Palladium Complexes of Thio/Seleno-Ether Containing N-Heterocyclic Carbenes: Efficient and Reusable Catalyst for Regioselective C-H Bond Arylation

Article abstract of DOI:10.1002/ejic.201901259

The synthesis of the novel S,C_NHC type half-pincer ligand precursors (L1 and L2) is described herein by using the atom economy reactions of 1-(2-(phenylthio)ethyl)-1H-imidazole with benzyl bromide and bromodiphenylmethane, respectively. The analogous reaction of 1-(2-(phenylselanyl)ethyl)-1H-imidazole with 2-bromoethyl phenyl sulfide has also resulted in a imidazolium bromide (L3) which is a precursor of novel S,C_NHC,Se type pincer ligand. The route of silver-NHC transmetalation was employed to get the palladium complexes [Pd(L1/L2–HBr)Cl₂] (C1 and C2) and [Pd(L3–HBr)Cl]BF₄ (C3). The imidazolium bromide (L1–L3) and palladium complexes (C1–C3) were characterized by using multinuclear NMR and HR-MS. The structure and bonding in the complexes C1 and C2 were validated by X-ray crystallography. Thermally robust and moisture/air insensitive palladium complexes C1–C3 have been explored in the catalysis of C–H bond arylation of imidazoles. The protocol operates under mild reaction conditions in air with an excellent regioselective C–H bond arylation at C-5 position in imidazoles. All the complexes were found to be efficient (yield up to 97 % in 12 h) in the catalysis; however, the activating pincer ligand framework containing Pd catalyst C3, was found to be utmost effective among the three catalysts. Only 0.5 mol-% catalyst loading is required to achieve admirable yield of the desired cross-coupled products. A wide range of substrates was examined, and the developed protocol was applicable to all derivatives with high functional group tolerance and greater efficiency. More importantly, the catalyst C3 has also been found recyclable up to five cycles with minor decrease in efficiency which is highly desirable feature for the development of economical and sustainable industrial reaction processes. The PPh₃ and Hg poisoning tests have established the complete homogeneous nature of the catalysis.

Full text of DOI:10.1002/ejic.201901259

A Journal of
Accepted Article
Title: Palladium Complexes of Thio/Seleno-Ether Containing N-
HeterocyclicCarbenes: Efficient and Reusable Catalyst for
Regioselective C-H Bond Arylation
Authors: Ramprasad Bhatt, Nattamai Nattamai Bhuvanesh, Kamal
Nayan Sharma, and Hemant Joshi
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201901259
Link to VoR: http://dx.doi.org/10.1002/ejic.201901259

European Journal of Inorganic Chemistry
10.1002/ejic.201901259
Chalcogenated NHC
Palladium Complexes of Thio/Seleno-Ether Containing N-Heterocyclic
Carbenes: Efficient and Reusable Catalyst for Regioselective C-H Bond
Arylation
Ramprasad Bhatt, Nattamai Bhuvanesh, Kamal Nayan Sharma,* Hemant Joshi,*, [d]
[
a]
[b]
, [c]
Abstract: The synthesis of the novel S,CNHC type half-pincer ligand precursors (L1 and L2) is
described herein by using the atom economy reactions of 1-(2-(phenylthio)ethyl)-1H-imidazole
with benzyl bromide and bromodiphenylmethane, respectively. The analogous reaction of 1-(2-
(
phenylselanyl)ethyl)-1H-imidazole with 2-bromoethyl phenyl sulfide has also resulted in a
imidazolium bromide (L3) which is a precursor of novel S,CNHC,Se type pincer ligand. The route
of silver-NHC transmetalation was employed to get the palladium complexes
[
(
Pd(L1/L2−HBr)Cl
2
] (C1 and C2) and [Pd(L3−HBr)Cl]BF (C3). The imidazolium bromide
4
L1-L3) and palladium complexes (C1-C3) were characterized by using multinuclear NMR and
HR-MS. The structure and bonding in the complexes C1 and C2 were validated by X-ray
crystallography. Thermally, robust and moisture/air insensitive palladium complexes C1-C3
have been explored in the catalysis of C-H bond arylation of imidazoles. The protocol operates
under mild reaction conditions in open air with an excellent regioselective C-H bond arylation at
C-5 position in imidazoles. All the complexes were found to be efficient (yield up to 97% in 12
h) in the catalysis, however, the activating pincer ligand framework containing Pd catalyst C3,
was found to be utmost effective among the three catalysts. Only 0.5 mol% catalyst loading is
required to achieve admirable yield of the desired cross-coupled products. A wide range of
substrates was examined and developed protocol was applicable to all derivatives with high
functional group tolerance and greater efficiency. More importantly, the catalyst C3 has also
been found recyclable up to five cycles with minor decrease in efficiency which is highly
desirable feature for the development of economical and sustainable industrial reaction
processes. The PPh
3
and Hg poisoning tests have established the complete homogeneous nature
of the catalysis.
Keywords: N-heterocyclic carbenes, pincer ligand, reusable catalyst, regioselective C-H bond
arylation, imidazole, chalcogenoether
[a]
Department of Chemistry, Birla Institute of Technology and Science, Pilani Campus, Pilani
3
33031, India
[
b]
Department of Chemistry, Texas A&M University, PO Box 30012, College Station, Texas
7
7842-3012, USA
[
c]
Department of Chemistry, Malaviya National Institute of Technology Jaipur, J.L.N. Marg,
Jaipur 302017, Rajasthan, India, Email: kamalnayaniitd@gmail.com,
https://kamalnayaniitd.wixsite.com/kamalnayan
[
d]Department of Chemistry, School of Chemical Sciences and Pharmacy, Central University of
Rajasthan, NH-8, Bandarsindri, Ajmer, Rajasthan 305817, India, Email:
hemant.joshi@curaj.ac.in, https://sites.google.com/view/joshisresearchgroup
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
■
Introduction
The direct C-H bond arylation of aromatic heterocycles is a prominent protocol for the
industrial synthesis of polymers, functional materials, pharmaceuticals, and natural products.[1]
The palladium catalyzed arylation at C-4 carbon in imidazole is less favored[2] as compared to
the C-2 and C-5 positions.[2-3] The use of polar solvents and base (acetate or pivalate) selectively
activates the C-5 position for arylation, whereas the C-2 position is most likely to be arylated in
the presence of non-polar solvents in combination with a strong base such as potassium or
sodium tert-butoxide and a copper salt. The most commonly used catalytic systems for arylation
of imidazole typically involve palladium precursors with bulky, sterically hindered, and electron-
rich phosphines such as XPhos,[4] PCy
,[5] PPh
,[6] Xantphos,[7] AsPh ,[8] and 1,10-
3
3
3
[
9]
phenanthroline as supporting ligand to make the metal center electron-rich which in turn glides
the oxidative addition step. In contrast to the direct C-H activation, there are only few
examples[1a, 10] reported for the palladium mediated C-H bond activation of imidazole and most
of these suffer from drawbacks like: (i) high catalyst loading (>2 mol% Pd), (ii) long reaction
times and high reaction temperature (~140 ºC), (iii) indiscrimination of C-2 and C-5 position,
(
iv) limited substrate scope, and (v) requirement of moisture free inert atmosphere. Direct
arylation of imidazole under aerobic conditions is thought-provoking due to the formation of
peroxo species [LPd(O) ], which prevents the oxidative addition of the substrates. The N-
2
heterocyclic carbenes (NHCs) are considered advantageous due to the availability of two
electrons, strong σ-donating and week π-accepting coordination behavior which provides their
metal complexes a unique balance of stability versus reactivity, highly desirable for catalysis.[11]
Due to the strong soft donor properties of organochalcogen ligands and their applicability under
mild reaction conditions in open air, their metal complexes have also emerged as better
alternative of phosphine based catalytic systems in the organic transformations.[12] The
combinatory activating effect of NHC and chalcogen (S/Se) on the catalytic activity was also
proven beneficial in improving the potential of palladium catalyst of chalcogen containing
NHCs.[13] Therefore, due the unique features of these complexes discussed above, we herein plan
-
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
to explore the Pd(II) complexes of novel NHC based pincer (S,CNHC,Se) and half pincer (S,CNHC
)
organochalcogen ligands for the catalysis of regioselective C-H bond activation in imidazoles.
To the best of our knowledge, no metal complex of S,CNHC,Se pincer type has been reported yet.
Moreover, the effect of pincer and half-pincer coordination of ligand on the catalytic activity of
the palladium metal is also investigated for the regioselective C-5 arylation of imidazole. Owing
to the novel combination of coordination sites and protective environment of the pincer ligand in
the Pd(II)-S,CNHC,Se complex, it was found to be more efficient as compared to the half-pincer
(
S, CNHC) ligated Pd(II) complexes for the catalysis of C-H arylation in imidazole working in
open air along with mild reaction conditions.
■
Results and discussion
The functionalization of the 1H-imidazole with a thioether moiety to prepare (2-
phenylthio)ethyl)-1H-imidazole was achieved by employing an analogous procedure reported
(
earlier for the synthesis of the benzimidazole derivative.[13c] 1-(2-(Phenylthio)ethyl)-1H-
imidazole on atom economy reaction with benzyl bromide and bromodiphenylmethane in
acetonitrile has resulted in thioether based NHC ligand precursors L1, and L2 respectively. The
palladium complexes of thioether-NHC (C1 and C2) were then synthesized by using the route of
silver carbene transmetalation[14] reaction in which the appropriate imidazolium bromide (L1 or
L2) was reacted with Ag
solution of Pd(CH CN)
2
O in CH
2
Cl
2
in inert atmosphere followed by the reaction with CH CN
3
3
2
Cl . The reaction proceeds through in situ generated silver-carbene
2
complex transmetalation route. The direct route for the preparation of C1 and C2 using a strong
base, potassium tert-butoxide, was also successfully employed; however, relatively lower yield
1
13
1
of the desired complexes was obtained. The H, C{ H} NMR, and mass spectra of newly
synthesized ligands and complexes L1, L2, C1, and C2 are provided in Figures s1-s12 in the
Supporting Information (SI) and are found to be in agreement with their structures as proposed in
Scheme 1.
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
Scheme 1. Syntheses of palladium(II) complexes of thio-ether containing N-heterocyclic carbene based half-pincer
ligand.
The most acidic H
9
proton attached to pre-carbene carbon (C ) appeared as deshielded
9
1
singlet at 10.21 and 9.60 ppm in the H NMR of each imidazolium bromide salt L1 and L2,
respectively. This is consistent with imidazolium salts reported earlier (δ 9.5-12.0 ppm).[13b] The
signal of this proton disappeared in the proton NMR of the complexes C1 and C2 which
indicates the successful deprotonation of C upon its coordination with Pd(II) during
9
1
13
1
complexation. Furthermore, two signals in H and C{ H} NMR of ligands (L1 and L2)
observed due to the presence of two adjacent –CH groups, were found to be deshielded and
2
appeared as four multiplets (or broad singlet) in the proton NMR spectrum of complexes (C1 and
C2), confirming diastereotopic nature of these protons arising due to the rigid six-membered
chelate ring formed by the coordination of thioether NHC with Pd(II) in bidentate half-pincer
(
S^CNHC) mode. Such phenomena was commonly observed in earlier reported six-membered
chelate ring containing Pd(II) pincer complexes.[13b, 15] In the mass spectrum of imidazolium
bromides (L1 and L2), the intense peaks observed at (m/z) 295.1261 and 371.1566, respectively,
+
correspond to imidazolium cations [M − Br] of L1 and L2 salts. The mass spectrum of complex
C1 shows a peak at (m/z) 295.1260 which is related to [(M + H) – PdCl
2
]+ cationic species,
while an intense peak observed at (m/z) 511.0207 in the mass spectrum of complex C2 is
-
4-
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
+
ascribed to [M − Cl] cationic species. Both of these observations further strengthen the
proposed structures of C1 and C2 given in Scheme 1.
In addition, the third new 3-(2-(phenylselanyl)ethyl)-1-(2-(phenylthio)ethyl)-1H-
imidazolium bromide (L3) ligand, which is precursor to Se,CNHC,S type pincer ligand, was also
prepared (yield ~88%) through an analogous atom economy reaction of selenoether of imidazole;
1
1
-(2-(phenylselanyl)ethyl)-1H-imidazole[16] with 2-bromoethyl phenyl sulfide in acetonitrile at
20 °C under N atm. The Pd(II) complex of this unique Se,CNHC,S type pincer ligand [Pd(L3-
2
HBr)Cl]BF
4
(C3) was then prepared through the reaction of imidazolium bromide L3 with
Cl in presence of silver oxide followed by counter anion exchange with silver
Pd(CH CN)
3
2
2
1
13
1
tetrafluoroborate. The H, C{ H} NMR and HR-MS of L3 and C3 authenticating their
molecular structures are provided in Scheme 2 and in Figures s13-s18 in the SI. A singlet at 9.65
1
ppm in the H NMR spectra of L1 is ascribed to highly deshielded H
9
proton of imidazolium salt
L3, which on the complex (C3) formation, disappears owing to the removal of the acidic proton.
1
Furthermore, the presence of four deshielded broad singlets in the H NMR spectrum of C3
confirmed the complex formation and the coordination of ligand to Pd in a tridentate Se,CNHC,S
type pincer mode forming two six-membered chelate rings. Herein, the Pd(II) complex C3 is the
first example of metal complex of Se,CNHC,S type pincer ligand.
Scheme 2. Synthesis of palladium(II) complex of (Se,CNHC,S) type pincer.
Single crystal X-ray structures. The X-ray quality single crystals of complexes C1 and C2
were grown by its CH
structures of these complexes (C1 and C2) were solved as outlined in crystallographic section
see SI, and Table s1) and are presented in Figures 1 and 2 along with key bond angles/distances
provide in Table s2 of SI. However, the repeated attempts to get X-ray quality crystals of C3
3
CN:Et
2
O (4:1) solution under ambient conditions. The molecular
(
-
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
were failed. In complex C1, a molecule of acetonitrile was found solvated (Figure 3). In both the
complexes, C1 and C2, the metal center Pd acquires a distorted square planar geometry with
bond angles nearly 90° and 180°. The ligands (L1 and L2) coordinate with Pd in a bidentate
fashion through sulfur and carbene carbon forming a six-membered chelate ring in each case.
Moreover, in both the Pd complexes, the imidazole ring is observed to be twisted at an angle of
4
7.7° and 43.9° with the coordination square plane of Pd center in C1 and C2, respectively. Such
kind of twisting in the imidazole ring and coordination plane of Pd also accounts that the
overlapping of Pd d orbital with the p orbital of the NHC carbon is actually insignificant which
π
z
in turn confirms very low π-accepting property of NHC ligands with metal center.[17] The
palladium-sulfur bond lengths in complexes C1 and C2 are 2.290(2) and 2.2737(15) Å
respectively, which is consistent with the earlier reported thioether-carbene−palladium
complex.[13c] The palladium-carbon bond length values are 1.992(5) and 1.978(7) Å respectively,
are in good corroboration with earlier reported values 1.973(3) for thioether consisting NHC Pd
complex.[13c] Figures 3 and 4 show the unit cell representations of C1 and C2 and each complex
contains four molecules. The unit cell of C1 also has four CH CN molecules solvated within
3
this.
Figure 1: Crystal structure (50% probability) of C1.
-
6-
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
Figure 2: Crystal structure (50% probability) of C2.
Figure 3. Unit cell diagrams of complex C1.
Figure 4. Unit cell diagrams of complex C2.
Regioselective C-H bond arylation of imidazoles. Considering the air/moisture
sensitivity of the reported sterically hindered electron rich phosphine based metal catalytic
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
systems, the palladium complexes of phosphine-free ligands may be served as better alternatives
for the catalysis of direct regioselective arylation. The present palladium complexes of
chalcogenated N-heterocyclic carbene (C1-C3) were explored for their catalytic activity towards
direct C-5 bond arylation in imidazoles. The reaction of N-methyl imidazole and 1-bromo-4-
nitrobenzene was chosen as a model reaction for standardizations (Table 1). A reaction was first
carried out using 1.0 mol% of C1, Cs
2
CO in N,N-dimethylacetamide (DMA) solvent without
3
using any additive as shown in Table 1. However, in absence of additive, first entry resulted in
no conversion of the desired product 3a. Later, when benzoic acid was used as additive in next
reaction under similar reaction conditions as entry 1, only traces of arylated product 3a was
observed (Table 1, entry 2). The reaction under similar conditions but in the presence of pivalic
acid (PivOH) as an additive afforded 3a in 38% yield and with better selectivity (96%) of
arylation at the C-5 position over the C-2 position (Table 1, entry 3). The presence of PivOH is
essential during the reaction as it participates in the proton shuttle event during the C-H bond
cleavage. Greater steric bulk and higher pK
assists in driving the reaction to completion. Next, we screened different bases under similar
reaction conditions as in entry 3. Reaction in presence of K CO went smoothly with high yield
of 3a (86%) and excellent selectivity (>99%) of C-5 product (Table 1, entry 4). The yields of 3a
were lowered when bases like t-BuOK, t-BuONa, KOH, and KOAc were used instead of K CO
Table 1, entries 5-8). Screening of different solvents for C-H bond arylation was studied next
a
of PivOH (5.03) as compared to benzoic acid (4.20)
2
3
2
3
(
using a wide variety of polar and some non-polar solvents (DMA, DMF, DMSO, 1,4-dioxane,
NMP, toluene, and THF). The desired coupled product 3a was not formed in DMSO (Table 1,
entry 10) while solvents like DMF and NMP gave moderate to high yields (76% and 68%) of 3a
(
(
Table 1, entries 9 and 12). Solvents like 1,4-Dioxane, toluene, and THF resulted in poor yields
7-14%) of cross-coupled product (Table 1, entries 11, 13, and 14). Among all the screened
solvents, DMA gave excellent yield as well as selectivity of the desired product (Table 1, entry
), and hence was used in rest of the catalytic investigations.
4
-
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European Journal of Inorganic Chemistry
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Table 1. Screening of reaction conditions for palladium catalyzed regioselective C-5 arylation of imidazolea
Entry
No.
Catalyst
(mol%)
Base
Solvent
(3 mL)
Additive
Yieldb Selectivityc
(%)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C1 (1.0)
C2 (1.0)
C3 (1.0)
C3 (2.0)
C3 (0.5)
C3 (0.1)
C3 (0.5)
—
Cs
Cs
Cs
2
2
2
CO
CO
CO
3
3
3
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMF
—
nd
07
38
86
23
21
32
56
76
nd
13
68
14
07
89
97
98
95
67
78
nd
68
—
—
1
2
3
4
5
6
7
8
9
PhCO H
2
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
96
K
2
CO
3
>99
92
t-BuOK
t-BuONa
KOAc
92
94
KOH
88
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
3
3
3
3
>99
—
DMSO
1,4-dioxane
NMP
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
94
98
Toluene
THF
93
92
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
>99
>99
>99
>99
>99
>99
—
d
2
2
2
0
1
2
e
f
C3 (0.5)
>99
a
Reaction conditions: 1-bromo-4-nitrobenzene (1.0 mmol), N-methyl imidazole (2.0 mmol), additive
(
0.30 mmol), base (2.0 mmol), solvent (3 mL), temperature (100 °C), time (12 h), under open air
b
c
d
e
conditions; Isolated yield; selectivity calculated by proton NMR; reaction time (8 h); Control
experiment without catalyst; temperature (70 °C).
f
The choice of catalyst (C1-C3) and catalyst loading were subsequently screened (Table
, entries 15-19). A slight increase (3%) in the yield of 3a was observed when the reaction in
1
entry 4 was repeated with C2 (1.0 mol%, Table 1, entry 15). The higher yield (97%) of 3a was
obtained using 1.0 mol% of C3 under similar reaction conditions as entry 4 (Table 1, entry 16).
On increasing the catalyst loading of C3 to 2.0 mol%, there is no significant change in the yield
of 3a (Table 1, entry 17). Upon lowering the loading of catalyst C3 to 0.5 mol%, 3a was isolated
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
in 95% yield (Table 1, entry 18). On further decreasing the amount of C3 to 0.1 mol%, yield of
a was significantly lowered (67%, Table 1, entry 19). When the reaction in entry 4 was stopped
3
after eight hours, the yield was decreased (78%, Table 1, entry 20). The control experiments
without the catalyst suggested no cross-coupled product (Table 1; entry 21). On decreasing the
reaction temperature to 70 °C, yield of coupled product was decreased (Table 1; entry 22)
Among all screened conditions, 1.0 mmol of aryl bromide, 2.0 mmol of N-methyl
imidazole, 2.0 mmol of K
2
CO , 0.3 mmol of PivOH as an additive, 1.0 mol% of catalyst C3 in 3
3
mL of DMA at 100 ºC under aerobic conditions provided the maximum yields of cross-coupled
product 3a in 12 h of shorter reaction time. A catalyst loading of 0.5 mol% of C3 was used in
further studies as it gave a good balance between low catalyst loading and high yield. We were
intrigued that no C-2, C-4 arylated, or even homo-coupled products were observed during any of
the reactions.
With the optimized reaction conditions in hand, we could now explore the substrate scope
with different aryl bromides using catalyst C3 (Table 2). It was observed that aryl bromides with
a wide selection of functional groups, e.g. nitro, cyano, acetyl, chloro, aldehyde, and methoxy
groups reacted smoothly to give admirable yields (82-97%) of cross-coupled products (Table 2,
entry 3a-3h and 3m-3q). In addition, using the heteroaryl derivative (3-bromopyridine) with 1-
methyl-1H-imidazole and 1,2-dimethyl-1H-imidazole resulted in the desired products in
excellent yield (81-84%) (Table 2, entry 3l and 3t).
Introducing electron-donating groups on aryl bromide did not significantly impact the
yields (82-83%) (Table 3, entries 3h and 3q). Even the position (ortho vs. para) of functional
groups on aryl bromide did not affect the efficacy of the catalyst C3. The ortho substituted cyano
and aldehyde groups resulted in excellent yields (86-90%, Table 2, entries 3f, 3g, and 3p).
Sterically bulky derivatives such as 1-bromonapthalene and 4-bromo-1,1'-biphenyl resulted in
excellent yields of cross-coupled products (84-88%, Table 2, entries 3j, 3k, and 3s). Some of
these derivatives 3b, 3e, 3j, and 3k are important intermediates in making bioactive
compounds[1],[18] and by using the present protocol these can be easily synthesized in one pot
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
with high yields and greater regioselectivity. The yields for both 1,2-dimethyl-1H-imidazole and
-methyl-1H-imidazole are in the same range. Using aryl chloride instead of the bromide
1
derivative, decreased rate of the reaction (18 h) and gave desired cross-coupled product in
moderate yield of 47% (Table 2, entry 3m in parenthesis).
Table 2. Substrate scope of direct C-5 arylationa
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
a
Reaction conditions: imidazole derivative (2.0 mmol), aryl bromide/chloride (1.0 mmol), PivOH (0.30 mmol),
b
DMA (3 mL), C3 (0.5 mol%), temp (100 °C), time (12 h), isolated yield; 1-chloro-4-nitrobenezene (1.0 mmol), C3
(
1.0 mol%), time (18 h).
We have also performed sequential C-H arylation and Suzuki-Miyaura coupling reaction
to access biaryl motifs having two phenyl rings, which are important intermediates in many
functional and biologically active molecules.[19] We were particularly interested in synthesizing
imidazole-bearing biaryl moieties which have only few reported protocols in literature.[10a] We
successfully synthesized these imidazole containing biaryl motifs in one step by reacting 1-
methyl-1H-imidazole with 4-bromo-1,1'-biphenyl in 87% yield (Table 2, entry 3k). We then
attempted to synthesize these compounds by a sequential arylation and Suzuki coupling route
(
Scheme 3). Suzuki-Miyaura coupling reaction of compound 3e with phenylboronic acid in
presence of C3 successfully resulted in imidazole containing biaryl motifs 3k in moderate yield
23%). It was earlier reported that Suzuki-Miyaura coupling reactions of imidazole containing
(
substituent are less feasible due to their deactivating nature.[10a] A reverse reaction sequence,
where the Suzuki coupling reaction was performed first followed by the arylation reaction with
1
-methyl-1H-imidazole gave 32% yield of 3k.
B(OH)2
Cl
N
C3 (0.5 mol%), K CO3
2
N
C3 (1.0 mol%), K2CO3
N
Cl
N
PivOH, DMA, 100 °C
N
DMF, 110 °C
N
CH3
CH3
3e
CH3
Br
9
2%
23%
3k
B(OH)2
Cl
Br
N
N
CH3
C3 (0.5 mol%), K CO3
C3 (1.0 mol%), K CO3
2
2
N
Cl
DMF, 80 °C
PivOH, DMA, 100 °C
N
CH3
96%
4
32%
3k
Reaction conditions for C-H bond arylation: N-methyl imidazole (2.0 mmol), aryl bromide/chloride (1.0 mmol), PivOH
0.30 mmol), DMA (3 mL), time (12 h), isolated yields are given; for Suzuki-Miyaura coupling: Phenyl boronic acid (1.2
(
mmol), aryl bromide/chloride (1.0 mmol), DMF (3 mL), time (12 h), isolated yields are given.
Scheme 3. Sequential arylation and Suzuki-Miyaura reactions.
A comparison of the performance of catalyst C3 with previously reported literature using
palladium catalysts for the arylation of imidazoles has been made. Palladium-PEPPSI complexes
of imidazolium carbene ligands were reported as efficient catalyst for arylation of imidazole,
however, they used a rather large catalyst loading (1.0 mol%, 33-98%).[1a] Recently, the same
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
group synthesized sterically bulky Pd-PEPPSI complexes of bis(imino)acenaphthene and used as
catalysts for the direct arylation of azoles with aryl halides. Here, 0.5 mol% of the catalyst was
required to achieve >90% yield of coupled product using comparable reaction times and at
substantially higher temperatures (12 h at 130 °C).[10e] The trans-[PdBr
(CH
C, 2.5 mol%, 18 h, inert atm)[10c] and phosphine functionalized NHC palladium complexes (140
C, 2.5 mol%, 18 h, inert atm)[10d] are less efficient compared to C3. A NHC Im-palladium
CN)]
(μ -ditz) (140
2
3
2
°
°
complex was reported to be better for C-H arylation of benzimidazole and imidazole with aryl
chlorides, but 2-4 mol% of catalyst is required to achieve acceptable yields under inert
conditions.[10b] Palladium (II) complexes having both NHC and phosphine ligands were found to
be efficient catalysts (140 °C, 2.5 mol%, 18 h) for arylation of imidazole.[10a] However, their
protocol required higher catalyst loading, longer reaction times, and substantially higher reaction
temperature compared to our optimized protocol. A tridentate C,N,O-donor palladium(II)-NHC
was also reported to catalyze the direct C-H functionalization of heterocycles with aryl bromides.
This protocol also required longer time, harsher conditions and a higher catalyst loading (2
mol%, 18 h, 140 °C).[20] Recently, palladium (II) complexes of organochalcogen bearing NHC
ligands were used as efficient catalyst for the direct arylation of imidazoles. The catalyst loading
was 0.5 mol% and good yields were obtained in 10 h and at a slightly lower reaction temperature
than aforementioned studies (110 °C).[10f]
The reusability of C3 was studied for arylation of 1-bromo-4-nitrobenzene (2a) with 1a.
After achieving maximum conversion in first reaction cycle, a fresh batch of 1a, 2a, K
2
CO , and
3
pivalic acid were added to the reaction pot (see experimental section in SI). The catalyst C3
showed appreciable recyclability up to next five reaction cycles with minor decrease in
efficiency later on. The results of this experiment are presented in Figure 5, which shows
respective yields after each reaction cycle.
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
Figure 5. Results of reusability experiments of C3 under standard reaction conditions.
Homogeneous vs heterogeneous catalysis.
To gain an insight about the type of catalytic process (homogeneous vs heterogeneous), PPh
Hg poisoning experiments were performed.[21] The availability of excess amount of PPh
or Hg
5.0 mol% PPh or Pd/Hg 1/300) under standard reactions conditions (Table 1, entry 18) showed
3
and
3
(
3
negligible effect on the yield of desired product (see experimental section for procedure and
results). This confirms the homogeneous nature of present catalytic process for the regioselective
arylation of imidazole.
■
Conclusions
Three moisture/air insensitive palladium(II) complexes, two of them bearing novel
S,CNHC half-pincer ligand and one bearing Se,CNHC,S type first pincer ligand, have been
successfully synthesized in high yield and fully characterized with multinuclear NMR, HR-MS
and FT-IR. Single crystal X-ray studies of the two Pd(II) complexes revealed the distorted square
planar geometry around Pd in each complex. All three complexes were screened for the catalysis
of regioselective C-H bond arylation of imidazole in open air conditions. Owing to the unique
S,CNHC,Se pincer coordination mode, the air/moisture stable complex C3 showed excellent
activity and remarkable C-5 selectivity during C-H bond arylation of imidazoles derivatives with
aryl bromides/chlorides, yet running under mild conditions with air and moisture tolerance. This
work shows a promising outlook for the use of chalcogenated N-heterocyclic carbenes in
catalysis by enabling unique selectivity paradigms. The catalyst showed high functional group
tolerance and only 0.5 mol% of the catalyst loading was found to be sufficient in achieving
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
excellent yields. More importantly, the catalyst can also be reused efficiently up to five reaction
cycles with minor decrease in the catalytic efficiency. Mercury and PPh
3
poisoning tests suggest
homogeneous nature of the catalysis process.
■
Experimental section
General. All the reactions to synthesize imidazolium salts (L1-L3) and carbene
complexes (C1-C3) were performed under inert atmosphere using standard Schlenk techniques.
Direct arylation reactions were cunducted in pressure tube under aerobic conditions. HPLC grade
1
,2-dichloeoethane, EtOH, CH
2
Cl
2
, toluene, hexane, CH
3
CN, THF, DMF, DMSO, NMP and 1,4-
dioxane were used directly for the reactions. CDCl
3
(Sigma Aldrich), thiophenol (Spectrochem),
diphenyldiselenide (TCI Ltd.), NaBH
4
(Fisher Scientific), Na
2
SO
4
(EMD), PdCl (Alfa Aesar,
2
9
9.9%), 1-methyl-1H-imidazole (Alfa Aesar), and 1,2-dimethyl-1H-imidazole (Alfa Aesar),
were used as purchased. The other reagents used for arylation reaction were procured from local
sources and used as purchased. The proton and carbon NMR spectra were obtained on Bruker
1
4
00 MHz instrument at ambient temperature. The solvent peaks are referenced as (δ, ppm): H,
1
3
1
CHCl (7.26); C{ H}, CDCl (77.00). HRMS were recorded using Agilent Technologies 6545
3
3
Q-TOF LC/MS. The melting points of complexes were taken in open capillary and reported as it
is. IR spectrums of all ligands and complexes were taken on a Nicolet Protége 460 FT-IR
spectrometer on KBr pellets.
3
-Benzyl-1-(2-phenylthio-ethyl)-3H-imidazolium bromide (L1). A round bottom
reaction flask was used and filled with 1-(2-(phenylthio)ethyl)-1H-imidazole (0.408 g, 2.00
mmol), benzyl bromide (0.342 g, 2.00 mmol), and acetonitrile (4 mL) and attached with a
condenser under inert atmosphere. The mixture was heated on refluxing for 48 h while stirring.
After 48 h, the reaction was cooled to room temperature and solvent was removed using rotary
evaporation. The residue was washed with n-pentane and dried to give L1 as a dark red viscous
oil (0.713 g, 1.90 mmol, 95%).
1
NMR (CDCl
3
, δ/ppm): H (400 MHz) 10.21 (s, 1H, H
9
), 7.53 – 7.52 (m, 1H, H
8
), 7.45 –
7
.42 (m, 2H, H and H
7
3
), 7.37 – 7.35 (m, 3H, H
3
and H12), 7.32 – 7.29 (m, 2H, H ), 7.27 – 7.18
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
(
m, 4H, H13, H14 and H
1
), 5.49 (s, 2H, H10), 4.51 (t, 3JH−H = 6.1 Hz, 2H, H
); C{ H} (100 MHz) 137.0 (s, C ), 133.3 (s, C
), 129.0 (s, C13), 127.3 (s, C14), 123.1 (s, C
6
), 3.47 (t, 3JH−H = 6.1
1
3
1
Hz, 2H, H
5
9
4
), 132.8 (s, C11), 130.3 (s, C
3
), 129.5
),
(
s, C
1
), 129.5 (s, C12), 129.4 (s, C
2
7
), 121.6 (s, C
8
−
1
5
1
3.3 (s, C10), 49.2 (s, C6), 34.3 (s, C
5
). IR (cm , powder film): 3040 (w), 2963 (s), 1558 (s),
+
265 (m), 1026 (m), 725 (s). Mass (CH
3
CN) [M − Br] (m/z) Found: 295.1261; Calc. value for
+
[
C
18
H
19
N
2
S] : 295.1263.
3
-Benzhydryl-1-(2-phenylthio)-ethyl)-3H-imidazolium bromide (L2). Acetonitrile (4
mL), 1-(2-(phenylthio)ethyl)-1H-imidazole (0.409 g, 2.00 mmol), and (bromomethylene)dibenz-
ene (0.494 g, 2.00 mmol) were taken in a round bottom flask using procedure same as L1. A
similar workup as L1, gave L2 as a dark red viscous oil (0.834 g, 1.85 mmol, 92%).
1
NMR (CDCl
3
, δ/ppm): H (400 MHz) 9.60 (s, 1H, H
9
), 7.89 (s, 1H, H
7
), 7.39 – 7.36 (m,
7
6
H, H
3
, H12 and H
8
), 7.24 – 7.17 (m, 9H, H
1
, H
2
, H13, and H14), 7.11 (s, 1H, H10), 4.57 (t, 3JH−H
=
), 3.49 (t, 3JH−H = 6.0 Hz, 2H, H
); C{ H} (100 MHz) 137.0 (s, C
13
1
), 136.3 (s,
.0 Hz, 2H, H
6
5
9
C11), 133.1 (s, C
4
), 130.1 (s, C
3
), 129.5 (s, C
2
), 129.4 (s, C12), 128.3 (s, C13), 127.3 (s, C14 and C
1
−
1
merged), 123.5 (s, C
7
), 121.4 (s, C
8
), 66.8 (s, C10), 49.0 (s, C
6
), 34.2 (s, C ); IR (cm , powder
5
film): 3078 (s), 2945 (m), 1551 (s), 1450 (s), 1327 (m), 1141 (s), 864 (s), 733 (s); Mass
+
+
(
CH
3
CN) [M − Br] (m/z) Found: 371.1566 ; Calc. value for [C18
Complex 1. Path A: A round bottom two necked flask taken to combine L1 (0.454 g,
.21 mmol), Ag O (0.280 g, 1.21 mmol), and CH Cl (25 mL). The reaction mixture was stirred
H
19
N
2
S] : 371.1576.
1
2
2
2
overnight at rt under nitrogen atmosphere. Pd(CH
3
CN)
2
Cl
2
(0.314 g, 1.21 mmol) in CH
2
Cl (5
2
mL) was then mixed in the reaction mixture and further stirred at rt. After 8 h, reaction mixture
was passed through celite pad and filtrate was concentrated on a rotary evaporator. The
remaining residue was then purified by using silica column (2 × 12 cm) with MeOH/CH
1:20 v/v) solvents. Complex C1 was collected by removing solvent fractions containing
product, using rotary evaporator. The C1 was isolated as yellow solid (0.502 g, 1.06 mmol,
8%). mp: 192-194 °C.
Path B: A round bottom two necked flask taken to combine L1 (0.454 g, 1.21 mmol), t-
2
Cl
2
(
8
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
BuOK (0.168 g, 1.50 mmol), Pd(CH
3
CN)
2
Cl
2
(0.314 g, 1.21 mmol) and THF (20 mL). The
reaction mixture was stirred at reflux under nitrogen atmosphere. After 10 h, reaction mixture
was passed through celite pad and filtrate was concentrated on a rotary evaporator. A similar
workup as C1 (Path A), gave C1 as a yellow solid (0.371 g, 0.84 mmol, 65%).
1
NMR (Path A) (CDCl
3
, δ/ppm): H (400 MHz) 7.52 – 7.33 (m, 14H) 5.69 – 5.41 (m, 2H,
1
3
1
H
6
), 4.59 (br s, 2H, H
), 130.0 (s, C ), 129.2 (s, C12), 129.1 (s, C
3.7 (C10), 49.7 (s, C
5
); C{ H} (100 MHz) 137.1 (s, C
9
), 136.6 (s, C
4
), 132.9 (s, C11), 130.9 (s,
, C ),
C
3
1
2
), 128.9 (s, C
1
3), 128.6 (s, C14), 123.7 (s, C
7
8
−
1
5
6
), 23.5 (s, C
5
). IR (cm , powder film): 3095 (w), 2924 (m), 1419 (s), 1211
(
[
m), 717 (s); Mass (CH
3
CN) [(M + H) – PdCl
2
]+ (m/z) Found: 295.1260; Calc. value for
+
C
18
H
19
N
2
S] : 295.1263.
Complex 2. Path A. CH
2
Cl
2
(25 mL), L2 (0.546 g, 1.21 mmol), Ag O (0.280 g, 1.21
2
mmol) and Pd(CH
3
CN)
2
Cl
2
(0.314 g, 1.21 mmol) were taken in a round bottom flask by using
procedure same as C1 (Path A). A similar workup as C1, gave C2 as a yellow solid (0.523 g,
0
.96 mmol, 79%). mp: 196-198 °C.
Path B: THF (20 mL), L2 (0.546 g, 1.21 mmol), t-BuOK (0.168 g, 1.50 mmol), and
Pd(CH
3
CN)
2
Cl (0.314 g, 1.21 mmol) were taken in a round bottom flask by using procedure
2
same as C1 (Path B). A similar workup as C1 (Path A), gave C2 as a yellow solid (0.411 g,
0
2
.75 mmol, 62%).
NMR (Path A) (CDCl
1
3
, δ/ppm): H (400 MHz) 7.84 – 7.08 (m, 18H), 4.85 – 4.36 (m,
1
3
1
H), 3.82 – 3.50 (m, 2H); C{ H} (100 MHz) 138.8 (s, C
9
), 138.6 (s, C11), 134.9 (s, C
4
), 130.1
), 67.2
(
(
(
s, C
3
2
), 129.9 (s, C ), 129.3 (s, C12), 127.9 (s, C13), 126.5 (s, C
1
), 124.1 (s, C ), 121.8 (s, C
7
8
–
1
s, C10), 49.8 (s, C
6
), 31.9 (s, C
5
). IR (cm , powder film): 3094 (m), 2924 (m), 2854 (m), 1411
+
m), 1072 (s), 879 (w), 732 (m); Mass (CH
3
CN) [M – Cl] (m/z) Found: 511.0207; Calc. value
PdS] : 511.0222, [M – 2Cl]+ (m/z) Found: 475.045; Calc. value for
+
for [C24
H22ClN
2
2
+
[
C
24
H
22
N
2
PdS] : 476.0539.
-(2-(phenylselanyl)ethyl)-3-(2-(phenylthio)ethyl)-3H-imidazolium bromide (L3).
Acetonitrile (4 mL), 1-(2-(phenylselanyl)ethyl)-1H-imidazole (0.502 g, 2.00 mmol), and (2-
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
bromoethyl)(phenyl)sulfane (0.434 g, 2.00 mmol) were taken in a round bottom flask using
procedure same as L1. A similar workup as L1, gave L3 as a off white viscous oil (0.829 g, 1.77
mmol, 88%).
1
NMR (CDCl
.36 (m, 2H, H
.43 (t, 3JH−H = 6.3 Hz, 2H, H
3
, δ/ppm): H (400 MHz) 9.65 (s, 1H, H
9
), 7.43 (br s, 2H, H
7
, H
8
), 7.38 –
7
3
), 7.25 (d, 3JH−H = 7.5 Hz, 2H, H13), 7.18 – 7.05 (m, 6H, H14, H15, H
1
, and H
2
),
4
6
), 4.37 (t, 3JH−H = 6.0 Hz, 2H, H10), 3.38 (t, 3JH−H = 6.0 Hz, 2H,
), 3.31 (d, 3JH−H = 2H, H11); C{ H} (100 MHz) 136.5 (s, C
), 129.4 (s, C14), 127.9 (s, C12), 127.7 (s, H
), 49.9 (s, C ), 49.1 (s, C10), 33.9 (C ), 27.1 (s, C11). IR (cm , powder film):
13
1
), 133.5 (s, C
), 132.9 (s, C
),
H
5
9
4
3
1
30.1 (s, C13), 129.5 (s, C
2
1
), 127.1 (s, H15), 122.8 (s,
–
1
C
7
), 122.5 (s, C
8
6
5
3
042 (w), 2951 (s), 2845 (s), 1442 (m), 1264 (m), 1028 (s), 732 (m); Mass (CH
CN) [M − Br]+
3
+
(
m/z) Found: 389.0557 ; Calc. value for [C19
Complex 3. CH Cl (15 mL), CH
.21 mmol) and Pd(CH CN) Cl (0.314 g, 1.21 mmol) were taken in a round bottom flask by
H
21
N
2
SSe] : 389.0585.
2
2
3
CN (10 mL), L3 (0.567 g, 1.21 mmol), Ag O (0.280 g,
2
1
3
2
2
using procedure same as C1. The reaction mixture was stirred overnight under reflux in nitrogen
atmosphere. Thereafter, the reaction mixture was cooled to rt and filtered through G4 crucible.
Solid AgBF (0.233 g, 1.21 mmol) was added to the filtrate and the reaction mixture was further
4
stirred under reflux in N
remove the precipitate of AgCl and filtrate was concentrated on a rotary evaporator. The
remaining residue was then purified by using silica column with 5% CH OH in CH Cl
2
atm. After 2 h, reaction mixture was passed through celite pad to
3
2
2
.
Complex C3 was collected by removing solvent fractions containing product, using rotary
evaporator. The C3 was isolated as an off white solid (0.567 g, 0.92 mmol, 76%). mp: 215-217
°C.
1
3
NMR (CDCl
, and C ), 7.60 – 7.41 (m, 6H, C
s, 2H, C ), 3.08 (s, 2H, C11); C{ H} (100 MHz) 148.3 (s, C
31.3 (s, C ), 131.2 (s, C ), 130.8 (s, C15), 130.4 (s, C ), 130.2 (s, C14), 128.9 (s, C12), 124.1 (s,
), 124.1 (s, C ), 49.6 (s, C ), 48.8 (s, C10), 37.3 (s, C
3
, δ/ppm): H (400 MHz) 7.96 (d, JH−H = 7.3 Hz, 3H, H
, C , C14, and C15), 4.86 – 4.35 (m, 4H, C10, and C
), 134.7 (s, C ), 134.2 (s, C13),
3
, and H13), 7.62 (s,
2
H, C
7
8
1
2
6
), 3.32
1
3
1
(
5
9
3
1
4
1
2
–
1
C
7
8
6
5
), 32.5 (s, C11) . IR (cm , powder film):
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
3
007 (m), 2947 (s), 1439 (m), 1048 (s), 868 (w), 744 (m); Mass (CH
3
CN) [M – BF
]+ (m/z)
4
+
Found: 528.9227 ; Calc. value for [C19
H20ClN
2
PdSSe] : 528.9236.
General procedure for arylation reaction. A 10 mL pressure tube is taken to combine
imidazole derivatives (2.0 mmol), additive (0.30 mmol), aryl bromide/chloride (1.0 mmol),
catalyst (C1-C3), base (2.00 mmol), and solvents (3 mL). The reaction mixture was stirred and
heated at 100 °C in open air conditions for 12 h. The progress of reaction was monitored by
using TLC to achieve the maximum conversion of product. After that, the reaction mixture was
cooled to rt and 25 mL distilled water was added. The mixture was washed with CH
mL) and dried with Na SO
mixture was purified using silica gel column (1 × 12 cm) chromatography with CH
2
Cl
2
(2 ×20
2
4
(anhyd.). The solvent was then removed using rotary evaporator and
2
Cl
2
/MeOH
(
20:1 v/v) solvents. The products as isolated were then authenticated with the help of proton and
carbon NMR spectroscopy and data are presented in SI.
Reusability experiment of C3. In a 10 mL pressure tube, 1-methyl-1H-imidazole (2.0
mmol), 1-bromo-4-nitrobenzene (1.0 mmol), C3 (0.5 mol %), PivOH (0.30 mmol), DMA (3
mL), and K
2
CO
3
(2.0 mmol), were combined. The tube was then stirred for 12 h at 100 °C. After
1
1
2 h, the mixture was cooled to rt and an aliquot (60 μL) was pipette out for H NMR analysis.
Thereafter, a fresh batch of reactants was added without catalyst and mixture was allowed to stir
for 12 h under similar reaction conditions. This procedure was repeated for three more times and
the results were summarized in the Figure 5.
Triphenylphosphine poisoning test. In a pressure tube, 1-methyl-1H-imidazole (2.0
mmol), 1-bromo-4-nitrobenzene (1.0 mmol), PivOH (0.30 mmol), K
2
CO (2.0 mmol), and DMA
3
(
3 mL) were combined. Thereafter, catalyst C3 (0.5 mol%) and triphenylphosphine (PPh
3
/Pd,
1
:5) were added into the reaction mixture. Mixture was stirred for 12 h under optimal reaction
conditions. The analogues workup of reaction mixture resulted in 89% of 3a.
Hg poisoning test. In a pressure tube, 1-methyl-1H-imidazole (2.0 mmol), 1-bromo-4-
nitrobenzene (1.0 mmol), K
2
CO
3
(2.0 mmol), PivOH (0.30 mmol), and DMA (3 mL) were
combined. Thereafter, an excess of Hg and catalyst C3 (0.5 mol%, Hg/Pd 300/1) were added into
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
the reaction mixture. Mixture was stirred for 12 h under optimal reaction conditions. The
analogues workup of reaction mixture resulted in 86% of 3a.
Notes
The authors declare no competing financial interest.
Acknowledgement
HJ is thankful to Department of Science and Technology (DST) Inspire
(
[
DST/INSPIRE/04/2017/002162). Authors are also thankful to DST for DST-FIST grant
SR/FST/CSI-270/2015] and Central Analytical Facility, BITS Pilani. KNS thanks to SERB,
New Delhi for financial support. Authors thank Dr. Shobha Sharma for insightful discussion.
Supplementary Information (SI)
CCDC 1963719 and 1963720 contain the supplementary crystallographic data for C1 and
C2 respectively. These data can be obtained free of charge via http://www.ccdc.
cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB21EZ, UK; Fax: (+44)1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Corresponding authors
*
E-mail: hemant.joshi@curaj.ac.in, kamalnayaniitd@gmail.com
ORCID
Hemant Joshi: 0000-0002-7616-0241
Kamal Nayan Sharma: 0000-0001-5919-3861
-
20-
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
■
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European Journal of Inorganic Chemistry
10.1002/ejic.201901259
TOC
Palladium Complexes of Thio/Seleno-Ether Containing N-Heterocyclic
Carbenes: Efficient and Reusable Catalyst for Regioselective C-H Bond
Arylation
Ramprasad Bhatt, Nattamai Bhuvanesh, Kamal Nayan Sharma,* Hemant Joshi,*, [d]
[
a]
[b]
, [c]
Three Pd(II) catalysts, two bearing S,CNHC half-pincer and one bearing Se,CNHC,S type pincer,
have been synthesized and applied in the regioselective C-H bond arylation of imidazole. All the
complexes were found to be efficient under mild conditions. The Se,CNHC,S pincer ligated Pd(II)
complex, which is the first example of this type, was found to be utmost effective amongst them.
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24-
This article is protected by copyright. All rights reserved.