Rhodium(III)-Catalyzed Oxidative Annulation of 2,2′-Bipyridine N-Oxides with Alkynes via Dual C-H Bond Activation

Article abstract of DOI:10.1021/acs.orglett.8b01434

Rh(III)-catalyzed switchable annulation of 2,2′-bipyridine N-oxides with internal alkynes via dual C-H bond activation has been developed. Tuning the reaction conditions enabled the reaction pathway to be switched between rollover and nonrollover annulation, delivering 5,6-disubstituted-1,10-phenanthrolines and 5,6,7,8-tetrasubstituted-1-(pyridin-2-yl)isoquinoline 2-oxides in high yields, respectively. The procedures feature excellent regioselectivity, broad substrate scope, and high tolerance of functional groups. The synthetic utilities of these obtained products were demonstrated in the catalytic reactions.

Full text of DOI:10.1021/acs.orglett.8b01434

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rhodium(III)-Catalyzed Oxidative Annulation of 2,2′-Bipyridine
N‑Oxides with Alkynes via Dual C−H Bond Activation
Xin Xu, Haoqiang Zhao, Jianbin Xu, Changjun Chen, Yixiao Pan, Zhenli Luo, Zongyao Zhang,
Huanrong Li, and Lijin Xu*
Department of Chemistry, Renmin University of China, Beijing 100872, China
S
* Supporting Information
ABSTRACT: Rh(III)-catalyzed switchable annulation of 2,2′-bipyridine N-
oxides with internal alkynes via dual C−H bond activation has been developed.
Tuning the reaction conditions enabled the reaction pathway to be switched
between rollover and nonrollover annulation, delivering 5,6-disubstituted-1,10-
phenanthrolines and 5,6,7,8-tetrasubstituted-1-(pyridin-2-yl)isoquinoline 2-
oxides in high yields, respectively. The procedures feature excellent
regioselectivity, broad substrate scope, and high tolerance of functional groups. The synthetic utilities of these obtained
products were demonstrated in the catalytic reactions.
metalation, and the halogenated products could be easily
deoxygenated.4b
he past decade has witnessed the tremendous advances of
Ttransition-metal-catalyzed direct conversion of inert C−H
bonds into C−C and C−X bonds for the synthesis and
functionalization of various heterocyclic compounds.1 It is not
surprising that the wide application of 2,2′-bipyridines in
coordination chemistry, transition-metal catalysis, supramolec-
ular chemistry, and material science has encouraged the
synthetic chemists to explore the feasibility of direct
functionalization of these useful compounds under transition-
metal catalysis via direct pyridyl C−H bond activation.2
However, attempts at direct C−H functionalization of 2,2′-
bipyridines has been met with limited success, mainly because
of their low electrophilicity and the catalyst poisoning by the
strong coordination of N atoms.3 In 2009, Miura and co-
workers first reported the regioselective direct alkenylation of
2,2′-bipyridines with terminal silylacetylenes under Rh(I)
catalysis.3a Later, Chang et al. described the hydroarylation of
alkenes and alkynes with 2,2′-bipyridines in the presence of a
Rh(NHC) catalytic system.3b,c It is believed that the
introduction of the NHC ligand allows the facile decom-
plexation of one of the N atoms from N,N-chelate complex of
2,2′-bipyridines, thereby facilitating rollover cyclometalation
that results in pyridyl C−H activation. Recent studies of the
stoichiometric reaction of 2,2′-bipyridine N-oxide with a
dimethylplatinum(II) complex by Puddephatt and co-workers
revealed a rollover cyclometalation of the N-oxide ring with
pyridine as a directing group, and no coordination of the N-
oxide oxygen was observed.4a Therefore, it is natural to expect
that 2,2′-bipyridine N-oxides would be more suitable substrates
for direct C−H functionalization under transition-metal
catalysis. Unfortunately, the chemistry about the direct
functionalization of 2,2′-bipyridine N-oxides has been less
investigated, with only one report known in the literature.
Tzschucke and co-workers reported the Pd(II)-catalyzed direct
halogenation of 2,2′-bipyridine N-oxides via rollover cyclo-
Recently, Rh(III)-catalyzed C−H bond annulation with
alkynes has evolved into a powerful tool for the preparation
of highly substituted polycyclic aromatic compounds.5−7 In
particular, several novel fused polycyclic (hetero)arenes could
be readily synthesized by annulation reactions of (hetero)-
arenes with alkynes under Rh(III) catalysis via double C−H
activation involving chelation-assisted ortho C−H activation
and subsequent rollover or nonrollover C−H activation.6,7
Notably, the reaction pathway between rollover and nonroll-
over annulation can be switched by varying the reaction
conditions, thereby delivering different types of products from
the same substrates. For example, Miura and Satoh et al. found
that Rh(III)-catalyzed rollover annulation of 1-phenyl-1H-
pyrazoles with 1 equiv of alkynes could give 4,5-disubstituted
pyrazolo[1,5-a]quinolones (Scheme 1a, right), while the
nonrollover annulation of 1-phenyl-1H-pyrazoles with 2 equiv
of alkynes afforded 1-(5,6,7,8-tetrasubstituted naphthalen-1-yl)-
1H-pyrazoles as the products under slightly modified
conditions (Scheme 1a, left).6a In a similar way, Dong and
co-workers described the selective synthesis of different 7-
azaindole derivatives in the annulation of 7-azaindoles with
alkynes (Scheme 1b).6b More recently, Choudhury et al.
demonstrated the rollover and nonrollover annulations of
imidazoles with alkynes (Scheme 1c).6h Inspired by these
elegant studies, we envisioned that tuning the reaction
conditions may allow the structurally similar 2,2′-bipyridine
N-oxides to undergo chelation-assisted rollover or nonrollover
annulation with alkynes to give novel 2,2′-bipyridine derivatives
that cannot be readily prepared using known methods.
Following our continued interest in catalytic direct C−H
functionalization of heteroarenes,8 herein, we demonstrate that
Received: May 7, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01434
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Under the optimized reaction conditions, we then examined
the coupling reaction of 1a with various internal alkynes (see
Scheme 2a). Symmetric diarylacetylenes 2b−2j bearing differ-
Scheme 1. Rh(III)-Catalyzed Rollover and Nonrollover
Annulation of Heteroarenes with Alkynes via Dual C−H
Activation
Scheme 2. Annulation of 2,2′-Bipyridine N-Oxides with 1
a
equiv of Internal Alkynes
switchable rollover and nonrollover annulation of 2,2′-
bipyridine N-oxides with internal alkynes to access 5,6-
disubstituted-1,10-phenanthrolines and 5,6,7,8-tetrasubsti-
tuted-1-(pyridin-2-yl)isoquinoline 2-oxides, respectively, can
be achieved by tuning the reaction conditions in the Rh(III)
catalytic systems (Scheme 1d).
a
Reaction conditions: 1 (0.2 mmol), 2 (0.22 mmol), [Cp*RhCl2]2
(2.0 mol %), Cu(OAc)2 (2.0 equiv), PivOH (1.0 equiv), DCE (3.0
mL), 120 °C, 18 h. Isolated yield in a gram scale reaction.
b
ent para-substituents on the aryl ring all smoothly underwent a
reaction with 1a to afford the corresponding products (3ab−
3aj) in 71%−91% yields; the electronic nature of these
substituents has little effect on the reaction’s efficiency. The
structure of 3ab was confirmed by single-crystal X-ray
diffraction (CCDC 1563903). Bis(3-substituted phenyl)
acetylenes 2k−2m also exhibited good reactivity, delivering
the target products (3ak−3am) in good yields. Similarly,
unsymmetrical diarylacetylenes (2n,2o) proceeded well,
allowing for the isolation of 3an and 3ao in 76% and 72%
yields, respectively. 1,2-Di(thiophen-2-yl)ethyne (2p) has been
proven to be a viable coupling partner to generate 3ap in 76%
yield. The reaction also tolerated the use of 1,4-diphenylbuta-
diyne (2q), 1-phenyl-1-propyne (2r), and dialkylacetylenes (2s
and 2t). To showcase the scalability of the current trans-
formation, a gram-scale reaction of 1a and 2a was performed to
give 3aa in 85% yield under the standard reaction conditions.
We next investigated the scope of 2,2′-bipyridine N-oxides
(Scheme 2b). We found that 5,5′- and 6,6′-disubstituted 2,2′-
bipyridine N-oxides (1b−1f) reacted smoothly with 2a to
provide the corresponding products (3ba−3fa) in 58%−81%
yields. In the case of 6-cyclohexyl-[2,2′-bipyridine] 1-oxide
(1g), the product 3ga was isolated only in 43% yield, probably
because of the steric effect. Delightfully, 2,2′-biquinoline 1-
oxides (1h,1i) could also engage in the reaction to deliver the
expected products (3ha, 3ia) in good yields.
We commenced our studies by investigating the reaction of
2,2′-bipyridine N-oxide (1a) with 1,2-diphenylethyne (2a) (see
the Supporting Information (SI)). We first attempted the
reaction of 1a (0.5 mmol) and 2a (1.0 mmol) with
[Cp*RhCl2]2 as the catalyst precursor and Cu(OAc)2 as the
oxidant in DCE at 120 °C. To our surprise, the reaction only
gave a mixture of 5,6-diphenyl-1,10-phenanthroline (3aa) and
2,2′-bipyridine in 29% and 20% yields, respectively, and the
expected product, 5,6-diphenyl-1,10-phenanthroline 1-oxide
(3aa′) was not observed (see Table S1, entry 1, in the SI).
This result suggests that in situ deoxygenation might occur
during the process. The yield of 3aa could be increased to 45%
upon the addition of HOAc as the additive (Table S1, entry 2).
Subsequent screening of additives revealed that PivOH turned
out to be optimal and provided the product 3aa in 90% yield
(Table S1, entry 5). It appears that deoxygenation of 1a was
suppressed in the presence of acid, as no 2,2′-bipyridine was
observed in these reactions (Table S1, entries 2−7). Further
optimization showed that the yield of 3aa dropped significantly
when replacing DCE with other solvents (Table S1, entries 8−
21), and other oxidants were inferior to Cu(OAc)2 (Table S1,
entries 22−30). The reaction did not occur when omitting the
oxidant, thereby ruling out the possibility that the N-oxide
moiety acts as an internal oxidant in the reaction process
(Table S1, entry 43).9 No reaction proceeded in the absence of
[Cp*RhCl2]2, suggesting the indispensability of a rhodium
catalyst for this reaction (Table S1, entry 44). Notably, neither
a dried solvent, nor an inert atmosphere is required in this
reaction. Thus, the optimal reaction conditions were set as
[Cp*RhCl2]2 (2.0 mol %), PivOH (1.0 equiv), and Cu(OAc)2
(2.0 equiv) in DCE at 120 °C for 18 h.
During the course of optimization of the reaction conditions
for the production of 3aa, we serendipitously found that the
reaction of 1a and 2a in DCE at 100 °C with [Cp*RhCl2]2 as
the catalyst precursor and Cu(OAc)2 as the oxidant gave
5,6,7,8-tetraphenyl-1-(pyridin-2-yl)isoquinoline 2-oxide (4aa)
(23% yield, 18 h) as the solo product, in the absence of any
B
DOI: 10.1021/acs.orglett.8b01434
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
additive. This observation prompted us to further explore this
transformation. After extensive optimization, we found that the
combination of [Cp*RhCl2]2, AgSbF6 and Cu(OAc)2·H2O
could work effectively to catalyze the reaction of 2,2′-bipyridine
N-oxide with 2 equiv of internal alkynes in DMF at 80 °C to
give the corresponding 5,6,7,8-tetrasubstituted-1-(pyridin-2-
yl)isoquinoline 2-oxides (4aa−4ai, 4al, and 4am) in good to
excellent yields with good tolerance of functional groups in
both coupling partners (see Scheme 3). We noted that
was then synthesized via N-oxidation of 4aa with m-CPBA to
give racemic 5aa and subsequent optical resolution of racemic
5aa with (−)-dibenzoyl-L-tartaric acid (Scheme 4b). With
(−)-5aa (10 mol %) as the catalyst, the allylation of
benzaldehyde with trichloroallylsilane delivered the target
product 1-phenylbut-3-en-1-ol in 75% yield and 72% ee
(Scheme 4c).3b,7c In addition, products 4 could be readily
deoxygenated by PCl3, and the measurement of absorption and
emission spectra of these obtained 2,2′-bipyridines were
performed (see the SI). Notably, 5,6,7,8-tetrakis(4-chlorophen-
yl)-1-(pyridin-2-yl)isoquinoline (6ad) exhibited strong fluo-
rescence emission at a broad range of 360−475 nm.
Scheme 3. Annulation of 2,2′-Bipyridine N-Oxides with 2
a
equiv of Internal Alkynes
To address the question of in situ deoxygenation in the
formation of products 3, several control experiments were
conducted (see the SI). No reaction was observed when
treating 2,2′-bipyridine with 2a under the standard reaction
conditions. The HRMS analysis of the reaction system of 1a
and 2a under the standard conditions confirmed the generation
of 5,6-diphenyl-1,10-phenanthroline 1-oxide (3aa′) during the
reaction process. Furthermore, when 3aa′ was subjected to the
standard reaction conditions, 3aa was obtained in 95% yield.
Further study indicated that this catalytic deoxygenation of 3aa′
proceeded equally well in the absence of Cu(OAc)2 and
PivOH, but the reaction did not occur without using
[Cp*RhCl2]2. These results show that DCE could act both as
a solvent and a reductant in this Rh(III)-catalyzed deoxygena-
tion reaction.
In order to gain more insight into the reaction mechanisms,
additional experiments were performed. The reversibility of C−
H activation was first examined (see the SI). The observed H/
D scrambling in the deuterium labeling experiments between
1a and CD3CO2D under the standard conditions in the absence
of alkyne suggested that a reversible C−H activation step may
be involved in the two transformations. However, no
deuteration of 1a, 3aa, and 4aa was observed when performing
the same reactions in the presence of 2a, revealing the possible
involvement of an irreversible alkyne insertion.11 The reaction
of 1a with [Cp*RhCl2]2 led to the formation of a five-
membered cyclometalated rhodium(III) intermediate 7, the
structure of which was confirmed by single-crystal X-ray
diffraction (XRD) studies (CCDC 1563904). The coordination
of pyridine N-oxide oxygen atom to Rh(III) center was not
observed.12 The stoichiometric reactions of 2a with 7 afforded
the products 3aa and 4aa in high yields in shorter reaction
times (Scheme 5a). Further studies showed that 7 could
catalyze the annulation reactions of 1a with 2a to give the
expected products 3aa and 4aa in essentially the same yields to
those obtained under the standard reaction conditions (Scheme
5b). These results clearly indicate the potential intermediacy of
a
Reaction conditions: [Cp*RhCl2]2 (2.5 mol %), AgSbF6 (10 mol %),
1 (0.2 mmol), 2 (0.5 mmol), Cu(OAc)2·H2O (2.0 equiv), DMF (3.0
mL), 80 °C, 18 h. Isolated yield in a gram-scale reaction.
b
unsymmetrical alkyne 2o was also applicable, but a mixture
of two inseparable regioisomers was isolated in an ∼1:1 ratio
(4ao and 4ao′). When 1,2-di(thiophen-2-yl)ethyne (2p) was
used, 4ap was obtained in 66% yield. Reactions of 1c and 1j
with 2a proceeded smoothly to produce 4ca and 4ja in 80%
and 76% yields, respectively. Further study indicated that 1a
could be transformed to 4aa in a gram-scale with 87% yield,
demonstrating the scalability of this transformation.
To showcase the utility of these annulation products, we first
attempted the application of 3aa as the ligand in an Rh(III)-
catalyzed transfer hydrogenation of quinoxaline in water, and
the desired 1,2,3,4-tetraquinoxaline was obtained in 92% yield
(Scheme 4a).10 Considering the potential of 2,2′-bipyridine
N,N′-dioxides as promising chiral organocatalysts,3b (−)-5aa
Scheme 4. Synthetic Application of 2,2′-Bipyridine N-Oxide
Derivatives
Scheme 5. Mechanistic Studies
C
DOI: 10.1021/acs.orglett.8b01434
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the five-membered rhodacycle species in the two catalytic
processes.
ASSOCIATED CONTENT
■
S
* Supporting Information
Based on these observations and the literature precedents,5−7
possible catalytic cycles for both transformations are given in
Scheme 6. First, cyclometalation of 1a generates a five-
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01434.
Detailed experimental procedures, characterization data,
Scheme 6. Plausible Mechanism
1
and copies of H and 13C NMR spectra of products
(PDF)
Accession Codes
CCDC 1563903 and 1563904 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: 20050062@ruc.edu.cn
ORCID
Lijin Xu: 0000-0003-4067-8898
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
membered rhodacyclic intermediate A. In the presence of
PivOH (X = OPiv), path a is favored to deliver a rhodacyclic
intermediate B via a rollover cyclometalation, because of the
strongly basic and coordinating nature of OPiv anion. Insertion
of alkyne 2a gives a seven-membered rhodacycle C (path a),
which undergoes reductive elimination to deliver product 3aa′
and a Rh(I) species. The Rh(I) is reoxidized by Cu(OAc)2 to
regenerate the active Rh(III) catalyst for the next catalytic cycle,
and Rh(III)-catalyzed deoxygenation of 3aa′ affords the
product 3aa. Alternatively, intermediate A possibly first reacts
with 2a to afford a seven-membered rhodacycle D, and the
presence of strongly basic and coordinating OPiv anion
facilitates the following rollover cyclorhodation to produce C
(path b). With weakly basic and less coordinating SbF6 anion,
the second intramolecular C−H activation of D occurs
preferably to furnish a five-membered rhodacycle E (path c).
After insertion of 2a once again and subsequent reductive
elimination, product 4aa is obtained with the regeneration of
the active Rh(III) catalyst.
In summary, we have achieved Rh(III)-catalyzed chelation-
assisted switchable annulation of 2,2′-bipyridine N-oxides with
internal alkynes via double C−H activation. The neutral
Rh(III) catalytic system favored rollover annulation to give 5,6-
disubstituted-1,10-phenanthrolines, whereas the cationic Rh-
(III) catalytic system worked well for nonrollover annulation to
deliver 5,6,7,8-tetrasubstituted-1-(pyridin-2-yl)isoquinoline 2-
oxides. The protocols have the advantages of high efficiency
and functional group tolerance, excellent regioselectivity and
broad substrate scope, enabling them to become attractive
complements to the existing methods for the synthesis of useful
2,2′-bipyridine derivatives.
We are grateful for financial support from the NSFC (No.
21372258), the Fundamental Research Funds for the Central
Universities, and the Research Funds of Renmin University of
China (Program No. 16XNLQ04).
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DOI: 10.1021/acs.orglett.8b01434
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