Regioselective synthesis of pyrrole and indole-fused isocoumarins catalysed by N ∧ O chelate ruthenium(II) complex

Article abstract of DOI:10.1007/s12039-018-1519-z

Abstract: Heterocyclic compounds containing pyrrole and indole-fused isocoumarins were obtained from a one-pot synthesis via annulations of N-heterocyclic carboxylic acids and alkynes with N^∧O chelate ruthenium(II) complex, [RuCl(PySO ₃

Full text of DOI:10.1007/s12039-018-1519-z

J. Chem. Sci. (2018) 130:120
© Indian Academy of Sciences
https://doi.org/10.1007/s12039-018-1519-z
REGULAR ARTICLE
Regioselective synthesis of pyrrole and indole-fused isocoumarins
catalysed by N∧O chelate ruthenium(II) complex
KEISHAM S SINGHa,∗, SNEHA G SAWANTa and WERNER KAMINSKYb
aBioorganic Chemistry Laboratory, CSIR-National Institute of Oceanography, Dona Paula, Goa 403 004, India
bDepartment of Chemistry, University of Washington, Seattle, WA 98195, USA
E-mail: keisham@nio.org; keisham.sarjit@gmail.com
MS received 25 May 2018; revised 2 July 2018; accepted 4 July 2018
Abstract. Heterocyclic compounds containing pyrrole and indole-fused isocoumarins were obtained from a
one-potsynthesisviaannulationsofN-heterocycliccarboxylicacids andalkynes withN∧Ochelateruthenium(II)
complex, [RuCl(PySO3)(p-cymene)] (PySO3 = 2-pyridinesulfonate) in the presence of Cu(OAc)2H2O. The
reaction proceeds efficiently in DMF but can also be performed in other solvents such as dichloroethane,
t AmOH, and water. The annulations reaction is regioselective when performed 1-methylpyrrole-2-carboxylic
acid with 1-phenyl-1-propyne.
Keywords. Annulation reaction; isocoumarins; alkynes; ruthenium(II) catalyst; regioselection.
1. Introduction
various functional molecules. Initially, catalysts based
on transition metals such as Pd and Rh have been used in
these C–H bond functionalization of heterocycles.19–24
In recent years, sp2-C–H bond functionalization using
less expensive ruthenium(II) compounds were rapidly
developed and their utility in several reactions such
as arylation,24–27 alkenylation13,14,28 and annulation29–35
reactions have been explored. Annulations reaction
involving C–H/N–H and C–H/O–H bond cleavage
resulting in varieties of the heterocyclic compounds
have been reported with Cu,36 Pd,37 Rh38–40 and Ru.29–35
Specifically, Ackerman29 and Jeganmohan’s10 groups
have studied ruthenium(II)-catalysed annulations of
amide or carboxylic acid with alkynes but the cata-
lyst used was mainly based on [(p-cymene)RuCl2]2 and
requires additives such as KPF6 or AgSbF6.
Notably, as mentioned above, ruthenium-catalysed
annulations of N-heterocyclic acids and alkynes were
based on [(p-cymene)RuCl2]2 catalyst and the reac-
tions have employed organic solvents, although a few
reports mentioned also in water.32 Carboxylate assisted
catalysis is well documented in arene ruthenium(II)
systems30,41–43 involving mostly [RuCl2(p-cymene)]2
catalysts. A limited number of reports describe on C-
H bond functionalization catalysed by ruthenium(II)
Pyrrole and indole-fused N-heterocyclic compounds are
found in several pharmacophores and bioactive com-
pounds.1,2 They are widely distributed in terrestrial and
marine organisms and show various biological prop-
erties such as anti-microbial, antibiotic, antioxidant,
anticancer and antitumor activities.3–7 Interest for the
development of synthetic methodologies of these hete-
rocyclic scaffolds is inspired by their promising biologi-
cal activities. There are several methods available for the
synthesis of N-heterocyclic compounds. Among them,
the transition metal-catalysed C–H bond functionaliza-
tion of N-heterocyclic ring has emerged as an attractive
and environmentally benign method for a ready access-
ing to various heterocyclic compounds.8–11 Over the
decades, tremendous progress has been made on the
metal-catalysed direct C–C bond formation of aromatic
compounds involving the activation of a normally un-
reactive aromatic C–H bond, specifically in terms of
synthesis efficiency and minimizing chemical waste.12
A wide range of functional groups such as ketones,13,14
amines,15 carboxylic acids16 and amides17,18 have been
successfully used as a directing group in transition
metal-catalysed C–H bond functionalization leading to
*
For correspondence
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0123456789().: V,-vol

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J. Chem. Sci. (2018) 130:120
compounds containing a sulfonate ligand.44 Herein, 2.3 Preparation of [RuCl (p-cymene)(PySO3)](1)
we wish to explore annulations of 1-methylpyrrole-
2-carboxylic acid (2) or 1-methylindole-3-carboxylic
A dried Schlenck tube was charged with pyridine sulfonic
acid (PySO3H,0.342 mmol) and KOt Bu (0.342 mmol). After
acid (3) and substituted alkynes (4a–e) using ruthe-
addition of methanol, the reaction mixture was allowed to stir
nium(II) catalyst containing a pyridine sulfonate ligand,
for 30 min then [RuCl2(p-cymene)]2 (0.1 g, 0.163 mmol) was
[RuCl(PySO3)(pcymene)](1) (PySO3 = 2-pyridine-
added. The reaction mixture was stirred under nitrogen for 24
h. The yellow suspension became a clear red solution within
a few minutes. As the reaction progress, the solution became
sulfonate) without additive but in the presence of an oxi-
dant. Spectroscopic characterization of [RuCl(PySO3)
(p-cymene)] (1) and regioselective annulations of 1-
methylpyrrole-2-carboxylic acid (2) or 1-methylindole-
3-carboxylic acid (3) with unsymmetrical alkyne has
been described.
cloudy then an orange-yellow solid appeared. The solid was
centrifuged and washed with diethyl ether (2 x 10 mL).
The additional compound was obtained from the super-
natant by subsequently drying, then dissolving the residue
in dichloromethane followed by precipitation with hexane.
Orange yellow solid; yield: 106 mg (76%); Anal. Calcd. for
C15H18ClNO3RuS: C 41.96; H 4.22; N 3.26; S 7.45. Found: C
41.86; H 4.11; N 3.58; S 7.24. FTIR (KBr, cm−1): 3051, 2974,
1591, 1288, 1436; 1H NMR (CDCl3, δ): 8.76 (d, 1H, J= 5.1),
8.01(m, 1H), 7.88(d, 1H, J=7.5), 7.57(t, 1H), 5.78(d, 1H, J=
5.7), 5.69 (d, 1H, J= 5.7), 5.59 (d, 1H, J 6.0), 5.49 (d, 1H, J=
6.0), 3.05 (m, 1H), 2.22 (s, 3H), 1.36 (t, 6H, J= 7.2); 13C NMR
(CDCl3, δ): 157.34, 153.57, 140.57, 127.84, 122.79, 103.97,
97.65, 82.10, 81.92, 81.64, 80.54, 30.96, 22.91, 21.65, 18.39;
ESI-MS: 394.0051 (M-Cl)+ calculated for C15H18NO3RuS,
found 394.0026.
2. Experimental
2.1 General remarks
All reactions were performed under air in closed Schlenck
tubes unless otherwise noted. The solvent used were of
analytical grade and used as received. RuCl33 · H2O was
purchased from Arrora Matthey Ltd., India. Diphenylacety-
lene, 1-phenyl-1-propyne, 3-hexyne, 4-octyne, 2-butyne, 1-
methylpyrrole-2-carboxylic
acid,
1-methylindole-3-
carboxylic acid, Cu(OAc)2H2O, KOAc and Pivalic acid were
obtained from Sigma Aldrich, India. Distilled water was
degassed with nitrogen prior to use. NMR spectra were
recordedonaBrukerAvance300MHzspectrometerat300.13
(1H), 75.47 MHz (13C) with SiMe4 as internal references
and coupling constants were given in Hertz. Mass spec-
tra were recorded using QSTAR-TOF MS/MS of Applied
Biosystems instrument or Waters UPLC-MSMS(Xevo TQD)
mass spectrometers. Microanalysis (CHNS) was performed
using Vario micro Cube Elemental Analyser. The precursor
complex [RuCl2(p-cymene)]2 was prepared according to a
published procedure.45
2.4 General procedure for ruthenium(II)-catalysed
annulations of 1-methylpyrrole-2- carboxylic acid or
1-methylindole-3-carboxylic acid with alkynes in DMF
A dried Schlenck tube was loaded with [RuCl(p-cymene)
(PySO3)] (0.022 g, 0.05 mmol, 10 mol%), heterocyclic acid
(2 or 3) (0.5 mmol), alkyne (0.65 mmol), Cu(OAc)2H2O
(0.149g, 0.75 mmol). The reaction mixture was stirred in
◦
DMF at 80 C for 16 h under air. After which the reaction
mixture was rotary evaporated. The residue was purified over
silica gel column using petroleum ether and ethyl acetate as
eluant to give the compounds 5 and 6 in up to 80% of isolated
yield. Spectroscopic data of the compounds 5 and 6 are in
accordance with those reported values.23,32,34
2.2 Crystal structure analysis and refinement
X-ray quality crystals of the complex 1 were grown by slow
diffusion of hexane into a dichloromethane solution of 1. The
X-ray diffraction data were collected at 100(2)◦K on a Non-
ius Kappa CCD FR590 single crystal X-ray diffractometer,
using MoKα radiation (λ = 0.71073 Å). Crystal-to-detector
2.5 General procedure for ruthenium(II)-catalysed
distance was 30 mm and exposure time was 20 seconds annulations of 1-methylpyrrole-2- carboxylic acid or
per degree. Data collection was 99.6% complete to 25◦ in
θ. The structure was solved by direct methods (SHELXS,
1-methylindole-3-carboxylic acid with alkynes in water
SIR97),46 and refined by full matrix least-squares base on F2 A dried Schlenck tube was loaded with [RuCl (p-cymene)
using (SHELXL 97).47 All non-hydrogen atoms were refined (PySO3)] (0.022 g, 0.05 mmol), heterocyclic acid 2 or 3
anisotropically while hydrogen atoms were placed in geo- (0.5 mmol), alkyne (1 mmol), Cu(OAc)2H2O (0.199g, 1.0
◦
metrically idealized positions and constrained to ride on their mmol). The reaction mixture was stirred in water at 100 C
parent atoms with C–H distances in the range 0.95–1.0 Å. for 20 h under nitrogen. After which the reaction mixture was
Refinement converged at a final R = 0.0165 (for observed extracted with ethyl acetate (3 x 20 mL). The organic fractions
data F), and wR2 = 0.0415 (for unique data F2). The data were combined and treated with sodium sulfate and filtered,
collection parameters, selected bond lengths and angles are then dried in a rotary evaporator. The residue was purified over
presented in Table S1 and Table S2 (Supplementary Informa- silica gel column using petroleum ether and ethyl acetate as
tion), respectively.
eluant to give the compound 5 or 6.

J. Chem. Sci. (2018) 130:120
Page 3 of 9 120
3. Results and Discussion
Annulations of N-methylpyrrole-2-carboxylic (2) acid
and N-methylindole-3-carboxylic acid (3) with substi-
tuted alkynes (4a–e) was carried out using ruthenium(II)
catalyst, [RuCl(PySO3)(p-cymene)] (1) in the presence
of an oxidant, Cu(OAc)2H2O. Complex 1 was obtained
in 76% yield by the reaction of [RuCl2(p-cymene)]2
with 2-pyridinesulfonic acid in the presence of KOt Bu
(Scheme 1). Complex 1 was fully characterized by
NMR spectroscopy, ESI-MS data and its solid state
structure has been established by single crystal X-ray
1
crystallography (Figure 1). The H-NMR spectrum of
1 shows signals for a pyridine ring in the region of
7.88–8.75 ppm in addition to the signals for the coordi-
nated p-cymene ligand. ESI-MS spectrum of 1 showed
a peak at m/z 394.0026 corresponding to [M-Cl]+. An
ORTEP diagram of complex 1 is shown in Figure 1.
Figure 1. ORTEP diagram of [RuCl(p-cymene)(PySO3)]
(1) with thermal ellipsoid at 50% probability.
(Table 1, entry 6) indicating the reaction proceeds
through carboxylate assisted ruthenium(II)-catalysed
route.33 The reaction performed in other organic sol-
ventssuchasDCEandt AmOH, gavethedesiredproduct
in lower yields (Table 1, entries 1 & 5).
¯
Complex 1 crystallizes in triclinic space group P1 and
adopts the well-known piano stool configuration. The
geometry around the ruthenium atom is regarded as
pseudo-octahedral with the p-cymene ligand occupy-
ing three coordinate sites while the remaining three
coordination sites are occupied by nitrogen, oxygen and
chlorine atoms.
Notably, the majority of annulations of alkynes
and heterocyclic acids were performed in organic sol-
vents using an additive.10,29 Recently, we have reported
annulations between heterocyclic acid and alkyne in
water using the [RuCl2(p-cymene)]2 as catalyst which
proves to be highly efficient and regioselective.32 This
prompted us to evaluate efficacy of this ruthenium(II)
complex, [RuCl(PySO3)(p-cymene)] (1) towards the
annulations of heterocyclic acids and alkynes in water.
In order to find the optimized reaction condition, we
have tested the reaction for various reaction parame-
ters in water (Table 1, entries 7–11). When the reaction
was carried out using complex 1 (5 mol%) as catalyst,
1-methylpyrrole-2-carboxylic acid (0.5 mmol), alkyne
(1.2 equiv.) and Cu(OAc)2H2O (1.5 equiv.) in water
(2 mL) under air for 20 h, gave the desired product 5a
in 25% isolated yield (Table 1, entry 7). An increased in
catalyst loading to 10 mol% with 2 equiv. of dipheny-
lacetylene boost the reaction giving upto 58% of 5a
(Table 1, entry 11). However, employing additives such
as PivOH or KOPiv (20 mol%) did not make any signifi-
cant change in the yield of the product (Table 1, entries 9
The annulations of 2 or 3 with various alkynes have
been performed in both DMF and water as a sol-
vent and the efficiency of the ruthenium(II) complex
1 was compared to that of [RuCl2(p-cymene)].32 When
the reaction was performed using 1-methylpyrrole-2-
carboxylic acid 2 (0.5 mmol), complex 1 (10 mol%),
diphenylacetylene (1.1 equiv), and Cu(OAc)2H2O (1.5
equiv.) in DMF under air afforded the desired prod-
uct, 1-methyl-4,5-diphenylpyrano[3,4-b]pyrrol-7(1H)-
one (5a) in 60% isolated yield (Table 1, entry 3). Under
the similar reaction condition, the reaction performed
with KOAc (20 mol%) gave a lower yield (Table 1,
entry 4). Employing silver salt such as AgSbF6 as addi-
tive did not boost the efficacy of the reaction, instead
of decreased the catalytic efficacy (Table 1, entry 2).
The presence of Cu(OAc)2H2O as oxidant is essen-
tial since attempting the reaction with another oxidant
such as (NH4)2S2O8 did not give any desired product
Scheme 1. Reaction pathway for the synthesis of complex 1.

120 Page 4 of 9
J. Chem. Sci. (2018) 130:120
Table 1. Optimization of reaction for ruthenium (II)-catalysed annulation of 1-methylpyrrole-2-carboxylic acid and dipheny-
lacetylene in organic solvents and water.[a]
Entry
Oxidant
Additive
Solvent
Yield[b]
[1]
[2]
[3]
[4]
Cu(OAc)2H2O
Cu(OAc)2H2O
Cu(OAc)2H2O
Cu(OAc)2H2O
Cu(OAc)2H2O
(NH4)2S2O8
Cu(OAc)2H2O
Cu(OAc)2H2O
Cu(OAc)2H2O
Cu(OAc)2H2O
Cu(OAc)2H2O
Cu(OAc)2H2O
AgSbF6
AgSbF6
—
KOAc
—
—
—
—
PivOH
KOPiv
—
DCE
DMF
DMF
DMF
tAmOH
DMF
H2O
H2O
H2O
H2O
H2O
H2O
54
20
60
58
35
NR
25
54
53
35
58
27
[5]
[6]
[7]c
[8]
[9]
[10]
[11]d
[12]
AgSbF6
[a]Reaction conditions: Substrate (0.5 mmol), catalyst (0.05 mmol), diphenylacetylene (0.65 mmol, 1.2 equiv.), Cu(OAc)2.H2O
(1.5 equiv.), Additive (0.1 mmol), Solvent (2 mL), Temp. 80 ◦C/16 h (organic solvents) and 100 ◦C/20 h (water), under air.
[b]Isolated yield after column chromatography; [c]Reaction performed using 5 mol% of catalyst 1; [d]Reaction performed under
N2 atmosphere and 2 equiv. of diphenylacetylene.
Scheme 2. Annulation of 2 or 3 with substituted alkynes into isocoumarins 5 or 6.
& 10). Even use of silver salt did not boost the reaction
We have carried out the annulations of 1
as in DMF instead, decreased the yield of the desired -methylpyrrole-2-carboxylicacid(2)or1-methylindole-
product (Table 1, entry 12). Thus, the optimized reaction 3-carboxylic acid (3) with various alkynes in the pres-
conditions in DMF and water are as follows: a) [Ru-Cat] ence of 1 under the optimized condition as found for
(10 mol%), substrate (0.5 mmol), Cu(OAc)2H2O (1.5 5a and evaluated the catalytic efficacy of 1 first in
◦
equiv.), alkyne (1.1 equiv.), DMF (2 mL) at 80 C for DMF then in water (Scheme 2; Table 2 & 3). Thus,
16 h under air; and b) [Ru-Cat] (10 mol%), substrate annulations of 2 with 4-octyne, 3-hexyne, and 2-butyne
(0.5 mmol), Cu(OAc)2H2O (2 equiv.), alkyne (2 equiv.), gave 41, 50 and 48% of the desired products, respec-
water (2 mL) at 100 ◦C for 20 h under nitrogen.
tively (Table 2, entries 4–6). On the other hand, 3 gave

J. Chem. Sci. (2018) 130:120
Page 5 of 9 120
Table 2. Ruthenium(II)-catalysed annulations of 1-methylpyrrole-2-carboxylic acid or 1-methylindole-3-carboxylic acid
with alkynes in DMF.[a]
Entry Substrates
Alkynes
Products
Isolated Yield
(%) [b]
2 & 3
2
5a–e
1
2
2
2
60
80
53
R1 = R2 = Ph (5a)
2
R1 = Me; R2 = Ph (5b)
R1 = Me; R2 = Ph (5b)
3[c]
2
41
4
5
R1 = Pr; R2 = Pr (5c)
R1 = Et; R2 = Et (5d)
50
2
6
R1 = Me; R2 = Me
48
2
(5e)
3
6a–e
R1 = R2 = Ph (6a)
R1 = Me, R2 = Ph
(6ba)
7
8
3
3
69
7
21
R1 = Ph, R2 = Me
(6bb)
9
3
3
3
45
64
R1 = R2 = Pr (6c)
10
11
R1 = R2 = -Et (6d)
Traces
R1 = R2 = -CH3 (6e)
[a]Reaction conditions: Substrate (0.5 mmol), catalyst (0.05 mmol), alkynes (0.65 mmol, 1.2 equiv.), Cu(OAc)2H2O (1.5
equiv.), DMF (2 mL), Temp. 80 ◦C, Time 16 h; [b]Isolated yield after column chromatography; [c]Reaction performed in the
presence of KOAc.

120 Page 6 of 9
J. Chem. Sci. (2018) 130:120
Table 3. Ruthenium (II)-catalyzed annulations of N-methylpyrrole-2-carboxylic acid and N-methylindole-3-carboxylic acid
in water[a]
.
Entry
Substrates
Alkynes
Products
Isolated Yield
(%)[b]
2&3
2
5a–c
1
2
3
R1 = R2 = Ph (5a)
58
86
12
2
2
2
R1 = Me; R2 = Ph (5b)
R1 = R2 = -(CH2)2CH3
(5c)
4
R1 = R2 = -(CH2)CH3
16
2
(5d)
3
3
6a–c
R1 = R2 = Ph (6a)
5
6
42
13
22
23
R1 = Me, R2 = Ph
(6ba)
R1 = Ph, R2 = Me
(6bb)
R1 = R2 = -(CH2)2CH3
(6c)
3
7
8
3
3
R1 = R2 = -(CH2)CH3
20
(6d)
[a]Reaction conditions: Substrate (0.5 mmol), catalyst (0.05 mmol), alkynes (1.0, mmol, 2 equiv.), Cu(OAc)2H2O (2 equiv.),
Water (2 mL), under N2 atmosphere, Temp., 100 ◦C, Time 20 h; [b]Isolated yield after column chromatography.
an isolated yield of 45% and 64% with 4-octyne and N-methylindole-3-carboxylic (3) acid resulted in two
3-hexyne, respectively but only a trace product with 2- products, 6ba and 6bb in low yields giving 7% and
butyne (Table 2, entries 9–11).
21%, respectively in DMF (Scheme 3, Table 2, entry
Annulationof N-methylpyrrole-2-carboxylicacid(2) 9). Interestingly, 6bb was formed with the 1 in favour of
with 1-phenyl-1-propyne is highly regioselective form- isomer 6ba irrespective of the solvent used in contrast
ing 5b, as the only product in 80% isolated yield to [RuCl2(p-cymene)]2 in which 6ba was formed as the
(Table 2, entry 2). Regioselectivity is also observed favour isomer32 (Table 2, entry 9 and Table 3, entry 6).
when performed in water as well where the desired
Several ruthenium(II)-catalysed reactions were per-
product 5b was formed with 86% yield, in the reac- formed in water and in some cases catalyst activity
tion of 2 with 1-phenyl-1-propyne (Table 3, entry enhances in water.25,26 As many of ruthenium(II) cata-
2). However, the reaction of 1-phenyl-1-propyne with lysts tolerate water, the efficacy of 1 towards annulations

J. Chem. Sci. (2018) 130:120
Page 7 of 9 120
Scheme 3. Reaction pathway for the annulation of 1-methylin-
dole-3-carboxylic acid with 1-phenyl-1-propyne, producing 6ba and
6bb.
4. Conclusions
Annulations of 1-methylpyrrole-2-carboxylic acid and
1-methylindole-3-carboxylic acid with alkynes in the
presence of ruthenium(II) complex, [RuCl(PySO3)(p-
cymene)] (1) as a catalyst, and Cu(OAc)2H2O as oxidant
but without an additive is described. Reactions were per-
formed in DMF giving excellent to modest yields of the
desired products and even the reaction could be per-
formed in water, though proved to be less effective then
catalyst [RuCl2(p-cymene)]2, but competitive for cer-
tain alkynes such as diphenylacetylene and 1-phenyl-1-
propyne. The reaction is regioselectively forming exclu-
sively (5b) in the annulation of 1-phenyl-1-propyne
with 2 irrespective of solvent used. However, regios-
electivity is less significant when performed 3 with
1-phenyl-1-propyne for which two isomers 6ba and
6bb were formed in low yields. Thus, 1 is efficient for
catalytic sp2C–H bond annulations of 1-methylpyrrole-
2-carboxylic acid and 1-methylindole-3-carboxylic acid
with alkynes in DMF in the presence of Cu(OAc)2H2O.
Scheme 4. Plausible reaction mechanism for the formation
of 5 and 6.
of 2 or 3 with alkynes was tried as well in water. The
reactions in water were not as competitive as in DMF
except in the case of diphenylacetylene and 1-phenyl-
1-propyne which gave isolated yields of 58% and 86%,
respectively (Table 3, entries 1 & 2) for the substrate 2
but a modest yield of 42% with 3 and diphenylacety-
lene, whereas, two products 6ba and 6bb occurred in
13 and 22% when 1-phenyl-1-propyne was employed
in the annulation with 3 (Table 3, entries 5 & 6). How-
ever, annulations of all the other tested alkynes viz.,
4-octyne and 3-hexyne with 3 gave low yields, 23 and
20%, respectively, when performed in water (Table 3,
entries 7 & 8). Thus, the complex 1 is not as productive
and effective as [RuCl2(p-cymene)]2 in water.32
Supplementary Information (SI)
CCDC No. 1055202 contains the supplementary crystallog-
raphy data for this paper. Copies of this information may be
obtained free of charge from the Cambridge Crystallogra-
phy Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Additional information pertaining to preparation and charac-
terization of compounds (1H, 13C NMR and ESI-MS spectral
1
data), copies of H and 13C-NMR spectra of all the com-
pounds are available as supplementary information at www.
ias.ac.in/chemsci.
Since the Cu(OAc)2H2O is essential for the reaction,
the annulations are expected to occur via carboxy-
late directed reaction by formation of RuLn(OAc)2
(Scheme 4). The reaction probably initiated by the
deprotonationoforthoC–Hbondofthesubstrate(2or3)
with acetate arising from Cu(OAc)2H2O48 resulting in
the formation of cyclometalate A. Coordinated insertion
of alkyne into RuI I –C bond expected to give inter-
mediate B which then undergoes reductive elimination
leading to products 5 and 6. The produced ruthenium(0)
species is then reoxidized by Cu(OAc)2H2O into the
active ruthenium(II) catalyst, RuLn(OAc)2 (Scheme 4).
Acknowledgements
KSS thanks the Department of Science and Technology, India
for financial support and the Director, CSIR-NIO for provid-
ing necessary facilities.
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