Rhodium(III)-Catalyzed Oxidative Annulation of Amidines with Alkynes via Sequential C?H Bond Activation

Article abstract of DOI:10.1002/ejoc.202001634

In this paper, a rhodium-catalyzed sequential two-fold ortho-C?H functionalization of N-phenylbenzimidamide with internal alkyne is reported. The double C?H activations proved viable in a one-pot fashion with the assistance of C=N and C?N bonds, providing a series of benzimidazoisoquinolines with high levels of positional selectivity control. The operationally simple transformation showed high functional group compatibility and featured the cleavage of C?H bonds located on different a moiety of the N-phenylbenzimidamide substrates. Detailed mechanistic studies provided strong support for C?H bond cleavage on the N-phenyl ring to be preferential compared with C?H bond cleavage on C-phenyl ring. As a multifunctional catalytic platform, the rhodium catalyst conducted two independent and compatible catalytic cycles in one pot.

Full text of DOI:10.1002/ejoc.202001634

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Rhodium(III)-Catalyzed Oxidative Annulation of Amidines
with Alkynes via Sequential CÀ H Bond Activation
Yan-Yu Meng+,[a] Wen-Jing Zhu+,[b] Yuan-Yuan Song,[b] Gang-Gang Bu,[b] Li-Juan Zhang,[b] and
Fen Xu*[b]
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In this paper, a rhodium-catalyzed sequential two-fold ortho-
CÀ H functionalization of N-phenylbenzimidamide with internal
alkyne is reported. The double CÀ H activations proved viable in
a one-pot fashion with the assistance of C=N and CÀ N bonds,
providing a series of benzimidazoisoquinolines with high levels
of positional selectivity control. The operationally simple trans-
formation showed high functional group compatibility and
featured the cleavage of CÀ H bonds located on different a
moiety of the N-phenylbenzimidamide substrates. Detailed
mechanistic studies provided strong support for CÀ H bond
cleavage on the N-phenyl ring to be preferential compared with
CÀ H bond cleavage on C-phenyl ring. As a multifunctional
catalytic platform, the rhodium catalyst conducted two inde-
pendent and compatible catalytic cycles in one pot.
Introduction
Nitrogen-containing heterocyclic compounds are ubiquitous
features in a plethora of natural products, synthetic compounds
with conspicuous biological activities,[1] and functional
properties.[2] Among them, π-conjugated polycyclic heteroaryl
scaffolds have received special attention owing to their photo-
luminescence properties,[3] electrochemical properties,[4] and
pharmaceutically activities[5] (Scheme 1a). Benzimidazoisoquino-
lines, as classical π-conjugated polycyclic heteroaryl com-
pounds, are widely found in biologically active molecules,[6] and
can be potentially utilized as fluorescent labels due to the
aggregation-induced emission (AIE) phenomenon.[7] Due to its
importance, there exists a continued imperative demand for the
highly efficient and selective construction of benzimidazoiso-
quinoline derivatives.
Traditionally, synthesis of benzimidazoisoquinolines has
thus far largely relied on oxidative CÀ H/NÀ H annulation of
phenylbenzimidazole with alkyne[8] or multistep methods[9]
(Scheme 1b). As a paradigm, Miura and co-workers[10] succes-
sively demonstrated chelation-assisted CÀ H activation to syn-
thesize imidazoisoquinoline from 2-phenyl-1H-benzimidazole
and alkyne, wherein an additive (1,2,3,4-tetraphenylcyclo-
pentadiene) was required. Following this pioneering work,
cobalt(III)-catalyzed or nickel-catalyzed CÀ H activation of sub-
stituted (benz)imidazoles were independently reported by
Sen[11] and Chatani.[12] Very recently, the group of Ackermann
Scheme 1. Representative examples of benzimidazoisoquinolines synthesis.
developed the ruthenium-catalyzed dehydrogenative annula-
tion reaction of imidazoles with alkynes through an electro-
chemical CÀ H/NÀ H annulation.[13] While these strategies are of
solid reliability and have been frequently used, the multiple
CÀ H bond activations for the rapid construction of benzimida-
zoisoquinolines from readily available substrates is quite
promising, in which several new bonds were formed in a single
operation.
Amidine has been used as a flexible reaction partner for
direct CÀ H activation reaction with alkynes,[14] which include: 1)
the C=N-aided monoannulation of amidines with alkynes to
easily fabricate isoquinoline, 2) the incorporation of two alkyne
units into specific substrates to generate indole-functionalized
[a] Y.-Y. Meng+
Department of College of Science,
Henan Agricultural University,
Zhengzhou 450002, P. R. China
[b] W.-J. Zhu,+ Y.-Y. Song, G.-G. Bu, L.-J. Zhang, Dr. F. Xu
Department of Material and Chemical Engineering
Zhengzhou University of Light Industry
Zhengzhou 450002, P. R. China
E-mail: fenxu_zzuli@163.com
[+] Y.-Y. Meng and W.-J. Zhu contributed equally to this work.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202001634
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isoquinoline[15] and 1H-benzo[de][1,8]naphthyridine derivate,[16]
Table 1. Optimization of Reaction Conditions.[a]
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3) the fourfold CÀ H activation of amidines with alkynes, leading
to π-conjugated polycyclic heteroaryl scaffolds, 4) ortho-
alkenylation of aromatic amidines with alkynes,[17] and others.
However, the twofold annulation of amidines with alkynes to
furnish benzimidazoisoquinolines has generally remained un-
developed. One of the reasons for this is the absence of a
general and reliable catalytic system for simultaneous activation
of CÀ H bonds at different positions. It was observed that the
oÀ C(sp2)À H bond of N-phenyl ring is specifically activated under
identical conditions rather than C- phenyl ring of
amidine.[14a,15,18] Thus, it offers the possibility to exploit double
CÀ H bond activation conducted by sequential difunctionaliza-
tion of N-phenylbenzimidamide in one pot. Herein, we reported
Rh-catalyzed oxidative CÀ H/NÀ H annulation of amidines with
alkynes to assemble benzimidazoisoquinoline skeletons. In this
cascade reaction, one CÀ C and two CÀ N bonds are rapidly
formed to build the molecular complexity. Moreover, this
protocol also features easily accessible starting materials and
mild reaction conditions.
Entry
Catalyst
Additive 1
Additive 2
Yield[b]
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[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
–
Na2CO3
NaF
(CH3)4NOH.5H2O
CH3COONa
–
–
–
–
–
–
–
–
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27
N.D.
45
N.D.
N.D.
N.D.
N.D.
N.D.
55
25
75
N.D.
trace
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NBu4F (1 equiv)
NBu4F (2 equiv)
NBu4F (3 equiv)
NBu4F (4 equiv)
NBu4F (2 equiv)
NBu4F (2 equiv)
NBu4F (2 equiv)
NBu4F (2 equiv)
NBu4F (2 equiv)
NBu4F (2 equiv)
NBu4F (2 equiv)
–
9[c]
10[d]
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–
–
AgSbF6
AgOTf
AgBF4
Ag2CO3
pivalic acid
pivalic acid
pivalic acid
pivalic acid
pivalic acid
NBu4F (2 equiv)
NBu4F (2 equiv)
NBu4F (2 equiv)
[Cp*RhCl2]2
Results and Discussion
[a] Reaction conditions: [Cp*RhCl2]2 (2.5%), alkyne 2a (0.125 mmol,
22.8 mg), amidine 1a (1.5 equiv), Cu(OAc)2 (2 equiv), additive 1 (x equiv),
additive 2 (20%), toluene/DCE (v/v=1:4, 4 mL), 80 C, 48 h. [b] Isolated
°
To begin our studies, we chose N-phenylbenzimidamide 1a and
diphenylacetylene 2a as the model substrates. Inspired by our
previous work,[19] the reaction was performed in the presence of
2.5 mol% [Cp*RhCl2]2 and 1.0 equiv. of Na2CO3 in toluene/DCE
yield. [c] DCE (3 mL). [d] Toluene (3 mL). [e] PIFA (1 equiv) and CF3CH2OH
°
°
(2 ml) were added after reaction for 80 C. [f] 40 C.
°
at the temperature of 80 C. The results are summarized in
Table 1. Gratifyingly, the desired product 5,6-diphenylbenzo
[4,5]imidazo[2,1-a]isoquinoline 3aa was obtained in 40% yield
after 48 h (Table 1, entry 1). Inspired by this initial result, we
checked the effect of various additives like NaF,
(CH3)4NOH·5H2O, CH3COONa, and NBu4F (entries 2–5). The
results showed that NBu4F was proven to be the better choice,
affording 3aa in 46% yield (entry 5). A slight improvement of
the yield was observed upon increasing the loading of NBu4F to
2 equiv, whereas an obvious decrease was detected on
increasing the amounts of NBu4F to 3 or 4 equiv. (entries 7 and
8). The reaction gave a lower yield (45%) when DCE was
employed as a single solvent, while no desired product was
obtained in toluene (entries 9 and 10). Further optimization was
performed using different silver salts such as AgSbF6, AgOTf,
AgBF4, and Ag2CO3, but in all these cases, no product was
observed (entries 11–14). The yield was improved to 55% with
the assistance of pivalic acid, and byproduct 1-aminoisoquino-
line 3a was obtained in 29% yield (entry 15). Notably, when the
reaction was conducted without NBu4F, the yield decreased to
25%, which suggested that NBu4F played an important role in
the reaction (entries 15 versus 16). To our delight, a further
increase in the yield was achieved by post-treatment of the
reaction mixture to CF3CH2OH and PIFA, then stirred for 4 h,
delivering a satisfactory yield of 3aa in 75% yield (entry 17). We
have elaborated the reaction between 1a and 2a, and the
result demonstrated that the starting material 2a remained in
10% yield before the PIFA treatment. A control experiment
demonstrated that no product was formed in the absence of
[Cp*RhCl2]2 (entry 18), and further investigation revealed that
decreasing the reaction temperature to 40 C afforded a trace
amount of inseparable mixture (entry 19).
°
With the optimal reaction conditions in hand, we started to
investigate the scope of the cascade between amidines 1a–1p
and alkyne 2a. As summarized in Scheme 2, we were pleased to
find that a range of amidines, regardless of the electronic
nature and the positions, could be successfully employed in the
reaction. The desired products 3aa–3pa were furnished with a
yield up to 81%, wherein the structures of 3ja and 3ma were
verified unambiguously by X-ray crystallography (CCDC:
2039498 and 2039499, see the Supporting Information). More-
over, amidine bearing a meta moiety on C-phenyl ring was
adopted to this protocol, as expected, the CÀ H bond with less
steric hindrance was selectively transferred to the target
product 3ka. Furthermore, 2-fluoro-N-phenylbenzimidamide 1j
was attempted in the reaction and underwent the reaction,
albeit with 21% yield. We next turned our attention to the
substitutions on the N-phenyl ring. When the reaction was
extended to N-(3-chlorophenyl)benzimidamide 1l and N-(3-
methoxyphenyl) benzimidamide 1m, products 3la and 3ma
were exclusive obtained in similarly yields. This phenomenon
indicated that the second CÀ H activation on N-phenyl favored
the pathway in a less hindered manner. Notably, the electronic
nature of the 4-position substituted N-phenyl moiety in amidine
has a distinguished effect on the reaction efficiency (Scheme 2).
The opposite is in the case that substrates with electron-
donating groups delivered two regioisomers in higher yields
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Scheme 3. Scope of alkynes.
ated, furnishing the desired products 3aa–3aq in reasonable
yields. Alkyl, halogen, and methoxy functionalities were com-
patible with the reaction conditions. Especially, a variety of
alkynes with electron-donating groups at the para-positions on
the aromatic ring reacted smoothly with 1a to construct the
corresponding products (3ab–3ae) in good yields (63%–73%).
The methyl substituent at the ortho-position of the phenyl ring
failed to provide the desired product, which may be attributed
to steric effects. The electron-withdrawing substituents were
also tolerated in this protocol but furnished the products in
absolutely lower yields (3af–3ah). The cascade reactions of
asymmetrically disubstituted alkynes proceeded smoothly, and
two possible regioisomers were achieved in moderate yields.
However, terminal alkynes could not react with 1a under the
standard reaction conditions. When 3-hexyne was employed,
no desired product was obtained. Moreover, the reaction of 1a
with a terminal alkyne such as phenylacetylene under the
standard conditions failed to afford the desired product, where-
as homocoupling product 1,4-diphenylbuta-1,3-diyne was ob-
tained as a major product.
Scheme 2. Reaction scope of amidines.
(3qa–3xa), while amidines bearing electron-withdrawing
groups gave products in lower yields (3na and 3oa).
Having identified the optimal template, we next focused
our attention on the substrate scope with respect to alkynes. As
shown in Scheme 3, electron-neutral, electron-donating, or
electron-withdrawing substituents were generally well-toler-
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To elucidate the reaction mechanism, the reaction of
mechanism, we performed an additional study. Treatment of 4c
with 2a afforded the desired product 3aa in 73% yield under
standard conditions (Scheme 5). The reaction may be carried
out according to path A, or it may be conducted simultaneously
with two possible paths.
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amidine 1q and 2a was re-analyzed, and two possible paths
are proposed (Scheme 4). For path A, CÀ H functionalization
occurs preferentially at the CÀ H bond on ring A, and
subsequent CÀ H functionalization on ring B gives 3qa. As a
more reasonable alternative Path B, the reaction of 1q readily
generates 6-methoxy-2-phenyl-1H-benzo[d]imidazole 4b, which
then undergoes CÀ H activation/[4+2] annulation to form 3qa
and 3ra. In addition, the reaction towards to amidines 1s, 1u,
and 1w bearing electron-donating group at 4-position also
delivered strong support for CÀ H bond cleavage on N-phenyl
ring is preferential than CÀ H bond on C-phenyl ring. This would
be premature to provide an explanation of the reactivity and
selectivity. To develop a better understanding of the reaction
On the basis of the above mechanistic studies and previous
reports,[20] we proposed a plausible mechanism as depicted in
Scheme 6. As a multifunctional catalytic platform, the rhodium
catalyst conducted two independent and compatible catalytic
cycles in one pot. The catalytic cycle was commenced by
coordination of amidine to Cp*Rh(III) to form intermediate C,
which undergoes CÀ H activation and release of HCl to provide
intermediate D. Sequential release of a second HCl from D
offers rhodacycle E. Finally, reductive elimination of E releases
the intermediate F and regenerates the rhodium(I) species
Cp*Rh(I). Next, (benz)imidazole-assisted CÀ H rhodiumation
towards to C-phenyl ring was achieved. The cyclometalated
rhodium complex G undergoes ligand coordination and alkyne
insertion to give intermediate I. Then, the desired product and
Cp*Rh(I) were released simultaneously via reductive elimination
of I. Finally, reoxidation of Cp*Rh(I) regenerates catalytically
competent Cp*Rh(III) again.
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Conclusion
In summary, we have reported on the twofold ortho-CÀ H
functionalization between N-phenylbenzimidamide and alkynes
by a single Cp*Rh(III) catalyst that enables the synthesis of
benzimidazoisoquinolines in moderate to good yields. The
cascade strategy is believed to proceed through sequential CÀ H
bond activation/[4+2]annulation. The catalytic system proved
broadly applicable with wide scope and good functional group
tolerability, thus reflecting the robustness of the CÀ H activation
regime. This approach eliminates the need to isolate/store
prefunctionalized starting materials, and occurred with remark-
able site selectivity. Detailed mechanistic studies provided
strong support for CÀ H bond cleavage on N-phenyl ring is
preferential than CÀ H bond on C-phenyl ring. Current efforts
are focused on the further exploration of multiple CÀ H
activation of amidine for novel CÀ C bond formation in our
laboratory.
Scheme 4. Proposed mechanism.
Scheme 5. Control experiment.
Experimental Section
A mixture of [Cp*RhCl2]2 (2.5%, 2 mg), alkyne (0.125 mmol), amidine
(1.5 equiv), Cu(OAc)2 (2 equiv, 49.9 mg) NBu4F (2 equiv), pivalic acid
°
(20%) and toluene/DCE (v/v=1:4, 4 mL) was stirred at 80 C for
48 h. After cooling the reaction to room temperature, PIFA (1 equiv)
and CF3CH2OH (2 ml) were added, and then the mixture was stirred
for 4 hours. After that, the solvent was removed under vacuum and
the residue was purified by silica gel chromatography using ethyl
acetate/petroleum ether (1:10-1:4) to afford desired products.
Deposition Numbers 2039498 (for 3ja), 2039499 (for 3ma), and
2039500 (for 3ra) contain the supplementary crystallographic data
for this paper. These data are provided free of charge by the joint
Cambridge Crystallographic Data Centre and Fachinformationszen-
Scheme 6. Proposed mechanism
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Acknowledgements
We are grateful for the support from the National Natural Science
Foundation of China (21701148), and Henan Province Key
Research
and
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and
Promotion
Project
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Conflict of Interest
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The authors declare no conflict of interest.
Keywords: Amidine · Alkynes · CÀ H activation · Cascade
reactions · Dual catalysis
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Manuscript received: December 17, 2020
Revised manuscript received: January 5, 2021
Accepted manuscript online: January 7, 2021
Eur. J. Org. Chem. 2021, 1290–1294
www.eurjoc.org
1294
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