of αꢀacyloxy ethers.
In the past decades, the direct activation and cleavage of C–H bonds
has been a very intense research field in synthetic organic
R3
R2
O
chemistry.1 Traditional formation of CꢀC bonds usually needs the
prefunctionalization of the reactants. By comparison, the direct
activation of CꢀH bonds and subsequent coupling reaction avoids
this laborious process and can be a more atomꢀeconomical and
potent strategy.2 Most of the researches reported have concentrated
on the activation of CꢀH bond adjacent to an amine nitrogen atom.3
However, only relatively few studies involved the activation of CꢀH
bond adjacent to an ethereal oxygen atom.4
O
esterification
(a)
+
R1
Y
HO
O
(Y=OCOR4, OH, Cl, Br)
R3
R2
R3
R2
nucleophilic
substitution
O
O
O
traditional method
+
R1
O
R1
OH
X
(X=Cl, Br)
O
O
addition
+
R3
R1
OH
OH
H
R2
O
αꢀAcyloxy ethers are ubiquitous structural unit in many natural
products and functional molecules, such as antimalarial drug
artemisinin (a), heterocyclic cage compounds (b) and the
R3
R3
O
O
O
TBAI catalyst
new method
this work
+
(b)
(c)
R1
R1
O
R2
R3
H
R2
R3
ellagitannin natural product sanguiin Hꢀ5 (c) (Scheme 1).5 The CꢀO
bonds in these important building blocks were usually produced by
the methods of the esterification of a hemiacetal,6 the nucleophilic
substitution of a carboxylic acid and an αꢀhalo substituted ether,7a, 7b
or the addition of a carboxylic acid to an alkenyl ether.7c Besides the
traditional methods (Scheme 2a), very few effective synthesis
method for it has be reported.8 Recently, wan and coꢀworkers
reported a Bu4NI catalyzed oxidative esterification reaction between
acids and ethers for the construction of various αꢀacyloxy ethers with
Bu4NI (20 mol %) as the catalyst (Scheme 2b).8a Although metalꢀ
free catalysts have many advantages such as higher activity and
excellent selectivity, the search for more effective and inexpensive
catalyst remains a considerable challenging task.
O
O
O
O
copper catalyst
+
R1
R1
O
R2
H
R2
Scheme 2 Different pathways for the synthesis of αꢀacyloxy ethers
As an inexpensive and easily obtainable catalyst, copper has
been increasingly applied in many oxidative reactons and
showed high catalytic activity.9 In 2004, Li and coꢀworker
reported an efficient CuBr catalyzed CDC reaction of tertiary
aliphatic amines and terminal alkynes for the formation of CꢀC
bonds.3a Recently, Nicholas and coꢀworker developed
a
Cu(OAc)2 catalyzed amidation of 2ꢀphenylpyridine for the
formation of C–N bonds with molecular oxygen as the
oxidant.10 However, there is no precedent of effective CꢀO
bond formation from aldehydes and ethers catalyzed by metal
catalyst has been reported to date. We herein present an
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DOI: 10.1039/C4OB00862F
efficient copper catalyzed CꢀO bond formation for the synthesis
After screening different solvents and temperature we got the
of αꢀacyloxy ethers (Scheme 2c). The reactions proceeded optimal reaction conditions (entry 3, Table 1). Then a variety of
through double CꢀH activations of ethers and aldehydes, and aldehydes and ethers were subjected to the optimal conditions
subsequent crossꢀcoupling reactions.
to explore the applicability of this protocol. Gratifyingly, 1,4ꢀ
We began our investigation by the reaction of benzaldehyde dioxane smoothly underwent reactions with most of aldehydes
(1 equiv) with 1,4ꢀdioxane (20 equiv) catalyzed by to give the corresponding αꢀacyloxy ethers in satisfactory yields
Cu(OAc)2.2H2O (20 mol %) and used aqueous TBHP (4 equiv) (63ꢀ93%, 1a
ꢀ
15a
,
Scheme 3). Among them, 4ꢀ
as the oxidant. This approach gave the expected product (1a), methylbenzaldehyde gave the best yield (93%, 1b, Scheme 3).
but the yield was very poor (18%, entry 1, Table 1). At the The aldehydes with electronꢀdonating substituents provided
same time a great deal of benzoic acid as the byproduct was higher yields of the αꢀacyloxy ethers (2a
also detected. When the 1,4ꢀdioxane quantity was increased to Scheme 3) than those with electronꢀwithdrawing substituents
40 equiv, a complete disappearance of benzaldehyde and the 5a 6a 7a and 13a, Scheme 3). We suspected that electronꢀ
, 8a, 9a and 10a,
(
,
,
byproduct benzoic acid was observed with an isolated 92% donating substituents in the aryl ring enhanced the stability of
yield (entry 2). In addition we found that Cu(OAc)2.H2O was the radical intermediates and easily led to homolytic cleavage
not completely dissolved in the reaction solvent, and perhaps, a reaction than electronꢀwithdrawing substituents. The sterically
smaller amount of catalyst worked with the same result. When hindered effect of the substituents in benzaldehydes hardly
we decreased catalyst amount to 2 mol %, the solution of affected the product yields. No matter what substitution
reactants became transparently blue and up to 91% yield was positions, orthoꢀ, metaꢀ, and paraꢀmethoxy substituted
obtained in the end (entry 3). When the experiments were benzaldehydes reacted with 1,4ꢀdioxane to give the similar
carried out with either a copper catalyst or TBHP alone, no good results (8a
desired product could be detected (entry 12 and 13). Other were also examined and moderate yields of 63ꢀ70% were
divalent copper(II) salts such as CuBr2, CuCl2, CuSO4, and obtained (15a 16a and 17a, Scheme 3). In addition, this
Cu(ClO4)2 were also tried, but led to relatively low yields of methodology was also applicable to the reactions of
1a) (34ꢀ65%, entry 5ꢀ8). In particular, no desired product was heteroarenes, such as furan and thiophene with 78ꢀ80% yields
, 9a and 10a, Scheme 3). Aliphatic aldehydes
,
(
obtained when copper(I) salts such as CuBr, CuCl was used as
catalyst (entry 9 and 10). This result indicated that the use of
(
11a
, 12a, Scheme 3).
To further determine the scope of application, other specific
Cu(II) salts seems to be critical for efficient transformation. ethers were also explored. Most ethers underwent coupling
When H2O2 was used in place of TBHP as the oxidant, no reactions easily to offer the desired products (1b 10e, Scheme 3)
expected product was detected (entry 11). If the aqueous TBHP in 58ꢀ89% yields. However, tetrahydrofuran (1c 2c 10c
ꢀ
,
,
,
quantity was further decreased, the product yield declined Scheme 3) showed slightly lower activity than other ethers,
dramatically. All the reactions were conducted under open air. which may be attributed to the instability of fiveꢀmembered
Contrary to most traditional transitionꢀmetalꢀcatalyzed ring radical intermediate. Notably, when acyclic ether, such as
reactions, the approach was unaffected by the presence of 1ꢀbutoxybutane(
moisture.
d
)
or 1ꢀ(2ꢀchloroethoxy)ꢀ2ꢀchloroethane(
was used as a substrate, orthoꢀmethoxybenzaldehyde showed
quite negligible activity compared to meta or para
methoxybenzalde under the same reaction conditions. The
phenomenon can be ascribed to the larger steric effect of ortho
e)
ꢀ
Table 1 Optimization of the reaction conditionsa
ꢀ
O
O
H
O
H
O
methoxy group on benzaldehyde when acyclic ethers were used
as coupling partners.
catalyst, oxidant
80) , 24h
O
+
O
O
(1)
(a)
(1a)
R3
R3
R2
Yield (%)e
O
Cu(OAc)2.H2O(2 mol %)
O
O
O
Entry
Catalyst
Oxidant
+
1b
2c
3
4
5
6
7
8
9
10
11
12
13
14d
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2.H2O
Cu(OAc)2
CuCl2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
18
92
91
90
36
34
51
65
nd
nd
nd
nd
nd
90
R1
H
R1
O
R2
t-BuOOH, 80) , 24h
H
(1-15)
(a-e)
(1a-10e)
O
H
H
O
H
O
O
O
c :
e :
H
Cl
b :
d :
a :
H
Cl
O
CuBr2
CuSO4
O
O
O
O
O
O
O
O
O
O
O
O
(1a, 91%)
Cu(ClO4)2
CuBr
(3a, 81%)
(2a, 93% )
CuCl
O
O
O
O
O
O
O
O
Cu(OAc)2.H2O
_
Cu(OAc)2.H2O
Cu(OAc)2.H2O
O
O
O
O
TBHP
_
TBHP
(4a, 85% )
(6a, 71% )
(5a, 65%)
Cl
F
O
O
OMe
O
O
O
O
O
MeO
O
O
O
O
O
O
a Reaction conditions: benzaldehyde (1 mmol), 1,4ꢀdioxane (40
mmol), catalyst (2 mol %), and TBHP (4 equiv, 70% aqueous
(7a, 74% )
(9a, 85% )
(8a, 89% )
Br
O
for 24 h unless otherwise noted. b Catalyst (20
O
O
O
O
O
solution) at 80
℃
O
O
c
S
mol %), 1,4ꢀdioxane (20 mmol). Catalyst (20 mol %), 1,4ꢀ
dioxane (40 mmol). 100 mmol of benzaldehyde was used.
O
O
O
O
(11a, 80%)
d
e
(12a, 78%)
(10a, 91% )
MeO
Yield of isolated product.
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4OB00862F
radical (Scheme 5b). Moreover, deuterated 1,4ꢀdioxane was
employed to test the kinetic isotopic effect (KIE) of the
experiment under the optimized conditions (Scheme 5c). A
significant kH/kD of 9.0 was detected, implying that the rate
determining step of this conversion should be the cleavage of
CꢀH bond adjacent to an ethereal oxygen atom.
O
O
O
O
O
O
O
O
O
O
O
O
O
(15a, 66% )
(14a, 71%)
(13a, 63%)
O2N
O
O
O
O
O
O
O
(16a, 63%)
(17a, 70%)
O
O
O
O
O
H
O
O
O
Cu (OAc)2.H2O (2 mol %)
O
O
O
O
O
+
Ph
O
O
(a)
O
Ph
O
Ph
O
TBHP (4 equiv), 80)
O
O
O
(1a, 33% )
(3b, 75%)
(2b, 88%)
O
(1b, 89%)
H
O
Cu (OAc)2.H2O (2 mol %)
O
O
O
O
O
+
O
(b)
(c)
+
+
O
Ph
O
O
O
Ph
O
Ph
Ph
H
O
O
TBHP (4 equiv), 80)
O
MeO
trace
(1a, 91% )
O
O
O
O
Cu (OAc)2.H2O (2 mol %)
TBHP (4 equiv), 80) ,24h
(2c, 61%)
(9b, 78%)
(1c, 58%)
1a
+
[D]1a
+
H
O
O
O
O
O
O
O
O
ratio:9/1
[D8]1,4-dioxane
O
O
O
(2d, 75%)
Scheme 5 Investigation into the reaction mechanism.
(10c, 63%)
(1d, 78%)
MeO
O
O
Cl
In consideration of these control experiments, we proposed a
plausible reaction pathway as shown in Scheme 6. Initially,
O
O
O
O
O
Cl
O
O
(9d, 81%)
both tertꢀbutoxyl radical
generated in the catalytic cycle by the aid of copper (Scheme
6a).12 Then
extracts H from the CꢀH bond adjacent to an
oxygen atom to produce the intermediate (Scheme 6b). On
the aldehyde, traps H to form the acyl radical
bonds with to afford the tertꢀbutyl perester
decomposes to the acyloxy radical and tert
(Scheme 6d). Finally the coupling of
A and tertꢀbutylperoxy radical B were
(10d, 77%)
(1e,73% )
MeO
Cl
OMe
A
Cl
Cl
O
O
O
O
O
O
C
Cl
Cl
MeO
Cl
O
O
O
A
D.
(2e, 69%)
(10e, 79%)
(9e, 78%)
Subsequently
(Scheme 6c).
D
E
B
E
ꢀ
MeO
F
Scheme 3 Substrate scope for αꢀacyloxy ethers. Reaction
conditions: aldehyde (1 mmol), ether (40 mmol),
Cu(OAc)2.H2O (2 mol %), TBHP (4 equiv, 70% aqueous
butoxyl radical
A
F
and
C
affords the expected product (Scheme 6e). A similar radical
pathway can be applied to explain the reaction of carboxylic
solution), 80
℃, 24 h.
acid with 1,4ꢀdioxane (Scheme 4b). The acyloxy radical
F
was
formed by the reaction of tertꢀbutoxyl radical
acid (Scheme 6f).
A
and benzoic
To further understand these coupling reactions, we carried
out several elementary experiments to probe the reaction
mechanism. The yield of (1a) declined sharply to 6% when 2,6ꢀ
diꢀtertꢀbutylꢀ4ꢀmethylphenol (BHT) was introduced into the
reaction solution (Scheme 4a). This result suggested that the
reaction may proceed through a radical intermediate. Then
benzoic acid instead of benzaldehyde was used directly as the
coupling partner and the identical product (1a) was obtained in
92% yield (Scheme 4b).8a No expected product was found when
sodium benzoate was brought into the reaction system (Scheme
4c).
O
(a)
O
H
O
.
H2O
-
+
O
+
+
HO
Cu
B
+
Cu
2+
O
.
O
O
H
-
HO
O
A
O
H
.
O
.
r.d.s.
OH
(b)
+
+
O
O
A
C
O
O
Cu (OAc)2.H2O (2 mol %)
TBHP (4 equiv)
O
O
O
O.
O
O
.
O
H
O
O
(a)
+
Ph
O
O
.
B
Ph
H
R
(c)
(d)
R
R
BHT (1equiv), 80)
+
O
(1a, 6%)
O
A
D
OH
O
O
E
O
H
O
O
Cu (OAc)2.H2O (2 mol %)
TBHP (4 equiv), 80)
O
(b)
H
+
Ph
O
Ph
O
O
O
(1a, 92% )
O
.
R
R
O
+
O
O
O
O
.
O
O
Cu (OAc)2.H2O (2 mol %)
TBHP (4 equiv), 80)
A
F
E
O
(c)
+
Ph
O
Ph
ONa
O
O
O
(1a, not observed)
.
O
O
+
R
(e)
(f)
O
O
.
O
R
O
O
Scheme 4 Investigation into the reaction mechanism.
F
C
O
OH
We suspected that the coupling reaction proceeded via an
acyloxy radical intermediate, generated in situ from
benzaldehyde or benzoic acid. We employed benzoyl peroxide
which can offer acyloxy radicals to react with 1,4ꢀdioxane. This
reaction afforded the anticipated product (1a) in 33% yield
(Scheme 5a).11 Besides we found trace tertꢀbutyl peroxy ester
by LCꢀMS in the template reaction, which may been provided
by the reaction of the acyl radical with the tertꢀbutyl peroxy
O
.
R
+
R
+
O
.
O
H
A
F
Scheme 6 Proposed reaction mechanism.
Conclusions
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4OB00862F
In conclusion, we have disclosed a convenient and efficient
construction of CꢀO bonds via oxidative crossꢀcoupling
reaction of aldehydes and ethers. When 2 mol % copper
catalyst was used, the expected product αꢀacyloxy ether was
obtained with up to 93% yield. Moreover, these reactions
proceeded smoothly under open air and the presence of
moisture did not affected the yields. This is very different from
traditional transitionꢀmetalꢀcatalyzed reactions. Ongoing
investigations focus on the extension of substrate scopes and
asymmetric versions of the reaction in our laboratory.
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4
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Acknowledgements
This work was supported by grants from National Nature
Science Foundation of China (No. 21302013), Nature Science
Foundation of Jiangsu Province (No. BK2012207) and Nature
Science Foundation of Jiangsu Educational Department (No.
12KJA150001 and 12KJB150001).
Y. Cui, L. A. Villafane, D. J. Clausen, P. E. Floreancig, Tetrahedron
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,
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2
For representative examples on CꢀC or CꢀX bond formation by using a
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,
6
7
M. A. Huffman, J. H. Smitrovich, J. D. Rosen, G. N. Boice, C. Qu, T.
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DOI: 10.1039/C4OB00862F
Chem. Eur. J., 2010, 16, 13226; (c) E. Spier, U. Neuenschwander, I.
Hermans, Angew. Chem. Int. Ed., 2013, 52, 1581.
a
School of Chemistry and Materials Engineering, Jiangsu Key
Laboratory of Advanced Functional Materials, Changshu Institute of
Technology, 99 South 3rd Ring, Changshu, 215500, P. R. China. Email:
renzhongfujs@126.com; yuanrx@cslg.edu.cn
†
Electronic Supplementary Information (ESI) available. See
DOI: 10.1039/c000000x/
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