Nickel-Catalyzed Arylation of C(sp3)-O Bonds in Allylic Alkyl Ethers with Organoboron Compounds

Article abstract of DOI:10.1021/acs.orglett.1c01879

A nickel-catalyzed cross-coupling of allylic alkyl ethers with organoboron compounds through the cleavage of the inert C(sp3)-O(alkyl) bonds is described. Several types of allylic alkyl ethers can be coupled with various boronic acids or their derivatives to give the corresponding products in good to excellent yields with wide functional group tolerance and excellent regioselectivity. The gram-scale reaction and late-stage modification of biologically active compounds further prove the practicality of this synthetic method.

Full text of DOI:10.1021/acs.orglett.1c01879

pubs.acs.org/OrgLett
Letter
Nickel-Catalyzed Arylation of C(sp3)−O Bonds in Allylic Alkyl Ethers
with Organoboron Compounds
Xiaowei Li,§ Yuxiu Li,§ Zhong Zhang, Xiaolin Shi, Ruihua Liu, Zemin Wang, Xiangqian Li,
and Dayong Shi*
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ABSTRACT: A nickel-catalyzed cross-coupling of allylic alkyl
ethers with organoboron compounds through the cleavage of the
inert C(sp3)-O(alkyl) bonds is described. Several types of allylic
alkyl ethers can be coupled with various boronic acids or their
derivatives to give the corresponding products in good to excellent
yields with wide functional group tolerance and excellent
regioselectivity. The gram-scale reaction and late-stage modifica-
tion of biologically active compounds further prove the practicality
of this synthetic method.
sulfonates4 and phosphates5 were first used to construct C−C
bonds in cross-coupling reactions, while moisture instability
and high cost hindered their application. In recent years,
considerable efforts have been made to explore some more
stable and readily available C−O electrophiles, such as esters,6
ethers,7 and alcohols.8 In comparison with esters and alcohols,
ethers usually act as solvents and protecting groups in organic
synthesis owing to their higher chemical stability, and it is
more challenging to utilize ethers as electrophiles in transition-
metal-catalyzed cross-coupling reactions. The key challenges
are related to the high activation energy required for the
cleavage of the very electron-rich C(sp3)−O(alkyl) bonds and
the low propensity of alkoxy residues to act as leaving groups.
Thus, it is strategically important for C−O bond cleavage from
allylic alkyl ethers to construct C−C bonds.
ransition-metal-catalyzed cross-coupling reactions have
Temerged as a powerful tool in the formation of C−C and
C−heteroatom bonds for decades.1 Organohalides as good
electrophilic cross-coupling partners have been widely studied
and applied.2 With the development of high-efficiency catalytic
systems, oxygen-based electrophiles have been shown to be a
viable alternative to organohalides considering their natural
abundance, ready availability, low toxicity, and excellent
economic profile (Figure 1a).3 Based on their high reactivity,
Over the last decades, some efforts to investigate the
arylation of allyl alkyl ethers with nucleophilic coupling
partners via transition-metal catalysis have been made (Figure
1b). In 2005, Kobayashi and co-workers reported an example
of palladium-catalyzed allyl−aryl coupling between allyl methyl
ether and phenylboronic acid.9 Then, Oshima and co-workers
developed a phenylation reaction of allylic ethers with
phenylmagnesium bromide (PhMgBr) catalyzed by cobalt.10
An example of iron-catalyzed phenylation of allyl methyl ether
has also been shown by Li and co-workers with a low yield.11
Recently, nickel-based catalysts have been attracting consid-
Received: June 6, 2021
Figure 1. Transition-metal-catalyzed arylation of allylic ethers.
https://doi.org/10.1021/acs.orglett.1c01879
© XXXX American Chemical Society
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A

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a
erable attention in the transformations of C(sp3)−O bonds
because of their low cost, wide availability, and unique reaction
properties. Under nickel-catalyzed conditions, Hoveyda and
Wang et al. employed PhMgBr or arylzinc chlorides (ArZnCl)
as the nucleophiles to couple with allyl ethers, respectively.12
However, to the best of our knowledge, nickel-catalyzed allyl−
aryl cross-coupling between allylic alkyl ethers and organo-
boron compounds was still unknown.
Scheme 2. Scope of Boronic Acids
Compared with organozinc or Grignard reagents, organo-
boron compounds have emerged as more attractive nucleo-
philic coupling partners because of their advantages, such as
commercial availability, stability to air and moisture, and low
toxicity.13 Herein, we develop a highly efficient protocol for the
direct cross-coupling between allylic alkyl ethers and organo-
boron compounds (Figure 1c). In the presence of an
inexpensive nickel/triphenylphosphine (PPh3) catalytic system
and sodium acetate (NaOAc), a series of coupling products
were generated though C(sp3)−O bond activation and
C(sp3)−C(sp2) bond formation in good to excellent yields.
Initially, 2-(methoxymethyl)-N-methyl-N-(p-tolyl)-
acrylamide (1a) and phenylboronic acid (2a) were chosen as
the model substrates, and a series of experiments aimed at
identifying optimal conditions for selective arylation were
conducted (see the Supporting Information). To our delight,
we found that in the presence of bis(1,5-cyclooctadiene)nickel
(Ni(cod)2) (10 mol %), triphenylphosphine (20 mol %), and
sodium acetate (2.0 equiv) the desired product 3aa could be
obtained in 95% yield in 1 mL of toluene at 100 °C (Scheme
1). In addition, under these conditions, by using 2,4,6-
a
b
Isolated yield. (p-OMe-C6H5)3P (20 mol %) was used for 24 h.
c
Reaction performed at 120 °C for 24 h.
bearing dibenzothiophene, carbazole heterocycles, thiophene,
and benzothiophene groups could also be successfully applied
in this process, manufacturing 3an−aq in 53−89% yields.
These results further expanded the scope of boronic acids. The
trans-β-styreneboronic acid 2u also proved to be a suitable
substrate, to afford the desired product 3ar in good yield
(88%).
Next, the broad scope of this protocol with respect to
various allylic ethers was observed (Scheme 3). By employing
a
Scheme 3. Scope of Allylic Ethers
a
Scheme 1. Optimized Conditions and Scope of
Organoboron Compounds
a
b
Isolated yield. GC yield with adamantane as the internal standard.
c
(p-OMe-C6H5)3P (20 mol %) was used instead of PPh3 for 24 h.
d
Reaction performed at 120 °C for 24 h.
triphenylboroxin 4 and phenylboronic acid esters 5 and 6
instead of 2a, the product 3aa could also be generated in high
isolated yields (65−88%).
With the optimized conditions in hand, the substrate scope
of boronic acids was explored (Scheme 2). Both electron-rich
and electron-poor arylboronic acids could react with 1a to
afford the corresponding products 3aa−ak in good to excellent
yields with excellent regioselectivities (60−99%). A wide range
of functional groups, such as methyl (3ab−ad), methoxyl
(3ae), fluoro (3af and 3aj), chloro (3ag), trifluoromethyl
(3ah), trifluoromethoxyl (3ai), and even acetal (3ak), were all
well tolerated. Notably, the steric hindrance did not affect the
high efficiency of this method, and p-, m-, and o-tolylboronic
acids were converted to their desired products with
comparable reactivities (3ab−ad, 77−99% yields). Both 2-
and 1-naphthylboronic acids could serve as the nucleophiles to
give the corresponding 3al and 3am in 93% and 61% yield,
respectively. Furthermore, some (hetero)arylboronic acids
a
b
Isolated yield. (p-OMe-C6H5)3P (20 mol %) was used for 24 h.
c
Reaction was carried out at the 1 mmol scale.
arylboronic acid 2a as the nucleophilic coupling partner,
acrylamide substrate 1b with an unprotected N-H group was
efficiently transformed into the desired product 3ba in 85%
yield. Notably, a series of 2-(methoxymethyl)acrylamides with
alkyl and aryl groups located at the nitrogen core could be
readily applied in this reaction, generating the corresponding
products 3ca−ga in good to excellent yields (77−92%). In
addition, the substrate scope toward heterocyclic unsaturated
B
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dialkylamides was also screened under the optimized
conditions. The formation of 3hl, 3ia−ka, and 3lk indicated
that these substrates containing five-, six-, and seven-
membered heterocycles could be effectively transformed to
their desired products with satisfactory results (48−78%
yields). Subsequently, various 2-(methoxymethyl)acrylates
were tested. Benzyl 2-(methoxymethyl)acrylamide 1m and
(tetrahydrofuran-2-yl)methyl 2-(methoxymethyl)acrylate 1n
were well-tolerated in our conditions, producing the arylation
products in high yields. Substrates with a bulky group were
compatible reaction partners and could generate coupling
products 3oa and 3pa in 82% and 96% yield, respectively. To
demonstrate the potential utility of this coupling reaction, we
evaluated the method in the late-stage modification of complex
and/or biologically active compounds, including the deriva-
tives of camphanol, estradiol benzoate, mexiletine, L-menthol,
and fructose. The results showed that these substrates were
transformed into the target molecules successfully with good
yields (3qa−ua, 67−90%). Moreover, this transformation was
scaled up to 1 mmol to generate 3ta in 80% yield without
sacrificing yield.
arylboronic acid, yielding the desired products 8f and 8g in
good yields.
To further demonstrate the practicality of this method, a
gram-scale reaction of 1a (5 mmol) with 2a was carried out
under the standard reaction conditions, affording product 3aa
(1.2 g) in 90% yield (Scheme 4, top). Furthermore, several
Scheme 4. Gram-Scale Synthesis and Transformation of 3aa
To broaden the generality of the method, we turned our
attention to other C−O electrophiles. As shown in Table 1,
a
Table 1. Scope of Other C−O Electrophiles
transformations of 3aa were performed (Scheme 4, bottom).
In the presence of various coupling partners and oxidants, the
carbocyclization of 3aa proceeded successfully, providing a
series of functionalized 3,3-disubstituted oxindoles 9−14 in
moderate to good yields. Azido,14 benzoyl,15 cyclohexyl,16
trifluoromethyl,17 iodo,18 and benzyl groups19 were well-
tolerated. Notably, when 3aa was treated with iodine (I2) and
iodobenzene diacetate (PhI(OAc)2) in acetonitrile at room
temperature, an unexpected difunctional product 13 was
formed in 57% yield.
To gain insights into the reaction mechanism, a control
experiment was carried out. Under the standard reaction
conditions, chiral allylic alkyl ether 15a can react with 2a,
yielding racemic products 16 in 43% yield (er = 51:49)
(Scheme 5, top). It is very likely that an allylic nickel
intermediate was formed. Therefore, according to the previous
reports3c and the result of our experiments, we proposed a
possible mechanism for this reaction (Scheme 5, bottom).
a
b
Isolated yield. 2a (3.0 equiv) and (p-OMe-C6H5)3P (20 mol %)
were used for 24 h.
Scheme 5. Mechanistic Study and Proposed Catalytic Cycle
various allylic substrates were tested with arylboronic acid 2
under the optimized reaction conditions. We were pleased to
find that allylic isopropyl ether 7a was well applicable for the
current transformation, delivering 3aa in 76% yield. Moreover,
a range of cinnamyl methyl ethers were also suitable substrates
for this transformation (7b−e). In contrast to primary ether,
methyl-substituted secondary and tertiary ethers reacted more
efficiently with 2a, affording the corresponding products (8b
and 8c) in 78% and 83% yield, respectively. In addition, when
there is a methyl substituent on the alkene of the cinnamyl
methyl ether, moderate yield was obtained (8d). It is noted
that a branched allylic ether substrate 7f coupling with 2a was
found to produce the linear product 8e, albeit with somewhat
low yield. Alkyl-substituted allylic ethers could also react with
C
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Assisted by the coordination with alkene 1, a key allylic
intermediate A is formed by oxidative addition of nickel(0) to
a C−O bond. Transmetalation followed by reductive
elimination gives the desired product 3.
In summary, we have established an efficient and practical
nickel-catalyzed cross-coupling of allylic alkyl ethers with
organoboron compounds through the cleavage of the inert
C(sp3)−O bond. This protocol is distinguished by its wide
substrate scope, providing the desired products in good to
excellent yields with excellent regioselectivity and good
functional group compatibility. The application in the late-
stage modification of biologically active molecules, gram-scale
synthesis, and product transformation proves the synthetic
utility of this method.
Taishan scholar project, Shandong Provincial Natural Science
Foundation for Distinguished Young Scholars (JQ201722),
Qingdao Science and Technology Benefit People Demon-
stration Guide Special Project (20-3-4-20 nsh), and the
Fundamental Research Funds of Shandong University
(2020GN033). Haiyan Sui and Xiaoju Li from Shandong
University Core Facilities for Life and Environmental Sciences
are thanked for their help with the NMR.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01879.
Experimental procedures and characterization of all
compounds (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
Dayong Shi − State Key Laboratory of Microbial Technology,
Shandong University, Qingdao 266237, P. R. China;
Laboratory for Marine Biology and Biotechnology, Pilot
National Laboratory for Marine Science and Technology,
Qingdao 266237 Shandong, P. R. China; orcid.org/
0000-0002-5807-5633; Email: shidayong@sdu.edu.cn
Authors
Xiaowei Li − State Key Laboratory of Microbial Technology,
Shandong University, Qingdao 266237, P. R. China;
orcid.org/0000-0001-7586-2850
Yuxiu Li − State Key Laboratory of Microbial Technology,
Shandong University, Qingdao 266237, P. R. China;
orcid.org/0000-0002-5185-7501
Zhong Zhang − State Key Laboratory of Microbial
Technology, Shandong University, Qingdao 266237, P. R.
China; orcid.org/0000-0001-7056-0060
Xiaolin Shi − State Key Laboratory of Microbial Technology,
Shandong University, Qingdao 266237, P. R. China
Ruihua Liu − State Key Laboratory of Microbial Technology,
Shandong University, Qingdao 266237, P. R. China
Zemin Wang − State Key Laboratory of Microbial Technology,
Shandong University, Qingdao 266237, P. R. China
Xiangqian Li − State Key Laboratory of Microbial Technology,
Shandong University, Qingdao 266237, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01879
Author Contributions
§V.B. and V.M. contributed equally to this work.
Notes
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M.; Matsunaga, S. Cobalt-Catalyzed Allylic Alkylation Enabled by
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Funded by National Program
for Support of Top-Notch Young Professionals, Fund of
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