Palladium-catalyzed aerobic regio- and stereo-selective olefination reactions of phenols and acrylates: Via direct dehydrogenative C(sp2)-O cross-coupling

Article abstract of DOI:10.1039/c8cc01226a

An efficient olefination protocol for the oxidative dehydrogenation of phenols and acrylates has been achieved using a palladium catalyst and O₂ as the sole oxidant. This reaction exhibits high regio- and stereo-selectivity (E-isomers) with mod

Full text of DOI:10.1039/c8cc01226a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Palladium-Catalyzed Aerobic Regio- and Stereo-Selective
Olefination Reactions of Phenols and Acrylates via Direct
Dehydrogenative C(sp )−O Cross-Coupling
2
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
,a,b
DOI: 10.1039/x0xx00000x
Yun-Bin Wu, Dan Xie, Zhong-Lin Zang, Cheng-He Zhou and Gui-Xin Cai*
www.rsc.org/
An efficient olefination protocol for the oxidative (1) Kinart, Sarrafi and Cook, respectively
dehydrogenation of phenols and acrylates has been achieved
base
OR3
CO2R1
R2
O
CO2R1
2
using palladium catalyst and O as the sole oxidant. This reaction
R2
[
Cu] cat.
exhibits high regio- and stereo-selectivity (E-isomers) with
I
CO2R1
R3 = SnBu3 or H
moderate to excellent isolated yields and wide substrate scope
(
2) Liu and Wan, respectively
(
32 examples) including ethyl vinyl ketone and endofolliculina.
O
Ar
I
OTf +
R2
CO
2
R3
CO
PPh3
3
R
Ar
OH
[Cu] cat.
R1
Aryl alkenyl ethers, represented by the phenoxy-acrylates,
have been considered as versatile building blocks in organic
and medicinal chemistry. Several elegant examples for the
R
2
O
O
2
R2
CO
2
R1
+
+
R3
1
Cl
synthesis of aryl alkenyl ethers have been reported over the (3) Li, 2017
last decade (Scheme 1). Kinart reported the addition reactions
R1 Ts
2
a
of organotin phenoxides and ethyl propiolate and the similar
OH Ts
cinchoninederivatives
1
+
2
O
2
2
b
R
CO R
CO R
2
-20 C~0 C
o o
2
catalytic system could be used for the activation of phenols.
Copper (I)-catalyzed cross coupling reaction of (E)- or (Z)-3-
2
c,2d
2
This work: the direct dehydrogenative C(sp )
iodoacrylates with phenols has been pioneered by Cook.
Liu and Wan respectively extended the substrate scope to coupling
diaryliodonium salts and the multi-component reaction
-
O cross-
2
e
2f
involving phenols and Wittig reagents. Li also detailed an
asymmetric addition reaction of substituted phenols with
allenes catalyzed by the cinchonine derivatives.
2
g
Scheme 1. Synthetic strategy
Although the chemical community has made fantastic
progress in this field, current methods still possess some
limitations such as stereoisomers, the lack of atom and step
economy as well as the harsh reaction conditions and fairly
toxic substrates. Thus it is a topic of crucial importance to
explore more green and efficient approaches to constructing
the scaffold of aryl alkenyl ether. C−H functionalization
due to its atom- and step-economy, and this logic has
underpinned4 the process of synthetic planning and
applications. Compared to the developed dehydrogenation
couplings concerning phenols, the direct olefination of phenols
faces the following pre-existing challenges: (1) the addition
5
reaction of phenols and alkenes; (2) the metal-catalyzed
directed ortho, meta or para-selective C-H functionalization of
strategies, to be versatile and efficient, play an important role
3
in organic chemistry. Direct functionalization of inactive C
-
H
bonds to construct new C-C or C-heteroatom (C-N, C-O, C-P)
6
phenols; (3) the homo-coupling of phenols under an aerobic
7
system. In brief, the direct dehydrogenation of phenols and
bonds has become one of the most concise and
straightforward methods
electron-deficient alkenes for the highly efficient synthesis of
aryl alkenyl ethers through C-H bond activation of acrylates is
quite rare, except for the intramolecular process for
benzofurans and the alkoxylation of α,β-unsaturated
4m
carbonyl compounds using alcohols for the synthesis of enol
6b
ethers. Thus, we herein report a palladium-catalyzed regio-
-O cross-coupling
2
and E-selective direct dehydrogenative C(sp )
reaction of phenols and acrylates using O
under mild conditions.
2
as the sole oxidant
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Journal Name
8
On the basis of our previous work, we attempted to and 3xa). To our surprise, the reaction did occur in the
develop a synthetic protocol for the formation of vinyl C
presence of benzylalcohol, affording 3ya in 12% isolated yield.
bond via direct cross-dehydrogenation coupling (CDC) . Our H o w e v e r , t h e s u c c e s s o f t h i s t r a n s f o r m a t i o n
-O
9
investigation started with phenol (1a) and n-butyl acrylate (2a
as the model substrates to optimize the reaction parameters
and the results are summarized in Table 1. When the reaction
)
a
Table 1. Optimization of the reaction conditions
was treated using Pd(OAc)
CH Cl
2 2
2
catalyst under an O
at 40 C, 3aa was isolated in 12% yield (Table 1, entry 1).
Several bases as well as KHCO were then tested (Table 1,
entries 2-7) and K CO was chose as the optimal candidate for
2
atmosphere in
o
Additive/
Oxidant
3aa
(%)
12
14
32
35
24
37
28
3
Entry
Catalyst
Base
-
Solvent
b
2
3
the following optimization. Various additives were further
screened to boost the transformation. Fortunately, the
product 3aa was obtained with moderate yield in the presence
of TBAC (tetrabutylammonium chloride) (Table 1, entry 8).
Other tetrabutylammonium-based additives, such as TBAI
1
2
3
4
5
6
7
8
9
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
-
-
-
-
-
-
-
CH
CH
CH
CH
CH
CH
CH
2
2
2
2
2
2
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
2
2
2
2
2
2
NEt
DABCO
Cs CO
Na CO
CO
KHCO
K2CO3
3
2
3
2
3
(
(
tetrabutyl-ammonium
iodide)
and
TBAB
tetrabutylammonium bromide), were also screened and
K
2
3
3
d
experimental results showed that TBAI was inferior in
promoting this reaction, whereas TBAB showed a positive and
cheerful result in 81% isolated yield (Table 1, entries 9-10). The
dosage of TBAB was investigated and the results indicated that
Pd(OAc)2
Pd(OAc)
Pd(OAc)
TBACe
TBAI
TBAB
CH Cl2
2
51
trace
81
2
K
K
2
CO
CO
3
CH
2
Cl
2
f
10
2
2
3
CH
2
Cl
2
g
1
1
2
Pd(OAc)
Pd(OAc)
2
2
TBAB
K
K
2
2
CO
CO
3
3
CH
CH
2
2
Cl
2
2
trace
19
3
equiv. of TBAB was optimum (see SI). Meanwhile, it is
noteworthy that the existence of iodide ions would
1
KI
Cl
h
1
0
13
Pd(OAc)2
TBAB
K2CO3
CH Cl2
20
2
enormously inhibit this reaction (Table 1, entries 11-13).
Surprisingly, the omission of K CO caused the significantly
diminished yield, which might indicate a synergistic effect of
TBAB and K CO (Table 1, entry 14). Other additives with
14
15
16
17
18
19
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
-
2
2
2
2
2
2
2
2
2
TBAB
LiBr
NaBr
KBr
-
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
20
trace
7
2
3
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
2
3
bromide anions were evaluated and those results showed
weaker reactivity (Table 1, entries 15-17). Furthermore,
commonly used oxidants in the palladium catalyst system,
22
i
CuBr
Cu(OAc)
2
trace
trace
i
2
i
c
such as CuBr
2
, Cu(OAc)
2
and PhI(OAc)
2
were tested (Table 1,
20
21
PhI(OAc)
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
TBAB
2
NP
i
entries 18-20). However, none of them displayed positive
effects on this transformation. Compared to the results of air
and Ar atmosphere (Table 1, entries 21-22), molecular oxygen
71
29
j
22
23
24
25
26
27
28
29
30
31
32
33
c
NP
(O
2
) was confirmed that it had a central role in the
PdBr
PdCl
Pd(PPh
Pd(dppf)
Pd
2
32
34
8
1
1
regeneration of palladium catalyst. In the absence of
palladium catalyst totally restrained the reaction, which
indicated its core role in the catalytic system (Table 1, entry
2
3
)
2
Cl
Cl
2
2
2
13
35
41
78
36
42
53
50
2
3). Other palladium catalysts such as PdBr2, PdCl2,
Pd(PPh Cl , Pd(dppf) Cl and Pd (dba) were also evaluated
Table 1, entries 24-28), and Pd(OAc) was superior to them.
2
(dba)
3
)
2
2
2
2
2
3
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
DCE
CHCl
THF
3
(
2
2
2
2
2
3
2
When other solvents were used instead of CH
product was obtained in moderate yields and therefore CH Cl
2 2
Cl , the desired
CH
dioxane
CH Cl
3
CN
2
2
was the optimum solvent (Table 1, entries 29-33). Finally, we
treated the reaction at room temperature (25 C), but the
result was undesired (Table 1, entry 34).
o
k
34
2
2
a
Reaction conditions: phenol 1a (0.4 mmol), n-butyl acrylate 2a (0.2 mmol),
catalyst (0.02 mmol), additive or oxidant (3 equiv.), base (0.06 mmol), solvent (1
o
b
c
With optimized conditions in hand, phenols bearing mL) in a schlenk tube under O balloon at 40 C for 36 h. Isolated yields. NP =
various substituents on the phenyl ring were tested (Table 2). no desired product.
Due to the effect of steric hindrance, 2-substituted phenols
afforded the corresponding products in poor to moderate
yields (3ba-3fa). No desired product 3ca was observed in the
2
d
e
TBAC
=
tetrabutylammonium chloride.
tetrabutylammonium iodide. TBAB = tetrabutylammonium bromide. Extra
TBAI (3 equiv.) was added. Extra KI (3 equiv.) was added. Under air. Under Ar
TBAI =
f
g
h
i
j
k
balloon. RT.
presence of 2-methoxyphenol, which meant that palladium
catalyst might be inactive under this condition because of 2-
methoxyphenol as a bidentate ligand coordinating to
palladium. With regard to 3- or 4-substituted phenols,
electron-rich phenols generally gave better yields of the
couldn’t be extended to pyridin-4-ol and quinolin-4-ol et al.
due to the strong coordination effect of heterocyclic phenols
on Pd catalyst.
6
f
The scope of various substituents on the acrylate partner
desired products than that of their electron-deficient was then explored (Table 3). Satisfyingly, a range of functional
counterparts (3ga
3ta). In addition, multi-substituted groups, groups were compatible, including n-butyl, methyl, ethyl, tert-
-
owing to the steric effect, led to corresponding products in butyl, benzyl, n-hexyl, cyclohexyl, trifluoroethyl, 2-methoxy-
lower yields (3ua and 3va). Naphthols were also compatible ethyl, (tetrahydrofuran-2-yl)methyl and phenyl groups, to
under standard conditions and the performance of afford the corresponding products (3aa-3ak) in modest to
naphthalen-2-ol was better than that of naphthalen-1-ol (3wa excellent yields (up to 84% isolated yield). While methyl at
α
-
2
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or position of ethyl acrylate respectively gave a low
COMMUNICATION
β
-
conversion under the standard conditions, which meant that
the steric hindrance had obvious inhibitory effect on this
t
r
a
n
s
f
o
r
m
a
t
i
o
n
a
Table 2. Reactions of various phenols and n-butyl acrylate
a
Reaction conditions: phenol 1a (0.4 mmol), acrylates 2 (0.2 mmol), Pd(OAc)
0.02 mmol), TBAB (3 equiv.), K CO (0.06 mmol), CH Cl (1 mL) in a schlenk tube
balloon at 40 C reflux for 36 h with isolated yields.
2
(
2
3
2
2
o
under O
2
the intermediate II assisted by base resulted in butyl (E)-3-
phenoxyacrylate 3aa exclusively and released a HPd X complex.
II
II
Then under the effect of base, the Pd -hydride complex
0
experienced reductive elimination to afford a Pd species,
2
which could be oxidized by O as a sole oxidant to regenerate
II
11
the Pd catalyst. The excellent regio- and stereo-selectivity
observed in this transformation might be attributed to the
favorable formation of the intermediate II. In addition, the role
of excess TBAB (3 equiv.) in the reaction system was to avoid
II
a
the deactivation of the Pd catalyst arising from the strong
coordination by phenol, and therefore promoting the catalytic
cycle (see SI).
Reaction conditions: phenols 1 (0.4 mmol), n-butyl acrylate 2a (0.2 mmol),
(0.02 mmol), TBAB (3 equiv.), K CO (0.06 mmol), CH Cl (1 mL) in a
balloon at 40 C reflux for 36 h with isolated yields. NP
Pd(OAc)
schlenk tube under O
=
2
2
3
2
2
1
6
o
b
2
c
no desired product. TBAB (5 equiv.).
Furthermore, to demonstrate the synthetic application of
our approach, we investigated the transformation of natural
product (8S,9R,13R,14R)-3-hydroxy-13-methyl-6,7,8,9,11,12,13,
(
3al and 3am). Fortunately, the treatment with pent-1-en-3- 14,15,16-decahydro-17H-cyclopenta[a]phenanthren-17-one
one also proceeded well to obtain the desired product (3an) in with n-butyl acrylate 2a to prepare a novel functionalized
3% isolated yield, whereas electron-rich alkenes such as molecule and gave the corresponding target product
in 82%
4
8
5
styrene showed a poor performance and gave the trace isolated yield.
amount of product (3ao).
We further had insights into the mechanism by control
experiments. In many cases, the reactions of phenols involve
1
2
free radicals, while the results of our experiments using
TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) or ascorbic acid
1
3
could exclude the free radicals in this transformation
Scheme 2). We also found that 3ab could not be obtained
(
coming from methyl 3-phenoxypropanoate, which indicated
this reaction not involved the process of the addition reaction
of acrylate followed by α,β-dehydrogenation elimination of
methyl 3-phenoxypropanoate. A plausible reaction mechanism
was depicted in Scheme 3. First, the reaction was initiated by
Scheme 2. Control experimental results
the coordination of Pd(OAc)
2
to n-butyl acrylate 2a to give a
which underwent nucleophilic
1
4
palladium-olefin complex
attack by phenol 1a to generate an intermediate II
I
,
1
5
.
-hydride elimination from
Subsequent deprotonation and syn
β
a
Table 3. Reactions of phenol and various acrylates
Scheme 3. Plausible mechanism
Scheme 4. The application of our approach
Conclusions
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In conclusion, we have developed an efficient and mild
Journal Name
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2
Pd-catalyzed direct dehydrogenation C(sp )-O cross-coupling
reaction of phenols and acrylates under oxygen atmosphere,
providing a highly regio- and stereo-selective approach to
synthesize (E)-alkenyl aryl ether compounds. A wide variety of
products could be synthesized in moderate to excellent yields.
And the optimized catalytic system can be used for
functionalization of ethyl vinyl ketone and endofolliculina to
afford the desired products with high stereo-selectivity and
excellent isolated yields.
S. Zhang, Z. W. Li, Z. J. Shi, Org. Chem. Front., 2014,
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We are grateful to the National Natural Science
Foundation of China (Grants 21372186 and 21672173), Beijing
National Laboratory of Molecular Sciences (BNLMS)
6
1
8
7
,
(
20140130), the Doctoral Fund of Southwest University
SWU111075), and the Research Funds for the Central
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(
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Conflicts of interest
There are no conflicts to declare.
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The Direct Dehydrogenative C(sp )-O Cross-Coupling
EWG
OH
O
Pd(OAc)2, TBAB, K2CO3
EWG
CH2Cl2, 40 oC, O2
EWG = esters and ketone
Substituted Phenols
Naphthols, Endofolliculina
32 examples
O2 as the sole oxidant
Regioselectivity and stereoselectivity (E-isomers)
Inexpensive and readily available substrates
View Article Online
DOI: 10.1039/C8CC01226A