Nickel-Catalyzed oxidative coupling of unactivated C(sp3)-H bonds in aliphatic amides with terminal alkynes

Article abstract of DOI:10.1021/acs.organomet.6b00529

In this work, we demonstrated Ni-catalyzed oxidative coupling of unactivated C(sp3)-H bonds with terminal alkynes for construction of C(sp3)-C(sp) bonds to synthesize alkyl-substituted internal alkynes. Different amides exhibited good compatibility. Preliminary mechanistic studies were conducted to account for this alkynylation.

Full text of DOI:10.1021/acs.organomet.6b00529

Communication
pubs.acs.org/Organometallics
Nickel-Catalyzed Oxidative Coupling of Unactivated C(sp3)−H Bonds
in Aliphatic Amides with Terminal Alkynes
Fei-Xian Luo,† Zhi-Chao Cao,† Hong-Wei Zhao,† Ding Wang,† Yun-Fei Zhang,† Xing Xu,†
and Zhang-Jie Shi*,†,‡
†Beijing National Laboratory of Molecule Sciences (BNLMS) and Key Laboratory of Bioorganic Chemistry and Molecular
Engineering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
‡State Key Laboratory of Organometallic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: In this work, we demonstrated Ni-catalyzed oxidative coupling
of unactivated C(sp3)−H bonds with terminal alkynes for construction of
C(sp3)−C(sp) bonds to synthesize alkyl-substituted internal alkynes. Different
amides exhibited good compatibility. Preliminary mechanistic studies were
conducted to account for this alkynylation.
xidative C−H/C−H coupling is an ideal and environ-
C(sp) bonds to synthesize alkyl alkynes is more challenging.13
Omentally attractive strategy for construction of C−C Continuous efforts have been contributed in this field in the
past decades. Radical alkynylation of the C(sp3) center adjacent
bonds by avoiding time-consuming prefunctionalization of
substrates.1 The concept of cross dehydrogenative coupling
to a heteroatom or decarboxylative alkynylation with a
nucleophilic or electrophilic alkynyl reagent provided successful
tools for construction of C(sp3)−C(sp) bonds.14 Transition-
metal-catalyzed alkynylation of alkyl halides/peusudohalides
with alkynyl reagents is another efficient and direct strategy.15,16
In 2003, the first example of direct alkynylation of primary alkyl
bromides and iodides with terminal alkynes was disclosed by Fu
and co-workers (Scheme 1).17 Subsequently, other groups
(CDC) was conceived in the late 1990s and applied to
construct different types of carbon−carbon bonds. Afterward,
pioneering work on oxidative coupling between two different
C−H bonds were reported.1−3 However, those examples
mainly involved the oxidative coupling of C(sp2)−H/C-
(sp2)−H bonds.2 From the viewpoint of C(sp3)−H bonds,
CDC was conducted with at least one relatively active C(sp3)−
H bond in most cases.1e Besides, developments on oxidative
coupling of an unactivated C(sp3)−H bond with the other
“inert” C−H bond were far behind.4 In 2008, Li’s group
reported the Ru-catalyzed oxidative coupling of C(sp3)−H/
C(sp2)−H between 2-arylpyridines and cycloalkanes.5 In the
same year, Fagnou’s group reported the Pd-catalyzed intra-
molecular coupling of arenes with unactivated alkanes.4a
Recently, Chatani’s group demonstrated an elegant work for
direct oxidative coupling of aromatic C(sp2)−H bonds with
C(sp3)−H bonds of toluene by using a bidentate directing
strategy.6 Later on, they extended this strategy to CDC of
unactivated C(sp3)−H bonds with benzylic C(sp3)−H bonds
of toluene derivatives.7 Recently, Ge’s group reported a
protocol for cross dehydrogenative alkylation of substituted
arenes.8 Zhang’s group reported a beautiful work on oxidative
coupling/cyclization of aliphatic amides and terminal alkynes
by using a Co/Ag cocatalyst.9 However, the reaction failed to
give the alkynylated product. When we were preparing this
paper, Lei’s group developed an elegant work on Cu/Ni/Ag
multimetallic catalyzed radical oxidative cross-coupling of
unactivated C(sp3)−H in alkanes with terminal alkynes.10
Alkynes are among the most versatile building blocks in
organic synthesis and widely applied in chemical biology and
material science as precursors.11 The Sonogashira reaction is a
reliable and efficient method for construction of a C(sp2)−
C(sp) bond.12 In comparison, the construction of C(sp3)−
Scheme 1. Previous Works and Our Strategy for Alkyl
Alkynes Synthesis
extended this chemistry with a special ligand set18 or
catalyst19,20 or to broaden the substrate scope.21,22 Very
recently, Li and co-workers reported an example of performing
a photopromoted transition-metal-free coupling between
alkyne and alkyl iodide in water.23 On the other hand,
transition-metal-catalyzed coupling of unactivated C(sp3)−H
bonds with alkynyl halides provided another route to synthesize
alkyl alkynes.24−26 From the viewpoint of atom- and step-
economy, the oxidative coupling of unactivated C(sp3)−H
bonds with terminal alkynes would be more ideal and attractive.
Herein, we disclose the site-selective C(sp3)−C(sp) bond
Special Issue: Hydrocarbon Chemistry: Activation and Beyond
Received: July 25, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00529
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Communication
formation to synthesize alkyl alkynes through Ni-catalyzed
oxidative coupling of unactivated C(sp3)−H bonds with
terminal alkynes by using the bidentate directing strategy.
To pursue this goal, one of the most challenging points is to
avoid the homocoupling of terminal alkynes, which is prone to
proceed under oxidative conditions. Thus, a proper oxidant
should be intensively investigated to oxidize the low valence of
catalytic metal species while suppressing the homocoupling of
terminal alkynes. Another challenge is to find a proper
transition-metal catalyst, which could efficiently promote both
the activation of the unactivated C(sp3)−H bonds and the
reductive elimination to construct the C(sp3)−C(sp) bond.
Inspired by recent exploration of Ni-catalyzed C(sp3)−H
functionalization27 and the successful bidentate directing
strategy in unactivated C(sp3)−H functionalization,28 we set
out to investigate the oxidative coupling of an unactivated
C(sp3)−H bond with terminal alkynes with a bidentate
directing group via Ni catalysis (Table 1). Initial screenings
A number of ligands were further extensively tested with NiBr2
and Me2S·CuBr (Table S7). Davephos improved the efficacy
significantly with 35% NMR yield (entry 5).
Decreasing the loading of Me2S·CuBr (entry 6) further
improved the efficacy and gave the desired product in 40%
NMR yield. Gratifyingly, increasing the catalyst and ligand
loading also improved the activity (entry 9). Next the cosolvent
effect was examined. A trace amount of PhCN and N-
methylpyrrolidone (NMP) as cosolvent slightly improved the
activity (entry 10). Finally, increasing the Cs2CO3 amount from
3.0 equiv to 5.0 equiv gave the alkynylated product in 52%
isolated yield (entry 11). Notably, other transition-metal salts,
such as Co(acac)2, CoBr2, or Fe(acac)2, were not suitable
(Table S8). Control experiments indicated that the catalyst
(entry 11), ligand (entry 13), transmetalating reagent (entry 8),
and oxidant (entry 14) were all necessary. Unfortunately, other
efforts could not promote the efficacy at this stage.
With the optimized conditions in hand, we set out to expand
the substrate scope of aliphatic acid (Table 2). To our delight,
Table 1. Optimization
Table 2. Substrate Scope of Aliphatic Amides
a
entry
[M] (x)
ligand (x)
additive (y)
results
13%
1
2
3
4
5
6
7
8
9
Ni(acac)2 (10) DPPBz (10)
Ni(acac)2 (10) DPPE (10)
Cu2O (20)
Cu2O (20)
Cu2O (20)
13%
18%
NiBr2 (10)
NiBr2 (10)
NiBr2 (10)
NiBr2 (10)
NiBr2 (10)
NiBr2 (10)
NiBr2 (20)
NiBr2 (20)
NiBr2 (20)
−
DPPE (10)
DPPE (10)
Me2S·CuBr (20) 28%
Davephos (10) Me2S·CuBr (20) 35%
Davephos (10) Me2S·CuBr (10) 40%
Davephos (10) Me2S·CuBr (5)
Davephos (10) Me2S·CuBr (0)
Davephos (20) Me2S·CuBr (10) 45%
Davephos (20) Me2S·CuBr (10) 48%
35%
21%
b
10
c
d
11
Davephos (20) Me2S·CuBr (10) 55% (52% )
Davephos (20) Me2S·CuBr (10) <5%
12
13
NiBr2 (20)
−
Me2S·CuBr (10) <5%
a
b
e
Isolated yield. brsm, based on the recovered starting material.
14
NiBr2 (20)
Davephos (20) Me2S·CuBr (10) 15%
a
b
NMR yield, using dibromomethane as the internal standard. PhCN/
NMP (10 μL). PhCN/NMP (10 μL), Cs2CO3 (5 equiv). Isolated
c
d
e
different alkyl-, phenyl-, or benzyl-substituted amides were
tolerated well. Both cyclic amides (1a) and acyclic amides (1b−
d) performed well to produce the alkynylated products in
moderate to good isolated yields. In addition, the reaction
preferred to occur at the methyl group beyond the methylene
groups in substrate 1c and 1d due to steric hindrance. More
importantly, only a monoalkynylated product was observed for
substrates 1b−d, which might be attributed to the steric effect
of the generated alkynylated products. We further screened
phenyl-substituted amides (1e−i). The amides containing a
phenyl ring equipped with electron-donating substituents, for
example, tert-butylphenyl (1f) and a methoxy group (1g and
1h), proceeded well and delivered the corresponding
alkynylated products in moderate yields. To our interest,
substrate 1i, with an electron-withdrawing trifluoromethyl
group, exhibited better activity, and the desired product was
isolated in 63% yield. The structure of 1i was further confirmed
by X-ray crystallography data (Figure 1). This result
unambiguously solidified the success of oxidative coupling of
unactivated C(sp3)−H bonds with terminal alkynes. It should
be mentioned that the reaction showed a predominant
preference for the C(sp3)−H bonds of the methyl group over
the C(sp2)−H bonds of the phenyl group (1e−i), indicating
yield. In the absence of Cu(OAc)2.
were carried out with 1a and ethynyltriisopropylsilane 2, in
combination with Ni(acac)2 as catalyst, DPPBz as ligand, Cu2O
as transmetalating reagent, Cu(OAc)2 as oxidant, and Cs2CO3
as base in 1,2-dimethoxyethane (DME) at 140 °C under N2. To
our delight, we observed the desired product in 13% NMR
yield with the use of CH2Br2 as internal standard (entry 1).
More importantly, compared to the previous report,9 the
cascade cyclization of the alkynylated product did not occur. A
remarkable solvent effect was observed and only 1,2-dichloro-
ethane (DCE) gave good results; other solvents were not
suitable (Table S1). A number of bases were examined. Cs2CO3
was found the best choice (Table S2). Notably, only Cu(OAc)2
was workable as an oxidant (Table S3). The phosphine ligands
were also tested. DPPE exhibited comparable activity with
DPPBz (entry 2), while others gave worse results (Table S4).
Further screening on different nickel salts (Table S5) indicated
that NiBr2 outperformed the others to give the product in 18%
NMR yield (entry 3). Gratifyingly, after intensively screening
transmetalating reagent Cu(I) salts (Table S6), Me2S·CuBr
significantly improved the efficacy to 28% NMR yield (entry 4).
B
DOI: 10.1021/acs.organomet.6b00529
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Organometallics
Communication
by the treatment of equivalent amide 1a and 1a-d3 in a vessel
(2.03) (eq 5).
Both of them (KIE > 2.0) are large, suggesting that the
cleavage of the C−H bond was involved in the rate-determining
step. Moreover, when we added TEMPO in our standard
reaction to determine potential radical species (eq 6), the
reaction efficacy decreased significantly, and 15% isolated yield
of desired product was obtained, suggesting that the radical
might be involved.
On the basis of current results, we proposed a plausible
catalytic cycle as shown in Scheme 2. The substrate 1a
Figure 1. ORTEP of drawing of internal alkyne 3i.
that in the metalation step a 5/5-membered fused ring
intermediate was favored over a 5/6-membered intermediate.
For benzyl-substituted substrates (1j−m), the regioselectivity
preferred to proceed at the β-methyl group over the more
active β-benzyl group, arising from steric effects. Gratifyingly,
the chloro (1k) or bromo (1l) functional group survived well,
albeit in low yields. Finally, diphenyl acids (1n−p) can be
smoothly converted to the alkynylated product in acceptable
yields. This reaction exhibited good mass balance, and the
starting materials were recovered in all cases. It is noteworthy
that only triisopropylsilyl acetylene 2 can serve as the alkynyl
reagent in this reaction, which is consistent with a previous
report.29
Scheme 2. Plausible Catalytic Cycle
Removing the protecting silyl group afforded the terminal
alkyne 4b-1 in 75% isolated yield, which can be used to conduct
diverse transformations (eq 1). The amide 3b can easily remove
the 8-aminoquinoline directing group in refluxing EtOH with
the base NaOH to give alkynyl acid 4b-2 in 70% isolated yield.
coordinated to Ni(II) species A through a ligand exchange
under alkaline condition to generate B, which gave the
intermediate C through C−H activation with the assistance
of base. Subsequently, a single-electron oxidation of C by
oxidant Cu(OAc)2 afforded Ni(III) complex D (path a),27,30
which was further converted to E with an alkynyl ligand
through a transmetalation process with alkynyl Cu(I) species 4,
which was in situ generated from a Cu(I) complex and alkynyl
reagent. Finally, reductive elimination of E generated the
alkynylated product 3a with the release of Ni(I) complex F,
which was further oxidized to the Ni(II)species A to fulfill the
catalytic cycle. In addition, an alternative pathway (path b) to
generate the species E from C through oxidative addition of
alkynyl radical 5, which would be generated from the alkynyl-
copper species, cannot be ruled out.10,31
To unveil the catalytic pathway, preliminary mechanistic
studies were conducted. The deuterated 1a-d3 was submitted to
standard reaction conditions in the absence of terminal alkynes,
and the scrambling of proton−deuterium was not observed in
the recovered starting material 1a-d3 (eq 2). This observation
In conclusion, we disclosed the oxidative coupling of
unactivated C(sp3)−H bonds with terminal alkynes by using
a synergetic Ni/Cu cocatalyst. Different amides exhibited good
compatibility in this reaction. The reaction preferred to occur at
the methyl group over the secondary C(sp3)−H bond and
benzyl methylene group. Confirmation of the crystal structure
of alkynylated product 3i further confirmed the success of this
direct alkynylation. From the preliminary mechanistic studies, a
plausible pathway was proposed. Further studies to completely
understand the reaction mechanism, to promote the reaction
efficacy, and to expand the substrate scope are under way.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00529.
■
S
indicated that the C−H activation step was irreversible. An
intermolecular kinetic isotope effect (KIE) experiment was
conducted independently to calculate their initial rate constant
(kH for 1a and kD for 1a-d3) to determine the intermolecular
KIE (2.28) (eq 3 and eq 4). A competition KIE was performed
Experimental procedures, detailed optimization data, and
characterization data of all compounds (PDF)
Crystallographic data for 3i (CCDC-1432770) (CIF)
C
DOI: 10.1021/acs.organomet.6b00529
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Communication
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AUTHOR INFORMATION
Corresponding Author
*E-mail: zshi@pku.edu.cn.
Notes
■
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Support of this work by the “973” Project from the MOST of
China (2013CB228102, 2015CB856600) and NSFC (No.
21332001) is gratefully acknowledged.
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D
DOI: 10.1021/acs.organomet.6b00529
Organometallics XXXX, XXX, XXX−XXX