One-pot synthesis of highly substituted polyheteroaromatic compounds by rhodium(III)-catalyzed multiple C-H activation and annulation

Article abstract of DOI:10.1002/anie.201405183

A new method for the synthesis of highly substituted naphthyridine-based polyheteroaromatic compounds in high yields proceeds through rhodium(III)-catalyzed multiple C-H bond cleavage and C-C and C-N bond formation in a one-pot process. Such highly substituted polyheteroaromatic compounds have attracted much attention because of their unique π-conjugation, which make them suitable materials for organic semiconductors and luminescent materials. Furthermore, a possible mechanism, which involves multiple chelation-assisted ortho C-H activation, alkyne insertion, and reductive elimination, is proposed for this transformation.

Full text of DOI:10.1002/anie.201405183

Angewandte
Chemie
DOI: 10.1002/anie.201405183
À
Multiple C H Activation
Hot Paper
One-Pot Synthesis of Highly Substituted Polyheteroaromatic
À
Compounds by Rhodium(III)-Catalyzed Multiple C H Activation and
Annulation**
Jayachandran Jayakumar, Kanniyappan Parthasarathy, Yi-Hsiang Chen, Tai-Hua Lee,
Shih-Ching Chuang,* and Chien-Hong Cheng*
Abstract: A new method for the synthesis of highly substituted
naphthyridine-based polyheteroaromatic compounds in high
À
yields proceeds through rhodium(III)-catalyzed multiple C H
À
À
bond cleavage and C C and C N bond formation in a one-pot
process. Such highly substituted polyheteroaromatic com-
pounds have attracted much attention because of their unique
p-conjugation, which make them suitable materials for organic
semiconductors and luminescent materials. Furthermore, a pos-
sible mechanism, which involves multiple chelation-assisted
À
ortho C H activation, alkyne insertion, and reductive elimi-
nation, is proposed for this transformation.
À
T
ransition-metal-catalyzed C H bond activation is an excel-
lent method for the construction of various heteroaromatic
molecules from readily available starting materials, which is
generally difficult to achieve by using traditional synthetic
methods.[1] Numerous examples of the rhodium(III)- or
À
À
À
ruthenium(II)-catalyzed cleavage of C H/N H or C H/
À
O H bonds followed by annulation reactions with alkynes
to form diverse heterocyclic compounds have been
reported.[2,3,4] Thus far, only a few examples of double C H
À
À
À
bond activation and subsequent C C and C X (X = N or O)
bond formation reactions have been described (Scheme 1a).
For example, Miura, Satoh et al. reported the synthesis of
polyarylated naphthyl-/anthrylazole,[5a] isoquinolinoisoquino-
linone,[5b] and acene[5c] derivatives by Rh -catalyzed C H
III
À
Scheme 1. Previous and current work.
activation. In 2010, Li and co-workers reported a [RhCp*]-
cles[9f–j] prompted us to explore the synthesis of polyheter-
À
catalyzed facile synthesis of polycyclic amides by double C H
À
activation of benzamides and coupling with alkynes (Cp* =
pentamethyl cyclopentadienyl).[6] Very recently, efficient
syntheses of complex aza-fused polycyclic quinolines[7] and
naphthapyrans[8] by RhIII catalysis have also been described.
oaromatic compounds through multiple C H activation and
annulation. Herein, we wish to report an efficient RhIII-
catalyzed reaction of (Z)-N-hydroxybenzamidines (1) with
alkynes to afford benzo[de](isoquinolino[2,1-a])[1,8]naphthy-
ridines and 1H-benzo[de][1,8]naphthyridines (Scheme 1b).
Treatment of 1a (0.25 mmol) with diphenylacetylene (2a;
À
Our continuous interest in metal-catalyzed multiple C H
activation processes[9a–e] and in the synthesis of heterocy-
[10]
0.80 mmol) in the presence of [Cp*Rh(CH3CN)3](SbF6)2
[*] J. Jayakumar, Dr. K. Parthasarathy, Y.-H. Chen, Prof. Dr. C.-H. Cheng
Department of Chemistry, National Tsing Hua University
Hsinchu 30013 (Taiwan)
(6.0 mol%) and Cu(OAc)2 (4.5 equiv) in tert-amyl alcohol at
1308C for 18 hours afforded benzo(isoquinolino)naphthyri-
dine 3a in 83% yield (Table 1, entry 1). The structure of 3a
E-mail: chcheng@mx.nthu.edu.tw
Homepage: http://mx.nthu.edu.tw/%7Echcheng
1
was confirmed by H and 13C NMR spectroscopy and mass
spectrometry. To the best of our knowledge, this is the first
synthesis of benzo(isoquinolino)naphthyridines by catalytic
T.-H. Lee, Prof. Dr. S.-C. Chuang
Department of Applied Chemistry, National Chiao Tung University
Hsinchu 30013 (Taiwan)
À
multiple C H bond activation and annulation in a one-pot
process. Highly substituted polyheteroaromatic compounds
are useful p-conjugated functional materials, which have been
used as organic semiconductors or luminescent materials.[11]
Furthermore, some heteroaromatic derivatives are versatile
building blocks for natural products.[12]
E-mail: jscchuang@faculty.nctu.edu.tw
[**] We thank the Ministry of Science and Technology of the Republic of
China (NSC 102-2628-M-007-005) for support of this research.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201405183.
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À
Table 1: Rhodium-catalyzed multiple C H bond activation of
efficiency of this process. tert-Amyl alcohol was found to be
the ideal solvent affording 3a in 83% yield. Other solvents,
such as DMF, ortho-xylene, dichloroethane (DCE), and
CH3CN, were less suitable for this reaction, giving 3a in 70,
64, 58, and 45% yield, respectively (Table 1; see also the
Supporting Information).
N-hydroxybenzamidines and annulation with alkynes.[a]
Under similar reaction conditions, various para-substi-
tuted (Z)-N-hydroxybenzamidines (1b–k) reacted with
diphenylacetylene (2a) to give the corresponding products.
À
The benzamidine derivatives 1b–d underwent multiple C H
activation and annulation with 2a to afford 3b–d in 78, 70, and
66% yield, respectively (Table 1, entries 2–4). This trans-
formation is also compatible with halogen substituents on the
aromatic ring of 1. Thus, the reaction of the fluoro-, chloro-,
bromo-, or iodo-substituted benzamidines 1e–h with 2a gave
3e–h in 86, 75, 72, and 78% yield, respectively (entries 5–8).
Both electron-poor and electron-rich benzamidines (1i,j)
were suitable substrates (entries 9 and 10). Furthermore,
N-hydroxy-4-phenylbenzamidine (1k) reacted with alkyne 2a
to provide 3k in 81% yield (entry 11). In a similar manner,
N-hydroxy-3,4,5-trimethoxybenzamidine 1l reacted with 2a
to give 3l in 81% yield (entry 12). However, the reaction of
meta-methoxy-substituted N-hydroxybenzamidine 1m with
2a provided two regioisomeric products 3m and 3m’ in
a 60:40 ratio in a combined yield of 70% (entry 13). The
structures of 3c and 3i were confirmed by single-crystal X-ray
diffraction.[13]
1
2
Product 3
Yield
[%][b]
1
2
3
4
5
6
7
8
9
1a 2a
1b 2a
1c 2a
1d 2a
1e 2a
1 f 2a
1g 2a
1h 2a
1i 2a
3a: R=H
83
78
70
66
86
75
72
78
63
68
81
3b: R=4-Me
3c: R=4-tBu
3d: R=4-OMe
3e: R=4-F
3 f: R=4-Cl
3g: R=4-Br
3h: R=4-I
3i: R=4-CF3
3j: R=4-NMe2
3k: R=4-Ph
10 1j 2a
11 1k 2a
12 1l 2a
13 1m 2a
3l: R=3,4,5-(OMe)3 81
3m: R=3-OMe/
3m’: R=5-OMe
70[c]
Aside from 2a, other symmetric alkynes (2b–e) were also
tested in the present reaction. Thus, the di-aryl-substituted
acetylenes 2b–2e reacted with 1a to afford the corresponding
products 3n–q in good yields (entries 14–17). Interestingly,
di(2-thienyl)acetylene 2 f also reacted smoothly with 1a to
give the annulated product 3r in 60% yield (entry 18).
To further expand the substrate scope of this rhodium-
catalyzed reaction, aliphatic alkynes 2g–i were also subjected
to the reaction with benzamidine 1b (Scheme 2). However,
when 1b was treated with hex-3-yne (2g) under the standard
reaction conditions (see Table 1), only a trace amount of the
cyclization product benzonaphthyridine 5b was observed.
Fortunately, 5b was isolated in 72% yield when the reaction
time was increased to 48 hours. Under similar reaction
conditions, oct-4-yne (2h) and dec-5-yne (2i) also reacted
smoothly with 1b to give substituted benzonaphthyridines 5c
and 5d in 80 and 88% yield, respectively (Scheme 2).
14 1a 2b
15 1a 2c
16 1a 2d
17 1a 2e
3n: R1 =Me
3o: R1 =OMe
3p: R1 =Br
76
80
73
68
3q: R1 =CF3
18 1a 2 f
3r
60
To account for the present catalytic reaction, a possible
[a] Unless otherwise mentioned, all reactions were carried out using
1 (0.25 mmol), 2 (0.80 mmol), [Cp*Rh(CH3CN)3](SbF6)2 (6.0 mol%),
Cu(OAc)2 (1.12 mmol), and tert-amyl alcohol (3.5 mL) at 1308C for 18 h.
[b] Yields of isolated products. [c] 3m/3m’=60:40.
III
À
mechanism involving Rh -catalyzed multiple C H bond
activation and annulation is proposed for 1b and 2a as the
model substrates (Scheme 3).[1,2,4] The first step likely
involves coordination of the oxime nitrogen atom of 1 to
À
Other metal complexes, such as [{RhCp*Cl2}2] and
[{RuCl2(p-cymene)}2] were also effective catalysts, giving 3a
in 68% and 43% yield, respectively. The presence of an
oxidant was crucial for this transformation. In the absence of
an oxidant, 3a was not formed. Cu(OAc)2 appeared to be the
best oxidant, giving 3a in 83% yield. The addition of other
copper(II) salts, such as Cu(OAc)2·H2O or Cu(OTf)2, also led
to the formation of 3a, but a longer reaction time of 24 hours
was required, and 3a was only formed in 62% and 15% yield,
respectively. The choice of solvent was also crucial for the
the RhIII center, followed by ortho C H activation to form
five-membered rhodacycle I. Coordinative insertion of alkyne
2a into the rhodium–carbon bond of I gives seven-membered
rhodacycle II. Reductive elimination affords 1-aminoisoqui-
noline 4 and a RhIII species. Further coordination of the amine
nitrogen atom of 4 to the RhIII species followed by C H
À
activation, insertion of 2a, and reductive elimination affords
5a and a RhI species. The latter is reoxidized by Cu(OAc)2 to
regenerate the active RhIII species. Similar steps that include
À
C H bond activation, alkyne insertion, reductive elimination,
2
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acts as a redox-neutral directing group.[4,9h] As a result, the
formation of 4 from 1b and 2a does in principle not require
an oxidant.
Compound 4 (0.10 mmol) reacted with 2a (0.21 mmol)
under the standard reaction conditions to give product 3b
(96%; Scheme 4). Similarly, 4 also reacted with diaryl and
dialkyl ethynes 2c and 2g to give 3s and 3t in 94 and 90%
yield, respectively (Scheme 4, bottom). We were unable to
isolate intermediate 5, even when 4 was treated with aryl or
alkyl alkynes 2 in a 1:1 ratio. The reactions afforded only the
corresponding products 3 in 45–48% yield. The structures of
3s and 3t were determined by single-crystal X-ray diffrac-
tion.[13] It is noteworthy that the reaction of 4 with different
alkynes provides an efficient method for the introduction of
different substituents onto the final product 3. There are two
nitrogen atoms in 4 that can coordinate to the RhIII species to
Scheme 2. Synthesis of substituted naphthyridines. Reaction condi-
tions: 1 (0.25 mmol), 2 (0.80 mmol), [RhCp*(CH3CN)3](SbF6)2
(6.0 mol%), Cu(OAc)2 (1.10 mmol), t-AmylOH (3.5 mL), 1308C, 48 h.
À
undergo C H bond activation and annulation with alkyne.
The two corresponding intermediates are III and III’
(Scheme 3), which can both give product 3.
The solubility of 3 is reasonably good in dichloromethane
and toluene. For example, the solubilities of the representa-
tive species 3i, 3j, 3l, and 3r are all greater than 10À3 m in
these solvents at ambient temperature. To further understand
the physical properties of these products, we measured the
absorption and emission spectra of these species in dichloro-
methane (Figure 1; see also the Supporting Information,
page S19). The fluorescence spectra of 3i, 3j, and 3r show
emission maxima at 500–530 nm with broad bandwidths (full
width at half maximum ꢀ 100 nm) and weak intensities. On
the other hand, product 3l (R = 3,4,5-(OMe)3) shows very
strong green photoluminescence with a narrower bandwidth
of 75 nm. Furthermore, we also measured the absorption and
emission spectra of 3t and 5d (page S20). Strong deep-blue
emission was observed for 5d, but only weak emission was
found for 3t.
Scheme 3. Proposed mechanism for the formation of 3b. L=Cp*.
and oxidation of the RhI intermediate, afford the final
product 3b and a RhIII species as the catalyst of the next cycle.
Substrate 1 has two possible sites (the amine and oxime
nitrogen atoms) for coordination to the RhIII species. To
determine which atom is coordinating and to support the
mechanism that is proposed in Scheme 3, we isolated 4 from
the reaction of 1b (0.25 mmol) and diphenylacetylene 2a
(0.30 mmol) under the standard conditions in 80% yield
(18 h; Scheme 4, top). It is noteworthy that a relatively small
amount of 2a was used in this reaction. Based on this result, it
appears that the oxime nitrogen atom coordinates to the
rhodium center first to form intermediate I as shown in the
proposed mechanism. Furthermore, the oxime group in 1 also
In conclusion, we have successfully developed a simple
and effective method for the synthesis of highly substituted
naphthyridine-based polyheteroaromatic compounds in high
yields from N-hydroxybenzamidines and alkynes. This trans-
formation appears to proceed through multiple rhodium(III)-
À
catalyzed C H bond activation steps and annulation in a one-
Figure 1. Absorption (1.0ꢀ10À5 m) and fluorescence (1.0ꢀ10À4 m, exci-
tation at 400 nm) spectra of 3i (R=4-CF3), 3j (R=4-NMe2), 3l
(R=3,4,5-(OMe)3), and 3r in dichloromethane.
Scheme 4. Isolation of intermediate 4 and its reaction with alkynes.
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pot fashion. Further applications of this method for the
synthesis of organic materials and a detailed investigation of
the reaction mechanism are currently in progress.
Experimental Section
General procedure for the rhodium-catalyzed synthesis of
benzo[de](isoquinolino[2,1-a])[1,8]naphthyridine. A sealed tube con-
taining N-hydroxybenzamidine 1 (0.25 mmol), alkyne 2 (0.80 mmol),
[Cp*Rh(CH3CN)3](SbF6)2 (6.0 mol%), and Cu(OAc)2 (1.12 mmol)
was evacuated and purged with nitrogen gas three times. Then, tert-
amyl alcohol (3.5 mL) was added to the system via syringe under
nitrogen atmosphere, and the reaction mixture was stirred at 1308C
for 18 hours. The mixture was cooled and diluted with CH2Cl2
(10 mL). The mixture was filtered through a Celite pad, which was
washed with CH2Cl2 (50 mL). The filtrate was concentrated, and the
residue was purified by column chromatography on silica gel using
hexane/EtOAc (5–15%) as the eluent to afford 3 as a pure product.
Received: May 11, 2014
Revised: June 12, 2014
Published online: && &&, &&&&
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Keywords: alkynes · annulation · benzamidines ·
.
À
C H activation · rhodium
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Communications
À
Multiple C H Activation
Activated and annulated: A rhodium-
catalyzed one-pot synthesis of highly
substituted polyheteroaromatic com-
pounds from N-hydroxybenzamidines
and alkynes is described. This reaction
J. Jayakumar, K. Parthasarathy, Y.-H. Chen,
T.-H. Lee
&&&& — &&&&
À
One-Pot Synthesis of Highly Substituted
Polyheteroaromatic Compounds by
likely proceeds through multiple C H
bond activation and annulation.
À
Rhodium(III)-Catalyzed Multiple C H
Activation and Annulation
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!