Water as a green solvent for efficient synthesis of isocoumarins through microwave-accelerated and Rh/Cu-catalyzed C-H/O-H bond functionalization

Article abstract of DOI:10.1039/c3ra43175d

Green chemistry that uses water as a solvent has recently received great attention in organic synthesis. Here we report an efficient synthesis of biologically important isocoumarins through direct cleavage of C-H/O-H bonds by microwave-accelerated and Rh/

Full text of DOI:10.1039/c3ra43175d

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Water as a green solvent for efficient synthesis of
isocoumarins through microwave-accelerated and
Cite this: RSC Adv., 2013, 3, 23402
Rh/Cu-catalyzed C–H/O–H bond functionalization†
Qiu Li,ac Yunnan Yan,ad Xiaowei Wang,ae Binwei Gong,ae Xiaobo Tang,a JingJing Shi,a
ab
a
*
*
and Wei Yi
H. Eric Xu
Green chemistry that uses water as a solvent has recently received great attention in organic synthesis. Here
we report an efficient synthesis of biologically important isocoumarins through direct cleavage of C–H/O–H
bonds by microwave-accelerated and Rh/Cu-catalyzed oxidative annulation of various substituted benzoic
acids, where water is used as the only solvent in the reactions. The remarkable features of this “green”
methodology include high product yields, wide tolerance of various functional groups as substrates, and
excellent region-/site-specificities, thus rendering this methodology a highly versatile and eco-friendly
alternative to the existing methods for synthesizing isocoumarins and other biologically important
derivatives such as isoquinolones.
Received 25th June 2013
Accepted 18th September 2013
DOI: 10.1039/c3ra43175d
www.rsc.org/advances
various pharmacophores bearing isocoumarin unit have
attracted particular attention because of their broad ranges of
interesting biological properties including anti-diabetic, anti-
allergic, anti-fungal, anti-bacterial, anti-inammatory, anti-
angiogenic, and differentiation inducing-activities against
leukemic cells.4
Introduction
In recent years, much effort has been devoted towards the
development of sustainable reaction media, especially, the use
of water as solvent has aroused considerable interest in the eld
of organic synthesis.1 Indeed, water as a solvent has many
advantages over conventional organic solvents, because it is
cheap, readily available, non-toxic, non-polluting, and non-
ammable. Thus, the water-mediated organic synthesis is very
attractive from both an economical and environmental point of
view.2 As a result, intensive effort has been invested into the
development of catalytic processes by employing water as a
medium to accomplish greener syntheses of many biologically
important pharmacophores.3 An uncertainty that exists is how
far the catalytic processes used the water as the green solvent
could be engineered to benet the synthesis of biologically
important pharmacophores. To speak of bioactive molecules,
The potential of isocoumarins has led to the development of
various synthetic protocols for making these compounds. So
far, one of the most general strategies for their synthesis
involves palladium-catalyzed annulations of alkynes by ortho-
halo-substituted carboxylic acids.5 However, this approach
requires pre-functionalized benzoic acids as substrates, which
result in formation of stoichiometric amounts of salt wastes as
by-products, thus limiting the utility and applicability of this
method in practical use. To circumvent these disadvantages,
Miura,6 Jeganmohan7a and Ackermann7b have recently devel-
oped a more atom- and step-economical process, in which
transition-metal Rh or Ru/Cu was used to catalyze oxidative
annulations of alkynes by non-halogenated carboxylic acids via
a direct C–H bond cleavage with organic solvent DMF, DCE or
t-BuOH as the reaction medium. This process improved the
reaction conditions but produced environmentally non-
compatible solvents that limit its utility. Taking into account
this point, Ackermann and co-workers also investigated the
Ru/Cu-catalyzed reaction, where the water was used as the
solvent. However, the lower reaction yield was found in their
catalysis.7c In addition, Chiba and co-workers showed that the
treatment of Cu2+ in DMF resulted in the disappearance of Cu2+
band in the UV/vis region, implying that DMF might reduce Cu2+
to form Cu+.8 This result suggests that the use of DMF reduced the
synthetic efficiency in the Rh/Cu-catalyzed oxidative annulation
aVARI/SIMM Center, Center for Structure and Function of Drug Targets, CAS-Key
Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese
Academy of Sciences, Shanghai 201203, P.R. China. E-mail: yiwei.simm@simm.ac.
cn; Fax: +86-21-20231000-1715; Tel: +86-21-20231000-2507
bLaboratory of Structural Sciences, Program on Structural Biology and Drug Discovery,
Van Andel Research Institute, Grand Rapids, Michigan 49503, USA. E-mail: eric.xu@
vai.org
cNano Science and Technology Institute, University of Science and Technology of
China, Suzhou, Jiangsu 215123, P.R.China
dCollege of Pharmaceutical Sciences, Gannan Medical University, Ganzhou, Jiangxi
341000, P.R. China
eCollege of Pharmaceutical and Biological Engineering, Shenyang University of
Chemical Technology, Shenyang 110142, P.R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra43175d
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system. Therefore, it is important to develop economically and Aer many trials (Table 1, entries 1–8), we found that treatment
environmentally more viable procedures for the synthesis of of 1a with 2a in the presence of 5.0 mol% of [Cp*RhCl2]2 and
isocoumarins.
Cu(OAc)2$H2O in water at 120 ꢀC for 0.5 h gave the desired
On the other hand, microwave-assisted reaction strategy has isocoumarin product 3aa in the best yield of 85% (Table 1, entry
now been recognized as one of the most powerful and envi- 5). Comparison with the present example (Table 1, entry 8),
ronmental friendly tools in synthetic chemistry and has which resulted in 62% yield of 3aa for 1 h under the conven-
attracted much attention due to its specic effects such as tional heating conditions, it showed that the use of microwave-
efficient atomic utilization, improved temperature and reaction assisted heating provided a faster reaction rate and an improved
homogeneity, and possible modications of activation param- yield of the desired product. Moreover, the reaction could
eters (e.g., DHs and DSs).9,10 Indeed, many reports have conveniently be scaled up to gram levels (Table 1, entry 9).
demonstrated that a wide range of metal-catalyzed reactions
and cross-coupling reactions benet strongly from microwave essential for the success of the present catalytic reaction,
heating, exhibiting faster reaction kinetics.11
because the acetate anion and the copper cation is critical for
It is noteworthy that the addition of copper acetate was
Thus, we envisioned that the use of microwave-assisted the cyclometallation step and the regeneration of the catalyst
method in combination with water as a solvent could be (see below). No reaction was observed in the absence of the
developed as a versatile and sustainable methodology for oxidant Cu(OAc)2$H2O (Table 1, entry 10). Other anionic copper
expeditious synthesis of biologically important heterocyclic salts, such as Cu(BF4)2$6H2O and CuCl2$2H2O, gave lower
molecules. Here we report a “greener” route to isocoumarins via conversion (Table 1, entries 11 and 12). No desired product was
a water-mediated and Rh/Cu-catalyzed coupling reaction of formed in the absence of catalyst under otherwise identical
benzoic acids with alkynes under microwave-assisted heating conditions (Table 1, entry 13).
procedures. This method is simple, efficient, and fast, which
With the optimal “green” conditions in hand, we sought to
can be extended to the synthesis of many other biologically investigate the scope and generality of this methodology. The
important heterocyclic compounds.
results shown in Scheme 1 attest that this methodology is
compatible with a wide range of substituents in the carboxylic
acids. As shown in Scheme 1, the reaction of various substituted
benzoic acids with 2a proceeded well, which resulted in the
corresponding isocoumarin derivatives in good yields. Both
electron-rich and electron-decient benzoic acids were good
substrates in this reaction. Many important and valuable
Results and discussion
From “green chemistry” point of view, we initiated our study
with the coupling reaction of tolane and carboxylic acid under
the microwave heating condition using water as a green solvent.
Table 1 Optimization of reaction conditionsa
Entry
Catalyst
Oxidant
Temp (ꢀC)
Time (min)
Yieldb
1
2
3
4
5
6
7
[Cp*RhCl2]2
[RuCl2(p-cymene)]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
[Cp*RhCl2]2
—
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
—
110
110
110
100
120
130
120
120
120
120
120
120
120
120
30
30
30
30
30
30
15
60
30
30
30
30
30
30
79%
40%
61%
65%
85%
51%
35%
62%
81%
0
8c
9d
10
11
12
13
14e
Cu(BF4)2$6 H2O
CuCl2$2H2O
Cu(OAc)2$H2O
Cu(OAc)2$H2O
11%
0
0
[Cp*RhCl2]2
46%
a
Reaction conditions: 1a (0.25 mmol), 2a (0.3 mmol), Rh catalyst (5.0 mol%), and Cu(OAc)2$H2O (0. 5 mmol), H2O (1.0 mL). b Isolated yields. c The
reaction was carried out under the conventional heating conditions. d The reaction was scaled-up to a gram substrate level. e 1 mol% of Rh catalyst
was used.
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Scheme 1 Scope using symmetrical phenyl disubstituted alkyne 2a. Isolated yield were given. Reaction conditions: 1a–h (0.25 mmol, 1.0 eq.), 2a (0.3 mmol, 1.2 eq.),
Rh catalyst (5.0 mmol%), and Cu(OAc)2$H2O (0.5 mmol, 2.0 eq.), H2O (1.0 mL), 120 ꢀC, 30 min.
functional groups in the aromatic ring of carboxylic acids 1b–h,
Interestingly, extension of this reaction to heteroarenes 1i–j
such as uoro, chloro, bromo, iodo, methyl, and nitro substit- proved to be successful (Scheme 2, 3ia–ja). Moreover, the
uents (3ba–ha) were compatible in the present catalytic cationic rhodium(III) catalyst further allowed the oxidative
reaction. Notably, para-methyl benzoic acid afforded the iso- coupling of acrylic acid derivative 1k and alkyne 2a, providing a
coumarin product in good yield (3ca), whereas para-nitro or step-economical access to a-pyrone 3ka.
-iodo benzoic acid showed a slightly decreased efficiency (3ga
It is important to note that when asymmetrical alkynes were
and 3ha). The result suggested that the introduction of electron- employed, the insertion of an aryl alkyl-disubstituted alkyne
rich groups might be more favorable for the optimal “green” occurred in remarkably high region-selectivity with the aryl-
catalytic system.
substituted carbon center being installed at the 3-position
(Scheme 3, 3ab–hb, 3ib and 3kb).
Considering the remarkably broad substrate scope displayed
by the rhodium(III) catalysis in water, we performed mechanistic
studies to delineate its mode of action (Scheme 4). To this end,
an intramolecular competition experiment with meta-
substituted benzoic acid 1l exclusively delivered product 3la
through the site-selective functionalization, which occurred at
the sterically less congested C–H bond and displayed a higher
kinetic acidity. This result was in good agreement with that
found by Ackermann.7 Additionally, competition experiments
between differently substituted carboxylic acids indicated
that electron-rich benzoic acids were converted preferentially,
suggesting they were better substrates than electron-poor ben-
zoic acids.
Based on these observations and the known metal-cata-
lyzed C–H bond activation/annulations reactions,12 we
proposed a possible mechanism as illustrated in Scheme 5.
The rst step is an acetate-assisted irreversible C–H bond
activation, which yields a ve-membered rhodacycle interme-
diate A coupling with formation of acetic acid. Subsequently
the region-selective transfer of alkyne insertion forms seven-
membered rhodacycle intermediate B, which, upon reductive
elimination of C–H/O–H bonds, yields the desired iso-
coumarin derivative 3 with the reduction of rhodium center
Scheme 2 Oxidative annulations with heteroarenes 1i–j and 2-methyl acrylic
acid 1k. Isolated yield were given. Reaction conditions: 1i–k (0.25 mmol, 1.0 eq.),
2a (0.3 mmol, 1.2 eq.), Rh catalyst (5.0 mmol%), and Cu(OAc)2$H2O (0.5 mmol,
2.0 eq.), H2O (1.0 mL), 120 ꢀC, 30 min.
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Scheme 3 Scope using asymmetrical phenyl methyl-disubstituted alkyne 2b. Isolated yield were given. Reaction conditions: 1a–h, 1i and 1k (0.25 mmol, 1.0 eq.), 2b
(0.3 mmol, 1.2 eq.), Rh catalyst (5.0 mmol%), and Cu(OAc)2$H2O (0.5 mmol, 2.0 eq.), H2O (1.0 mL), 120 ꢀC, 30 min.
from Rh(III) to Rh(I). Finally, the resulting Rh(I) may be
oxidized by the Cu(II) salts to regenerate Rh(III) complex, which
Conclusions
In conclusion, we have developed a simple, versatile, efficient,
and high region-/site-selective methodology for the direct
synthesis of isocoumarins and their heterocyclic analogs
through microwave-accelerated and Rh-catalyzed oxidative C–
H/O–H bonds cleavages, where water was used as a green
solvent. In the present study, a Cu(OAc)2$H2O co-catalyst was
found to be crucial for the C–H bond activation step. A wide
range of substrates are compatible with this synthetic method,
including both electron-rich and electron-decient benzoic
acids, and other characteristic carboxylic acids. The identical
catalytic system was applicable to make biologically important
molecules such as isoquinolones. The use of a microwave-
accelerated protocol and water as the green reaction medium
makes this methodology as a benign alternative to the existing
methods.
is then released to reinitiate the catalytic cycle.
Developing a “greener” process as a synthetic strategy to
make biologically active scaffolds is one major impetus in the
eld of organic synthesis. We therefore sought to demonstrate
the application of our methodology in the synthesis of iso-
quinolones 5 and 7, indispensable structural motifs in bioac-
tive compounds of importance to medicinal chemistry.13
Although two protocols for preparing the isoquinolones were
reported by Ackermann that employed Ru(II) as the catalyst
and used water as the solvent,14 which represent important
advances, the development of novel catalysts is still necessary
to achieve elusive intermolecular reactivity. Therefore, the
reactions for synthesizing the isoquinolones 5 and 7 were
investigated in our Rh (III)-catalyzed system and typical results
were summarized in Scheme 6. We were pleased to nd that
the green catalytic system worked well on N-hydrox-
ybenzamide 4 and acetophenonoxime 6. In the present cases,
the desired products 5 and 7 were produced smoothly through
oxidative annulations of the C–H/N–O bonds cleavage in the
Experimental section
General procedure for activation/annulations reactions
same conditions and using water as the reaction solvent. All A biotage microwave vial was charged with Rh catalyst (5 mol%)
these results further illustrated the remarkable robustness of H2O (1 mL), carboxylic acids (0.25 mmol), alkynes (0.3 mmol)
our “green” catalyst system.
and Cu(OAc)2$H2O (0.5 mmol). The vial was capped and heated
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Scheme 4 Intramolecular competition experiment. Isolated yield were given. Reaction conditions: 1l, 1c–d (0.25 mmol, 1.0 eq.), 2a (0.25 mmol, 1.0 eq.), Rh catalyst
(5.0 mmol%), and Cu(OAc)2$H2O (0.5 mmol, 2.0 eq.), H2O (1.0 mL), 120 ꢀC, 30 min.
Scheme 5 Proposed mechanism of this “green” catalyst system.
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Scheme 6 Isoquinolone synthesis using this “green” catalyst system.
ꢀ
in the microwave reactor at 120 C for 30 min. A sample of the
cool reaction mixture was extracted with ethyl acetate. The
organic phase was dried over anhydrous sodium sulfate, ltered
and evaporated. The crude product was puried by chroma-
tography on a silica gel column to afford the corresponding
product.
Acknowledgements
7 For selected examples on Ru-catalyzed oxidative annulations
of alkynes by non-halogenated carboxylic acids, see: (a)
C. G. Ravi Kiran and M. Jeganmohan, Chem. Commun.,
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The authors thank the Jay and Betty Van Andel Foundation,
Amway (China) and the Chinese Postdoctoral Science Founda-
tion (2012M511158 and 2013T60477) for nancial support on
this study.
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