Regio- and Stereoselective Synthesis of Enynyl-Aryl Ethers Enabled by Copper/Iodide Tandem Catalysis

Article abstract of DOI:10.1002/adsc.201900626

An approach to preparing enynyl-aryl ethers from phenols and phenylacetylenes is described. This method without extra ligands, overcoming the favored Glaser-Hay dimerization of alkyne, features a wide substrate scope (38 examples including endofolliculina

Full text of DOI:10.1002/adsc.201900626

Title: Regio- and Stereoselective Synthesis of Enynyl-Aryl Ethers
Enabled by Copper/Iodide Tandem Catalysis
Authors: Yun-Bin Wu, Lin Xiao, Chun-Li Mao, Zhong-Lin Zang, Cheng-
He Zhou, and Gui-Xin Cai
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10.1002/adsc.201900626
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201######.
Regio- and Stereoselective Synthesis of Enynyl-Aryl Ethers
Enabled by Copper/Iodide Tandem Catalysis
Yun-Bin Wu,a Lin Xiao,a Chun-Li Mao,a Zhong-Lin Zang,a Cheng-He Zhou*,a and Gui-
Xin Cai*,a,b
a
Key Laboratory of Applied Chemistry of Chongqing Municipality, Institute of Bioorganic & Medicinal Chemistry,
School of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China
E-mail of corresponding author: gxcai@swu.edu.cn; zhouch@swu.edu.cn.
Beijing National Laboratory for Molecular Sciences, Beijing 100190, China
b
Received:######.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
are still scarce. Copper catalysis, owing to the low
from phenols and phenylacetylenes is described. This toxicity and cost-effective nature of copper, has
Abstract: An approach to preparing enynyl-aryl ethers
method without extra ligands, overcoming the favored
Glaser−Hay dimerization of alkyne, features a wide
substrate scope (38 examples including endofolliculina and
indole) and the merits of high atom and step economy,
good regio- and stereoselectivity (Z-isomers major) in
moderate to good isolated yields. Mechanistic studies show
that the reaction is enabled by distinctive copper/iodide
tandem catalysis.
witnessed significant momentum since its
emergence[3a,6]. But the success of these protocols
generally relies on the auxiliary of extra ligands[6,7], in
particular of nitrogen- and oxygen-carrying
compounds, and the study of other factors boosting
copper catalysis is underdeveloped. Although organic
transformations involving iodine reagents are quite
elegant and particularly useful[8], a limited knowledge
has been acquired when employing the copper/iodide
combination for tandem catalysis without additional
ligands. Thus, driven by our previous works
constructing the scaffold of aryl alkenyl ether[9] and
copper-catalyzed tandem reactions[10] promoted by
equivalent iodides[11], we herein report a regio- and
stereoselective route using unactivated phenols and
phenylacetylenes via the copper/iodide tandem
catalysis[12] without extra ligands to efficiently
prepare enynyl-aryl ether compounds (Scheme 1).
Keywords: copper/iodide tandem catalysis; enynyl-aryl
ethers; regio- and stereoselectivity; cross-electrophile
coupling
Enynyl-aryl ether derivatives possessing conjugated
double and triple bonds are valuable and privileged
scaffolds widely embedded in organic materials,
pharmaceuticals and agrochemicals.[1,2d] Great efforts
have been made to gain access to this intriguing
structural unit. Published synthetic methods include
the addition of phenols to conjugated diynes[2a]
,
Sonogashira
couplings[2b-2d]
and
multistep
procedures[2e-2i]. Although the chemical community
has made significant progress in this field, current
methods still suffer from various challenges, such as
the limited substrate scope, expensive catalysts,
prefunctionalized starting materials and/or the
requirement of multistep procedures. Ideally,
employing the commercially available and cheap
phenols and phenylacetylenes to directly construct
C(sp2)-O/C(sp2)-C(sp) bonds can effectively avoid
the separation of halides, and therefore promoting the
elegance of synthesis. However, major challenges
happen, due to the high reactivity of phenols and
alkynes[3], especially the oxidation of phenols and
unsaturated C-C bonds by high-valent iodine
reagents[4] along with the homo-/heterocoupling of
phenylacetylenes in the presence of transition
metals[5], and straightforward methods for the
efficient synthesis of enynyl-aryl ether compounds
Scheme 1. Synthetic strategies of enynyl-aryl ethers.
Initially, we began our investigation by
examining phenol (1a) and phenylacetylene (2a) as
model substrates to optimize the reaction conditions
(Table 1). Testing of 1a and 2a did lead to 30%
isolated yield of enynyl-aryl ether products (Z)-
3aa/(E)-3aa’ with good Z:E selectivity (Z:E = 86:14)
(Table 1, entry 1). In consideration of the auxiliary
effect of ligands[6,7], we next investigated the
commonly used N-donor ligand and unexpectedly,
the ligand played
a
negative role in this
transformation (Table 1, entry2). Then switching to
1
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10.1002/adsc.201900626
Advanced Synthesis & Catalysis
other base revealed that Cs2CO3 was suitable one and
under the standard conditions, but the performance of
a cheerful result could be achieved to deliver 3aa/3aa’ naphthalen-2-ol was much better than that of
in 83% isolated yield (Table 1, entry 3). In
comparison with TBAI, TBAB showed an analogous
performance, whereas TBAC seemed to be much less
effective (Table 1, entries 4-5). Finally, a slight
decrease in yield was observed when the reaction was
naphthalen-1-ol ((Z)-3za and (Z)-3ya, respectively).
However, the success of this transformation could not
be extended to phenols containing heterocycles such
as pyridin-4-ol and quinolin-4-ol because of these
heterocyclic phenols as strong bidentate ligands
blocking the catalytic cycle of copper catalyst (not
shown in Table 2).
o
conducted at 100 C, while the trace amount of
o
products could be obtained at 90 C (Table 1, entries
6-7). Further screening the reaction conditions
including copper catalysts, oxidants, other iodide
sources, bases and solvents has been shown in
Supporting Information (SI) Table 1.
Table 2. Substrate scope of phenols[a].
Table 1. Optimization of the reaction conditions[a].
yield
[%]
yield
[%]
product
product
iodide
entry
base
yield [%](Z:E) [b]
source/additive
TBAI[c]
TBAI
1
2[d]
3
4
K3PO4
K3PO4
30 (86:14)
26 (82:18)
83 (86:14)
42 (86:14)
74 (86:14)
72 (86:14)
trace
TBAI
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
TBAC[e]
TBAB[f]
TBAI
5
6[g]
7[h]
TBAI
[a]
Reaction conditions: 1a (0.4 mmol), 2a (0.4 mmol), CuI
(0.02 mmol), iodide source (0.02 mmol), PhI(OAc)2 (1
equiv), base (2 equiv), DMSO (1 mL) in a sealed tube
under air at 110 oC for 22 h.
Isolated yields (Z-3aa isomer and E-3aa isomer can be
completely separated by flash column chromatography, and
the yield in Table 1 is the sum of the isolated yield of Z-3aa
and that of E-3aa). The ratio of Z isomer to E isomer is
calculated by the Z and E isomers’ isolated yields.
[b]
[a]
[b]
[c] TBAI = tetrabutylammonium iodide.
[d] 1,10-Phenanthroline monohydrate (0.1 mmol) was added.
[e] TBAC = tetrabutylammonium chloride.
[f] TBAB = tetrabutylammonium bromide.
[g] 100 oC.
Reaction conditions: 1 (0.4 mmol), 2a (0.4 mmol), CuI
(0.02 mmol), TBAI (0.02 mmol), PhI(OAc)2 (1 equiv),
Cs2CO3 (2 equiv), DMSO (1 mL) in a sealed tube under air
at 110 oC for 22 h with isolated yields.
Because the E-isomer is too little to separate and purify,
only Z-isomer was isolated and analysed.
[h] 90 oC.
[c] NP = no desired product.
With the optimized protocol in hand, we next
examined the substrate scope. First, phenols bearing
various substituents on the phenyl ring were tested
(Table 2). Pleasingly, both electron-rich and electron-
deficient phenols undergo effective transformations
and this reaction appears to be relatively insensitive
to steric effects: ortho, meta and para substitution can
be well-tolerated, except for the methoxyl group and
the nitro group. Due to 2-methoxyphenol serving as a
strong bidentate ligand on the copper catalyst[6], no
desired product (Z)-3ga was observed in this
transformation. And in the case of 4-methoxyphenol,
only a very minor amount of the product (Z)-3va was
formed. Notably, the structure of (Z)-3wa was
Phenylacetylenes bearing different substituents
were also explored to further expand the substrate
scope of our catalytic system (Table 3). Similarly,
except for these functional groups including 2-OCH3,
4-OCH3 and 4-NO2, a range of phenylacetylenes
participated smoothly in this reaction and provided
the corresponding products with high stereo-
selectivity in moderate to good isolated yields. And
the treatment with 2-ethynylthiophene and 3-
ethynylpyridine proceeded well to obtain the desired
products ((Z)-3aq/(E)-3aq’ and (Z)-3ar/(E)-3ar’) in
moderate isolated yields, albeit in lower
stereoselectivity. However, aliphatic alkyne such as
1-ethynylcyclohex-1-ene showed a poor performance
giving a trace amount of the products.
confirmed by X-ray crystallography (Please refer to
[13]
SI for details).
Naphthols were also compatible
2
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10.1002/adsc.201900626
Advanced Synthesis & Catalysis
Table 3. Substrate scope of phenylacetylenes[a].
in the presence of terminal alkynes and copper
catalyst.[15]
yield
[%]
yield
[%]
product
product
Scheme 3. Experimental results of 4 and 5.
The role of each relevant chemical reagent (such
as CuI, TBAI, PhI(OAc)2 and Cs2CO3) in this
reaction was next investigated (Table 4). Although
the base was necessary, the influence of Cs2CO3 only
existed when it combined with the other three
partners (Table 4, entry 1) and any two or three
factors of them would lead to a poor performance
(Table 4, entries 2-9), which might indicate a
synergistic effect among them.
[a]
[b]
Reaction conditions: 1a (0.4 mmol), 2 (0.4 mmol), CuI
(0.02 mmol), TBAI (0.02 mmol), PhI(OAc)2 (1 equiv),
Cs2CO3 (2 equiv), DMSO (1 mL) in a sealed tube under air
at 110 oC for 22 h with isolated yields.
Because the E-isomer is too little to separate and purify,
only Z-isomer was isolated and analysed.
Table 4. The role of each relevant chemical reagent.
[c] NP = no desired product.
[d] The Z/E ratio was determined by 1H NMR analysis.
yield [%]
(Z:E)
entry
control conditions
To elucidate the reaction mechanism, a series of
control experiments has been conducted (Schemes 2-
4 and Table 4). Radical trapping experiment was
carried out at first by adding 1 equivalent of TEMPO
(2,2,6,6-tetramethyl-piperidine-1-oxyl) under the
standard conditions and the result indicated that this
transformation was not involved in free radicals
(Scheme 2).
[a] [Cu] = 10 mol% CuI.
[b] [I-] = 10 mol% TBAI.
[c] 1 eq. PhI(OAc)2.
[d] 2 eq. Cs2CO3.
[e] The Z/E ratio was calculated by the Z and E isomer's
isolated yields.
[f] The Z/E ratios were determined by 1H NMR analysis.
Scheme 2. Radical trapping experiment.
The reactivities of intermediates 6, 7 and 8 were
further investigated (Scheme 4). When the model
reaction was conducted for 2 h, phenylethynyl copper
intermediate 6 was obtained (Scheme 4, Eq 1), which
proved that 6 was an important reaction intermediate
in our reaction system. What’s more interesting was
that the treatment of 2a with TBAI and PhI(OAc)2 in
DMSO at 110 oC for 22 h successfully gave
(iodoethynyl)benzene 7 in 45% isolated yield
(Scheme 4, Eq 2). 7 reacting with 1a in the presence
of CuI and Cs2CO3 also afforded (Z)-3aa/(E)-3aa’ in
Then experimental results of benzoquinone 4
and 1,4-diphenylbuta-1,3-diyne 5 instead of 1a and
2a, respectively, excluded 4 or 5 as the reaction
intermediate, and therefore demonstrated that this
work differed from Yamamoto’s[2a] and Yao’s work[14]
via palladium catalysis (Scheme 3). Probably due to
the high reaction temperature and the lack of extra
nitrogen-containing ligand, our reaction did not
involve the Glaser−Hay dimerizations via the
homocoupling process of the alkynyl-copper complex
3
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10.1002/adsc.201900626
Advanced Synthesis & Catalysis
been an increasingly popular approach via the
combination of metal catalysis and external
reductants, which has enabled the advancement of
sorts
of
new
cross-coupling
processes[19]
.
Unexpectedly, we obtained an exciting finding on the
cross-electrophile coupling of different C(sp2)-
I/C(sp)-I iodine intermediates without homocoupling
product 5 in the absence of metal catalyst and
reductant in the transformation (Scheme 4, Eq 7),
which opened an entry to the cross-electrophile
coupling enabled by iodine reagents. Screening
experiments of 8 and 2a under a series of control
conditions testified that our transformation did
undergo a copper/iodide tandem catalytic procedure
in this reaction (Scheme 4, Eq 8).
Scheme 5. Plausible mechanism.
Based on the control experimental results and
the previous publications[8,18,19]
,
we tentatively
propose a plausible mechanism in Scheme 5. First,
the transformation is initiated by the reaction of an
iodide source (TBAI or CuI) and PhI(OAc)2 to
generate IOAc[20], which works as a strong
electrophile
(iodoethynyl)benzene 7 in the company of a
hydrogen/iodine exchange equilibrium process[21]
against
alkyne
2a
to
give
.
Scheme 4. The reactivities of intermediates 6, 7 and 8.
Then intermolecular nucleophilic addition of phenol
1a to 7 assisted by base delivers 8 with excellent
regio- and stereoselectivity[2f]. Subsequently, 8
rapidly reacts with 6 which is mainly produced by the
reaction of 7 and CuI[16] to form a CuIII intermediate I
via oxidative addition, and then intermediate I
undergoes reductive elimination to give (Z)-3aa and
release CuI[7d,7e], which can regenerate IOAc assisted
by PhI(OAc)2. Since the alkene isomerization in
intermediate I may occur before the reductive
elimination process, a trace amount of (E)-3aa’ is
obtained.
62% isolated yield along with a certain amount of
compound 6, which probably meant that 7 was
another intermediate in our reaction involving
umpolung of 7 (Scheme 4, Eq 3)[16]. On the contrary,
we found that compound 8[17] was obtained as a main
product[2f] in the absence of CuI and no desired
products 3aa/3aa’ were obtained (Scheme 4, Eq 4),
which illustrated the important role of copper catalyst
in the reaction. Subsequently, 1a reacted with 6
instead of 7 under similar conditions and the result
showed that desired products 3aa/3aa’ and 5 were
not produced (Scheme 4, Eq 5). Further exploration
of 8 demonstrated that our reaction probably involved
a high-valent copper intermediate[18], but regrettably,
owing to the fast reductive elimination of a four-
coordinated CuIII intermediate, we cannot obtain the
corresponding high-valent copper complex (Scheme
4, Eq 6). Recently, cross-electrophile coupling has
To demonstrate the broad applicability of our
method, different coupling partners such as
structurally
complex
bioactive
compound
endofolliculina and indole were tested for this
transformation (unoptimized) as shown in Scheme 6.
And the reactions proceeded smoothly under standard
conditions, respectively yielding the expected
products (Z)-9/(E)-9’ and (Z)-10/(E)-10’ in good
yields, albeit in slightly lower stereoselectivity.
4
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10.1002/adsc.201900626
Advanced Synthesis & Catalysis
Acknowledgements
We are grateful to the Research Funds for the Central Universities
(XDJK2017B015 and XDJK2018C040), the Doctoral Fund of
Southwest University (SWU111075 and SWU117056), the Beijing
National Laboratory of Molecular Sciences (BNLMS) (20140130)
and the National Natural Science Foundation of China (Grants
21672173) for the support of this research.
Scheme 6. Applications of our approach.
References
In conclusion, we have developed an efficient
extra-ligand-free approach to the enynyl-aryl ether
scaffold via distinctive copper/iodide tandem catalysis.
The reaction features broad functional group tolerance
(38 examples including endofolliculina and indole) with
good regio- and stereoselectivity (Z-isomers major) in
moderate to excellent isolated yields. More important,
the present findings on copper/iodide tandem catalysis
not only open a distinct dimension of Cu catalysis, but
also enrich the known catalytic mechanism of cross-
electrophile coupling involving iodine compounds
without reductant. Currently, further studies of our
desired products are still underway and will be reported
in due course.
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Experimental Section
General Information
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5
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10.1002/adsc.201900626
Advanced Synthesis & Catalysis
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6
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10.1002/adsc.201900626
Advanced Synthesis & Catalysis
COMMUNICATION
Regio- and Stereoselective Synthesis of Enynyl-
Aryl Ethers Enabled by Copper/Iodide Tandem
Catalysis
Adv. Synth. Catal.Year, Volume, Page – Page
Yun-Bin Wu,a Lin Xiao,a Chun-Li Mao,a Zhong-
Lin Zang,a Cheng-He Zhou*,a and Gui-Xin Cai*,a,b
7
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