Rhodium(III)-catalysed decarbonylative annulation through C-H activation: Expedient access to aminoisocoumarins by weak coordination

Article abstract of DOI:10.1039/c8cc07167e

Rhodium-catalysed decarbonylative annulation of isatoic anhydrides with alkynes through C-H activation for the synthesis of aminoisocoumarins was developed. This enables the gram-scale transformation to iodoisocoumarin which is a vital building block in transition-metal-catalysed cross couplings. These compounds exhibit blue-emitting luminescence properties.

Full text of DOI:10.1039/c8cc07167e

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DOI: 10.1039/C8CC07167E
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Rhodium(III)-Catalysed Decarbonylative Annulation through C-H
Activation: Expedient Access to Aminoisocoumarin by Weak
Coordination†
Received 00th January 20xx,
Accepted 00th January 20xx
Sivakalai Mayakrishnan,a Yuvaraj Arun,ab Narayanan Uma Maheswari*a and Paramasivan
Thirumalai Perumal*ac
DOI: 10.1039/x0xx00000x
www.rsc.org/
Rhodium-catalysed decarbonylative annulation of isatoic weak Lewis basicity, which decreases the coordination power of
anhydrides with alkynes through C-H activation for the synthesis of a metal thus diminishes the stability at the transition state.7 The
aminoisocoumarins was developed. This enables the gram-scale evaluation of carboxylate directing group could potentially lead
transformation to iodoisocoumarin which is a vital building block in to the most consistent method for direct C-H functionalization
transition-metal-catalysed cross couplings. These compounds of readily accessible synthetic building blocks.8
exhibit blue-emitting luminescence properties.
In recent years, transition metal catalysed divergent C-H bond
functionalization highlights its potential in organic synthesis,
and profoundly impacted in synthetic strategy.1 Expanding the
synthetic tool box to allow for the selective functionalization of
different C-H bonds in a given substrate has been a long-
standing goal in methodology development.2 Rh-catalysed C-H
activation has attracted many research groups with a great
interest in the construction of C-C and C-X bonds.3
Even though huge development in the C-H activation, only a
few metal-catalyzed decarbonylative annulations were
reported. Initially, decarbonylative annulations were reported
in the strained four-membered cyclobutanone systmes.4
Subsequently, Matsubara et al. reported the first Ni-catalyzed
decarbonylative annulation of less-strained five-membered
anhydrides or phthalimides with alkynes for isoquinolones or
isocoumarins [Scheme 1, Eq. (1)].5 Recently, a directing-group-
Scheme 1 Transition-metal-catalysed decarbonylative/decarboxylative annulations.
assisted decarbonylative annulation of five-membered isatin
with alkyne for 2-quinolinone was reported by Dong et al.
In this context, we report the first Rh-catalyzed
[Scheme 1, Eq. (2)].6 In addition, albeit growing developments
decarbonylative annulation of less-strained six-membered
in Rh-catalyzed reactions, still very limited reports are available
isatoic anhydrides with alkynes assisted by weakly coordinating
anhydride group for the synthesis of aminoisocoumarins
[Scheme 1, Eq. (3)]. Previously, Matsubara et al. reported the
Ni-catalyzed decarboxylative annulation of isatoic anhydrides
[Scheme 1, Eq. (4)].9 Interestingly, we report decarbonylative
in C–H activation/alkyne annulation by weakly coordinating
directing groups. Moreover, weakly coordinating, anhydrides
add value as an ideal alternative for carboxylates due to their
a.Organic & Bioorganic Chemistry Laboratory, CSIR-Central Leather Research
Institute, Chennai 600 020, Tamil Nadu, India. Email: ptperumal@gmail.com,
umamaheswari@clri.res.in.
annulation of isatoic anhydride using Rh catalyst. Importantly,
this developed decarbonylative annulation delivers a feasible
synthetic route for the synthesis of aminoisocoumarins. The
naturally abundant isocoumarin products exhibit a wide range
of biological properties10 like anticancer, anti-HIV, antibacterial,
antifungal, anti-inflammatory, antiangiogenic, antioxidative,
and immunomodulatory activities.11 To explore this framework
b.Institute for Drug Research, School of Pharmacy, Faculty of Medicine, The Hebrew
University of Jerusalem, Jerusalem 91120, Israel.
c. Department of Chemistry, B. S. Abdur Rahman Crescent University, Vandalur,
Chennai 600 048, Tamil Nadu, India.
†
Electronic Supplementary Information (ESI) available: CCDC 1815391.
Experimental section and characterization. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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against new biological targets, further research efforts devoted 1H-isocoumarin 3a in merely 18%. To improve the yield, we
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towards the synthesis of isocoumarins by new methodologies.12 screened with polar protic solvents such as t-AmOH and MeOH,
DOI: 10.1039/C8CC07167E
it did not improve the yield (Table 1, entries 2 and 3). When
water was used, the reaction went in vain due to the inhibition
Table 1 Reaction optimization of isatoic anhydride with diphenylacetylenea
of C-H activation (Table 1, entry 4). When the polar aprotic
solvents such as toluene, Cl-benzene, 1,4-dioxane, MeCN,
DMSO, and DMF (Table 1, entries 5-10) were used, the yield
substantially increased to 85% in DMF. Either decreasing the
Entry
1
Catalyst (mol%)
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[RhCp*Cl2]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[RuCl2(p-cymene)]2
Pd(OAc)2
Oxidant (equiv.)
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
AgOAc
Solvent (mL)
1,2-DCE
t-AmOH
MeOH
H2O
Yield (%)b
18
temperature to 80 °C or prolonged heating diminished the yield
to 40% (Table 1, entry 11). Whereas, no product was formed in
DMF/H2O (1:1) mixture (Table 1, entry 12). The replacement of
oxidants such as AgOAc, NaOAc, CsOAc, and NaHCO3, decreases
the yield (Table 1, entries 13-16). Incorporation of the additive
like AgSbF6 and PivOH did not improve the yield (Table 1, entries
17 and 18). Reaction did not proceed when switching over to
[Rh(COD)Cl]2, [RuCl2(p-cymene)]2 and Pd(OAc)2 (Table 1, entries
19-22). Finally, we inferred that the best optimized reaction
condition is [RhCp*Cl2]2 as a catalyst, Cu(OAc)2 as an oxidant
and DMF as a solvent at 110 oC for 24 h (Table 1, entry 10).
2
20
3
20
4
---
5
Toluene
Cl-Benzene
1,4-Dioxane
MeCN
DMSO
DMF
25
6
25
7
45
8
30
9
trace
85
40c
10
11
12
13
14
15
16
17
18
19
20
21
22
DMF
DMF/H2O
DMF
---
Table 3 Substrate scope of substituted alkynes
trace
10
NaOAc
DMF
CsOAc
DMF
24
NaHCO3
DMF
---
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
DMF
35d
---e
---
DMF
DMF
Toluene
DMF
---f
trace
trace
DMF
a
Reaction conditions: 1a (0.3 mmol), 2a (0.3 mmol), catalyst (2.5 mol%), and oxidant (1 equiv.) in
solvent (3.0 mL) at 110 °C or refluxed for 24 h. b Isolated yields. c Performed at 80 °C. d AgSbF6 or e PivOH
is used as an additive. f PCy3 is used as a ligand. Bold letters indicate the optimized condition.
To best of our knowledge, there is no report for the
synthesis of 8-aminoisocoumarin by the elimination of CO from
isatoic anhydride. In connection with our interest,13 we herein
describe Rh-catalysed regioselective C-H activation and
cyclization of readily available isatoic anhydrides with alkynes to
afford 8-aminoisocoumarins.
Table 2 Scope of deferent isatoic anhydrides with diphenylacetylene
As given in Table 2, we explored the scope of the isatoic
anhydride in this catalytic system. We demonstrated the
consonance of the isatoic anhydrides with the electron-
donating and withdrawing substituents (including alkyl, halides
substituents) at 6th and 7th-position, thus reacted well with
excellent yields (63−89%). In addition, the reaction was
extended to naphthyl fused isatoic anhydride, which coupled
with a moderate yield 62% (3k). Subsequently, the scope of
alkynes was envisaged with various symmetrical
diarylacetylenes bearing electron-donating groups such as
methyl (3l) and methoxy (3m) coupled smoothly with 1a to give
better yield with 88% and 84% respectively (Table 3). Diaryl
acetylenes bearing halogens well tolerated and afforded
A preliminary reaction was carried out using isatoic
anhydride 1a and diphenylacetylene 2a with 2.5 mol%
[RhCp*Cl2]2 and Cu(OAc)2 (1.0 equiv) in 3.0 mL of 1,2-DCE at 110
oC for 24 h (Table 1, entry 1). This led to 8-amino-3,4-diphenyl-
2 | J. Name., 2012, 00, 1-3
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products (3n−p) in moderate to good yields (56%−71%) and followed by coordination of alkyne 2a offer an alkyVnieewsAprtieclceiOensliCne
.
DOI: 10.1039/C8CC07167E
making a substitution with electron-withdrawing nitro groups Further migratory insertion of the aryl ring into alkyne afford a
provided 3q in lesser yield (33%). The reaction exhibited great seven-membered intermediate D. This effectively delivering the
tolerance when alkyne was extended to dinaphthylcetylenes, desired aminoisocoumarin 3a through the decarbonylation and
which provided 3r in good yield (80%) and was also compatible regeneration of Rh(III) catalyst.
with o-methoxy (3s) and m-methyl diarylacetylene (3t), to give The mechanistic pathway of this hypothesis can be deduced
desired products in good yield 81% and 85% respectively. from competitive experiment between the electron-rich alkyne
Notably, p-methyl diarylacetylene reacted with 1i to afford 3u 2c and electron-poor alkyne 2d with 1a and the results revealed
in 76% yield. Unsymmetrical alkyl-aryl alkynes underwent that 2c reacts faster than 2d, which provided 3m and 3n in
smoothly with excellent regioselectivity (3v and 3w) (ESI). The 1.8:1.0 ratio (Scheme 3). In similar fashion, the reaction
present catalytic system was successfully extended to between aromatic diphenylacetylene 2a and aliphatic
symmetrical aliphatic alkynes (2n
product (3x ac) in moderate to good yields. The structure of the than 2p, which offered 3a and 3z in 2.0:1.0. Additionally, the
synthesized compound was unambiguously conformed by the reaction between electron-rich isatoic anhydride 1g and
-2s), which resulted in the dipropylacetylene 2p with 1a exhibited that 2a reacts faster
-
single crystal X-ray analysis of 3m (Scheme 2).
electron-poor isatoic anhydride 1i with 2a afforded 3g and 3i in
0.5:1.0. Non-symmetrical aromatic alkynes such as R1 = PhOMe;
R2 = PhMe and R1 = PhOMe; R2 = F are used as a precursors,
which provide mixture of products in ratio of 1:1 and 1:2 with
78% and 72% yield respectively. The results are included in the
supporting information (ESI). In addition, the elimination of CO
gas from the reaction mixture was confirmed by
phosphomolybdic acid/PdCl2 test (PMA/PdCl2) (ESI).15
a
Scheme 4 Gram scale synthesis and synthetic transformation.
To demonstrate the effectiveness of this methodology, the
gram-scale reaction between 1a and 2a was performed and 3a
was isolated in 74% yield. Furthermore, aminoisocoumarin 3a
was readily converted to the corresponding iodoisocoumarin 4a
using Sandmeyer reaction with excellent yield (94%), which
would be a useful building block in transition-metal-catalysed
cross coupling reactions (Scheme 4).
Scheme 2 Plausible mechanism for the decarbonylative annulation and ORTET diagram.
0.4
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
(a)
(b)
60
40
20
0
0.3
0.2
0.1
0.0
400
450
500
550
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
(d)1.0
0.8
(c)1.0
0.8
3a
3b
3h
3j
3k
3r
3a
3b
3h
3j
3k
3r
Scheme 3 Competitive experiments.
3z
0.6
0.6
3z
0.4
0.4
Based on the previous literature reports,14 a plausible
catalytic cycle is proposed in Scheme 2. The reaction begins with
the removal of Cl- from the [RhCp*Cl2]2 by copper acetate to
generate active [Rh] species. Intriguingly at first, oxygen of 1a
coordinates with Rh and followed by cyclometallation to deliver
0.2
0.2
0.0
0.0
400
450
500
550
600
650
700
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 1 a Absorption and b Fluorescence spectra of 3a in MeCN/H2O mixtures with varying
water fraction (fw): con. 5 μM; excitation wavelength 370 nm. c Normalized fluorescence
spectra of selected aminoisocoumarins in DCM solution and d in thin-film.
a five-membered rhodacyclic intermediate
protonation and cyclisation provides six-membered rhodacyclic
intermediate . The subsequent addition of CO to the Rh
A. The sequential
B
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Savage and G. Dong, Angew. Chem. Int. Ed., 20V1ie4w, A5rt3ic,le1O8n9li1ne-
1895; (d) P.-H. Chen, T. Xu and G. Dong,DAOnI:g1e0w.1.0C39h/eCm8C. ICn0t7.1E6d7.E,
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fluorescence in solution and thin-film state. In the pipeline of
developing new fluorophores,13 we have decided to investigate
the photophysical properties and potential applications of solid
fluorescent molecules in smart materials, optoelectronic
devices, fluorescent sensors, and biomedical imaging.16 We
measured the absorption and emission in DCM and thin-film
5
(Fig. 1). The fluorescence of 3a-3ac show emission maxima at
6
7
412-537 nm and 438-543 nm in DCM and thin-film, respectively
with broad bandwidths (ESI). To investigate the aggregation
induced emission (AIE) properties of 3a, we measured
photoluminescance in CH3CN/H2O mixtures. The emission was
greatly amplified progressively with a gradual increase of the
water content fw = 10 to fw = 80%. When we changed water
content (fw) from 80 to 90 vol%, PL spectra showed a
tremendous increase in emission due to the change in the
packing order of the aggregates from crystalline state.
In conclusion, the detailed investigation of Rh-catalysed
decarbonylative annulation for the regioselective synthesis of 8-
aminocoumarins with weekly coordinating isatoic anhydrides
was elaborated. Moreover, the present evaluation could be
useful for direct C-H functionalization to access important
synthetic building blocks. This reaction was compatible with
diversely substituted anhydrides and alkynes with preferential
regioselectivity. The mechanistic pathway was elucidated by
conducting competitive reactions. Gram scale synthesis was
successfully carried out with an excellent yield to prove this
viability. Importantly, the conversion to iodoisocoumarin could
be a facile building block in transition-metal-catalysed cross
couplings. Furthermore, we explored the optical behaviour of
aminoisocoumarins by investigating the absorption and
fluorescence properties in solution and thin-film.
8
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Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/C8CC07167E
wꢀꢁꢂꢃꢄꢅꢆLLLꢇꢈ/ꢉꢊꢉꢋꢌꢍꢎꢂ 5ꢎꢏꢉꢐꢑꢁꢒꢌꢋꢉꢊꢃꢓꢎ !ꢒꢒꢄꢋꢉꢊꢃꢁꢒ ꢊꢀꢐꢁꢄꢔꢀ /ꢈI
!ꢏꢊꢃꢓꢉꢊꢃꢁꢒꢕ 9ꢖ ꢎꢂꢃꢎꢒꢊ !ꢏꢏꢎꢍꢍ ꢊꢁ !ꢅꢃꢒꢁꢃꢍꢁꢏꢁꢄꢅꢉꢐꢃꢒ ꢑꢌ íꢎꢉ"
/ꢁꢁꢐꢂꢃꢒꢉꢊꢃꢁꢒ
{ꢀꢁꢂꢃꢂꢄꢂꢀ aꢂ ꢂꢃꢅꢀꢆꢇꢈꢂꢈꢉ òꢊꢁꢂꢅꢂꢋ !ꢅꢊꢈꢉ bꢂꢅꢂ ꢂꢈꢂꢈ Üꢌꢂ aꢂꢇꢍꢆꢎꢂꢅꢀꢉ tꢂꢅꢂꢌꢂꢆꢀꢁꢂꢈ Çꢇꢀꢅꢊꢌꢂꢄꢂꢀ tꢍꢅꢊꢌꢂꢄ
Rh-catalysed decarbonylative annulation of isatoic anhydrides with alkynes through C-H
activation for the direct synthesis of 8-aminoisocoumarins has been developed.