Trinuclear complexes of palladium(II) with chalcogenated N-heterocyclic carbenes: Catalysis of selective nitrile-primary amide interconversion and Sonogashira coupling

Article abstract of DOI:10.1039/c7dt02592k

3-Methyl-1-(2-(phenylthio/seleno)ethyl)-1H-benzo[d]imidazol-3-ium iodide (L1/L2), a precursor of sulfated/selenated N-heterocyclic carbene, was synthesized by the reaction of benzimidazole with 1,2-dichloroethane followed by treatment with PhS/SeNa and MeI. The reaction of L1/L2 with Ag₂O followed by treatment with [Pd(CH₃CN)₂Cl₂] (metal to ligand ratio 3:2), i.e. transmetallation, resulted in trinuclear palladium(ii) complexes [Pd₃(L1/L2-HI)₂(CH₃CN)Cl₆] (1-2). The complexes were characterized with ¹H, ¹³C{¹H} and ⁷⁷Se{¹H} NMR (2 only), elemental analyses, HR-MS and single-crystal X-ray diffraction. The geometry of three Pd atoms in each complex is nearly square planar. The Pd-S/Se, Pd-C, Pd-N and Pd-Cl bond distances (?) in 1/2 are 2.3179(19)/2.4312(10), 1.968(7)/1.952(4), 2.073(8)/2.079(4) and 2.2784(19)-2.298(2)/2.292(2)-2.3003(15), respectively. In both the complexes, all Cl are trans to each other. For the central Pd atom, two benzimidazole rings are also trans to each other. The C-H?Cl non-covalent interactions result in a three-dimensional network. The moisture and air insensitive trinuclear Pd(ii) complexes 1 and 2 are thermally stable and efficient as a catalyst for nitrile-amide interconversion and amine-free Sonogashira C-C coupling (in the presence of CuI). The optimum temperature is 80 °C for the interconversion and 110 °C for the coupling. The catalytic protocols are applicable to both aliphatic and aromatic amides/nitriles. The optimum catalyst loading is 1 mol% for the C-C coupling and 0.5 to 1 mol% for the interconversion. K₂CO₃ as a base gives the best result for Sonogashira C-C coupling. In the conversion of nitriles to amides, the formation of an acid was not detected. After using once, 1/2 can carry out the conversion of ten fresh lots of nitriles to amides with almost the same efficiency. The real catalytic species for the interconversion and coupling appear to be based on Pd(ii) and Pd(0), respectively.

Full text of DOI:10.1039/c7dt02592k

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Trinuclear complexes of palladium(II) with chalcogenated N‐
heterocyclic carbenes: catalysis of selective nitrile‐primary amide
interconversion and Sonogashira coupling
Received
Accepted
Pooja Dubey, Sonu Gupta, and Ajai K. Singh*
DOI: 10.1039/x0xx00000x
www.rsc.org/
3‐Methyl‐1‐(2‐(phenylthio/seleno)ethyl)‐1H‐benzo[d]imidazol‐3‐ium iodide (L1/L2), precursor of sulfated/selenated N‐
heterocyclic carbine, was synthesized by reaction of benzimidazole with 1,2‐dichloroethane followed by treatment with
PhS/SeNa and MeI. The reaction of L1/L2 with Ag2O followed by treatment with [Pd(CH3CN)2Cl2] (metal to ligand ratio 3:2),
i.e. transmetallation, resulted in trinuclear palladium(II) complexes, [Pd3(L1/L2‐HI)2(CH3CN)Cl6] (1‐2). The complexes were
1
characterized with H, 13C{1H} and 77Se{1H} NMR (2 only), elemental analyses, HR‐MS and single‐crystal X‐ray diffraction.
The geometry of three Pd atoms in each complex is nearly square planar. The Pd‐S/Se, Pd‐C, Pd‐N and Pd‐Cl bond
distances (Å) in 1/2 are 2.3179(19)/ 2.4312(10), 1.968(7)/1.952(4), 2.073(8)/2.079(4)and 2.2784(19)‐2.298(2)/2.292(2)‐
2.3003(15)respectively. In both the complexes, all Cl are trans to each other. For central Pd atom two benzimidazole rings
are also trans to each other. The C‐H∙∙∙∙Cl non‐covalent interactions result in three‐dimensional network. The moisture and
air insensitive trinuclear Pd(II) complexes, 1 and 2 are thermally stable and efficient as a catalyst for nitrile‐amide
interconversion and amine free Sonogashira C‐C coupling (in the presence of CuI). The optimum temperature is 80 °C for
the interconversion and 110 °C for the coupling. The catalytic protocols are applicable to both aliphatic and aromatic
amides/nitriles. The optimum catalyst loading is 1 mol% for the C‐C coupling and 0.5 to 1 mol% for the interconversion.
The K2CO3 as a base gives best result for Sonogashira C‐C coupling. In the conversion of nitrile to amide, formation of acid
was not detected. The 1/2 after using once can carry out conversion of ten fresh lots of nitrile to amide with almost same
efficiency. The real catalytic species for the interconversion and coupling appear to be based on Pd(II) and Pd(0)
respectively.
polyaromatic NHC‐based ligands13e‐h is also shown higher
compared to their monometallic analogues. The
organochalcogen ligands have emerged as an attractive
Introduction
The N‐heterocyclic carbenes (NHCs) have rich coordination
chemistry and diverse applications including those in catalysis
of organic reactions.1 The promising performance of transition
metal complexes of such ligands as catalysts in click reaction,2
hydroarylation of alkynes,3 C‐C cross‐coupling,4 oxidation5 and
transfer hydrogenation6 is well known. The photo‐physical
applications of some of such metal complexes were reported.7
The polydentate NHC‐ligands8 tailored with auxiliary donor
sites such as phosphines,9 nitrogen containing heterocycles10
and chalcogens11 form metal complexes which show better
catalytic efficiency than those of monodentate NHCs.12 The
multinuclear transition metal catalysts have drawn significant
attention in modern organic synthesis due to their enhanced
reactivity and selectivity over mononuclear systems.13a‐d
Recently catalytic activity of di‐ and tri‐metallic complexes of
alternative to phosphines for catalyst designing due to their
air/moisture insensitivity. Their transition metal complexes11
are also more air and moisture insensitive than their
phosphine counterparts. This has generated interest in
chalcogenated NHCs. However, reports on them are still
limited.14 The sulfur containing NHCs viz. (S, CNHC) and (S, CNHC
,
S) type ligands have been used to design palladium(II)
complexes found to catalyze C‐C coupling reactions
efficiently.15 The selenium containing imidazolium NHC ligands
and their Pd(II) complexes were first reported in 2013 by our
group and employed for Suzuki−Miyaura C−C coupling.14a
Current status is that selenium‐containing NHCs and their
metal complexes are scantly explored14 than their sulfur
analogues. Recently we have observed that coordination of a
Se ligand with Pd can be strong enough so that PPh3 fails to
cleave Pd‐Se bond.15 Thus with catalysts designed using
organochalcogen ligands desired stability is achievable. The
use of Pd(II) complex of any chalcogenated polydentate NHC,
in the catalytic nitrile‐amide interconversion and Sonogashira
C‐C coupling is not in our knowledge. The amide bond essential
Department of Chemistry, Indian Institute of Technology, Delhi, New Delhi 110016,
India. *e‐mail, aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com,
Fax: +91‐011‐ 26581102; Tel: +91‐011‐26591379;
a. Electronic Supplementary Information (ESI) available: Spectral data of L1 and L2
and 1–2; single crystal data of complexes 1–2 (CCDC 1523310 and 1569993), See
DOI: 10.1039/b000000x/
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to sustain life is of great interest in organic chemistry, reaction of L1 or L2 with Ag2O in CH2Cl2 (in the absence of
biological processes, and pharmaceuticals.16 Numerous natural light) under N2 atmosphere at room temperature. The
products and modern pharmaceutical molecules have this transmetallation of these silver carbene complexes by reacting
bond.16a,16b The direct hydration of nitrile appears to be an them with [Pd(CH3CN)2Cl2] in CH3CN (in 2: 3 ratio) resulted in
1
easy option to convert it into amide, but selective hydration of and
2 which were found air and moisture insensitive. The
nitrile to amide is troublesome and yields are moderate due to monometallic Pd complexes were not obtained when
two reasons (i) further hydrolysis to carboxylic acid often takes Pd:ligand ratio in the reaction was kept 1:1 and 1:2.
place17 and (ii) harsh conditions required due to low reactivity Surprisingly in this condition also the product was
1 or 2,
of nitrile18 are not tolerated by many functional groups.17 obtained when Pd to ligand ratio was 3:2. Probably greater
Therefore, selective and high yield procedure for conversion of stability of trimetallic complexes than the monometallic one
nitrile to amide continues to be of interest to academia and leads to the formation of
1 and 2 in both cases. The solubility
industry.16a Metal‐catalysed approaches to amide formation of these complexes in CH3CN, and DMF and DMSO was found
have been reviewed.16a However, examples of Pd(II) complexes good. In CH2Cl2 and CHCl3 they were soluble moderately and in
reported to catalyze hydration of nitriles to primary amides19 CH3OH, diethyl ether, THF and hexane sparingly. The solutions
are few. There is one report on reversible dehydration of of
primary amides and that too catalyzed with 10 mol% of PdCl2 conditions for several months (as evidenced by 1H NMR of the
in aqueous acetonitrile.20
solution). The elemental analyses, multinuclear NMR, IR data,
Sonogashira coupling catalyzed by palladium is one of the and HR‐MS of and are consistent with their single crystal
1 and 2 in CH3CN were found stable under ambient
1
2
most important method for preparing conjugated enynes and structures determined by X‐ray diffraction (Figs. 1 and 2).
arylacetylenes21 which are precursors for pharmaceuticals and
natural products.22 In this coupling catalyzed with Pd‐
H
Cl
E
PhE- Na+
80 o
N
K2CO3, TBAB
DCE, 80 oC
N
N
N
N
complexes, addition of Cu(I) as a co‐catalyst facilitates the
reaction by in situ generation of copper acetylide that also
promotes oxidative homocoupling of alkynes reducing the
yield of cross coupled product.23 The Sonogashira reaction
using NHC‐Pd complex as a catalyst has been carried out in Cu,
amine and/or solvent‐free conditions.24 However, Pd‐catalysts
designed with organochalcogen ligands are rarely reported for
such coupling.25
A
E = S;
E = Se; B
N
C
MeI RT
NCCH3
Pd
5
1
6
8
Cl
(i) Ag2O / DCM / 8h
RT / N2 atm
Cl
Ph
E
E
Cl
7
4
C
2
N
9
N
N
Pd
3
N
14
13
N
I
E
10
C
Cl
(ii) PdCl2(CH3CN)2
CH3CN
N
Ph
12
Cl
11
Pd
Cl
NCCH3
L1
E = S;
Complex 1; E = S
E = Se; L2
2
Complex ; E = Se
It was therefore thought worthwhile to explore
sulfated/selenated NHCs to design Pd(II) complexes for
catalytic nitrile‐amide interconversion and Sonogashira C‐C
coupling. Thus 3‐methyl‐1‐(2‐(phenylthio)ethyl)‐1H‐benzo[d]‐
imidazol‐3‐ium iodide (L1) and 3‐methyl‐1‐(2‐(phenylselenyl)‐
Scheme 1. Synthesis of L1 and L2 and trimetallic 1–2.
Spectral characterization
The 1H, 13C{1H} and 77Se{1H} NMR spectra of L1
,
L2
,
1
and
2
are
ethyl)‐1H‐benzo[d]imidazol‐3‐ium iodide
(
L2)
(Chart 1),
given in Figs. S1−S10 of ESI. The spectra of air insensitive
chalcogenated benzimidazolium iodide salts L1 and L2 and
precursors of new sulfated and selenated NHCs respectively
are reported herein. The trimetallic complexes of these NHCs,
[Pd3(L1/L2‐HI)2(CH3CN)Cl6] (1‐2) were found efficient catalysts
for nitrile‐amide interconversion and Sonogashira coupling.
Trimetallic Pd complexes for C‐C coupling are scantly
Pd(II) complexes of their carbenes,
1
and
2 were found
characteristic. The spectra of and
1
2 are also consistent with
1
their single‐crystal structures. In H NMR spectra of L1 and L2
proton attached to C14 has been observed at 10.83 and 10.46
ppm respectively, consistent with the values reported for
benzimidazolium halides (δ 9.0−12.0),27 which suggest high
reported.12a, 12d, 26 The Sonogashira coupling catalyzed with
1
and 2 is amine‐free (K2CO3 used as a base) and may be labelled
as environmental friendly.
acidity of this proton. In 1H NMR spectra of
1 and 2 there was
no signal in the range 9.0−11.0 ppm, indicaꢀng the
deprotonation of C14 and formation of Pd(II)‐carbene (NHC)
bond (Figs. S6 and S8 in ESI). In 77Se{1H} NMR spectrum of L2
one signal appears at 287.9 ppm. In the spectrum of palladium
complex of its carbene,
2
the signal shifts to 341.4 ppm (ESI:
to
Fig. S10). The shifting of the signal in 77Se{1H} spectrum of
2
a higher frequency (~53.0 ppm) in comparison to that of free
L2 supports the coordination of carbene with Pd through Se. In
HR‐MS of iodide salts L1 and L2 the peaks appearing at m/z
269.1100 and 317.0551 respectively, may be ascribed to
benzimidazolium cation (ESI, Figs. S69−S70). In mass spectra of
Chart 1.
Results and discussion
The syntheses of NHC ligand precursor iodide salts L1 and L2
and Pd(II)‐complexes and are summarized in Scheme 1.
1
2
1
and 2 the molecular ion or cation peak was not observed
First, Ag complex of sulfated or selenated NHC was formed by
(ESI, Figs. S71−S72). However, a peak in the mass spectrum of
2 | J. Name., 2012, 00, 1‐3
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1
noticed at m/z, 426.9863, may be ascribed to
C16H18ClN2OPdS (C) (Chart 2). Similarly in the mass spectrum of
2
peak observed at m/z, 456.9195 may be assigned to
C16H16ClN2PdSe (D in Chart 2).
Fig. 2 ORTEP of
2
. bond lengths (Å) Pd(1)–Se(1) 2.4312(10),
Cl(1)–Pd(1) 2.2971(12), Pd(2)–C(9) 1.952(4), Pd(2)–N(3)
2.079(4), Pd(2)–Cl(2) 2.2940(15), Pd(2)–Cl(3) 2.3003(15). Bond
angles (°): Se(1)–Pd(1)–Se(1) 180.00(2), Se(1)–Pd(1)–Cl(1)
96.50(4), C(9)–Pd(2)–N(3) 176.42(18), C(9)–Pd(2)–Cl(2)
87.67(13), C(9)–Pd(2)–Cl(3) 90.98(13), Cl(2)–Pd(2)–Cl(3)
178.64(4).
Chart 2
Crystal structures
The structures of
1
and 2 were determined by X‐ray diffraction
This lengthening may be ascribed to trans influence of carbene
(NHC) on Pd‐N bond in 1/2. The Pd−S bond length in complex 1
on their single crystals of suitable quality grown by diffusion of
diethylether into the solution of each complex made in
acetonitrile. The crystallographic data and refinement
parameters are given in the ESI (Table S1). The ORTEP
is 2.3178(19) Ǻ, consistent with the earlier reported values
2.273(11)−2.295(11) Å for Pd(II) complexes of thioether
ligands based on a pyrazole skeleton.28 The Pd−Se bond length,
diagrams of
1 and 2 are shown in Figs. 1 and 2 with selected
2.4312(10) Å, of
2.369(7)−2.390(5)
2
is consistent with the values13,28a
Å
observed for Pd(II) complexes of
bond lengths and bond angles (relevant for geometry of Pd).
The ellipsoids have 30% probability and H atoms are omitted
for clarity. The details of other selected bond angles and
lengths are given in the ESI (Table S2). Each palladium in
has a nearly square planar geometry, as revealed by selected
bond angles given in captions of Figs 1 and 2. Each complex
has two (S/Se, CNHC) ligands which coordinate with central Pd
through S/Se. The two remaining Pd atoms make a Pd‐C bond
with carbene. The coordination sphere of each Pd is completed
selenoether ligands having a pyrazole core. The Pd−Cl bond
lengths [2.2784(19)−2.3003(15) Å] of
1 and 2 are normal and
1 and
consistent with the values,13,28a 2.2868(19)−2.3221(14) Å found
2
for Pd(II) complexes of several carbene ligands. The Cl atoms
of complexes
1 and 2 are involved in C–H∙∙∙Cl secondary
interactions resulting in a three‐dimensional framework, as
depicted in Figs. 3 and 4.
by Cl or CH3CN and Cl. The Pd−C bond lengths of complexes
1
and
2 (1.968(7) and 1.952(4) Å respectively) are consistent
with the values13,27 1.955(4)−1.971(4) Å reported for Pd(II)
complexes of Se containing N‐heterocyclic carbenes and
related derivatives. The Pd−C bond lengths of
1 and 2 are also
consistent with the values found for their benzimidazolin‐2‐
ylidene based analogues.27 The Pd−N (CH3CN) bond lengths of
1
and 2 [2.073(8) and 2.079(4) Ǻ respectively] are similar to the
value, 2.0814(17) Å27b reported for Pd(II) complex of a
benzimidazole based N‐heterocyclic carbene but are
somewhat longer than the value 2.028(2) Å28a observed in case
of a pyrazole based ligand.
Fig. 3 3‐D Network due to non‐covalent C−H∙∙∙Cl interacꢀons in
the crystal of 1.
Fig. 1 ORTEP of 1. bond lengths (Å): Pd(1)–S(1) 2.3179(19),
Cl(1)–Pd(1) 2.2784(19), Pd(2)–C(9) 1.968(7), Pd(2)–N(3)
2.073(8), Pd(2)–Cl(2) 2.298(2), Pd(2)–Cl(3) 2.285(2). Bond
angles (°): S(1)–Pd(1)–S(1) 180.00, S(1)–Pd(1)–Cl(1) 85.20(7),
C(9)–Pd(2)–N(3) 178.4(3), C(9)–Pd(2)–Cl(2) 88.0(2), C(9)–
Pd(2)–Cl(3) 87.7(2), Cl(2)–Pd(2)–Cl(3) 175.70(9).
Fig. 4 3‐D Network due to non‐covalent C−H∙∙∙Cl interacꢀons in
the crystal of
The selected secondary interaction distances are given in ESI
(Table S3).
2
.
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Catalytic
Journal Name
conversion. In case reverse reaction i.e. amide to nitrile,
benzamide was employed as a substrate.
applications
in
nitrile‐primary
amide
interconversion
Table 3 Conversion of nitrile to primary amidep
The catalytic performance of complexes
1
and 2 for conversion
of nitrile to amide and vice versa (amides to nitriles) has been
explored.
Table 1 Optimization of conversion of nitrile to primary amidea
O
O
O
O
NH2
NH2
NH2
MeO
Entry No. H2O‐THF Temperature
Conversion (%)
4c
4a
1 - 95/86
2 - 92/82
4b
(v/v)
1:3
0:4
4:0
1:3
1:3
1:3
1:3
1:3
1:3
1:3
(o C)
80
80
80
80
r.t
1 - 84/76
2 - 82/72
1 - 88/80
2 - 86/78
1b
2
3
4
nd
nd
80
O
O
NH2
NH2
NH2
I
Br
95/86c
70
Cl
4d
1 - 95/87
2 - 88/80
4e
4f
1 - 94/85
2 - 87/80
1 - 92/82
- 85 78
5
O
O
2
/
O
6d
7e
8f
9g
10h
80
80
80
80
80
45
95
nd
62
NH2
NH2
NH2
O
O
F
4h
4g
Br
1 - 93/84
2 - 90/81
1 - 91/81
2 - 83/76
4i
1 - 85/76
2 - 80/72
66
NH2
aReaction conditions: benzonitrile (1 mmol), solvent: H2O‐THF, catalyst:
NH2
N
b
complex
of acetamide. Isolated yield (%). Catalyst: complex
eCatalyst : complex
1
(0.5 mol%), acetamide (250 mg), time 5 h. In the absence
4k
1 - 90/83
2 - 89/80
4j
c
d
1 - 68/58
2 - 63/54
1
(0.025 mol% ).
1
(1.0 mol% ). fIn the absence of Catalyst
gPd(CH3CN)2Cl2], h[Pd(PhS(CH2)2SPh)Cl2), Conversion: monitored with 1H
NMR; nd: not detected.
pReaction conditions: nitrile (1 mmol), acetamide (250 mg), THF:H2O
(3:1), temperature 80 °C, reaction time 5h, catalyst
1/2 (0.5 and
1mol%.). conversion(%) determined with 1H NMR/isolated yield (%)
below each structure.
Table 2 Optimization for conversion of primary amide to nitrilei
The conversions were monitored with 1H NMR. The THF : H2O
mixture (3:1) was found best as a solvent for conversion of
nitrile to amide (Table 1 and Entry 4), but for the reverse
reaction (amide to nitrile) mixture of CH3CN and H2O (1:1) gave
best results (Table 2 and Entry 5). Acetonitrile acts as a reagent
and solvent both in amide to nitrile conversion. In the absence
of water nitrile to amide conversion did not take place (Table 1
Entry 2). The reverse reaction occurs only 10% in pure
acetonitrile (Table 2, Entry 3). However in water only PhCN
and PhCH2CN could be converted to amide but yield was lower
than the one obtained in THF‐water mixture (Table 1 Entry 3).
Entry
No.
1j
2
3
H2O‐CH3CN
(v/v)
1:1
Temperature
Conversion(%)
(o C)
80
80
80
r.t
(%)
nd
nd
10
2:0
0:2
1:1
1:1
1:1
1:1
1:1
1:1
4
nd
5
80
80
80
80
80
90/82k
29
6l
The optimum loading of complex
amide/nitrile was 0.5/1.0 mol% whereas with
1
as a catalyst for preparing
it was 1.0/2.0
7m
8n
9o
90
nd
nd
2
mol%, indicating former as somewhat more efficient as a
catalyst The conversion of nitrile to amide did not increase on
adding more amount of complex than the optimized value
(Table 1/2, Entry 7), as a catalyst. However, on decreasing the
loading of the complex as a catalyst below the optimum value,
the interconversion diminished (Table 1/2, Entry 6). In absence
of acetamide there was no product, ruling out the possibility
that benzamide is formed by hydration of PhCN (Table 1 Entry
1) under the present conditions. On replacing acetamide with
benzamide in nitrile to amide transformation, no product was
formed. Similarly, CH3CN cannot be substituted with PhCH2CN
for a reverse transformation.
iReaction conditions: benzamide (1 mmol), H2O‐CH3CN (1:1), catalyst
1
j
k
l
(1.0 mol% ) time: 6h. No catalyst. Isolated yield (%). Catalyst
mol%). mCatalyst
Conversion estimated with 1H NMR, nd: not detected.
1
(0.5
1
(2.0 mol%) nPd(CH3CN)2Cl2], o[Pd(PhS(CH2)2SPh)Cl2),
The reaction conditions for both the conversions were
optimized in the presence of complex (Table 1 and 2).
Benzonitrile was used a substrate to optimize nitrile to amide
1
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also present as a solvent in the reaction mixture. The trimetallic
species is converted to [Pd(II)(L)Cl(H2O)]+. In ESI‐MS (Fig. S74 in ESI)
of reaction mixture after 1 h peak due to this species is observed at
m/z = 426.98. The ligand L in this species, expected to have square
planar palladium, is probably bidentate. Further structural
information on [Pd(II)(L)Cl(H2O)]+ eluded in the absence of its
isolation. The presence of water in its coordination sphere is
supported by the fact that it is essential for interconversion. This
species as a real catalytic one is also logical20 in view of analogous
[Pd(II)(H2O)]2+ reported for catalysis of such interconversion with
PdCl2. Water probably assists the nitrile and amide species to
interact with Pd and come closer for reaction.20 The palladium
complexes 1 and 2 may be said to be involved in the “water
transfer” similar to that of transfer hydrogenation process.29 In ESI‐
MS (See Fig. S75 in ESI) peak observed at m/z = 601.20 may be
attributed [E + Na + H]2+. It may be presumed as formed from F,
which in large excess of acetamide or acetonitrile leads to hydration
The scope of amide‐nitrile interconversion catalyzed with
1
and
2 is shown in Tables 3 and 4. Aliphatic as well as aromatic
primary amides and nitriles show good conversion with
optimized protocols. The conversion of nitriles to amide was
selective as no acid formation was noticed in any case.
Table 4 Conversion of primary amide to nitrileq
20
or dehydration. Thus the rate of conversion is almost same in
both direction.
[Pd(II)(L)(H2O)]2+
[Pd(II)(L)]2+
H
H
H
N
N
N
N
H
R/CH3
R/CH3
O
R/CH3
H3C/R
O
F
E
In E the proposed coordination of palladium with nitrogen is
supported by literature reports.20,30 The reaction mixture turned
black during interconversion reactions. The UV–Vis absorption
spectra (Fig. 5) were recorded for complex 1, the black particles and
reaction mixture after 1 and 3 h.
qReaction conditions: CH3CN:H2O (1:1), temperature 80 °C, reaction time 6 h,
amide (1 mmol), catalyst 1/2 (1 and 2 mol%). Conversion (%) determined
with 1H NMR/isolated yield(%) below each structure.
The presence of electron withdrawing or donating groups
marginally affects the reaction rate. However, in the presence of
former yield is more. In the absence of complex 1/2 no conversion
was noticed in either case (Table 1/2 Entry 8 and 1).
The interconversion was attempted with [Pd(CH3CN)2Cl2] (Table
1 entry no. 9 and Table 2 entry no. 8) and [Pd(PhS(CH2)2SPh)Cl2)]
(Table 1 entry no. 10 and Table 2 entry no. 9), expecting their
formation and functioning as real catalytic species in the catalysis
with 1/2. However, both these complexes promoted nitrile to
amide conversion (62‐66%) but not the reverse one. Thus their
formation by dissociation of 1/2 and functioning as a real catalyst in
the interconversion is ruled out. The PdCl2 alone as a real catalytic
species is also ruled out as this catalyzes such interconversion at a
loading of 10 mol%,20 much higher than the loading required for 1
and 2. The conversion of nitrile to amide using acetamide was
Fig. 5
UV–vis absorption spectra of the reaction mixture at
different stages, 1 and black particles
All were found to exhibit shoulder around 340 nm, which is
characteristic of Pd(II) species.31 This suggests that Pd is present in
+II oxidation state all along. Thus it appears that the trinuclear
complex decompose into mononuclear species and bridging Pd into
carried out in D2O in place of water.
The product was
predominantly undeuterated benzamide (C7H7NO) as revealed by
its mass spectrum which shows ~9000 counts in its molecular ion
peak at m/z 122.0571 while for deuterated benzamide (C7H5D2NO)
the figure is ~600 (See Fig. S73 in ESI). Thus acetamide is mainly
responsible for nitrile to amide conversion and not water which is
Pd(II) (as evidenced by UV‐Visible spectroscopy).
Probably
[Pd(II)(H2O)]2+ also contributes to catalysis. This results in better
catalytic efficiency. The rate of interconversion is drastically
reduced (yield become 30% in 5 h) if amount of acetonitrile or
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amide is taken equal to that of substrate. This suggests that one. Aerobic Sonogashira coupling is possible but gives low yield.
formation of F is likely to be a rate determining step.
Using PdCl2 as a catalyst under optimum conditions yield of the
desired
The negative mercury and triphenylphosphine poisoning tests32
rules out the formation of palladium(0) clusters. The Pd(II) moieties
derive interconversion reactions in aerobic condition due to their
greater stability in the presence of air. On adding fresh nitrile
substrate after completion of the conversion of nitrile to amide, it
was observed that catalyst was alive and converted ten fresh lot of
substrate with same efficiency to product. For reverse reaction
conversion of amide to nitrile, the same thing was possible up to 4
cycles.
Table 6 Sonogashira C‐C coupling reactionv
It is pertinent to compare the performance of catalytic activation
with 1 and 2 with those of other Pd(II) species known for this
purpose. However, only PdCl2 has been used so far as a catalyst for
such interconversion but required catalyst loading is 10 mol%.20
Catalytic application in Sonogashira C‐C coupling
The catalytic performance of Pd(II) complexes 1 and 2 was explored
for Sonogashira C–C coupling. To optimize conditions for the
coupling, reaction of 4‐bromobenzaldehyde with phenylacetylene
in various solvents and bases under ambient conditions using
complex 1 as a catalyst was studied. The conversion was monitored
1
with H NMR. The K2CO3 and DMSO were found as best base and
solvent respectively (Table 5).
vReaction conditions: aryl bromide (1 mmol), phenyl acetylene (1.2 mmol),
K2CO3 (2 mmol), DMSO (1 mL), bath temperature 110 °C, reaction time
20/8/6 h for ArCl, ArBr and ArI respectively, In case of ArCl and ArBr catalyst
1/2 (1 mol%, for electron withdrawing substrates), catalyst 1/2 (2 mol% for
R = OMe, Me substrate). CuI (5 mol%). under N2 atm, In case of ArI catalyst
1/2 = 0.1 mol% conversion(%) determined with 1H NMR/isolated yield(%)
below each structure.
Table 5 Optimization table for Sonogashira coupling reactionr
Entry No.
Base
Et3N
Solvent
DMF
DMF
DMF
DMF
1,4 dioxane
THF
DMSO
DMSO
DMSO
Conversion %
1
2
3
4
5
<5
<5
20
30
<5
K3PO4
Cs2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
product becomes 25%, which shows that even in privileged solvent
DMSO the fact, that catalyst is added as a trinuclear complex and
consequently its ligands has a role in functiong of catalyst, can not
be ignored. On reducing the loading of catalyst to half, the
conversion diminished (Table 5, Entry 8). On doubling the catalyst
amount to 2 mol% no significant change in the conversion was
noticed (Table 5, Entry 9). The cross‐coupling reactions of various
aryl halides with phenylacetylene under the optimized conditions
were attempted. The aryl bromides were easily coupled with
phenylacetylene resulting in good to excellent conversion (68‐92%)
in 8h (Table 6). For activated aryl bromides (Table 6, 9a, 9b, 9e and
9f) 1 mol% of 1/2 as a catalyst was sufficient while their 2 mol% was
required for the coupling of deactivated aryl bromides (Table 6, 9c
and 9d).
6
<5
7
92/83s
52
8t
9u
92
rReaction conditions: 1.0 mmol of 4‐bromobenzaldehyde, 1.1 mmol of
phenylacetylene, 2.0 mmol of base, catalyst 1 (1.0 mol%), CuI (5.0 mol%)
and solvent (1.0 mL), 110 °C, 8 h. sIsolated yield (%). Catalyst 1 (0.5 mol%).
t
uCatalyst 1 (2.0 mol%) nitrogen atmosphere.
There was no catalytic conversion with 1 mol% of Pd (added as 1 or
2) in the absence of CuI, as a co‐catalyst. If the coupling reaction
was performed in the presence CuI alone (no 1/2 present) homo‐
coupled product phenylacetylene (PhCCCCPh) was obtained
(Glaser‐type reaction).24a,33 In the presence of CuI (5 mol%),
complex 1 (1.0 mol%), K2CO3 and DMSO as a solvent maximum
conversion was achieved (Table 5, Entry 7, identified as optimum
conditions). Complex 2 (optimum loading 1.0 mol%) is somewhat
less efficient than complex 1 (Table 6). The nitrogen atmosphere
improves the conversion as co‐catalyst CuI is air sensitive and in air/
oxygen promotes homocoupled product rather than cross coupled
The 1/2 catalyzes Sonogashira coupling of ArCl with reasonably
good yield of the coupled product even when the substrate is less
reactive. 4‐Chlorobenzaldehyde and 4‐chloronitrobenzene, having
activating substituent, gave better results compared to those
having deactivating ones, i.e. 4‐chloroanisole (Table 6). However,
the conversion of ArCl into coupled product was difficult than that
of ArBr. With 1 mol% loading of 1/2 and carrying out reaction for
20h, the yield of coupled product was in the range 37‐82% for 4‐
6 | J. Name., 2012, 00, 1‐3
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nal Name
ARTICLE
chlorobenzaldehyde, 4‐chloronitrobenzene, 4‐chlorobenzene and 4‐
chloroanisole (Tabl 6, entries 9a, 9c, 9e and 9g). However, the
acti ation of aryl chlorides with present complexes is better
compared to imidazole based Pd NHC complexes and ferrocene
based Pd complex.
ee
vv
The time profile under optimum conditions shown in Fig. 6 for
Sonogashira coupling of 4‐bromobenzaldehyde with phenyl
acetylene reveals that conversion/yield increases almost linearly.
This indicates that the reaction is of zero order and rationalization
of such results with any mechanism is difficult. In view of earlier
reports35 the species formed on transmetalation may be proposed
as a rate determining step in present Sonogashira coupling also.
After completion of the first catalytic reaction a fresh lot of
reactants with base was added. It was converted to a coupled
product giving 40% conversion. Further addition of a fresh lot of
reactants with co‐reagents gave only 15% conversion.
Table 7 Sonogashira C‐C coupling reactionw
1/2
1-2 mol %, Cat
Y
R
Y
Br
110 oC, DMSO,
K2CO3, 8h
7
10
Y=MMe,NO2
11
Me
NO2
11a
11b
1 - 75/60
2 - 70/58
1
2
- 93/86
- 92/85
The two phase test36 (also called the three‐phase test when the
catalyst is in the solid form) as shown in Scheme 2 was applied (See
Experimental details in the ESI). 4‐Bromobenzaldehyde and 4‐
bromobenzoic acid (as amide) immobilized on silica were treated
with phenyl acetylene under optimum reaction conditions (given
below Table 6), using 1 mol% of complex 1 as a catalyst. In the case
of heterogeneous catalysis, the substrate immobilized on the solid‐
phase is not expected to be converted into a coupled product. In
the present case, ~77 % of the anchored substrate is converted to
the coupled product.
wRe
(1.2 mmol), K2CO3 (2 mmol), DMSO (1 mL), bath temperature 110 °C,
reac ion time 8h, catalyst 1/2 (1 mol%).
aaction conditions: aryl bromide (1 mmol), derivative of phenyl acetylene
tt
For 1 mol% loading of catalyst and 17‐24h reaction time the yield
was observed only up to 29 and 5% for deactivating and activating
aryl chlorides respectively.34a‐d In the case of aryl iodides viz. 4‐
iodobenzaldehyde, 4‐iodonitrobenzene and 4‐iodoanisole (Table 6,
9a,
9
9
c and 9e), the
after 6h eve
ling of 1‐ethynyl‐4‐nitrobenzene and 1‐ethynyl‐4‐methyl‐
ene with bromobenzene using 1.0 mol% of catalyst under the
optimized conditions gives 70‐93% yield (Table 7 structures 11a and
11b
c
c
onversions into cross‐coupled product were 76‐
97
%
%
nn when catalyst loading was 0.1 mol%. The
coupp
benzz
OR
O
Br
Si
O
)).
NH
O
Cat. 1 (1mol%)
Br
K2CO3, CuI (5 mol%)
110 oC, 8h, N2 atm
CHO
HOOC
Ratio
HOOC
OHC
Ph
Ph
Br
77 %
23 %
92 %
After Hydrolysis of immolized Silica
(condition: KOH, EtOH-H2O, 72h)
Scheme 2 Two‐phase test
Fig. 6 Time profile of Sonogashira coupling catalyzed with 1 and 2.
In onogashira coupling the reaction mixture turned black. Thus
This implies that discrete Pd(0)32,24h species formed from 1/2
drives predominantly the catalysis of present Sonogashira
coupling homogeneously. Palladium complexes such as
PdCl2(PPh3)2,37,38 (2‐3 mol%) are effective for this coupling
reaction, often in combination with copper(I) salts. They are
still the most preferred catalytic systems for it. The ligands
used in such a palladium complex are air sensitive. Some
earlier reported Pd‐NHC complexes are less efficient than the
present ones as the reported yield of coupled product is low
even after carrying out reaction for a long time.34a,b The few
Pd‐NHC24g,39a‐c catalysts reported as efficient give coupled
product with ArBr/ArI only and virtually fail with ArCl. Their
protocols are based on toxic amines (base) which have foul
smell.35 When Pd is coordinated with oxygen, the catalyst is
not efficient for Sonogashira coupling.40 The present NHC
ligands functionalized with soft donor S or Se, are easy to
prepare and reasonably stable. They form air and moisture
insensitive complexes 1/2, which are efficient for Sonogashira
SS
formation of colloidal Pd(0) is possible. The typical qualitative
methods to ascert in whether the catalytic activity is by colloidal
pall dium(0) cluster or not, are mercury and triphenylphosphine
poisoning tests.24h
hese poisoning tests were found negative for
the present complexes. This suggests that discrete Pd(0) is a real
cat lytic species a d not Pd(0) cluster in the present Sonogashira
cou ling. In earlier reports also such Pd(0) species are suggested as
real catalyst.24h On recording ESI‐MS (Fig. S76 in ESI) of the reaction
mix ure after allo ing the catalytic reaction to progress for 1 h a
pea
at m/z = 581.24 was observed. It may be attributed to [H+H]+.
Based on this G ap ears to be most plausible discrete Pd(0) species
deri ing the Sono ashira coupling and is its transmetalation
aa
aa
TT
aa
nn
pp
tt
ww
kk
p
p
vv
gg
H
product.
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coupling. Overall combination of NHC with S or Se is rewarding and reagents used were available commercially within the
for designing efficient catalyst for Sonogashira coupling.
country.
Conclusion
Synthesis of 1‐(2‐(phenylthio)ethyl)‐1H‐benzo[d]imidazole
(A). In a three necked round bottom flask of 100 mL containing
25 mL of EtOH and NaOH (0.880 g, 2.2 mmol), thiophenol
(0.220 g, 2.0 mmol) was added. The mixture was heated with
stirring at 80 oC for 1h under N2 atmosphere. On formation of
PhSNa, the solution of 1‐(2‐chloroethyl)‐1H‐benzo[d]imidazole
(0.361g, 2.0 mmol) made in 10 mL of ethanol was added
dropwise with constant stirring and the content was further
stirred for 3h. After completion of reaction, mixture was
The trinuclear Pd(II) complexes [Pd3(L1‐HI)2(CH3CN)Cl6] (1) and
[Pd3(L2‐HI)2(CH3CN)Cl6]
chalcogenated benzimidazolium iodide salt [L1: 3‐Methyl‐1‐(2‐
(phenylthio)ethyl)‐1H‐benzo[d]imidazol‐3‐ium iodide and L2
3‐Methyl‐1‐(2‐(phenylselanyl)ethyl)‐1H‐benzo[d]imidazol‐3‐
ium iodide] and the [Pd(CH3CN)2Cl2] through the silver carbene
transfer reaction route. The carbene precursors (L1 and L2
and the two complexes were authenticated with multinuclei
NMR and HR‐MS. The single crystal structures of and reveal
that the geometry around all Pd atoms in them is nearly
square‐planar. Each complex molecule has two (S/Se, CNHC
ligands which coordinate with central Pd through S/Se. Each of
the two remaining Pd atoms makes a bond with carbene. The
coordination sphere of each Pd is completed by Cl or CH3CN
(
2)
were
synthesized
from
:
)
extracted with CHCl3 (4
washed with water (3
×
20 mL). The organic layer was
1
2
×
30 mL) and dried over anhydrous
Na2SO4. The solvent was evaporated off under reduced
)
pressure to get 1‐(2‐(phenylthio)ethyl)‐1H‐benzo[d]imidazole
1
(
A
) as yellow oil. Yield 0.483 g, 95%. H NMR (CDCl3, 25 ºC, vs
Me4Si):
2H), 7.77‐7.91 (m, 2H,
δ
(ppm): 3.26 (t, 3JH‐H= 6 Hz, 2H), 4.31 (t, 3JH‐H = 7.2 Hz,
9), 7.22−7.46 (m, 9H).
H
and Cl. The complexes
1 and 2 efficiently catalyze selective
nitrile‐amide interconversion and Sonogashira coupling Synthesis of 1‐(2‐(phenylselenyl)ethyl)‐1H‐benzo[d]imidazole
reactions. A catalyst loading of 0.5–2.0 mol% is optimum for (B). In a three necked 100 mL round bottom flask 25 mL of
nitrile‐amide interconversion, while for Sonogashira coupling a EtOH was taken. Diphenyldiselenide (0.624 g, 2.0 mmol) was
loading of 1.0–2.0 mol% of Pd is essential. The catalytic added under N2 atmosphere and the mixture heated at 80 oC.
efficiency of complex
1, is somewhat higher relative to Solid NaBH4 (0.151 g, 4.0 mmol) was added and the mixture
complex . The Pd(II) and Pd(0) species derive interconversion further stirred till the solution become colorless due to the
2
and Sonogashira coupling respectively.
Experimental section
Physical measurement.
formation of PhSeNa. To this colorless solution, 1‐(2‐
chloroethyl)‐1H‐benzo[d]imidazole (0.361g, 2.0 mmol)
dissolved in 10 mL of ethanol was added dropwise with
constant stirring and the mixture stirred further for 3h. On
completion of reaction, the mixture was extracted with CHCl3
The C, H and N analyses were carried out with a C, H, N
analyzer. 1H, 13C{1H}, and 77Se{1H} NMR spectra were recorded
on a 300 MHz NMR spectrometer at 300.13, 75.47, and 57.24
MHz respectively. High‐ resolution mass spectral
measurements were performed with electron spray ionization
(10 eV, 180 °C source temperature, sodium formate as
reference compound) taking a sample in CH3CN. IR spectra in
the range 4000‐400 cm−1 in KBr pellets were recorded. The
(4
× 20 mL). The organic layer was washed with water (3 × 30
mL) and dried over anhydrous Na2SO4. The solvent was
evaporated off under reduced pressure to get 1‐(2‐
(phenylselanyl)ethyl)‐1H‐benzo[d]imidazole (
B
) as a yellow oil.
1
Yield 0.572 g, 95%. H NMR (CDCl3, 25 ºC, vs Me4Si):
δ
(ppm):
3.21 (t, 3JH‐H = 7.2 Hz, 2H), 4.37 (t, 3JH‐H = 7.2 Hz, 2H), 7.22−7.27
(m, 6H), 7.38−7.49 (m, 2H), 7.57−7.60 (m, 1H), 7.75–7.81 (m,
2H).
diffraction data on single crystals of
1 and 2 were collected on
a CCD diffractometer using Mo‐Ka (0.71073 Å) radiations at
298(2) K. The software SADABS41 was used for absorption
correction and SHELXTL for space group, structure
determination and refinements.42 All non–hydrogen atoms
were refined anisotropically. Hydrogen atoms were included in
idealized positions with isotropic thermal parameters set at 1.2
times that of the carbon atom to which they were attached.
The least‐square refinement cycles on F2 were performed until
the model converged. The melting points determined in an
open capillary are reported as such. Yields refer to isolated
yields of compounds which have purity ≥ 95%.
Synthesis of 3‐methyl‐1‐(2‐(phenylthio
benzo[d]imidazol‐3‐ium iodide (L1 L2). In a round bottom
flask equipped with a magnetic stirrer 2.0 mmol of (0.632 g)
or (0.502g)) was mixed with methyl iodide (0.342 g, 2.0
mmol). The mixture was stirred under N2 atmosphere for 8 h
at room temperature. The resulting solid was washed with dry
diethylether (2 × 40 mL) and dried in vacuo.
/selanyl)ethyl)‐1H‐
/
A
B
L1. Yield 0.802 g, 95%.1H NMR (CDCl3, 25 ºC, TMS); (δ, ppm):
3.70 (t, 3JH‐H= 6.0 Hz, 2H,
H
5), 4.13 (s, 3H,
6), 7.14‐7.17 (m, 2H), 7.18‐7.24 (m, 2H), 7.25‐7.27 (m,
2H), 7.61 (s, 3H), 10.83 (s, 1H). 13C{1H} NMR (CDCl3, 25 ºC,
TMS); (δ, ppm): 33.0 ( 5), 34.0 ( 13), 47.3 ( 6), 141.5 ( 14),
132.8 ( 12), 131.2 ( 7), 130.8 ( 1), 129.6 ( 3), 129.3 ( 4), 128.6
2), 126.9 ( 8), 132.8 ( 3), 126.5 ( 11), 112.9 ( 9), 112.6 ( 10).
HR‐MS: [M‐I]+(m/z) 269.11069.
H
13), 4.86 (t, 3JH‐H = 6.0
Hz, 2H,
H
Chemical and reagents
C
C
C
C
C
C
C
C
C
Thiophenol, diphenyldiselenide, benzimidazole, methyl iodide,
silver oxide and palladium(II) chloride procured from Sigma‐
Aldrich (USA) were used as received. 1‐(2‐Chloroethyl)‐1H‐
benzo[d]imidazole43 and [Pd(CH3CN)2Cl2]44 was prepared by
the earlier reported method. By standard procedures, all the
solvents were dried and distilled before use.45 Other chemicals
(
C
C
C
C
C
C
L2. Yield 0.842 g, 95%. 1H NMR (CDCl3, 25 ºC, TMS); (δ, ppm):
3
3.55 (t, 3JH‐H = 6.0 Hz, 2H,
H
5), 4.04 (s, 3H,
H
13), 4.89 (t, JH‐H
=
6.0 Hz, 2H,
H
6), 6.98−7.07 (m, 3H), 7.19–7.22 (m, 2H),
8 | J. Name., 2012, 00, 1‐3
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7.54−7.62 (m, 4H), 10.46 (s, 1H). 13C{1H} NMR (CDCl3, 25 ºC, water (3:1) was stirred at 80 oC for 5 h. The reaction was
TMS); (δ, ppm): 26.0 ( 5), 33.9 ( 13), 48.0 ( 6), 112.6 ( 10), monitored with TLC till maximum conversion of nitrile to the
112.8 ( 9), 126.9 ( 11), 126.9 ( 8), 127.1 ( 1), 127.2 ( 2), 128.7 amide occurred. After completion of reaction the mixture was
12), 130.6 ( 7), 131.2 ( 4), 132.4 ( 3), 141.5 (
14). 77Se{1H} extracted with EtOAc (2 × 15 mL). The organic layer was
NMR (CDCl3, 25ºC, Me2Se); (δ, ppm): 287.93. HR‐MS: [M ‐ I]+ washed with water and dried over anhydrous Na2SO4. Its
C
C
C
C
C
C
C
C
C
(
C
C
C
C
C
(m/z) 317.0551.
solvent was evaporated off on a rotary evaporator and the
residue was purified by column of silica gel using
a
General procedure for the synthesis of complexes [Pd3(L
HI)2(CH3CN)Cl6] (1
‐
EtOAc−ether as an eluent. All products were characterized by
−2).
1H and 13C{1H} NMR spectra (See, ESI Figure S11‐S32).
A solution of benzimidazolium salt (L1
mmol) made in dry CH2Cl2 (40 mL), was mixed with solid Ag2O
/
L2) (0.196 g/0.222 g, 0.5
Amide to nitrile conversion.
(0.173 g, 0.75 mmol) under N2 atmosphere. The mixture was Amide (1 mmol), complex
1 (1.0 mol%) or 2 (2.0 mol%) CH3CN
stirred at room temperature in the dark for 12h. On (19.15 mmol/1mL), water (1mL) were mixed and stirred at 80
completion of the reaction the mixture was filtered through a oC for 6 h. Thereafter the mixture was extracted with EtOAc (2
celite pad. The volume of filtrate reduced to 15 mL on a rotary × 15 mL). The organic layer was washed with water and dried
evaporator. To a suspension of [Pd(CH3CN)2Cl2] (0.194 g, 0.75 over anhydrous Na2SO4. Its solvent was evaporated off on a
mmol) in CH3CN, the above concentrate was added with rotary evaporator and the residue purified by a column of silica
constant stirring under N2 atmosphere at room temperature. gel using EtOAc−ether as an eluent. All products were
1
The mixture was stirred further for 8h and filtered through a characterized by H and 13C{1H} NMR spectra (See, ESI Figure
celite pad. The volume of filtrate was reduced to ~4 mL on a S33‐S54) .
rotary evaporator and mixed with diethyl ether (15 mL) to
Sonogashira coupling reaction.
obtain
1
or
2
as a yellow solid. The
and
1
/
2
was filtered and dried
Aryl bromide (1.0 mmol), phenylacetylene (1.2 mmol), K2CO3
(2.0 mmol), DMSO (1mL), complex or (1.0 mol%) and CuI (5
in vacuo. The single crystals of
1
2 were grown by diffusing
1
2
diethyl ether into their solution made in CH3CN−Et2O mixtures
mol%) were placed in 50 mL round bottom flask under N2
atmosphere. The reaction mixture was stirred at 110 oC for 8h.
After cooling its content to room temperature, the mixture
was extracted with EtOAc (20 mL) and washed with water (10
mL). The organic layer was separated and dried over Na2SO4.
Its solvent was evaporated off on a rotary evaporator and the
residue purified by column chromatography using EtOAc‐
(3:1).
1. Yield: 1.15 g, 80%. Anal. Calcd for C36H40Cl6N6Pd3S2 : C,
37.51; H, 3.50; N, 7.29. Found: C, 37.32; H, 3.47; N, 7.15. M.pt.
190 °C.1H NMR (300 MHz, DMSO‐
d
6, 25 °C): δ (ppm) 2.0 (s, 3H
H3 of CH3CN), 3.43 (s, 2H, H5 mixed with DMSO‐ 6 taken as
nmr solvent), 4.23 (s, 3H, 13), 4.96 (s, 2H, 6), 7.05–7.40 (m,
5H), 7.63–7.69 (m, 4H). 13C{1H} NMR (75 MHz, DMSO‐
6, 25
°C): δ (ppm) 1.66 (CH3 of H3CN), 35.4 ( 13), 40.8 (C5 mixed
with DMSO‐ 6 taken as nmr solvent), 46.9 ( 6), 111.7 (C9
10), 118.4 ( N of CH3CN), 124.3 (C8 11), 129.4 ( 4), 130.0 (C2
1), 133.6 C7 12), 134.6
[C16H18N2OPdS] (m/z) = 426.986313; calculated value for The optimized amounts of reactants (benzonitrile, benzamide
C16H18N2OPdS
426.986065. IR (KBr, cm−1): 2987 (m; or 4‐bromobenzaldehyde), other reagents for nitrile‐amide
νC−Haromaꢀc), 1621 (m; νC=Naromatic), 1409(m; νC=Caromatic), 749 (m; interconversion or Sonogashira coupling and catalyst were
C−Haromaꢀc).
added. The reaction was carried out under optimum
. Yield: 0.206 g, 84%. Anal.Calcd for C36H40Cl6N6Pd3Se2 : C,
34.68; H, 3.23; N, 6.74. Found: C, 34.59; H, 3.18; N, 6.63. M.pt.
180 °C. 1H NMR (300 MHz, DMSO‐
6, 25 °C): δ (ppm), 2.06 (s,
3H C 3 of CH3CN), 3.35 (s, 2H, 5 mixed with DMSO‐ 6 taken
as nmr solvent), 4.13 (s, 3H, 13), 5.26 (bs, 2H, 6), 7.05–7.11 PPh3 poisoning test.
(m, 2H), 7.27–7.34 (m, 5H), 7.43−7.51 (m, 1H), 7.69−7.85 (m,
1H). 13C{1H} NMR (75 MHz, DMSO‐
6, 25 °C): δ (ppm), 1.62
5), 47.9 ( 6), 111.6 ( 9), 111.9
N of CH3CN ), 124.4 (C8 11), 129.6 ( 4), 130.1
1), 132.1 (C7 12), 134.9 (
3). 77Se{1H} NMR (DMSO‐d6,
C
d
H
H
1
d
Hexane as an eluent. All products were characterized by H
and 13C{1H} NMR spectra (See, ESI Figure S55‐S68).
C
C
d
C
C
&
Hg poisoning test.
C
&
C
C
&
C
(
&
C
(C3). HR‐MS (CH3CN) An excess of Hg (Hg:Pd, 400:1) was taken in the reaction flask.
=
1
ν
conditions. After standardized work up detailed above 90, 82
and 79 % conversion was observed for formation of
benzamide from benzonitrile, reverse reaction and
Sonogashira coupling respectively.
2
d
H
H
d
H
H
PPh3 (5 mol%) was taken in the reaction flask before the
addition of reactants (benzonitrile, benzamide or 4‐
bromobenzaldehyde). Thereafter nitrile‐amide interconversion
reaction or Sonogashira coupling was carried out under
optimum conditions. After carrying out the reaction for
optimum time the yield of amide, nitrile and C‐C coupled
product was 93, 80 and 72 % respectively.
d
(
(
(
C
H3 of
C
H3CN), 27.2 (
C
13), 35.6 (
C
C
C
C
C10), 118.4 (
C
&
C
C2
&
C
&
C
C
25 ºC, Me2Se); 341.48. HR‐MS (CH3CN) [C16H16ClN2PdSe] (m/z)
= 456.9195; calculated value for C16H16ClN2PdSe = 456.9203. IR
(KBr, cm−1): 3055 (m; νC−Haromaꢀc), 1620 (m; νC=Naromatic), 1401
(m; νC=Caromatic), 745 (m; νC−Haromaꢀc).
Acknowledgements
Authors thank Department of Atomic Energy (BRNS), India and
Nano‐mission, Department of Science and Technology, India for the
financial support. PD and SG. thank University Grants Commission
(UGC), India for the award of Senior Research Fellowship.
Nitrile to amide conversion.
A mixture of nitrile (1 mmol), complex
mol%) and acetamide (4.24 mmol), in the presence of THF and
1 (0.5 mol%) or 2 (1.0
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Dubey, V. V. Singh, O. Prakash and A. K. Singh, RSC Adv,
2014, 4, 41659–41665; (f) D. Yuan, H. Tang, L. Xiao, H. V.
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TOC
Trinuclear complexes of palladium(II) with chalcogenated N-heterocyclic
carbenes: catalysis of selective nitrile-primary amide interconversion and
Sonogashira coupling
Pooja Dubey, Sonu Gupta, and Ajai K. Singh*
Department of Chemistry, Indian Institute of Technology, Delhi, New Delhi 110016, India
Interconversion of nitrile to amide and Sonogashira coupling were catalyzed with Pd(II) complexes (0.05-
2 mol%).