Halo-Bridged Abnormal NHC Palladium(II) Dimer for Catalytic Dehydrogenative Cross-Coupling Reactions of Heteroarenes

Article abstract of DOI:10.1021/acs.joc.8b01053

This work describes the dehydrogenative coupling of heteroarenes using a dimeric halo-bridged palladium(II) catalyst bearing an abnormal NHC (aNHC) backbone. The catalyst can successfully activate the C-H bond of a wide range of heteroarenes, which include benzothiazole, benzoxazole, thiophene, furan, and N-methylbenzimidazole. Further, it exhibited good activity for heteroarenes bearing various functional groups such as CN, CHO, Me, OMe, OAc, and Cl. Additionally, we isolated the active catalyst by performing stoichiometric reaction and characterized it as the acetato-bridged dimer of (aNHC)PdOAc by single-crystal X-ray study.

Full text of DOI:10.1021/acs.joc.8b01053

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Article
A Halobridged Abnormal NHC Palladium(II) Dimer for Catalytic
Dehydrogenative Cross-Coupling Reactions of Heteroarenes
Sreejyothi P, Samaresh Chandra Sau, Pavan K Vardhanapu, and Swadhin K. Mandal
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b01053 • Publication Date (Web): 17 Jul 2018
Downloaded from http://pubs.acs.org on July 17, 2018
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A Halobridged Abnormal NHC Palladium(II) Dimer for Catalytic Dehydrogenative
Cross-Coupling Reactions of Heteroarenes
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Sreejyothi P, ‡ Samaresh Chandra Sau, ‡ Pavan K. Vardhanapu and Swadhin K.Mandal*
Department of Chemical Sciences, Indian Institute of Science Education and Research,
Kolkata, Mohanpurꢀ741246, India. Email: swadhin.mandal@iiserkol.ac.in
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TOC/Abstract Graphic:
18 entries
Upto 95% yield
Abstract
This work describes the dehydrogenative coupling of heteroarenes using a dimeric halo
bridged palladium (II) catalyst bearing an abnormal NHC (aNHC) backbone. The
catalyst can successfully activate the C–H bond of a wide range of heteroarenes, which
include benzothiazole, benzoxazole, thiophene, furan, and Nꢀmethyl benzimidazole.
Further, it exhibited good activity for heteroarenes bearing various functional groups
such as CN, CHO, Me, OMe, OAc and Cl. Additionally, we isolated the active catalyst
by performing stoichiometric reaction and characterized it as the acetato bridged dimer of
(aNHC)PdOAc by single crystal Xꢀray study.
Keywords: abnormal Nꢀheterocyclic carbene, dehydrogenative coupling, biheteroaryls,
heterocoupling, dimeric palladium catalyst
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Introduction:
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Motifs bearing bonds between two heteroarenes are ubiquitously found in many natural
products, pharmaceuticals, and electronic materials.1 Development of new methods for
the synthesis of heteroaryl scaffolds possesses significant importance. The traditional
approach for the synthesis of biaryls has involved mainly the palladiumꢀcatalyzed crossꢀ
coupling reaction between two preꢀactivated substrates which include Heck, Suzuki–
Miyaura, Negishi, Stille or Kumada coupling type reactions (Scheme 1A).2,3,4 Owing to
the requirement of preꢀactivated substrates for such traditional catalytic methods which
involve several synthetic steps; development of an alternative pathway adopting atom as
well as step economic process has been very much desired for long. The most obvious
first step for such development towards this direction is the replacement of one preꢀ
activated species with a simple arene or heteroarene. This process has been accomplished
via C–H (bond) activation or C–H functionalization although the term direct arylation is
generally a preferred one (Scheme 1B).5 However, the most atom economic approach
would involve replacement of both preꢀactivated substrates by simple arenes or
heteroarenes. In fact, the coupling of two aryl C–H bonds to give the corresponding
biaryl product would seem ideal (Scheme 1C). Nonetheless, given the strength of the C–
H bond, such processes are often thermodynamically challenging.6 Earlier, Fagnou,7ꢀ8
Chang,9 Buchwald,10 Lu,11 Sanford12 and others13ꢀ15 have made a significant
breakthrough in the Pd(II)ꢀcatalyzed oxidative crossꢀcoupling of unactivated heteroaryls
with simple arenes. Nevertheless, the metalꢀcatalyzed oxidative crossꢀcoupling of two
heteroaryl CꢀH bonds to form unsymmetrical biheteroaryl molecules remains a topic of
significant challenge. Mori and coꢀworkers reported the palladiumꢀcatalyzed, highly
chemo and regioselective, dimerization of thiophenes to form biꢀthiophenes for the first
time for dehydrogenative cross coupling of heteroarenes.16,17 Later on, Sanford,18 You,19
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Dauglis,20 Yamaguchi,21 Zhang,22 Wang23 and other research groups24ꢀ26 have further
improved the strategies for dehydrogenative homocoupling of heteroarenes using
Pd(II)/Cu(II) catalysts.
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Scheme 1. General Schemes for Formation of Heterobiaryl Compounds: A) Traditional
CrossꢀCoupling Reaction, B) Direct CrossꢀCoupling Reaction and C) Dehydrogenative
CrossꢀCoupling Reaction
However, the dehydrogenative heterocoupling between two different heteroarenes
remained mostly unaddressed until 2010. In 2010, Hu and You and coworkers
demonstrated the Pd(II)ꢀcatalyzed CꢀH/CꢀH crossꢀcoupling of various
Nꢀcontaining
heteroarenes (e.g., azoles, and pyridine ꢀoxides) with fiveꢀmembered heterocycles (e.g.,
N
thiophenes, and furans) using Pd(OAc)2,and Cu(OAc)2.27 Later on, Ofial and coꢀworkers
demonstrated the dehydrogenative coupling between benzothiazoles and azoles to afford
the unsymmetrical 2,2
′
ꢀlinkage between the same.28 Following these work, You,29ꢀ30
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Wang,
Yang32 and others have made significant contributions in dehydrogenative
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crossꢀcoupling of heteroarene partners.33ꢀ45 It is to be noted that in all these reports,
Pd(OAc)2 was used as the catalyst in the presence of a sacrificial oxidant. There have
been few studies on understanding of the mechanism of this fascinating reaction with the
help of DFT studies27 along with controlled experiments under stoichiometric
conditions,28,32,41 however, understanding the catalytic cycle by isolating the active
catalyst and characterizing it by single crystal Xꢀray structure has been missing. In the
current study, we examined the catalytic activity of a wellꢀdefined halobridged palladium
(II) dimeric complex (Catalyst I)46 bearing an abnormal Nꢀheterocyclic carbene in the
crossꢀcoupling of two different heteroarenes. It may be noted that after the isolation of
the first abnormal Nꢀheterocyclic carbene by Bertrand and coꢀworkers in 201047 we have
developed this halobridged palladium (II) dimeric complex (Catalyst I)46 for carrying
SuzukiꢀMiyaura coupling of challenging aryl chloride substrates at ambient conditions.
Later on, we applied this abnormal Nꢀheterocyclic carbene for development of various
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Scheme 2. Summary of Present Catalytic Method for Dehydrogenative Heterocoupling
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transition metal complexes for a range of catalytic transformations.48ꢀ52 Herein we report
that the wellꢀdefined halobridged aNHC dimer can catalyze dehydrogenative crossꢀ
coupling of two heteroarene partners under 2 mol% catalyst loading (Scheme 2). By
performing stoichiometric reaction, an acetato bridged Pd(II)ꢀaNHC dimer was isolated
as the active catalyst and characterized by single crystal Xꢀray study and with the help of
further control experiments, a plausible mechanism for the dehydrogenative crossꢀ
coupling reaction was proposed.
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Results and discussion:
The chlorobridged aNHCꢀPd dimer (Catalyst I) was prepared according to the previously
reported method.46 At first, a series of optimization reactions for crossꢀcoupling between
two heteroarenes was performed using benzothiazole (1a) and 2ꢀformylthiophene (2a) as
model substrates in presence of Catalyst I (Table 1). The conversion was monitored by
1H NMR spectroscopy. To our delight, the reaction between one eqv. of benzothiazole
o
with 4 eqv. 2ꢀformylthiophene in DMSO at 110 C resulted in a nearly quantitative
1
conversion (by H NMR spectroscopy) into the desired heterocoupled product 3a which
was isolated in 95% yield after the column chromatography (Table 1, entry 1). However,
the use of 1 equiv. of 2a instead of 4 equiv. led to decrease in the NMR conversion of 3a
(66%) and resulted in significant homocoupling of 1a. It may be noted that earlier
27,32
studies
also reported use of one coupling partner in excess in related
dehydrogenative heterocoupling reactions. Further, the reaction was screened using
various solvents. Among these solvents, DMSO turned out to be the best solvent (Table
1, entry 1) whereas other solvents such as DMF, DMAc and NMP resulted in 92%, 55%
and 31% conversion, respectively (Table 1, entries 2ꢀ4). The reaction did not proceed in
presence of THF (Table 1, entry 5). To study the effect of oxidant in the reaction,
different oxidants were examined and AgOAc afforded the best conversion whereas
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AgNO3, Ag2CO3 and Ag2O delivered 55%, 50% and 10%, respectively (Table 1, entries
6ꢀ8). Another control reaction was carried out by using Cu(OAc)2 as an oxidant, which
gave 99% NMR conversion (Table 1, entry 14). The conversion was reduced to 90%
o
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when the reaction was carried out at 100 C keeping other conditions identical (Table 1,
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o
entry 9). The reaction did not proceed at all when it was carried out at 80 C (Table 1,
entry 10).
Table 1. Optimization of different reaction conditions for dehydrogenative crossꢀ
coupling of benzothiazole (1a) with 2ꢀformylthiophene (2a).[a]
CHO
Catalyst I
N
S
N
S
S
S
H
CHO
Oxidant, Solvent
temp., time
1a
2a
3a
Entry Solvent
Oxidant
Catalyst
Time (h) Temperature
NMR
Conversion (%)
(°C)
1
>99 (95[b])
DMSO
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgNO3
Ag2CO3
Ag2O
Catalyst I
Catalyst 1
Catalyst I
Catalyst I
Catalyst I
Catalyst I
Catalyst I
Catalyst I
Catalyst I
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24
24
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24
24
24
110
2
3
4
5
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7
8
9
DMF
DMAc
NMP
110
110
110
110
110
110
110
100
92
55
31
THF
N.R
55
DMSO
DMSO
DMSO
DMSO
50
10
AgOAc
90
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10 DMSO
11 DMSO
12[c] DMSO
AgOAc
AgOAc
AgOAc
AgOAc
Catalyst I
24
24
24
24
80
80
N.R
N.R
95
ꢀ
Catalyst I
Ia
110
110
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DMSO
90
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DMSO Cu(OAc)2
Catalyst I
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110
99
[a]
Reaction conditions: Benzothiazole (0.366 mmol), 2ꢀformylthiophene (1.464 mmol),
Oxidant (0.732 mmol), Catalyst I (0.0073 mmol, 2 mol% catalyst loading) and solvent
[b]
[c]
DMSO (3 mL).
Reaction”.
Isolated yield.
1 mol% catalyst loading. N.R stands for “No
A control reaction when performed without using the Catalyst I, did not result in the
formation of any desired product which establishes inevitable role of the catalyst in this
reaction (Table 1, entry 11). Experiments performed at catalyst loading as low as 1 mol
% yielded slightly less yet fairly good conversion (93%, Table 1, entry 12). For further
substrate scope, the reaction conditions corresponding to entry 1 in Table 1 was followed.
With the standardized conditions (entry 1, Table 1), we moved on to investigate further
substrate scope for dehydrogenative crossꢀcoupling of heteroarenes. A wide range of
substrates was investigated and moderate to excellent yields (55ꢀ95%) were obtained.
The present catalytic protocol was found suitable for coupling of benzothiazole,
benzoxazole, Nꢀmethyl benzimidazole with different functionalized thiophenes, furfural,
Nꢀmethyl imidazole. It is noteworthy that our reaction went smoothly and was able to
tolerate a wide range of functional groups in thiophenes (Table 2, 3a, 3i, 3l, 3k, 3p).
Moreover, imidazole and benzimidazole which are rather inactive than thiazoles based on
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Table 2: Catalytic cross dehydrogenative heterocoupling of heteroarenes using Catalyst
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5
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I[a]
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9
N
X
N
N
N
X
N
X
I
N
N
Catalyst (2 mol%)
X'
R2
or
or
R2 R1
+
R1
R1
X'
AgOAc, DMSO
110 oC, 24 h
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R1= OMe or Cl or H
X = S or O or NMe
X' = S or O
R2 = CHO or OAc or Me or Et or CN
CHO
CHO
N
N
N
N
S
N
O
S
S
S
3a (95%)
3b (82%)
3c (68%)
CHO
CN
CHO
N
N
O
CHO
N
N
S
N
S
O
3d
N
3e
3f
(88%)
(55%)
N
(58%)
S
CHO
S
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N
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S
MeO
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S
3g
%
3h
3i
(76%)
(77
)
(92%)
S
N
S
CN
N
S
N
N
S
S
3j
3k
3l
(84%)
N
(59%)
N
(80%)
O
CHO
N
N
S
N
S
N
Cl
O
N
Cl
3m
3n
3o
(86%)
(83%)
(60%)
S
N
N
N
S
S
N
N
S
O
3r[b]
3p
3q
(73%)
(79%)
(68%)
[a]
Reaction conditions: Benzothiazole/benzoxazole/Nꢀmethylbenzimidazole (0.366
mmol), thiophene/furfural/Nꢀmethylimidazole (1.464 mmol), AgOAc (0.732 mmol),
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Catalyst I (0.0073 mmol) and solvent DMSO (3 mL), 24 h, 110 C. [b] NMR conversion.
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Isolated yields are given in the parentheses.
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their pKa values4 and would often require harsh conditions for C2ꢀH activation have also
shown good to excellent activity (55ꢀ86%) with our conditions (Table 2, 3c, 3e, 3f, 3g,
3j, 3m, 3n, 3q, 3r). Benzothiazole showed more activity (95%, Table 2, 3a) towards
formylated thiophene coupling partner whereas with formylated furan coupling partner
delivered relatively less yield (88%, Table 2, 3d). Substituted benzothiazole coupling
partners also displayed good reactivity ranging from 60ꢀ92% (Table 2, 3h, 3n, 3o).
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Table 3. Catalytic cross dehydrogenative homocoupling with a variety of heteroarenes
using Catalyst I[a]
N
S
S
O
N
O
O
N
S
S
O
N
4c (95%)
4a (88%)
4b (80%)
N
N
N
O
N
S
S
S
N
N
S
O
4d (92%)
4e (93%)
4f (91%)
N
S
S
N
4g (80%)
Reaction conditions: Heteroarene (0.366 mmol), AgOAc (0.732 mmol), Catalyst 1
o
(0.0073 mmol) and solvent DMSO (3 mL), 24 h, 110 C. Isolated yield is given in the
parentheses.
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During the course of reaction, we have observed formation of homocoupling product in
varied yield as a side product along with formation of the desired heterocoupled product.
This obsevation further made us curious to check the homocoupling reactions using the
optimized conditions. We noticed under such conditions, dehydrogenative homocoupling
of heteroarenes also proceeded smoothly. As shown in Table 3, the reaction progressed
well with a various heteroarenes such as benzothiazole, benzoxazole, Nꢀmethyl
benzimidazole, furan as well as thiophene and delivered yield in the range of 80ꢀ95%.
The substrates with two heteroatoms displayed comparable activity (82ꢀ91%, Table 3,
4a, 4b, 4f and 4g) to those with a single heteroatom (92ꢀ95%, Table 3, 4c, 4d and 4e).
The substituted benzothiazoles showed less reactivity (80%, Table 3, 4g) in comparison
with naked benzothiazole (88%, Table 3, 4a). Further, this catalyst exhibited high
functional group tolerance as it activated a wide range of substrates with different
functional groups, which include CN, CHO, Me and OAc. Further, we moved to check
the reaction mechanism for this dehydrogenative coupling reactions. In order to
investigate the mechanism of the dehyrogenative crossꢀcoupling reaction, several
stoichiometric experiments were conducted. A stoichiometric reaction was carried out
between Catalyst I and AgOAc under nitrogen atmosphere (Scheme 3). NMR
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spectroscopic studies of the reaction mixture indicated appearance of a new peak in H
and 13C NMR spectra at 1.95 and 183.2 ppm, respectively. This observation suggests the
replacement of the chloride bridge with the acetate group. Delightfully, we were able to
generate suitable single crystals from the reaction mixture for Xꢀray analysis in
hexane/benzene 2:1 mixture, which unambiguously established the molecular structure of
an acetato bridged Pd(aNHC) dimer (Ia). This obsevation is reminiscent of the anion
metathesis reaction of chloride with a trifluoroacetate anion for a previously reported
chloro bridged palladium aNHC dimer.51 In complex Ia, the palladium center exhibited a
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Scheme 3. A) Reaction of the Chloro Bridged Palladium Dimer with AgOAc Leads to
the Formation of an Acetato Bridged Dimeric Complex Ia. B) Perspective ORTEP View
of the Molecular Structure of Ia. Thermal Ellipsoids are Drawn with 50% Probability.
The Lattice Held Benzene and Hexane as Well as Hydrogen Atoms have been Omitted
for the Sake of Clarity. Selected Bond Lengths (Å) and Bond Angles (deg): Pd(1A)ꢀ
O(2A), 2.148(3); Pd(1A)ꢀO(2B), 2.129(3); Pd(1A)ꢀC(5A), 1.962(4); Pd(1A)ꢀC(7A)
1.980(5); Pd(1A)ꢀPd(1B), 2.9618(5); C(5A)ꢀPd(1A)ꢀC(7A), 79.63(18); C(5A)ꢀPd(1A)ꢀ
O(2A), 98.08(16); C(5A)ꢀPd(1A)ꢀO(2B), 162.54(16)
distorted square planar geometry. The Pd1AꢀO2B bond distance in Ia was measured as
2.148 Å. The palladiumꢀC(carbene) bond exhibited a bond distance of 1.962 Å (Pd1Aꢀ
C5A) whereas, Pd1AꢀC7A (orthoꢀmetalated) showed a bond distance of 1.980 Å. These
distances were consistent with previously reported similar orthoꢀmetalated Pd dimers of
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Scheme 4. Plausible Mechanistic Cycle for the Dehydrogenative CrossꢀCoupling of
Heteroarenes
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abnormal NHC.46,
Next, the complex Ia was used as the catalyst for the
dehydrogenative crossꢀcoupling reaction following the same catalytic conditions and it
yielded the desired product in excellent yield (90%, Table 1, entry 13). This result
confirmed that the isolated acetato bridged dinuclear palladium complex Ia is an active
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catalyst for the reaction. Next, another stoichiometric reaction was performed by taking
Ia, AgOAc, and benzothiazole (1a) at 100 °C in DMSOꢀd6 for 24 h which resulted in the
missing of C2ꢀH peak of 1a as monitored by 1H NMR spectroscopy (see SI, Figure S53).
This clearly indicates the C2ꢀH abstraction of the substrate takes place during the
catalytic activation process.
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Additionally, we checked relative reactivity of two coupling partners 1a and 2a
separately in homocoupling reaction, over a duration of 1h which in turn showed that 2ꢀ
formylthiophene (2a) is more reactive than the corresponding benzothiazole (1a) (see SI,
Figures S54ꢀ55). Homocoupling of 1a in 1h shows only 10% NMR conversion, whereas
2a exhibited 28% NMR conversion for homocoupling reaction. Previously, Hu, You and
coworkers by DFT calculation also proposed that among the two heteroarene partners
namely thiophene and Nꢀmethylbenzimidazole, the abstraction of C2 proton in thiophene
is relatively faster than that of the other partner which prompted them to propose
thiophene activation as the first step in their catalytic cycle.27 Based on the above
observations, a plausible catalytic cycle is proposed for the dehydrogenative crossꢀ
coupling of heteroarenes in Scheme 4. In this proposed scheme, we considered that the
catalysis is initiated by the conversion of Catalyst I to Ia in presence of AgOAc, which
may be termed as catalyst activation step. The complex Ia was isolated in analytically
pure form and was well characterized by Xꢀray crystallography and NMR spectroscopy.
In presence of a coordinating solvent such as DMSO, Ia can be converted into a
mononuclear Pd complex Ib. Such dissociation of NHC containing Pd dimer into a
monomeric complex is wellꢀdocumented in the literature by Herrmann and coꢀworkers.53
The mononuclear complex Ib subsequently undergoes a rapid palladation of the
heteroarene (with more reactive heteroarene) by the formation of heteroarylpadallium(II)
intermediate Ic with release of an acetic acid molecule. It may be noted that earlier
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studies also suggested the formation of a HetAr–PdLn intermediate as the key step in the
initiation of the catalytic cycle.27ꢀ28 This intermediate further promotes the second CꢀH
activation of another heteroarene partner during which cleavage of ortho CꢀH activated
Pdꢀcarbene bond can takes place to make sure the Pd oxidation state remains in the most
favored bivalent state leading to the formation of a bisheteroaryl Pd intermediate (Id).
Such a cleavage of PdꢀC(orthometalated) bond in aNHCꢀPd dimeric complex has been
reported in earlier study.51 However, at this point, we cannot discard the possibility of
complete decoordination of Pd center by cleavage of the other PdꢀNHC bond as observed
in recent studies.54 Finally, the bisheteroaryl Pd intermediate (1d) upon reductive
elimination leads to formation of the crossꢀcoupled product and Pd(0) species (Ie), which
on oxidation by AgOAc regenerates the catalytically active species 1b.
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Conclusion:
In summary, it may be concluded that our catalytic conditions can successfully lead to
hetero dehydrogenative crossꢀcoupling for a wide range of heteroarenes such as
benzothiazole, benzoxazole, Nꢀmethyl benzimidazole, Nꢀmethyl imidazole, different
substituted thiophenes, and furfural with good to excellent yield. Additionally, this
catalyst exhibited high functional group tolerance as it activated a wide range of
substrates with different functional groups. The use of a wellꢀdefined catalyst enabled us
to understand the mechanistic details by performing a series of control reaction.
Additionally, we have been able to trap a catalytically active acetato bridged Pd(II)
organometallic dimer and characterized by single crystal Xꢀray study.
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Experimental Section:
General Consideration:
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Catalyst synthesis was performed in oven dried glasswares (130 °C) under dry and
oxygen free atmosphere (nitrogen) using standard Schlenk line technique. The solvents
were dried using Na/benzophenone mixture or CaH2 prior to use. All chemicals were
purchased from SigmaꢀAldrich, Alfa Aesar, Merck or Spectrochem and used as received.
Thinꢀlayer chromatography (TLC) was performed on a Merck 60 F254 silica gel plate
(0.25 mm thickness). Column chromatography was performed on a Merck 60 silica gel
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(100−200 mesh). The H and 13C NMR spectra were recorded on 400 and 500 MHz
spectrometers in CDCl3 with residual undeuterated solvent (CDCl3, 7.26/77.0) as an
internal standard. Chemical shifts (δ) and J values are given in ppm, and Hz, respectively.
All chemical shift values were reported in ppm using tetramethylsilane as a reference.
Chemical shifts (δ) downfield from the reference standard were assigned positive values.
Catalyst I was synthesized using the experimental procedure reported in literature.46
General procedure for catalytic dehydrogenative heterocoupling of heteroarenes: 10
mg (2 mol %) Catalyst I, heteroarene (for example 1a, 0.36 mmol), AgOAc (0.73 mmol),
heteroarene (for example, 2a, 1.46 mmol) and 3 mL of dry DMSO were taken in a 5 mL
o
pressure tube. The catalytic reactions were performed at 110 C in closed condition
using metal clip for 24 h. After completion, the reaction mixture was diluted with
ethylacetate and then transferred into a separating funnel. The organic layer was dried
using Na2SO4 and evaporated under reduced pressure and the residue was then purified
through a short column chromatography (silica gel 100ꢀ200 mesh) using appropriate ratio
of hexane and ethyl acetate to provide pure products.
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General procedure for catalytic dehydrogenative homocoupling of heteroarenes:
General procedure for catalytic dehydrogenative homocoupling of heteroarenes: 10 mg
(2 mol %) Catalyst I, heteroarene (for example 1a,0.36 mmol), AgOAc (0.73 mmol) and
3 mL of dry DMSO were taken in a 5 mL pressure tube. The catalytic reactions were
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performed at 110 C in closed condition using metal clip for 24 h. After completion, the
reaction mixture was diluted with ethylacetate and then transferred into a separating
funnel. The organic layer was dried using Na2SO4 and evaporated under reduced pressure
and the residue was then purified through a short column chromatography (silica gel 100ꢀ
200 mesh) using appropriate ratio of hexane and ethyl acetate to provide pure products.
Synthesis and characterization data of Ia: Under nitrogen atmosphere, a 50 mL
Schlenk flask was charged with 100 mg (0.07 mmol) of Catalyst I and 20 mg (0.12
mmol) of AgOAc in benzene (10 mL) and allowed to stir for 12 h at room temperature.
Reaction mixture was filtered and the solvent was evaporated under reduced pressure. X
ray quality crystals were grown in benzene/hexane mixture (2:1 v/v) at room
temperature. The resulting solid was washed with hexane (3 × 1 mL) and dried under
reduced pressure to afford Ia as yellow crystals: Yield 60%. 1H NMR (400 MHz, CDCl3,
25 °C): 7.57 (t, J = 8.0 Hz, 1H), 7.46 (t, J = 8.0 Hz, 1H), 7.36 (s, 1H), 7.33 (d, J = 8.0 Hz,
2H), 7.24 (d, J = 8.0 Hz, 1H), 7.19 (t, J = 8.0 Hz, 1H), 7.07 (t, J = 8.0 Hz, 2H), 6.95 (d, J
= 8.0 Hz, 2H), 6.76 (t, J = 8.0 Hz, 2H), 6.63 (t, J = 8.0 Hz, 2H), 5.68 (d, J = 8.0 Hz, 1H),
2.91ꢀ2.84 (m, 2H), 2.74ꢀ2.69 (m, 2H), 1.93 (s, 3H), 1.51 (d, J = 8.0 Hz, 6H), 1.07 (d, J =
8.0 Hz, 6H). 1.02 (d, J = 8.0 Hz, 6H), 0.97 (d, J = 8.0 Hz, 6H). 13C NMR (125 MHz,
CDCl3, 25 °C): 183.3, 152.1, 150.1, 145.2, 145.1, 144.3, 139.6, 138.7, 133.6, 132.2,
131.3, 130.6, 130.5, 130.0, 128.5, 128.3, 125.1, 124.9, 124.4, 123.6, 123.2, 119.2, 29.0,
28.9, 25.3, 24.9, 24.4, 22.9, 22.6. Anal. Calcd for C91H105N4O4Pd2: C, 71.36; H, 6.91; N,
3.66; Found: C, 70.85; H, 6.71; N, 3.61. The calculated molecular formula was
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determined from crystallographically determined asymmetric unit which contained lattice
held benzene molecule and half molecule of hexane (used as solvents for crystallization).
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Spectral Data of the Compounds:
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1) 5ꢀ(benzothiazolꢀ2ꢀyl)thiopheneꢀ2ꢀcarbaldehyde (3a):32 yellow solid, yield 83 mg,
95%; purified by column chromatography using 5% EtOAc in hexane;1H NMR
(400MHz, CDCl3): δ (ppm) 9.97 (s, 1H), 8.09 (d, J = 8.0 Hz, 1H), 7.90 (d, J =
8.0 Hz, 1H), 7.79 (d, J = 4.0 Hz, 1H), 7.73 (d, J = 4.0 Hz, 1H), 7.55ꢀ7.52 (m,
1H), 7.46ꢀ7.42 (m, 1H); 13C NMR (125 MHz, CDCl3): δ (ppm) 182.7, 159.9,
153.7, 145.6, 145.3, 136.1, 135.3, 128.4, 126.9, 126.3, 123.7, 121.8.
2) 2ꢀ(5ꢀformylthiophenꢀ2ꢀyl)ꢀbenzoxazole (3b):27 brown solid, yield 68 mg, 53%;
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purified by column chromatography using 5% EtOAc in hexane. H NMR (400
MHz, CDCl3): δ (ppm) 10.00 (s, 1H), 7.97 (d, J = 3.8 Hz, 1H), 7.83 (d, J = 3.8
Hz, 1H), 7.80ꢀ7.78 (m, 1H), 7.61ꢀ7.59 (m, 1H), 7.42 ꢀ7.38 (m, 2H); 13C NMR
(125 MHz, CDCl3): δ (ppm) 182.8, 157.4, 150.8, 146.2, 141.8, 137.9, 136.1,
130.0, 126.3, 125.4, 120.5, 111.0.
3) 2ꢀ(1ꢀmethylꢀ1Hꢀimidazolꢀ2ꢀyl)benzothiazole (3c):28 yellow solid, yield 53 mg,
1
68%; purified by column chromatography using 5% EtOAc in hexane; H NMR
(400 MHz, CDCl3): δ (ppm) 8.03 (d, J = 8.0 Hz, 1H), 7.93 (d, J = 8.0 Hz, 1H),
7.51ꢀ7.47 (m, 1H), 7.43ꢀ7.41 (m, 1H), 7.24 (s, 1H), 7.09 (s, 1H), 4.26 (s, 3H); 13C
NMR (125 MHz, CDCl3): δ (ppm) 158.3, 153.9, 140.3, 135.0, 128.6,126.4, 125.8,
125.0, 123.4, 121.7, 36.0.
4) 5ꢀ(benzothiazolꢀ2ꢀyl)furanꢀ2ꢀcarbaldehyde (3d):32 brown solid, yield 73 mg,
1
88%; purified by column chromatography using 10% EtOAc in hexane; H NMR
(400 MHz, CDCl3): δ (ppm) 9.77 (s, 1H), 8.13 (d, J = 8.0 Hz, 1H), 7.96 (d, J =
8.0 Hz, 1H), 7.58ꢀ7.54 (m, 1H), 7.49ꢀ7.45 (m, 1H), 7.40 (s, 2H); 13C NMR (125
MHz, CDCl3): δ (ppm) 177.9, 155.9, 153.7, 153.3, 152.8, 135.0, 126.9, 126.3,
123.7, 121.8, 121.5, 112.7.
5) 1ꢀmethylꢀ2ꢀ(5ꢀformylthiophenꢀ2ꢀyl)ꢀbenzimidazole (3e):27 yellow solid, yield 48
mg, 55%; purified by column chromatography using 10% EtOAc in hexane;1H
NMR (400MHz, CDCl3): δ (ppm) 9.99 (s, 1H), 7.86ꢀ7.84 (m, 2H), 7.81ꢀ7.80 (m
1H), 7.43ꢀ7.33 (m, 3H), 4.06 (s, 3H); 13C NMR (125 MHz, CDCl3): δ (ppm)
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182.3, 146.1, 145.2, 142.4, 141.7, 136.6, 136.0, 129.1, 124.2, 123.5, 120.1, 109.8,
32.2.
6) 1ꢀmethylꢀ2ꢀ(5ꢀformylfuranꢀ2ꢀyl)ꢀbenzimidazole (3f)27 yellow solid, yield 48 mg,
58%; purified by column chromatography using 10% EtOAc in hexane;
1H NMR (400MHz, CDCl3): δ (ppm) 9.78 (s, 1H), 7.82 (d, J = 8.0 Hz, 1H), 7.44–
7.32 (m, 5H), 4.23 (s, 3H); 13C NMR (125 MHz, CDCl3): 177.3, 152.8, 150.9,
143.0, 142.8, 136.0, 124.2, 123.2, 122.5, 120.1, 114.3, 109.8, 32.2.
7) 2ꢀ(1ꢀmethylꢀ1Hꢀbenzoimidazolꢀ2ꢀyl)benzothiazole (3g):41 pale yellow solid, yield
74 mg, 88 %; purified by column chromatography using 5% EtOAc in hexane;1H
NMR (400MHz, CDCl3): δ (ppm) 8.13 (d, J = 8.0 Hz, 1H), 7.99 (d, J = 8.0 Hz,
1H), 7.89 (d, J = 8.0 Hz, 1H), 7.56ꢀ7.34 (m, 5H), 4.45 (s, 3H); 13C NMR (125
MHz, CDCl3): δ (ppm); 159.8, 154.0, 145.1, 142.7, 137.1, 135.3, 126.4, 126.1,
124.5, 123.8. 123.3, 121.7, 120.5, 110.4, 29.7.
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8) 5ꢀ(6ꢀmethoxybenzothiazolꢀ2ꢀyl)thiopheneꢀ2ꢀcarbaldehyde (3h): yellow solid,
yield
92 mg, 92 %; purified by column chromatography using 5% EtOAc in
hexane;1H NMR (400 MHz, CDCl3): δ (ppm) 9.96 (s, 1H), 7.98ꢀ7.95 (m, 1H),
7.77ꢀ7.76 (m, 1H), 7.65ꢀ7.64 (m, 1H), 7.34ꢀ7.33 (m, 1H), 7.14ꢀ7.12 (m, 1H),
3.91 (s, 1H); 13C NMR (125 MHz, CDCl3): δ (ppm) 183.1, 159.0, 157.4, 147.9,
144.8, 137.0, 136.2, 127.6, 126.6, 124.1, 116.4, 104.0, 58.1. HRMS (ESIꢀTOF)
m/z: [M+H]+ Calcd for C13H10NO2S2 276.0153; Found 276.0147.
9) 5ꢀ(benzothiazolꢀ2ꢀyl)thiopheneꢀ2ꢀcarbonitrile (3i):32 yellow solid, yield 67 mg,
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76%; purified by column chromatography using 15% EtOAc in hexane; H NMR
(400 MHz, CDCl3): δ (ppm) 8.07 (d, J = 8.0 Hz, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.64
– 7.63 (m, 1H), 7.59ꢀ7.58 (m, 1H), 7.56ꢀ7.52 (m, 1H), 7.47ꢀ7.43 (m, 1H); 13C NMR
(125 MHz, CDCl3): δ (ppm) 158.6, 153.4, 143.9, 137.8, 135.1, 127.5, 127.0, 126.3,
123.7, 121.7, 113.7, 112.0.
10) 5ꢀ(1ꢀmethylꢀ1Hꢀbenzoimidazolꢀ2ꢀyl)thiopheneꢀ2ꢀcarbonitrile (3j): yellow solid,
yield 73 mg, 84%; purified by column chromatography using 10 % EtOAc in
hexane;1H NMR (400 MHz, CDCl3): δ (ppm) 7.80 (d, J = 8.0 Hz, 1H), 7.64 (d, J =
4.0 Hz, 1H), 7.50 (d, J = 4.0 Hz, 1H), 7.37ꢀ7.32 (m, 4H), 3.99 (s, 3H); 13C NMR
(125 MHz, CDCl3): δ (ppm) 145.2, 142.6, 139.7, 137.7, 136.6, 126.8, 124.1, 123.3,
120.2, 113.6, 117.7, 109.7, 31.7. HRMS (ESIꢀTOF) m/z: [M+H]+ Calcd for
C13H10N3S 240.0595; Found 240.0570.
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11) 2ꢀ(5ꢀethylthiophenꢀ2ꢀyl)benzothiazole (3k):32 yellow oil, yield 53 mg, 59%; purified
by column chromatography using 10% EtOAc in hexane;1H NMR (400 MHz,
CDCl3): δ (ppm) 8.00 (d, J = 8.0 Hz, 1H), 7.82 (d, J = 8.0 Hz, 1H), 7.49–7.45 (m,
2H), 7.36ꢀ7.34 (m, 1H), 6.83ꢀ6.82 (m, 1H), 2.90 (q, J = 8.0 Hz, 2H), 1.36 (t, J = 8.0
Hz, 3H); 13C NMR (125 MHz, CDCl3): δ (ppm) 161.7, 153.7, 152.3, 134.4, 128.7,
126.2, 124.9, 124.6, 122.7, 121.3, 24.1, 16.0.
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12) 2ꢀ(5ꢀmethylthiophenꢀ2ꢀyl)benzothiazole (3l):32 yellow oil, yield 67 mg, 80 %;
purified by column chromatography using 10% EtOAc in hexane;1H NMR (400
MHz, CDCl3): δ (ppm) 7.99 (d, J = 8.0 Hz, 1H), 7.83 (d, J = 8.0 Hz. 1H), 7.47ꢀ7.43
(m, 2H), 7.35ꢀ7.32 (m, 1H), 6.80ꢀ6.79 (m, 1H), 2.56 (s, 3H); 13C NMR (125 MHz,
CDCl3): 161.6, 153.6, 144.8, 134.9, 134.5, 128.8, 126.4, 126.3, 124.9, 122.7, 121.4,
15.7.
13) 2ꢀ(1ꢀmethylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)benzooxazole (3m): brown solid, yield 78
mg, 86%; purified by column chromatography using 10 % EtOAc in hexane;1H
NMR (400 MHz, CDCl3): δ (ppm) 7.94ꢀ7.92 (m, 1H), 7.87ꢀ7.85 (m, 1H), 7.73ꢀ7.70
(m, 1H), 7.51ꢀ7.39 (m, 5H); 13C NMR (125 MHz, CDCl3): 155.2, 150.1, 142.7,
141.5, 139.7, 136.3, 126.6, 125.1, 125.0, 123.6, 121.2, 120.6, 111.3, 110.1, 32.3.
HRMS (ESIꢀTOF) m/z: [M+Na]+ Calcd for C15H11N3ONa 272.0800; Found
272.0769.
14) 6ꢀchloroꢀ2ꢀ(1ꢀmethylꢀ1Hꢀimidazolꢀ2ꢀyl)benzothiazole (3n): brown solid, yield 75
mg, 83%; purified by column chromatography using 10 % EtOAc in hexane;1H
NMR (400 MHz, CDCl3): δ (ppm) 7.92ꢀ7.89 (m, 2H), 7.45ꢀ7.42 (m, 1H), 7.19 (m,
1H), 7.07 (m, 1H), 4.23 (s, 3H); 13C NMR (125 MHz, CDCl3): 160.0, 152.6, 140.2,
136.0, 131.4, 129.9, 127.0, 125.3, 123.9, 121.2, 35.6. HRMS (ESIꢀTOF) m/z: [M +
H]+ Calcd for C11H9ClN3S 250.0206; Found 250.0200.
15) 5ꢀ(6ꢀchlorobenzothiazolꢀ2ꢀyl)furanꢀ2ꢀcarbaldehyde (3o):55 yellow solid, yield 57
mg, 60 %; purified by column chromatography using 10 % EtOAc in hexane;1H
NMR (400 MHz, CDCl3): δ (ppm) 9.80 (s, 1H), 8.01 (d, J = 8.0 Hz, 1H), 7.94ꢀ7.93
(m, 1H), 7.52ꢀ7.49 (m, 1H), 7.40ꢀ7.39 (m, 1H), 7.36ꢀ7.35 (m, 1H); 13C NMR (125
MHz, CDCl3):177.9, 156.4, 153.4, 152.4, 152.3, 136.1, 132.3, 127.9, 124.4, 121.6,
121.5, 112.9. The compound was purified by column chromatography on silica gel
with hexane and ethyl acetate mixture (95:05 v/v) as eluent.
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16) 1ꢀ(5ꢀ(benzothiazolꢀ2ꢀyl)thiophenꢀ2ꢀyl)ethanone (3p):32 yellow solid, yield 74 mg,
79 %; purified by column chromatography using 10% EtOAc in hexane;1H NMR
(400 MHz, CDCl3): δ (ppm) 8.07 (d, J = 8.0 Hz, 1H), 7.89 (d, J = 8.0 Hz, 1H),
7.70ꢀ7.69 (m, 1H),7.60ꢀ7.65 (m. 1H), 7.54ꢀ7.49 (m, 1H), 7.45ꢀ7.40 (m, 1H); 13C
NMR (125 MHz, CDCl3): 190.1, 160.6, 154.3, 146.7, 144.3, 135.5, 132.1, 129.2,
127.2, 125.9, 123.1, 120.9, 27.0.
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17) 1ꢀmethylꢀ2ꢀ(5ꢀmethylthiophenꢀ2ꢀyl)ꢀ1Hꢀbenzoimidazole (3q):56 yellow oil, yield 70
mg, 73 %; purified by column chromatography using 10% EtOAc in hexane;1H
NMR (400 MHz, CDCl3): δ (ppm) 7.78ꢀ7.76 (m, 1H), 7.37ꢀ7.35 (m, 1H), 7.32ꢀ7.31
(m, 1H), 7.27ꢀ7.25 (m, 2H), 6.83 (m, 1H), 3.92 (s, 3H), 2.55 (s, 3H); 13C NMR (125
MHz, CDCl3): 148.0, 143.5, 142.6, 136.4, 129.9, 128.2, 126.1, 122.7, 122.5, 119.5,
109.2, 31.6, 15.3.
18) 2ꢀ(5ꢀethylthiophenꢀ2ꢀyl)ꢀ1ꢀmethylꢀ1Hꢀbenzoimidazole(3r):32 brown oil, yield 60 mg,
68 %, purified by column chromatography using 10% EtOAc in hexane;1H NMR
(400 MHz, CDCl3): δ (ppm) 7.79ꢀ7.77 (m, 1H), 7.39 (d, 3.8 Hz, 1H), 7.35ꢀ7.33 (m,
1H), 7.32ꢀ7.26 (m, 2H), 6.88 (d, 3.1 Hz, 1H), 3.96 (s, 3H), 2.93 (q, 8.0 Hz, 2H),
1.37 (t, 8.0 Hz, 3H); 13C NMR (125 MHz, CDCl3): 151.3, 148.3, 143.0, 137.0,
129.8, 128.1, 124.4, 122.7, 122.5, 119.7, 109.5, 32.0, 23.6, 15.9.
19) 2,2'ꢀbibenzothiazole (4a):21 colourless solid, yield 42 mg, 88%; purified by column
1
chromatography using 5% EtOAc in hexane; H NMR (400 MHz, CDCl3): δ (ppm)
8.17 (d, J = 8.0 Hz, 2H), 8.00 (d, J = 8.0 Hz, 2H), 7.59ꢀ7.48 (m, 4H), 13C NMR
(125 MHz, CDCl3): δ (ppm) 161.6, 153.4, 135.6, 126.9, 126.3, 124.1, 122.0.
20) 2,2'ꢀbibenzoxazole (4b):21 colourless solid, yield 34 mg, 80%; purified by column
chromatography using 5 % EtOAc in hexane;1H NMR (400 MHz, CDCl3): δ (ppm)
7.93 (d, J = 8.0 Hz, 2H), 7. 73 (d, J = 8.0 Hz, 2H), 7.53ꢀ7.46 (m, 4H), 13C NMR
(125 MHz, CDCl3): δ (ppm) 151.9, 151.1, 141.2, 127.6, 125.9, 121.6, 111.5.
21) 2,2'ꢀbithiopheneꢀ5,5'ꢀdicarbaldehyde (4c):16 yellow solid, yield 38 mg, 95 %;
1
purified by column chromatography using 5% EtOAc in hexane; H NMR (400
MHz, CDCl3): δ (ppm) 9.92 (s, 2H), 7.72 (d, J = 4.0 Hz, 2H), 7.43 (d, J = 4.0 Hz,
2H), 13C NMR (125 MHz, CDCl3): δ (ppm) 182.4, 144.9, 144.0, 136.9, 126.5.
22) 1,1'ꢀ(2,2'ꢀbithiopheneꢀ5,5'ꢀdiyl)diethanone (4d):16 yellow solid, yield 41.4 mg, 92
%; purified by column chromatography using 5% EtOAc in hexane; 1H NMR (400
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MHz, CDCl3): δ (ppm) 7.62 (d, J = 4.0 Hz, 2H), 7.30 (d, J = 4.0 Hz, 2H), 2.57 (s,
6H), 13C NMR (125 MHz, CDCl3): δ (ppm) 190.4, 144.3, 144.0, 133.2, 125.9, 26.9.
23) 2,2'ꢀbithiopheneꢀ5,5'ꢀdicarbonitrile (4e):57 yellow solid, yield 23 mg, 93 %;
1
purified by column chromatography using 5% EtOAc in hexane; H NMR (400
9
MHz, CDCl3): δ (ppm) 7.58 (d, J = 4.0 Hz 2H), 7.25 (d, J = 4.0 Hz, 2H), 13C NMR
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(125 MHz, CDCl3): δ (ppm) 141.6, 138.6, 126.0, 113.3, 110.1.
24) 1,1'ꢀdimethylꢀ1H,1'Hꢀ2,2'ꢀbibenzoimidazole (4f): colourless solid, yield 43 mg,
91%; purified by column chromatography using 5% EtOAc in hexane; 21 1H NMR
(400 MHz, CDCl3):
δ (ppm) 7.88 (d, J = 8.0 Hz, 2H), 7.49 (d, J = 8.0 Hz, 2H),
7.41ꢀ7.36 (m, 4H), 4.33 (s, 6H), 13C NMR (125 MHz, CDCl3):
δ (ppm) 143.1,
142.5, 136.2, 124.1, 123.0, 120.3, 110.2, 32.3.
25) 6,6'ꢀdimethylꢀ2,2'ꢀbibenzothiazole (4g):42 colourless solid, yield 42 mg, 80%;
1
purified by column chromatography using 15% EtOAc in hexane; H NMR (400
MHz, CDCl3):
δ (ppm) 8.01 (d, J = 8.0 Hz, 2H), 7.76 (s, 2H), 7.37ꢀ7.34 (m, 2H),
2.53 (s, 3H); 13C NMR (125 MHz, CDCl3):
δ (ppm) 160.7, 151.7, 137.2, 136.0,
128.6, 123.3, 121.3, 22.1.
X-ray crystallographic details:
Suitable single crystal of compound Ia was selected and intensity data were collected on
a SuperNova, Dual, Cu at zero, Eos diffractometer. The crystal was kept at 100.00(10) K
during data collection. Using Olex258 the structure was solved with the Superflip59
structure solution program using Charge Flipping and refined with the ShelXL60
refinement package using Least Squares minimization.
Crystal data for Ia: C91H105N4O4Pd2, Mr = 1531.59, monoclinic space group P21/n, a =
15.9861(5) Å, b = 22.1765(7) Å, c = 22.9883(8) Å, α = 90°, γ = 90°, β = 98.546(3)°, V =
8059.2(5) Å3, Z = 35, calculated density 1.264 g cmꢀ3, ꢁ(Mo Kα) = 0. 0.71073 mmꢀ1, =
100 K, 2θ range for data collection 4.972ꢀ65.63°,
95711 reflections measured, R1 =
0.0891 (I > 2σ(I)), wR2 = 0.2303 (all data). Crystallographic data for the structural
analysis of Ia have been deposited at the Cambridge Crystallographic Data Centre
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(CCDC) as file no: 1838603. This data can be obtained free of charge from the
Cambridge Crystallographic Data Centre.
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The Supporting Information is available free charge on the ACS Publications website, at
DOI:…..
X ray data for compound Ia (CIF)
X ray structure and spectroscopic data for all compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
*Email: swadhin.mandal@iiserkol.ac.in
Author Contribution
‡
These authors contributed equally to this work. The manuscript was written through
contributions of all authors. All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
SKM thanks CSIR, India (Grant No. 01(2779/14/EMRꢀII). SP thanks DSTꢀINSPIRE for
a research fellowship. SCS and PKV thank UGC for the research fellowships. The
authors are thankful to Dr. Debashis Adhikari, IISERꢀMohali, Gonela Vijay Kumar,
IISERꢀKolkata for their help in Xꢀray crystallographic measurements.
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