Rhodium(III)-Catalyzed Synthesis of Cinnolinium Salts from Azobenzenes and Diazo Compounds

Article abstract of DOI:10.1002/adsc.201800326

A Rh(III)-catalyzed C?H activation of azobenzenes in the coupling with diazo compounds has been realized, providing a straightforward strategy to access functionalized cinnolinium triflates in high yields. This protocol features silver free mild reaction conditions and compatibility with diverse functional groups. The coupling proceeds via initial Rh(III)-catalyzed C?H alkylation, followed by zinc triflate-mediated cyclization, where zinc triflate acts as a Lewis acid as well as a triflate source. (Figure presented.).

Full text of DOI:10.1002/adsc.201800326

Title: Rhodium(III)-Catalyzed Synthesis of Cinnolinium Salts from
Azobenzenes and Diazo Compounds
Authors: Xiaohong Chen, Guangfan Zheng, Guoyong Song, and
Xingwei Li
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10.1002/adsc.201800326
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201
Rhodium(III)-Catalyzed Synthesis of Cinnolinium Salts from
Azobenzenes and Diazo Compounds
Xiaohong Chen,†,‡ Guangfan Zheng,‡ Guoyong Song,*† and Xingwei Li*‡
a
† Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, China
b
‡ Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
E-mail: songg@bjfu.edu.cn; xwli@dicp.ac.cn
Received:
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
of imidazo[1,2-a]pyridines with alkynes, where
related fused salts could be formed under a controlled
condition.[7] Wang also explored the synthesis of
conjugated polycyclic quinoliniums by Rh(III)-
catalyzed multiple C−H activation and annulation of
arylpyridiniums with alkynes.[8] Under Rh(III)-
catalysis, the You group successfully extended the
directing group of arenes to neutral oxygen, where
various oxonium salts have been accessed either as an
intermediate or final products.[9] Besides relying on
rhodium(III) catalysis, Cheng and Wang also
independently realized Co(III) version of some of the
important annulation systems.[10]
Abstract.
A Rh(III)-catalyzed C−H activation of
azobenzenes in the coupling with diazo compounds has
been realized, providing a straightforward strategy to
access functionalized cinnolinium triflates in high yields.
This protocol features silver free mild reaction
conditions and compatibility with diverse functional
groups. The coupling proceeds via initial Rh(III)-
catalyzed C−H alkylation, followed by zinc triflate-
mediated cyclization, where zinc triflate acts as a Lewis
acid as well as a triflate source.
Keywords: rhodium; azobenzenes; C−H activation;
diazo compounds; cinnolinium salts
Cinnolinium salts are among the most important N-
heterocyclic quaternary ammonium salts that have
been widely found in the core structures of many
bioactive compounds, pharmaceuticals, alkaloids,
receptors, and inhibitors.[1] Therefore, as a step-
economic strategy, metal-catalyzed C−H activation
has received increasing attention in the synthesis of
functionalized cinnolinium and other quaternary
ammonium salts. So far, only limited C−H activation
strategies have been developed for the synthesis of
quaternary ammonium salts.[2-10] Mechanistically,
reductive elimination of a neutral nitrogen ligand and
an anionic carbon ligand consists of the predominant
pathway in these coupling systems. Moreover, the
coupling reagents are nearly always limited to internal
alkynes.
Scheme 1. Transition Metal-Catalyzed C−H Activation of
Azobenzene
Cp*Rh(III) complexes have been recently
recognized as competent catalysts for versatile C−H
functionalization owing to their high catalytic activity,
selectivity, and functional group compatibility.[11]
Under Rh(III) and Ir(III) catalysis, diazo compounds
have been widely used as coupling partners in C−H
activation systems because of their high reactivity and
electrophilicity.[12] Recently, Kim’s, Lee’s, Yao’s and
Lin’s groups independently reported Rh(III)-catalyzed
direct coupling of azobenzenes with diazo compounds
to afford alkylation products or neutral cinnolines.[13]
Despite the high reactivity of diazo compounds,
synthesis of salts via C−H activation has not been
explored. Inspired by these outcomes,[14] we reasoned
In 2012, Cheng reported the first oxidative
coupling of imines and alkynes for the synthesis of
isoquinolium salts.[3] Cheng and You also
independently realized oxidative coupling of
azobenzenes with alkynes for the synthesis of
cinnolinium salts.[4] In 2013, Huang disclosed the
synthesis of related salts via Rh(III)-catalyzed aerobic
annulation between 2-arylpyridines and alkynes.[5]
Later, Choudhury and Wang explored Rh(III)-
catalyzed
C−H
activation
of
imidazolium/benzimidazolium salts and annulative
coupling with alkynes for the synthesis of fused salts,
where an NHC acted as an efficient directing group.[6]
Our group reported the divergent annulative coupling
1
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10.1002/adsc.201800326
Advanced Synthesis & Catalysis
that synthesis of cinnolinium salts might be realized was used as a sole triflate source in the absence of
using diazo compounds as coupling reagents through AgOTf, 3aa was generated in high yield (92%), being
C−H activation. Herein, we report Rh(III)-catalyzed superior to than that from AgOTf as a sole triflate
mild and redox-neutral synthesis of cinnolinium salts source (64%) (entries 3-4). To our surprise,
with diazo compounds via C−H activation of decreasing the reaction temperature is applicable to
azobenzenes[11m,15] and related oxime ethers. In the current C−H annulated reaction, and 97% yield of
contrast to previous oxidative systems that rely on 3aa was obtained at room temperature (entries 6-9). A
N(neutral)−C reductive elimination, this system range of solvents was also tested. Obviously, the
follows a Lewis acid-assisted annulation pathway activity towards the formation of 3aa increased with
with the elimination of a hydroxy group under redox- the solvent polarity. For example, TFE and
neutral conditions.
hexafluoroisopropanol (HFIP) having high ET(30)
values (59.8 and 65.3 kcal mol-1, respectively)
showed higher activity than those from ethanol (ET(30)
= 51.9 kcal mol-1, 70%), dichloroethane (DCE, ET(30)
= 37.0 kcal mol-1, 36%) and PhCF3 (ET(30) = 38.5
kcal mol-1, 30%) (entries 10-14).[17] The molecular
structure of 3aa was unequivocally established by X-
ray crystallography (CCDC 1814875).[18]
Table 1. Optimization of Reaction Conditionsa
yieldb
（%）
catalyst
(mol %)
[Cp*RhCl2]2
(2.5)/AgOTf (10)
OTf salt
(mol %)
Zn(OTf)2
(60)
temp.
(ºC)
entry
1
solvent
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
120
120
120
120
120
80
89
31
64
92
93
95
96
94
97
76
70
36
90
30
[Cp*Co(CO)I2] Zn(OTf)2
2
(10)/AgOTf (20)
[Cp*RhCl2]2
(2.5)
(60)
AgOTf
(110)
Zn(OTf)2
(60)
Zn(OTf)2
(110)
Zn(OTf)2
(60)
Zn(OTf)2
(60)
Zn(OTf)2
(60)
Zn(OTf)2
(60)
Zn(OTf)2
(60)
Zn(OTf)2
(60)
Zn(OTf)2
(60)
Zn(OTf)2
(60)
Zn(OTf)2
(60)
3
[Cp*RhCl2]2
(2.5)
4
[Cp*RhCl2]2
(2.5)
5
[Cp*RhCl2]2
(2.5)
6
[Cp*RhCl2]2
(2.5)
7
60
[Cp*RhCl2]2
(2.5)
8
40
[Cp*RhCl2]2
(2.5)
9
25
[Cp*RhCl2]2
(2.5)
[Cp*RhCl2]2
(2.5)
[Cp*RhCl2]2
(2.5)
[Cp*RhCl2]2
(2.5)
10
11
12
13
14
MeOH 25
EtOH
DCE
HFIP
25
25
25
[Cp*RhCl2]2
(2.5)
PhCF3 25
Scheme 2. Substrate Scope of Azobenzenesa
aReaction conditions: 1a (0.2 mmol), 2a (0.3 mmol),
[Cp*RhCl2]2 (2.5 mol %), additive, solvent (2 mL),
25−120 °C, 10 h. bIsolated yield.
aReaction conditions: 1 (0.2 mmol), 2 (0.3 mmol),
[Cp*RhCl2]2 (2.5 mol %), Zn(OTf)2 (60 mol %), TFE (2
c
mL), 60 °C, 10 h, isolated yield. bAt room temperature. At
d
80 ºC. Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol),
[Cp*RhCl2]2 (2.5 mol %), Zn(OTf)2 (1.1 equiv), HOAc
(1.0 equiv) TFE (2 mL), 60 °C, 10 h.
First, the reaction of (E)-1,2-diphenyldiazene (1a)
with methyl 2-diazo-3-oxobutanoate (2a) was
examined under various reaction conditions (Table 1).
By using the combination of [Cp*RhCl2]2 and AgOTf
as a catalyst and Zn(OTf)2 as a triflate source,[16] the
C−H annulation product 3aa was obtained in 89%
yield in trifluoroethanol (TFE) at 120 ºC (entry 1).
Replacing the [Cp*RhCl2]2 with [Cp*Co(CO)I2] led to
a decreased yield of 3aa (entry 2). When Zn(OTf)2
With the establishment of the optimal reaction
conditions, we next explored the scope of azobenzene
substrate. Given the poor solubility of some
azobenzenes in TFE, the reaction was conducted at 60
ºC in most cases. As shown in Scheme 2, a range of
substituted azobenzenes was examined to react with
2
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10.1002/adsc.201800326
Advanced Synthesis & Catalysis
2-diazo-3-oxobutanoate (2a). Introduction of 78% yield. When diethyl 2-diazomalonate 2k was
electron-donating and -withdrawing groups at the C4- used as a coupling reagent, no target product 3ak was
position of the azobenzene was tolerated, providing observed, indicating that the ketone group is essential
the corresponding cinnolinium salts (3aa-3ha) in in this catalytic reaction. Unfortunately, no
good to excellent yields (68-99%). In the case of cinnolinium salt (3ai or 3am) was generated when the
azobenzene having a methyl group at the C3 position, electron-donating group was introduced into the diazo
the annulation reaction took place only at the less compounds, indicating the crucial role of EWG.
sterically demanding C−H bond, to give 3ia and 3na
The synthetic utility of the reaction system was
exclusively in excellent yield (both 90%). C2- then demonstrated in a scale-up reaction. Thus, a 4
functionalized azobenzenes were also tolerated, mmol-reaction of 1a and 2a afforded cinnolinium salt
affording the desired products 3ja-3la in consistently 3aa in 91% yield (Scheme 4a). Reduction 3aa with
good to excellent yields (68-97%). In order to Zn in wet acetonitrile under N2 atmosphere afforded a
understand the electronic effect, non-symmetrical azo mixture of 4 and 5 in 69% and 13% yield,
compound 1r was examined, yielding regioisomeric respectively (Scheme 4b). The possible pathway for
products 3ra/3ra’ (61%/24%), which indicates that the formation of product 5 involved a reductive
the C−H annulation is favored for a more electron-rich cleavage of 4, followed by condensation, which was
aromatic ring. Various (phenylazo)alkanes were also described in supporting information (Scheme S1).
suitable substrates to give 3sa–3ua in good yields.
When alkyl group was replaced with phenethyl group,
the cinnolinium salt 3va was successfully synthesized
in 65% yield. The arene substrate was further
extended to oxime ethers, and isoquinolium salts
3wa–3ya were isolated in good yield although they
were prone to decomposition in solution.
Scheme 4. Gram-Scale Synthesis and Synthetic
Applications
Several experiments have been conducted to probe
the mechanism of C−H annulation of azobenzenes
and diazo Compounds (Scheme 5). A competitive
reaction between 1b and 1g with diazo ester 2a
yielded a single product 3ba (> 20:1), suggesting that
the reaction is highly favored for a more electron-rich
azobenzene (Scheme 5a). This observation also
agrees with the outcome of the intramolecular
competitive reaction (3ra/3ra’). When 1b was
allowed to undergo H/D exchange under the standard
conditions with methanol-d4 and/or CD3COOD in the
absence of a diazo compound, no deuterium
Scheme 3. Substrate Scope of Diazo Compoundsa,b
incorporation was observed at ortho position (Scheme
aReaction Conditions: 1 (0.2 mmol), 2 (0.3 mmol),
5b). Furthermore, the catalytic reaction of 1b and 2a
[Cp*RhCl2]2 (2.5 mol %), Zn(OTf)2 (60 mol %), and TFE
in the presence of CD3COOD led to recovered 1b,
b
(2 mL) were stirred at room temperature for 10 h. Isolated
with no observable deuteration. These results
yield.
suggested that cyclorhodation is largely irreversible
under these conditions. A putative rhodacyclic
intermediate was synthesized from the reaction of 1b
Subsequently, the coupling of azobenzenes 1a and
1i with various diazo compounds was examined under
and [RhCp*Cl2]2 with NaOAc in TFE, which afforded
a five-membered rhodacycle 1b-I in 92% yield (see
the optimal conditions at room temperature (Scheme
SI). This complex was applied as a catalyst in the
3). Various symmetrical and nonsymmetrical diazo
reaction of 1b and 2a, which afforded cinnolinium
compounds coupled smoothly with azobenzenes, thus
salt 3ba in 97% yield. These results suggested a C−H
affording the cinnolinium salts in 76-99% yields. In
activation pathway. Moreover, another possible
the case of an -diazo phosphate 2f, the coupling also
organic intermediate 3id-IV was synthesized
according to the literature (Scheme 5e),[13b] and it was
cleanly converted to the target product 3id when
treated with Zn(OTf)2 in TFE at 60 ºC. This clearly
reacted smoothly to give the target product 3af. The
cyclic diazo compound 2j was a proper annulated
reagent, which resulted in polycyclic product 3aj in
3
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10.1002/adsc.201800326
Advanced Synthesis & Catalysis
suggests that the cyclization step is mediated by alkylated intermediate IV together with regeneration
Zn(OTf)2 and the rhodium catalyst is not necessary. of the active Rh(III) catalyst. Finally, ketone carbonyl
Kinetic isotope effect (KIE) value was measured as group is activated by the zinc salt and is
3.3 by two parallel reactions of 1a and 1a-d10 with 2a nucleophilically attacked by the nitrogen to furnish
(Scheme 5f), indicating that C−H bond cleavage is the final annulated product 3aa, where the
probably involved in the turnover-limiting step.
Zn(OH)(OTf) can promote this cyclization to
eventually give Zn(OH)2.
In summary, we have developed a Rh(III)-
catalyzed efficient C−H activation of azobenzenes
with diazo compounds, affording the target salts in
high yields. A variety of azobenzenes and diazo
compounds are amenable to the coupling systems.
This protocol features a relatively low catalyst
loading, mild and sliver-free reaction conditions, and
compatibility with diverse functional groups, thus
providing straightforward access to functionalized
cinnolinium salts.
Experimental Section
General procedure for the synthesis of compound 3:
To a pressure tube (35 mL) charged with azobenzene 1 (0.2
mmol), diazo compound 2 (0.3 mmol), [Cp*RhCl2]2 (2.5
mol%), and Zn(OTf)2 (60 mol %) was added TFE (2 mL),
and the result mixture was stirred at the reaction
temperature (r.t. – 80 ºC) for 10 h (monitored by TLC).
After that, the solvent was removed under reduced pressure
and the residue was purified by silica gel chromatography
to afford the target product.
Scheme 5. Mechanistic Investigations
Acknowledgements
Financial support from the NSFC (Nos. 21472186 and 21525208)
and the fund for new technology of methanol conversion of Dalian
Institute of Chemical Physics (Chinese Academy of Sciences) are
gratefully acknowledged.
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5
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10.1002/adsc.201800326
Advanced Synthesis & Catalysis
COMMUNICATION
Rhodium(III)-Catalyzed Synthesis of Cinnolinium
Salts from Azobenzenes and Diazo Compounds
Adv. Synth. Catal. Year, Volume, Page – Page
Xiaohong Chen,†,‡ Guangfan Zheng,‡ Guoyong
Song,*† and Xingwei Li*‡
6
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