Synthesis of imidazo and benzimidazo[2,1-a]isoquinolines by rhodium-catalyzed intramolecular double C-H bond activation

Article abstract of DOI:10.1039/c3ob27396b

The rhodium-catalyzed intramolecular direct arylation of imidazole and benzimidazole derivatives via double C-H bond activation is described. This approach provides new access to a wide range of imidazo and benzimidazo[2,1-a] isoquinoline derivatives in m

Full text of DOI:10.1039/c3ob27396b

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Synthesis of imidazo and benzimidazo[2,1-a]-
isoquinolines by rhodium-catalyzed intramolecular
Cite this: DOI: 10.1039/c3ob27396b
Received 10th December 2012,
Accepted 18th February 2013
double C–H bond activation†
DOI: 10.1039/c3ob27396b
Vutukuri Prakash Reddy, Takanori Iwasaki and Nobuaki Kambe*
www.rsc.org/obc
The rhodium-catalyzed intramolecular direct arylation of imida- benzimidazo[2,1-a]isoquinoline derivatives, the intramolecular
zole and benzimidazole derivatives via double C–H bond acti- oxidative cross-coupling via cleavage of two C–H bonds would
vation is described. This approach provides new access to a wide be
a more straightforward and efficient route to these
range of imidazo and benzimidazo[2,1-a]isoquinoline derivatives compounds.
in moderate to high yields. This reaction provides an alternative
It has long been known that Pd catalyzes the oxidative
method to the known Pd-catalyzed intramolecular oxidative homo-coupling of arenes8a,b and cross-coupling between
cross-coupling reactions.
arenes and olefins.8c,d This transformation has been success-
fully applied to cyclization by intramolecular oxidative cross-
coupling reactions.8e–m This successful Pd-catalyzed functiona-
lization of C–H bonds8n,o prompted the study of Rh catalyzed
systems leading to the recent remarkable development of
useful transformations via C–H bond cleavage9 such as oxi-
dative Heck-type reactions,10 oxidative aryl–aryl coupling,11 as
well as addition to carbon–carbon10c,d,k,q,12 or carbon–hetero-
atom13 unsaturated bonds.
Introduction
Nitrogen-containing heterocycles and their derivatives are
often found in natural products and in pharmaceuticals and
agrochemicals.1,2 Since imidazo and benzimidazo[2,1-a]iso-
quinoline derivatives show interesting biological activities such
as anticancer, anti-HIV-1, antimicrobial and antiviral proper-
ties,3,4 a variety of synthetic protocols have been developed for
the preparation of imidazo and benzimidazo[2,1-a]isoquino-
line derivatives based on condensation reactions5 and the
photocyclization of 1-styrylimidazoles.6 In addition, coupling
reactions catalyzed by transition metals, such as palladium
and copper, have recently been reported and successfully
applied to the preparation of such heterocyclic compounds
through C–N and C–C cross-coupling.7 However, these proto-
cols require the use of organohalides7a–d or boranes7e or both7f
as starting materials. These prefunctionalized starting
materials are not readily available from commercial sources
and are often difficult to synthesize. Therefore, the develop-
ment of more general and convenient processes using readily
accessible and inexpensive substrates is an important theme.
As a promising strategy for the synthesis of the imidazo and
During the course of our study on transition metal-cata-
lyzed cross-coupling reactions,14 we developed a straightfor-
ward procedure for the synthesis of imidazo and benzimidazo-
[2,1-a]isoquinoline derivatives. The method involves
a
rhodium-catalyzed cross-coupling reaction via double C–H
bond cleavage as the first example of the rhodium-catalyzed
intramolecular oxidative cross-coupling of heteroarenes with
arenes, although the corresponding aryl–aryl intramolecular
coupling reactions are known (Scheme 1).15
Department of Applied Chemistry, Graduate School of Engineering, Osaka University,
Suita, Osaka 565-0871, Japan. E-mail: kambe@chem.eng.osaka-u.ac.jp;
Fax: +81-6-6879-7390; Tel: +81-6-6879-7390
†Electronic supplementary information (ESI) available: Full experimental
details, characterization data, 1H and 13C NMR spectra for new products. See
DOI: 10.1039/c3ob27396b
Scheme 1 Rhodium-catalyzed
intramolecular
oxidative
cross-coupling
reaction.
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Table 1 Optimization of the rhodium-catalyzed intramolecular C–H arylationa
Table 2 Screening of oxidantsa
Entry Oxidant
Yieldb (%) Entry Oxidant
Yieldb (%)
Entry
Catalyst
Solvent
Additive
Yieldb (%)
1
2
3
4
5
6
7c
AgOAc
Ag2CO3
AgF
CuCl2
CuCO3
Cu(OAc)2
Cu(OAc)2·H2O 81
21
Trace
0
Trace
Trace
77
8d
Cu(OAc)2
Cu(OAc)2
(NH4)2S2O8
TBHP
47
65
0
0
0
9e
1
2
3
4
5
6
7
8
[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
[Rh(cod)Cl]2
[Rh(nbd)Cl]2
Rh2(OAc)4
DMA
Mesitylene
DMF
None
None
None
None
49
56
18
25
77
65
72
63
30
42
26
71
46
49
42
36
48
0
10
11f
12
13
Toluene
K2S2O8
—
Mesitylene
Mesitylene
Mesitylene
DMA
DMF
Toluene
PivOH
AcOH
1-AdCO2H
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
0
a Reaction conditions: 1a (0.25 mmol), [Cp*RhCl2]2 (3.0 mol%),
oxidants (0.6 mmol), PivOH (0.5 mmol), mesitylene (1.0 mL), 24 h,
140 °C. b Isolated yields. c [Cp*RhCl2]2 (5.0 mol%), Cu(OAc)2·H2O
(0.3 mmol) and AgSbF6 (20 mol%) in mesitylene (1.0 mL) without
using PivOH. d Cu(OAc)2 (0.3 mmol). e Cu(OAc)2 (0.45 mmol). f 2.0
equiv.
9
10
11
12
13c
14c
15c
16c
17d
18
19e
None
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Mesitylene
Rh(PPh3)3Cl
RhCl3
[Cp*RhCl2]2
None
similarly in the absence of PivOH to give a comparative yield
of product 2a (entry 7).
[Cp*RhCl2]2
31
a Reaction conditions: 1a (0.25 mmol), [Cp*RhCl2]2 (3.0 mol%), Cu-
To explore the scope of this rhodium-catalyzed intramole-
(OAc)2 (0.6 mmol), additive (0.5 mmol), solvent (1.0 mL), 24 h, 140 °C. cular oxidative cross-coupling, the reactions of 1a–n were exam-
b Isolated yields. c Rh (10 mol%). d [Cp*RhCl2]2 (1.5 mol%). e At 70 °C.
ined under optimized conditions. As shown in Table 3, the
Cp*: pentamethylcyclopentadienyl; cod: 1,5-cyclooctadiene; nbd:
bicyclo[2.2.1]hepta-2,5-diene.
corresponding imidazo and benzimidazo[2,1-a]isoquinoline
derivatives 2a–l were obtained in moderate to high yields. Alkyl
and methoxy substituents exerted little effect on the yield, but
substituents at the ortho position of the benzene ring slightly
reduced the yields due to the steric effect (2c and 2j). When 1h
carrying a 1,2-diphenylvinyl moiety was employed, the reaction
Results and discussion
We carried out the reaction under different conditions using proceeded efficiently to give the corresponding cyclized
product 2h in 62% yield, indicating that the phenyl group on
the vinylic tether carbon did not affect the reaction.16
(Z)-1-styryl-1H-benzimidazole (1a) as a model substrate and the
results are summarized in Table 1. Among the various solvents
tested, DMA and mesitylene afforded the best results (Table 1, However, 1m, having a thiophene ring, and 1n, having an
(E)-styryl group, did not afford coupling products. The pro-
entries 1, 2, 5, 8), whereas DMF and toluene gave lower yields
cedure using AgSbF6 and Cu(OAc)2·H2O in mesitylene gave
(entries 3, 4, 9, 10). PivOH was found to be an effective additive
(entry 5). Among the Rh complexes examined, [Cp*RhCl2]2 slightly better results than the procedure using PivOH as the
additive as shown in Table 3.
proved to be preeminent for this reaction (entries 5, 12–16).
A plausible catalytic cycle for the reaction is illustrated in
When the amount of [Cp*RhCl2]2 was reduced to half from
3.0 mol%, the yield decreased (entries 5 and 17). No reaction Scheme 2. The reaction starts from Cp*Rh(OPiv)2 or Cp*Rh
(OAc)2 which is derived from the ligand exchange between
rhodium dimer and the pivalic acid or Cu(OAc)2. Thus formed
Cp*Rh(OCOR)2 is coordinated with a nitrogen lone pair of
imidazole to give intermediate II in Path A. Rh then undergoes
insertion into the C–H bond at the C2 position assisted by the
took place in the absence of the metal catalyst (entry 18). The
reaction was slow at 70 °C and the coupling product was pro-
duced in low yield (entry 19).
Table 2 shows the effect of various oxidants on this reaction
and Cu(OAc)2 was found to be an excellent oxidant (entry 6).
The use of AgOAc gave products in low yield (entry 1) and carboxylate ligand leading to the formation of intermediate III
with the loss of carboxylic acid.17 The subsequent C–H bond
other oxidants such as Ag2CO3, AgF, CuCl2, CuCO3,
cleavage by Rh(III) generates a seven-membered rhodacyclic
ineffective (entries 2–5 and 10–12). In the absence of an intermediate IV which undergoes reductive elimination to give
(NH4)2S2O8, TBHP (tert-butyl hydroperoxide) and K2S2O8 were
product 2 and a Rh(I) species, and the latter is oxidized by
Cu(II) to complete the catalytic cycle. A possible alternative
oxidant, no reaction occurred (entry 13). The best yield was
obtained when 3.0 mol% amount of [Cp*RhCl2]2 was
employed in combination with Cu(OAc)2 (2.4 equiv.) as the pathway is shown by Path B, where the catalytic cycle is trig-
gered by the coordination of the nitrogen lone pair of imida-
zole to Cu(OCOR)2 affording intermediate V. The insertion of
oxidant and PivOH (0.5 mmol) as the additive in mesitylene
(1.0 mL) at 140 °C for 24 h (entry 6). This reaction proceeded
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Table 3 Rhodium-catalyzed intramolecular oxidative cross-coupling of various imidazolesa
a The same reaction conditions as in Table 2, entry 6. Results obtained under the conditions indicated in Table 2, entry 7 are in parentheses.
Acknowledgements
This work was supported by a GCOE and Grant-in-Aid for
Scientific Research (S) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
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We report herein on the rhodium-catalyzed synthesis of
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