Controllable Rh(III)-Catalyzed C-H Arylation and Dealcoholization: Access to Biphenyl-2-carbonitriles and Biphenyl-2-carbimidates

Article abstract of DOI:10.1021/acs.orglett.8b02915

A controllable Rh(III)-catalyzed C-H arylation and dealcoholization of benzimidates with arylboronic esters was developed, delivering various biphenyl-2-carbonitriles and biphenyl-2-carbimidates by simply tuning the reaction conditions. This approach features high efficiency, good functional group tolerance, and easy operation. It also provides an alternative pathway to thoroughly exploit the directing group in transition-metal-catalyzed C-H activations.

Full text of DOI:10.1021/acs.orglett.8b02915

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Controllable Rh(III)-Catalyzed C−H Arylation and Dealcoholization:
Access to Biphenyl-2-carbonitriles and Biphenyl-2-carbimidates
Bo Jiang,† Songxiao Wu,† Jing Zeng, and Xiaobo Yang*
Institute of Catalysis for Energy and Environment, College of Chemistry & Chemical Engineering, Shenyang Normal University,
Shenyang, Liaoning 110034, China
S
* Supporting Information
ABSTRACT: A controllable Rh(III)-catalyzed C−H aryla-
tion and dealcoholization of benzimidates with arylboronic
esters was developed, delivering various biphenyl-2-carbon-
itriles and biphenyl-2-carbimidates by simply tuning the
reaction conditions. This approach features high efficiency,
good functional group tolerance, and easy operation. It also
provides an alternative pathway to thoroughly exploit the
directing group in transition-metal-catalyzed C−H activations.
hodium (III)-catalyzed direct C−H transformation has
Even though several pioneering works on direct C−H arylation
reactions without any directing group have been reported,11 the
rational utilization of directing groups is still challenging.
Biphenyl-2-carbonitriles, as versatile synthetic intermediates,
are of great importance in the chemical synthetic industry.12
Traditional approaches to access these compounds were
accomplished via Pd-catalyzed Suzuki reactions of ortho-
halogenated aromatic carbonitriles (Scheme 2a).13 In contrast,
transition-metal-catalyzed C−H ortho-arylation is privileged in
the synthesis of them due to high atomic efficiency and low
cost.10 In 2011 and 2013, Sun’s group14 and Hsieh’s group15
independently reported a Pd-catalyzed C−H ortho-arylation
using cyano as the directing group for the synthesis of biphenyl-
2-carbonitriles (Scheme 2b). It has shown good yields and step-
R
emerged in the past few years as a powerful and versatile
strategy for the construction of valuable molecules.1 Among
them, readily available benzimidates, which act as both directing
groups and synthons for cyclization, are always employed as the
reaction partners.2 In previous works, sequential Rh(III)-
catalyzed direct C−H activations and intramolecular nucleo-
philic additions frequently occurred between benzimidates and
the coupling reactants such as α-diazocarbonyl compounds,3
azides,4 anthranils,5 nitrosobenzenes,6 dioxazolones,7 alkenes,8
sulfur ylides,9 etc., thus furnishing isoquinolines, quinazolines,
and other heterocycles (Scheme 1). These protocols provide a
Scheme 1. Previous Works on Rh(III)-Catalyzed Direct C−H
Transformation of Benzimidates
Scheme 2. Strategies for the Synthesis of Biphenyl-2-
carbonitriles
facile and step-economical strategy to access important N-
containing heterocycles. However, in these transformations, the
directing groups generally disappear, which deprives the chance
for further functionalization based on N-unsubstituted imines.
Recently, transition-metal-catalyzed C−H ortho-arylation of
arenes represents a promising tool to form aryl−aryl bonds,10 in
which the directing group is indispensable because of the
chelation-assisted strategy.10j Therefore, additional synthetic
operations to manipulate the directing group issue are desirable.
Received: September 12, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02915
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Organic Letters
Letter
economy, but corrosive strong acid of TFA was used as the
solvent, making this transformation less attractive. Recently,
Hong and co-workers disclosed a novel Pd−diimine complex for
the direct C−H ortho-arylation of simple arenes (Scheme 2c).16
Even though the corresponding biphenyl-2-carbonitrile was
obtained in moderate yield, the regioselectivity was still
unsatisfactory.
To address the above-mentioned issues and approach closer
to the rational utilization of directing groups, as the continuation
of our interest on Rh(III)-catalyzed C−H activations,17 herein,
we report a novel Rh(III) catalytic system for the direct C−H
ortho-arylation of benzimidates using common arylboronic
esters as the arylation sources. This practical catalytic system
allowed controllable synthesis of biphenyl-2-carbonitriles and
biphenyl-2-carbimidates through simple tuning of the reaction
conditions.
and 28% yields, respectively (entries 2 and 6). In comparison to
DME and other solvents, DME is the best solvent choice (entry
3). Three other Ag salts were screened in addition to AgF, all of
which provided lower yields (entries 7−9). Employing Cu-
(OAc)2 as the oxidant mainly led to an extensive decomposition
of 1a (entry 10). Switching the metal catalyst to [Cp*IrCl2]2/
AgSbF6 only produced a 17% yield of 3aa and afforded some
dual ortho-arylated product (entry 11 and see details in the
Supporting Information (SI) section V). When the Rh catalyst
was replaced with three other transition metal catalysts,
including Cp*Co(CO)I2, [RuCl2(p-cymene)]2, and
Mn2(CO)10, no target product 3aa or C−H ortho-arylation
product was detected (entries 12−14). Control experiments
verified the importance of proper reaction temperature (entries
15 and 16) as both lower and higher reaction temperature fell
short of high reaction efficiency. In addition, when the reaction
was performed under pure oxygen atmosphere, the yield of 3aa
decreased to 61% (entry 17).
To evaluate the feasibility of this Rh(III) catalytic system, as
shown in Table 1, we launched our study with the reaction of
With the optimized C−H arylation and dealcoholization
conditions in hand, various arylboronic acid pinacol esters were
selected to realize this sequential C−H arylation and deal-
coholization. As depicted in Scheme 3, a series of para-
a
Table 1. Optimization of Reaction Conditions
Scheme 3. Arylboronic Acid Pinacol Esters Scope in Rh(III)-
Catalyzed Sequential C−H Arylation and Dealcoholization
b
entry
catalyst
oxidants
AgF
AgF
AgF
AgF
solvent
DCE
yield (%)
1
2
3
4
5
6
7
8
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*IrCl2]2/AgSbF6
Cp*Co(CO)I2/AgSbF6
[RuCl2(p-cymene)]2
Mn2(CO)10
64
12
80
trace
64
28
trace
22
13
0
e
EGME
f
DME
EtOH
toluene
H2O
AgF
AgF
AgOAc
Ag2CO3
Ag2O
Cu(OAc)2
AgF
AgF
AgF
AgF
AgF
DME
DME
DME
DME
DME
DME
DME
DME
DME
DME
DME
9
10
11
12
13
14
17
0
0
0
c
15
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgSbF6
48
50
61
d
16
g
AgF
AgF
17
a
Reaction conditions: 1a (0.2 mmol), 2a (0.1 mmol), catalyst (2.5
mol %), AgSbF6 (10 mol %), oxidant (0.2 mmol), and NaOPiv·H2O
substituted phenylboronic acid pinacol esters participated in this
transformation successfully, giving the corresponding biphenyl-
2-carbonitriles in moderate to good yields (3aa−3af). Several
useful functional groups such as ether (3ac), fluoro (3ad), and
chloro (3ae) were well tolerated. Apart from para-substituted
phenylboronic acid pinacol esters, (3-(trifluoromethyl)phenyl)
boronic acid pinacol ester (1g) also reacted with 1a smoothly,
delivering 3ag in 73% yield. Notably, thiophen-3-ylboronic acid
pinacol ester (1h) as a valuable heterocyclic source was proven
as a viable substrate to produce 3ah in 72% yield.
Furthermore, the generality and compatibility of this
sequential C−H arylation and dealcoholization were then
scrutinized using of a wide variety of functionalized
benzimidates (Scheme 4). In most cases, the reaction proceeded
well, producing the desired biphenyl-2-carbonitriles efficiently.
Ethyl benzimidates, regardless of containing electron-donating
or electron-withdrawing groups, all resulted in good and
excellent yields (3bc−3dc, 3bi, 3ci, 3ba, 3bj, 3be, 3bh),
suggesting that this catalytic activity was not predominantly
b
(20 mol %) in solvent (1 mL) at 120 °C under air for 5 h. Isolated
c
d
yield. Reaction temperature is 80 °C. Reaction temperature is 150
e
f
°C. EGME = 2-ethoxyethanol. DME = 1,2-dimethoxyethane.
Under pure oxygen atmosphere.
g
ethyl benzimidate (1a) with phenylboronic acid pinacol ester
(2a), employing [Cp*RhCl2]2/AgSbF6 as the catalyst, DCE as
the solvent, in the presence of AgF and NaOPiv·H2O at 120 °C
for 5 h. Fortunately, the biphenyl-2-carbonitrile 3aa was
obtained in 64% yield through Rh(III)-catalyzed C−H arylation
and dealcoholization. To identify the optimized reaction
conditions, several other solvents such as DME or EGME
were tested, employing [Cp*RhCl2]2/AgSbF6 as the catalyst in
the presence of AgF. Toluene showed a similar performance,
delivering the desired product 3aa in 64% yield (entry 5). A trace
amount of 3aa was obtained in EtOH, and the major product
was the C−H ortho-arylation product (entry 4). EGME and
water were also capable of realizing this reaction, giving 3aa in 12
B
DOI: 10.1021/acs.orglett.8b02915
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Benzimidates’ Scope in Rh(III)-Catalyzed
Sequential C−H Arylation and Dealcoholization
Scheme 5. Rh(III)-Catalyzed C−H Arylation for the
Synthesis of Biphenyl-2-carbimidates
possible to synthesize biphenyl-2-carbimidate 4aa via traditional
nucleophilic addition from the corresponding aromatic nitriles
(Scheme 5b).
To further gain insight into the reaction mechanism,
preliminary experiments were performed, as demonstrated in
Scheme 6. At the outset, the simple benzonitrile was employed
Scheme 6. Experiments for the Mechanistic Study
affected by electronic variations. The other three alkyl
benzimidates also provided the sequential arylation and
dealcoholization products in good yields (3aa). Substituents at
different positions on the phenyl ring of benzimidates showed
no obvious influence on the yields, giving one regioisomer each
(3ic, 3ja). It is noteworthy that the reaction standard conditions
tolerated some useful functional groups such as fluoro (3ic,
3cd), chloro (3ka, 3be), iodo (3ec), and acetyl (3bj), providing
a potential possibility for further elaboration. Surprisingly, an
aldehyde group in benzimidate was well compatible during the
reaction, delivering the formyl product 3fc in 70% yield.
Moreover, imidates containing fused phenyl rings and
heterocyclic rings were also able to carry out this reaction with
good regioselectivity, affording the desired 2-carbonitriles (3gc,
3hc) in satisfactory yield.
In the above Rh(III)-catalyzed reactions, C−H arylation and
dealcoholization always occurred together, along with trans-
formation of the directing group into nitriles. Although aromatic
nitriles play a central role in the synthetics industry, under some
circumstances, the directing group still needs to remain. To
achieve this goal, further optimization was conducted (see
details in the SI section VI), and the biphenyl-2-carbimidates
4aa and 4la with the directing group reserved were successfully
obtained under very mild reaction conditions (Scheme 5a). This
transformation was accomplished easily by adjusting the
substrate ratio, the solvent, and the reaction temperature.
With the aid of the imidate, here is a chance to fulfill the second
C−H activation and cyclization for the synthesis of more useful
molecules. More importantly, after the verification, it was not
to achieve this transformation under the standard reaction
conditions (Scheme 6a), and no conversion was observed,
indicating that the cyano group could not coordinate with the
Rh(III) complex to furnish the direct C−H ortho-arylation. In
the above Rh(III)-catalyzed sequential C−H ortho-arylation and
dealcoholization, direct C−H ortho-arylation was deemed to be
the first step. The kinetic isotope effect (KIE) was determined to
be 3.5 through an intermolecular competitive reaction using 1a
and [D5]-1a as the substrates (Scheme 6b). The KIE result
reveals that the C−H bond cleavage probably involves in the
turnover-limiting step.
Based on the above preliminary experiments and previous
reports on Rh- or Ir-catalyzed direct C−H arylation,10,18,19
a
possible mechanism for this controllable Rh(III)-catalyzed C−
H arylation and dealcoholization is proposed as follows (Scheme
7). First, [Cp*RhCl2]2 reacts with AgSbF6, followed by the C−
H cleavage of 1a, to afford a cyclometalated Rh(III) species I.
Then a transmetalation occurs between the Rh(III) complex I
and an in situ generated arylboronic ester with silver fluoride to
produce intermediate II. Next, Rh(IV) intermediate III is
formed from the oxidation of intermediate II with Ag(I).
Subsequently, the desired product 4aa and Rh(II) species IV are
generated from the reductive elimination. Finally, the Rh(II)
species IV is oxidized by another equivalent of Ag(I) to
regenerate the Rh(III) complex I; meanwhile, the other product
3aa is probably obtained via dealcoholization.
In summary, we have developed a facile and easy-to-handle
protocol for the synthesis of biphenyl-2-carbonitriles and
C
DOI: 10.1021/acs.orglett.8b02915
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Environment-Major Platform for Science and Technology of the
Universities in Liaoning Province.
Scheme 7. Tentative Mechanism
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ORCID
Xiaobo Yang: 0000-0003-0684-8419
Author Contributions
†B.J. and S.W. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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The authors are grateful for support of this work from the
National Natural Science Foundation of China (Grant No.
21402126), Natural Science Foundation of Liaoning Province
(20180550882), the Basic Scientific Project of Universities in
Liaoning Province (LFW201701), the Program for Young
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