Rhodium(III)-catalyzed synthesis of cinnolinium salts from azobenzenes and alkynes: Application to the synthesis of indoles and cinnolines

Article abstract of DOI:10.1002/chem.201300922

Versatile salts: A new rhodium-catalyzed synthesis of cinnolinium salts from various azobenzenes and alkynes under air is described. These salts readily transform into three important classes of products, including indoles, indoloindoles, and cinnolines (see scheme). Copyright

Full text of DOI:10.1002/chem.201300922

COMMUNICATION
DOI: 10.1002/chem.201300922
RhodiumACHTUNGTRENNUNG(III)-Catalyzed Synthesis of Cinnolinium Salts from Azobenzenes
and Alkynes: Application to the Synthesis of Indoles and Cinnolines
Krishnamoorthy Muralirajan and Chien-Hong Cheng*[a]
Chem. Eur. J. 2013, 00, 0 – 0
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Substituted cinnolinium salts are flexible building blocks
for many natural products, alkaloids, receptors, inhibitors,
and pharmacologically active salts.[1] Consequently, great at-
tention has been paid to the synthesis of heterocyclic com-
pounds bearing a cinnoline moiety. Despite the high utility
of cinnoline derivatives, only few synthetic routes for cinno-
linium salts are available in the literature.[2a,b] Typical meth-
ods involve a direct alkylation of cinnolines to afford cinno-
linium salts. The direct synthesis of cinnolinium salts from
readily available starting materials in a one-pot manner was
hardly demonstrated.[2c–g] Previously, Heck and co-workers
described the stoichiometric annulation of cyclometalated
azobenzenes with alkynes to give cinnolinium salts
(Scheme 1).[3a–c] To the best of our knowledge, there is no
H activation of azobenzenes and the subsequent reaction
with alkynes. Herein, we report an RhCp*-catalyzed (Cp*=
Me5C5) annulation of azobenzenes with alkynes to give bio-
logically active cinnolinium salts. The reaction provides a
mild, atom- and step-economical method for the synthesis of
cinnolinium salts by using air as the co-oxidant.
The reaction of azobenzene (1a, 1.00 mmol) with diphe-
nylacetylene (2a, 1.20 mmol) in the presence of
[(RhCp*Cl2)2]
(1.00 mol%),
and
CuACHTGNUTER(NNUG BF4)2·6H2O
(0.50 mmol) in tBuOH under air at 708C for 16 h gave cin-
nolinium salt 3a in 91% isolated yield (Table 1, entry 1).
1
The structure of 3a was confirmed by its H, 13C, 19F, and
11B NMR spectra, as well as IR and mass data. Control ex-
periments revealed that in the absence of either Cu-
AHCTNUTGERGUN(NN BF4)2·6H2O or [(RhCp*Cl2)2] no product 3a was formed.
In addition to the above conditions, we also examined the
reaction with various oxidants. Among them, the combina-
tion of AgBF4 (1.00 mmol) and Cu
gave the best result, affording 3a in 96% yield. Using
AgBF4 (1.00 mmol) and Cu(OAc)2·H2O (0.30 mmol) under
ACHTUNGTREN(NUNG OAc)2·H2O (1.00 mmol)
AHCTUNGTRENNUNG
air was also effective and gave 3a in 94% isolated yield. It
is worth noting that the present reaction requires two equiv-
alents of the oxidant, such as CuII or Ag+, and one equiva-
À
lent of BF4 to reach completion. The solvent also plays a
Scheme 1. Synthesis of cinnolinium salts.
vital role for the reaction; the best solvent is tBuOH, afford-
ing 3a in excellent yield. Other solvents, such as t-amylOH,
MeOH, EtOH, 1,2-dichloroethane (DCE), THF, and acetic
acid, are less effective for the catalytic reaction, giving 3a in
65–86% yield (see the Supporting Information). Based on
the results above, we choose [(RhCp*Cl2)2] (1.00 mol%),
report on the synthesis of cinnolinium salts catalyzed by
metal complexes, despite the fact that a few reports are
available for the synthesis of cinnolines and its related deriv-
atives.[3d–g,4]
À
In recent years, transition-metal-catalyzed directed C H
functionalization offerd a great variety of useful transforma-
CuACHTGNUTERN(UNG BF4)2·6H2O (0.50 mmol), and tBuOH under air as the
standard reaction conditions for most of the following stud-
ies; these conditions use less metal oxidant and are thus
greener and less expensive.
tions.[5] Extensive studies have been devoted towards RhIII-
catalyzed aromatic ortho-C H bond activation[6] and annula-
À
tion reactions involving nitrogen-containing directing groups
to construct heterocyclic compounds.[7,8] In this context,
Jones and co-workers described a stoichiometric reaction of
N-benzylidenemethylamine, 2-phenylpyridine, and benzo[h]-
Under the standard reaction conditions, various substitut-
ed azobenzenes 1b–k reacted with diphenylacetylene (2a)
to give the corresponding cinnolinium salts in good to excel-
lent yields (Table 1, entries 2–11). Thus, azobenzenes 1b–e
with electron-rich 4,4’-diMe, 4-OMe, 4-tBu, and 4-nBu sub-
stituents afforded products 3b–e in 89–94% yield (entries 2–
5). Similarly, electron-withdrawing substituents, including F,
Br, and CO2Et, on the azobenzenes 1 f–h are also compati-
ble, providing functionalized cinnolinium salts 3 f–h in slight-
ly lower yields (74–79%, entries 6–8). To understand the
steric effects on the reaction, ortho- and meta-substituted
azobenzenes 1i–k were examined. The reactions with 2a
gave products 3i–k in 78–89% yield (entries 9–11). For (E)-
1,2-bis(3,4-dimethoxyphenyl)diazene (1j), the annulation re-
action gave single regioisomeric product 3j in 80% yield
À
quinoline with alkynes through C H bond activation medi-
ated by RhIII complexes to give various nitrogen-containing
salts.[9a]
Previously, we reported a nickel-catalyzed annulation of
2-halobenzaldimines[9b,c] or ortho-iodoketimines[9d] with al-
kynes to give isoquinolinium salts. Very recently we also
succeeded in developing a rhodium- and ruthenium-cata-
lyzed three-component reaction of aryl aldehydes, amines,
and alkynes to afford isoquinolinium salts.[9e,f,10] Our contin-
À
uous effort to develop synthetically versatile C H activation
reactions[11] prompted us to explore the metal-catalyzed C
À
À
(entry 10), although there are two possible C H bond acti-
À
vation sites at C2 and C6. The C H bond activation occurs
[a] K. Muralirajan, Prof. Dr. C.-H. Cheng
Department of Chemistry
only at C6, likely owing to the steric effect of the methoxy
group at C3. On the other hand, 1k and l with two different
aromatic rings afforded isomeric product pairs 3k/k’ and 3l/
l’ in a ratio of 58:42 and 63:37 with 89 and 82% combined
yield, respectively (entries 11 and 12). The isomers 3k or l
National Tsing Hua University
Hsinchu, 30013 (Taiwan)
Fax : (+886)3572-4698
E-mail: chcheng@mx.nthu.edu.tw
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300922.
À
with C H bond activation and cyclization at the p-anisyl
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Rhodium-Catalyzed Synthesis of Cinnolinium Salts
COMMUNICATION
Table 1. Results of annulation of azobenzenes 1 with alkynes 2.[a]
see Scheme 2 below), leading to higher yields of the
corresponding isomeric products 3k and l relative
to 3k’ and l’ after insertion of akyne 2a.
In addition to 2a, other symmetrical alkynes 2b–
g were tested (Table 1, entries 13–18). Thus, p-Me-,
MeO-, and Br-substituted diphenylacetylenes 2b–d
reacted with 1a to afford the corresponding salts
3m–o in 78–84% yield (entries 13–15). The present
catalytic reaction also proceeded smoothly with ali-
phatic alkynes, such as oct-4-yne (2e), but-2-yne
(2 f), and 1,4-dimethoxybut-2-yne (2g); treatment
of these alkynes with 1a afforded 3p–r in 73–81%
yield (entries 16–18).
Entry
1
2
Product 3
Yield [%][b]
1
2
3
4
1a
1b
1c
1d
1e
1 f
1g
1h
2a
2a
2a
2a
2a
2a
2a
2a
3a: R1 =H
91
89
93
94
90
79
75
74
3b: R1 =4-Me
3c: R1 =4-OMe
3d: R1 =4-tBu
3e: R1 =4-nBu
3 f: R1 =4-F
5
6[c]
7[c]
8[d]
3g: R1 =4-Br
3h: R1 =4-CO2Et
To understand the regioselectivity of the present
catalytic reaction with unsymmetrical alkynes, 1-
phenyl-1-propyne (2h) was used in the reaction
with 1a. The catalytic reaction appears to be sensi-
tive to the metal oxidant used. When only Cu-
9
1i
2a
3i
3j
78
AHCTNUGTERN(GNNU BF4)2·6H2O (0.50 mmol) was employed, the reac-
tion showed low regioselectivity with a regioisomer-
ic ratio of 85:15 and a combined yield of 85%. For-
tunately, when a combination of AgBF4 and Cu-
10
1j
2a
80
AHCTUNTGREN(NGUN OAc)2·H2O was used, the regioselectivity greatly
improved to 96:4. Under these conditions, other un-
ꢀ
symmetrical alkynes PhC CR in which R=Et (2i),
11
12
1k
1l
2a
2a
3k
3l
89 (58:42)[f]
82 (63:37)[f]
CH2OMe (2j), CO2Et (2k), and COMe (2l) also re-
acted in a highly regioselective manner with 1a to
afford products 3t–w in good to excellent yields
(entries 20–23). The regiochemistry of these prod-
ucts was confirmed by NOE experiments. The
scope of the catalytic reaction can be further ex-
tended to various (phenylazo)alkanes 1m–o.[12] The
reaction of these substrates with 2a afforded 3x–z
in 79–83% yield (entries 24–26).
13
14
15
16
17
18
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
2b
2c
2d
2e
2 f
2g
2h
2i
3m: R3 =4-MeC6H4
3n: R3 =4-OMeC6H4
3o: R3 =4-BrC6H4
3p: R3 =nPr
80
84
78
81
73
78
82 (96:4)
85 (99:1)
78 (96:4)
86 (99:1)
78 (92:8)
On the basis of known metal-catalyzed, directing-
3q: R3 =CH3
À
À
À
group-assisted C H activation/C C, C N bond for-
mation reactions,[5–8] a possible mechanism is pro-
posed to account for the catalytic reaction
(Scheme 2). The catalytic cycle starts with the coor-
dination of azobenzene to the rhodium center, fol-
3r: R3 =CH2OMe
3s: R3 =Me
19[e]
20[e]
21[e]
22[e]
23[e]
3t: R3 =Et
2j
2k
2l
3u: R3 =CH2OMe
3v: R3 =CO2Et
3w: R3 =COMe
À
lowed by ortho-C H bond activation to form five-
membered rhodacycle I. Coordination of alkyne 2a
to I gives intermediate II, and insertion of coordi-
nated 2a to the rhodium–carbon bond then leads to
seven-membered rhodacycle III. Subsequent reduc-
tive elimination affords the final cinnolinium salt
3a and RhI. The last was oxidized by CuII to regen-
erate the active RhIII species for the next catalytic
cycle. In the catalytic reaction, dioxygen acts as co-
oxidant to oxidize CuI to CuII. There are two possi-
ble pathways for the insertion of a coordinated
alkyne into rhodacycle II. One is the insertion into
24
25
26
1m
1n
1o
2a
2a
2a
3x: R2 =CH3
3y: R2 =nPr
3z: R2 =Cy
83
80
79
[a] Unless otherwise mentioned, all reactions were carried out by using azobenzene 1
(1.00 mmol), alkyne (1.20 mmol), [(RhCp*Cl2)2] (1.0 mol%), Cu(BF4)2·6H2O
(0.50 mmol), and tBuOH (3 mL) at 708C for 16 h under air. [b] Isolated yields. [c] At
808C. [d] At 1058C. [e] AgBF4 (1.00 mmol) and Cu
were used. [f] Ratio of isomers 3k/k’ or 3l/l’.
2
ACHTUNGTRENNUNG
ACHTUGNRTEN(NUGN OAc)2·H2O (0.30 mmol) under air
ring gave higher yields than the isomers 3k’ or l’ in which
the rhodium–carbon bond of II, the other is the insertion
into the rhodium–nitrogen bond. For most of the alkynes
without an electron-withdrawing group (2a–j), the insertion
À
the C H bond activation occurs at the phenyl or p-fluoro-
phenyl ring. It appears that the electron-rich p-methoxy
group on the phenyl ring can stabilize the corresponding
catalytic rhodacycle I better (an electrophilic RhIII species,
À
occurs more likely into the Rh C bond. However, for elec-
ꢀ
tron-deficient alkynes PhC CR in which R is an electron-
Chem. Eur. J. 2013, 00, 0 – 0
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C.-H. Cheng and K. Muralirajan
Scheme 2. A proposed mechanism for cinnolinium-salt formation.
withdrawing group, such as CO2Et (2k) or COMe (2l), in-
Scheme 4. Synthesis of indoles, indoloindoles, and cinnolines.
À
sertion into the Rh N bond likely becomes the major path-
way in view of the observed regiochemistry of products 3v–
w (entries 22–23). In these products, the nitrogen group of
azobenzene (1a) is bound to the b-carbon of the alkyne
moiety.[13] It is worth noting that the insertion may be
with pyridine in DMF afforded 3,4-diphenylcinnoline (6) in
84% yield (see the Supporting Information).
In conclusion, we have developed a new rhodium-cata-
À
À
viewed as addition of the Rh N bond to alkyne 2k or l in a
lyzed, chelate-assisted C H activation/annulation of azoben-
Michael-type addition, that is, the nitrogen atom adds to the
b-carbon of alkyne 2k or l. An opposite regiochemistry
would be obtained if the insertion occurs though the metal–
carbon bond of a metalacycle.[13a,c]
zenes with alkynes to give various substituted cinnolinium
salts in good to excellent yields. The reaction is compatible
with a wide substrate scope and highly atom- and step-eco-
nomic. The proposed mechanism is strongly supported by
the isolation of a five-membered rhodacycle. Finally, this
protocol has been successfully applied to the synthesis of
indole, indoloindole, and cinnoline derivatives. Further ap-
plication of this methodology in natural product synthesis is
in progress.
To shine light on the reaction mechanism, we prepared
the assumed rhodium intermediate I from the reaction of 1b
with [(RhCp*Cl2)2] in tBuOH at 708C for 2 h, which afford-
ed five-membered rhodacycle 1b-I in 88% yield (see the
Supporting Information). This complex was characterized by
its H and 13C NMR spectra, as well as single-crystal X-ray
1
diffraction. The crystal structure revealed that the complex
is an 18-electron system containing a Cp* moiety, a cyclome-
talated azobenzene and a chloro ligand. The reaction of 1b-
Acknowledgements
I (0.052 mmol) with 2a (0.062 mmol) and CuACTHNUGTRNEUNG(BF4)2·6H2O
(0.026 mmol) in tBuOH at 708C for 0.5 h gave 3b in 94%
yield (Scheme 3).
We thank the National Science Council of Republic of China (NSC-101-
2628-M-007-004) for support of this research.
À
Keywords: air · alkynes · azobenzenes · C H activation ·
cinnolinium salts · rhodium
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Scheme 3. Reaction of rhodium intermediate 1b-I with diphenylacetylene
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The importance of the present catalytic reaction was fur-
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Rhodium-Catalyzed Synthesis of Cinnolinium Salts
COMMUNICATION
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Received: March 11, 2013
Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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C.-H. Cheng and K. Muralirajan
Rhodium Catalysis
K. Muralirajan,
C.-H. Cheng* ................. &&&& — &&&&
RhodiumACHTUNGTRENNUNG(III)-Catalyzed Synthesis of
Cinnolinium Salts from Azobenzenes
and Alkynes: Application to the
Synthesis of Indoles and Cinnolines
Versatile salts: A new rhodium-cata-
lyzed synthesis of cinnolinium salts
from various azobenzenes and alkynes
under air is described. These salts
readily transform into three important
classes of products, including indoles,
indoloindoles, and cinnolines (see
scheme).
Rhodium Catalysis
An efficient RhCp*-catalyzed (Cp*=Me5C5) annulation
of azobenzenes with alkynes to give cinnolinium salts is
described. These salts are readily applied to the synthesis
of three important classes of compounds: indole, indoloin-
doles, and cinnoline derivatives. The background picture is
the Trichy Sri Ranganathaswamy Temple, India (left) and
the Taipei 101 Tower and National Chiang Kai-Shek
Memorial Hall, Taiwan (right).
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www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!