Stereo- and Regioselective Addition of Arene to Alkyne Using Abnormal NHC Based Palladium Catalysts: Elucidating the Role of Trifluoroacetic Acid in Fujiwara Process

Article abstract of DOI:10.1021/acs.organomet.7b00649

Hydroarylation of alkyne was reported by Fujiwara nearly two decades back. Interestingly, this reaction does not proceed in the absence of trifluoroacetic acid; however, the exact role of TFA has not been unambiguously established, in particular with the

Full text of DOI:10.1021/acs.organomet.7b00649

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX-XXX
Stereo- and Regioselective Addition of Arene to Alkyne Using
Abnormal NHC Based Palladium Catalysts: Elucidating the Role of
Trifluoroacetic Acid in Fujiwara Process
Pradip Kumar Hota, Anex Jose, and Swadhin K. Mandal*
Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur 741246, Nadia, India
S
* Supporting Information
ABSTRACT: Hydroarylation of alkyne was reported by Fujiwara
nearly two decades back. Interestingly, this reaction does not proceed
in the absence of trifluoroacetic acid; however, the exact role of TFA
has not been unambiguously established, in particular with the
support of X-ray crystallography by isolating the TFA-involved
catalytically active species. In this work, abnormal N-heterocyclic
carbene (aNHC) based Pd catalysts have been used for the efficient
hydroarylation of aromatic C−H bonds leading to new C−C bond
formation through regio- and stereoselective addition to alkynes. The
addition reaction has been realized by a catalytic amount of Pd (II)
compound (0.5 mol %) in trifluoroacetic acid (TFA) under ambient
conditions. Various arenes undergo transhydroarylation selectively
across the triple bond (containing functional groups CO2Me, CO2Et,
and CO2H), affording the kinetically controlled cis adduct
predominantly in good yields. A simple reaction condition through an intermolecular reaction has been outlined under
ambient conditions for the synthesis of coumarin derivatives, which are considered as an important class of bioactive compounds.
It was noted that the reaction does not proceed in the absence of TFA. Hence, the major emphasis was given to understand the
role of TFA in such hydroarylation reactions. A catalytically active reaction intermediate, [aNHCPd(CF3COO)]2, containing
trifluoroacetate anion was characterized as the first solid-state evidence by single-crystal X-ray crystallography, which helps to
understand the exact role of TFA in such a reaction. A detailed mechanistic understanding of this fascinating catalytic process has
been proposed by tandem experimental and computational experiments.
ion in situ via weakly coordinating trifluoroacetate anion,
INTRODUCTION
■
CF3CO2 (Scheme 1).1,2 According to the Fujiwara process,
−
Catalytic activation of aromatic C−H bonds leading to the
formation of new C−C bonds is of paramount interest for the
pharmaceutical industry. The direct C−H bond functionaliza-
tion method has several benefits in comparison to conventional
synthetic routes. This protocol does not require prefunction-
alization (such as halogenations, esterification, etc.) since it uses
only C−H bonds as the reaction center for the organic
transformation. Thus, the protocol not only cuts down the
reaction steps but also makes the whole process more cost
efficient. In particular, a hydroarylation reaction is the addition
of an electron-rich arene and a hydrogen atom across a C−C
multiple bond resulting in the formation of a vinylarene. In
2000, Fujiwara and co-workers in a series of publications1−3
had reported effective hydroarylation of activated alkynes using
palladium acetate ([Pd(OAc)2]) as a catalyst in trifluoroacetic
acid (TFA) medium. This catalytic protocol has been
successfully applied to the hydroarylation of arene to alkynes
in which addition of the C−H bond proceeds exclusively in a
trans manner. Later on, Nolan4 and others5 described the
transhydroarylation of alkynes using Pd catalysts with TFA as
the solvent. The key step of the proposed mechanistic pathway
(Fujiwara process) is the generation of an electrophilic metal
Scheme 1. Mechanistic Cycle on Addition Reaction
Proposed by Fujiwara1,2
the first step of the catalytic process involves an exchange
−
reaction between the acetate anion (CH3CO2 ) of Pd(OAc)2
−
with trifluoroacetate anion (CF3CO2 ). This is the catalyst
activation step generating the labile complex Pd(TFA)2, as
−
CF3CO2 is a weakly coordinating ion. The next step consists
of an equilibrium in the presence of excess TFA to form the
electrophilic [Pd(TFA)]+ by loss of an CF3CO2 ion. The
−
Received: August 26, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00649
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
formation of such cationic species encourages the electrophilic
attack on the aromatic C−H bond. This cationic palladium
complex activates the aromatic C−H bond and forms a σ-aryl−
Pd complex. The next steps are alkyne coordination and
insertion into the Pd−Ar bond followed by elimination of the
hydroarylation product. It has been reported that without TFA
the reaction does not proceed and thus it plays a major role in
such hydroarylation catalysis. Despite this mechanistic proposal
more than one and half decades back, there is no solid evidence
to support this proposed role of TFA during this important
catalytic process.
In this regard, we have utilized abnormal N-heterocyclic
carbene (aNHC) based ligands to prepare well-defined
palladium complexes (1 and 2). Further, the catalytic
hydroarylation process using these complexes was investigated,
with particular emphasis on elucidation of the role of TFA in
the catalytic process. It may be noted that aNHCs have been
demonstrated as excellent building blocks in the design of
numerous organometallic catalysts.6 Recently, we have reported
the isolated aNHC derived from 1,3-bis(2,6-diisopropylphen-
yl)-2,4-diphenylimidazolium salt7 as an impressive ligand for
convenient metal-free catalysts8 and organometallic cata-
lysts9−11 for a variety of organic conversions. For instance, we
recently reported the synthesis of aNHC-supported halo-
bridged C−H bond activated palladium complexes for Suzuki−
Miyaura cross-coupling9 of unactivated aryl chloride substrates
at room temperature. More recently, we have shown that the
acetate-bridged binuclear palladium complex is an effective
catalyst for oxidative Heck coupling in water at room
temperature.11 These results precisely showcased the utility of
aNHC in designing palladium-based catalysts for C−C bond
forming reactions. In the present study, we have used the two
palladium complexes 1 and 2 (Figure 1) for addition reactions.
RESULTS AND DISCUSSION
■
In the present work, 1,3-bis(2,6-dimethylphenyl)-2,4-dipheny-
limidazolium salt (L1HCl·Cl, Scheme 2) with two symmetrical
substituents on two nitrogen atoms was synthesized by
adopting a simplified synthetic method. The aNHC salt
L1HCl·Cl was prepared by the reaction of L1B and acetic
anhydride in HCl medium at room temperature for 12 h,
resulting in 76% yield.
L1B was prepared by the reaction between L1A12 and 2-
bromoacetophenone in 2-propanol at 120 °C following an
earlier method.11 L1HCl·Cl was characterized by spectroscopy
(mass and NMR spectrometry) and elemental analysis. The
imidazolium proton (C-5 proton) in L1HCl·Cl appeared as a
1
singlet at δ 8.87 ppm in H NMR spectrum measured using
CDCl3.8a The palladium complex 2 was synthesized by the
reaction between palladium acetate [Pd(OAc)2] and L1HCl·Cl
in 1,4-dioxane at 80 °C for 10 h, resulting in the formation of
the pale yellow chloro-bridged dinuclear palladium(II) complex
2 (68% yield, Scheme 2). Complex 2 was characterized by
NMR spectroscopy (1H and 13C), elemental analysis, and X-ray
crystallography. The C-5 resonance upon Pd(II) coordination
resonates at δ 155.4 ppm, as observed in the 13C NMR
spectrum of 2. This observation also supports our previous
report.9 The C-5 nucleus of previously reported halo-bridged
palladium(II) dimer resonates at δ 155.7 ppm.9 In the 1H NMR
spectrum, the doublet at δ 5.65 ppm corresponds to the aryl
proton close to the metal center (adjacent to the cyclo-
palladated carbon) and such a shielded chemical shift value for
an aryl proton may be attributed to the well-known agostic
interaction. This distinct proton with its unusual upfield shift is
a signature for ortho metalation, a phenomenon that was
previously reported in similar ortho-metalated Pd dimers of
abnormal NHC complexes.9,10d,11 Crystals suitable for an X-ray
diffraction study were grown from a hexanes/CH2Cl2 mixture
at room temperature. The ORTEP diagram of 2 is presented in
Figure 2, which confirms the formation of a chloro-bridged
dinuclear palladium complex. The solid-state structure of 2
established the abnormal mode of binding of the carbene.
Interestingly, the structure of 2 reveals that one of the ortho
C−H bonds of the phenyl group attached at C-4 of the
imidazole has undergone activation via an ortho-metalation
process, unlike the case observed for 1, where the ortho C−H
bond of phenyl attached to one of the imidazole nitrogen atoms
undergoes the ortho metalation. This difference may be
attributed to the fact that, in 2, the aryl ortho C−H bond is
only available with the phenyl, which is attached to C4. This
type of C−H bond activation during coordination to
palladium(II) with an NHC ligand is reminiscent of our earlier
observation.9 The geometry around the palladium center is
distorted square planar, where each palladium is bonded to the
C-5 center of the carbene, the C−H activated ortho aryl
carbon, and two chloride ions. The Pd−C(carbene) bond
distances were determined to be 1.969 Å (Pd1A−C5A) and
1.986 Å (Pd1B−C5B), which are comparable to those of our
previously reported halo-bridged aryl C−H bond activated
palladium dimers.9 The Pd−C(aryl) bond distances were
measured as 2.030 Å (Pd1A−C7A) and 2.004 Å (Pd1B−C7B),
respectively, indicating a shorter Pd−C (carbene) bond
distance in comparison to the Pd−C (ortho-metalated) bond
distance. Complexes 1 and 2 were subsequently tested as
catalysts for hydroarylation reactions.
Figure 1. C−H activated dimeric palladium complexes (1 and 2) used
in hydroarylation catalysis.
In this work, we utilized two bridged palladium complexes in
which one is an acetate-bridged binuclear complex (1) and the
other is a chloro-bridged binuclear complex (2). Catalyst 1 was
reported earlier,11 and 2 is a newly synthesized complex. The
catalytic activity of these dimers in the hydroarylation of
alkynes with arene is presented.
During the course of this study, it was found that the
dinuclear palladium complexes catalyze the stereoselective
addition of aromatic C−H bonds to unactivated alkynes at
room temperature in the presence of TFA. We further have
arrested the catalyst’s active state by reacting the starting dimers
(1 and 2) with TFA in stoichiometric fashion, and we
established the proposed Fujiwara type intermediate with the
help of molecular structures determined by single-crystal X-ray
diffraction. These two trifluoroacetate-bridged dinuclear
palladium complexes were further tested for aromatic C−H
bond activation over alkyne at room temperature, confirming
that they are active catalysts.
B
DOI: 10.1021/acs.organomet.7b00649
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 2. Synthesis of aNHC Ligand Salt and Its Palladium Complex 2
only resulted in low yields (see entries 7 and 8 in Table 1). The
hydroarylation reaction did not proceed at all in the absence of
either catalyst or TFA (see entries 9 and 10 in Table 1). Next,
the scope and generality of this reaction has been explored by
the reaction of various commercially available alkynes with
several arenes (Scheme 3 and Table 2). Here we focused on a
variety of alkynes bearing functional groups such as CO2Me,
CO2Et, and CO2H (eq 1 in Scheme 3 and entries 1−8 in Table
2). The reaction affords thermodynamically unfavorable cis-
arylalkenes predominantly in most cases (eq 1 in Scheme 3 and
entries 1−8 in Table 2), with good yields. The addition of the
arenes to methyl propiolate gave the cis product, (Z)-methyl 3-
arylacrylate (3a,c,e in Table 2) predominantly. The trans
isomer was also obtained in a minor amount (4a,c in Table 2).
Pentamethylbenzene, 1,2,4,5-tetramethylbenzene, and mesity-
lene with ethyl propiolate gave satisfactory results toward the
addition reaction (entries 2, 4, and 6 in Table 2). The reaction
was tried with a substrate having an acid functionality (such as
propiolic acid) as an alkyne partner and gave exclusively
thermodynamically unfavorable cis adduct in good yield (82%,
entry 7 in Table 2).
Figure 2. ORTEP diagram of 2 with 50% probability ellipsoids.
Hydrogen atoms have been omitted for the sake of clarity. Selected
bond lengths (Å) and bond angles (deg): Pd(1A)−C(5A), 1.969(9);
Pd(1A)−C(7A), 2.030(8); Pd(1A)−Cl(1A), 2.402(2); Pd(1B)−
C(5B), 1.986(9); Pd(1B)−C(7B), 2.004(8); C(5A)−Pd(1A)−C(7A),
79.2(4); C(5A)−Pd(1A)−Cl(1B), 100.6(2); C(7A)−Pd(1A)−Cl-
(1A), 95.9(3); C(7A)−Pd(1A)−Cl(1B), 178.2(2); Cl(1B)−Pd-
(1A)−Cl(1A), 84.28(8).
The reaction was also tested with internal alkyne, delivering
very good yield (eq 2 in Scheme 3; 85%, entry 8 in Table 2).
Despite the literature reports on hydroarylation of unactivated
alkynes to arenes, the reaction suffers from a number of
limitations such as high temperature13 and higher mole percent
loading of catalyst.14 Here we have shown that our synthesized
dinuclear palladium complex 2 results in the stereoselective
addition products (eq 3 in Scheme 3 and entries 9−16 in Table
2) at room temperature and lower mole percent loading (0.5
mol %). Further, this protocol was adopted for the synthesis of
coumarin derivatives. Coumarins are important structural
motifs in natural products and bioactive compounds (antico-
agulants, antifungal agents, and antioxidants or as anthelmintic,
hypnotic, and cytotoxic agents).15 Due to their fluorescent
nature, coumarins are also broadly used as agrochemicals,
optical brighteners, and additives in cosmetics.16 The traditional
methods for syntheses of coumarins, such as the Perkin, von
Pechmann, or Knoevenagel reaction, are usually conducted
under harsh reaction conditions.15a In addition to these
methods, coumarins can be synthesized via a hydroarylation
At first, the catalytic reactions were optimized to find out the
most efficient conditions for performing this fascinating
reaction (Table 1). We have considered an equivalent amount
of pentamethylbenzene and methyl propiolate as the substrates
for an optimization study. The Pd(II) catalysts were screened
for optimization of the hydroarylation reaction (see entries 1−4
in Table 1). Initially, we observed that the use of simple PdCl2
and Pd(OAc)2 in TFA afforded the addition products 3a/4a in
the % yield ratio of 21/5 and 52/12, respectively (see entries 1
and 2 in Table 1). The reaction was greatly enhanced by the
simple introduction of an aNHC backbone to the Pd center.
The acetate-bridged Pd dimer 1 and chloro-bridged Pd dimer 2
displayed significantly enhanced activity toward addition
reactions (the % yield ratio of 81/8 and 83/9 for 3a/4a, see
entries 3 and 4 in Table 1) in TFA as a solvent. However, these
two catalysts (1 and 2) did not produce any desired product in
the absence of TFA: for example, in AcOH medium (see
entries 5 and 6 in Table 1), the reaction did not proceed. Any
attempt to reduce the amount of catalyst used in the reaction
C
DOI: 10.1021/acs.organomet.7b00649
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 1. Pd(II)-Catalyzed Addition of Pentamethylbenzene to Methyl Propiolate
b
entry
Pd(II) (mol %)
acid (mL)
time (h)
yield (%) 3a/4a
1
PdCl2 (0.5)
Pd(OAc)2 (0.5)
1 (0.5)
TFA
TFA
TFA
TFA
AcOH
AcOH
TFA
TFA
none
TFA
12
3
21/5
52/12
81/8
83/9
0
2
3
4
5
3
2 (0.5)
3
1 (0.5)
12
12
12
24
12
24
6
2 (0.5)
0
7
2 (0.25)
2 (0.1)
48/8
32/13
0
8
9
2 (0.5)
10
none
0
a
b
All reactions were performed with pentamethylbenzene (1 mmol), methyl propiolate (1 mmol), and TFA (1 mL) at room temperature. Isolated
yields.
Scheme 3. Hydroarylation of Alkynes Using Catalytic
Amount of 2
change to acetic acid instead of TFA did not allow the reaction
to proceed, which confirms that CF3COO− plays a key role
during the catalytic cycle. To gain information on the exact role
of TFA during the catalytic process, stoichiometric reactions
were carried out between TFA and 1 or 2 (Figure 3a). The
acetate-bridged dimer 1 or chloro-bridged dimer 2 were treated
with TFA at room temperature under a nitrogen atmosphere.
For 1, the methyl signal of the acetate bridge disappeared, as
observed from 1H and 13C NMR spectroscopy. This
observation clearly supports the absence of any acetate bridge
in the resulting product and formation of the CF3COO−-
coordinated dimer 1a. The halo-bridged dimer 2 also resulted
in the formation of a similar trifluoroacetate-bridged dinuclear
palladium complex, 2a. These trifluoroacetate-bridged dimers
(1a and 2a) were thoroughly characterized by NMR (1H, 13C,
and 19F NMR) (see the Supporting Information) and IR
spectroscopy. To our delight, we have been successful in
growing suitable yellow crystals of 1a and 2a and the solid-state
structures were established unambiguously by single−crystal X-
ray studies. The single-crystal X-ray analysis of 1a and 2a
confirmed incorporation of trifluoroacetate groups by replace-
ment of the acetate bridge and chloro bridge, respectively. The
Pd centers in 1a and 2a both adopt a distorted-square-planar
geometry. The Pd1A−O1A bond distance in 1a was measured
as 2.162 Å, which is longer than the Pd−O bond distance in
catalyst 1 (2.143 Å). The Pd−C(carbene) bond distance in 1a
was determined as 1.952 Å (Pd1A−C5A), and in 2a, it is 1.958
Å (Pd1−C5). The C−F bond distance in 1a was found as 1.321
Å (C26A−F1A distance), whereas in 2a, it was measured as
1.343 Å (C35−F2 distance). To further establish 1a and 2a as
active catalysts, they were used as catalysts with arene and
alkyne in the presence of TFA, which afforded the
corresponding desired product in good yield (Table 4). The
catalytic activity of 1a and 2a in the hydroarylation reaction was
tested by using them as catalysts with 0.5 mol % loading. Both
1a and 2a resulted in good to excellent yields of the desired
products (Table 4). This result supports that these two TFA-
bridged dimers are active catalysts for this addition reaction.
However, the addition reaction did not proceed at all in the
absence of TFA when these trifluoroacetate-bridged Pd(II)
dimers (1a and 2a) were also used. Moreover, we performed
the hydroarylation reaction, taking pentamethylbenzene and
reaction, which may be intramolecular1,3 or intermolecular.17
The present literature reports on coumarin synthesis (by
intermolecular hydroarylation) suffer from a number of
limitations such as high temperature (120 °C),17d longer
reaction time (40 h),1 and use of a base.17c Here we report the
synthesis of various coumarin derivatives in a simpler way
(Table 3) at ambient temperature under base-free conditions
within a short reaction time with low catalyst loading. The
scope of this reaction has been explored through a reaction
between commercially available alkynes and several phenols
(Table 3). The products were obtained by regioselective
addition of arene to alkyne with good yields at room
temperature. The reaction was tested with several alkynes
such as propiolic acid (entries 1−5 in Table 3), methyl
propiolate (entry 6 in Table 3) and ethyl propiolate (entry 7 in
Table 3). Electron-rich phenols (entries 1−4, 6, and 7 in Table
3) and those with one additional −OH functional group (entry
5 in Table 3) were also used successfully for this reaction. For a
substrate containing an additional −OH functional group, a
mixture of products (5e and 5e′) was obtained. 5a was
obtained when methyl or ethyl propiolate was used instead of
propiolic acid (entries 6 and 7, Table 3). A similar observation
was reported by Trost’s group17a−c using an acid (HCOOH)
and catalytic amount of base (NaOAc). However, our protocol
is completely base free.
It was noted in the present optimization process (see Table
1), and was also observed in previous study,1 that the addition
of TFA is essential for this catalytic reaction to proceed. Even a
D
DOI: 10.1021/acs.organomet.7b00649
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 2. Catalytic Addition of Arenes to Alkynes Using 2
Table 3. Catalytic Reactions Leading to the Synthesis of
a
Coumarin Derivatives
a
All reactions were conducted with arene (1 mmol), alkyne (1 mmol),
b
2 (0.5 mol %), and TFA (1 mL) at room temperature. Isolated yields
are given in parentheses.
experiments and the results were substantiated with the help of
a detailed DFT calculation. When excess TFA was added to 1a
in CDCl3 under an argon atmosphere, there was a sharp change
1
1
in the H NMR spectrum. The H NMR spectrum of 1a (see
Figure S55 in the Supporting Information) on treatment with
excess TFA shows that the shielded signal around δ 5.92 ppm
vanished (see Figure S57 in the Supporting Information),
which indicates cleavage of the Pd−C (ortho-metalated) bond.
1
It may be recapitulated that this upfield peak in H NMR
spectrum is considered as a signature peak9,11 for ortho
metalation. Thus, it may be concluded that, after addition of an
excess amount of TFA to 1a, a new species is generated in the
reaction mixture, which has been assigned as the cationic
mononuclear complex 2c, [aNHCPd(solv)TFA]+ (see DFT
results below), similar to cationic monomeric Pd complexes
proposed by Fujiwara and co-workers.1,2 It has been noted in
an earlier study that the presence of a large excess of TFA is
necessary to keep this cationic Pd(II) species stable.2 It may be
recalled that the reaction does not proceed in acetic acid when
dimer 1 or 2 was used as catalyst. This fact supports that it is
essential to create a labile TFA-coordinated Pd complex 1a or
a
All reactions were conducted with arene (1 mmol), alkyne (1 mmol),
b
2 (0.5 mol %), and TFA (1 mL) at room temperature. Isolated yields
are given in parentheses.
methyl propiolate as substrates with 1a or 2a as catalyst in
AcOH medium, but we did not observe formation of the
desired product. This observation clearly shows that excess
TFA is needed for the reaction. Further, to gather more
information about the catalytic pathway we performed NMR
E
DOI: 10.1021/acs.organomet.7b00649
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 3. (a) Isolation of trifluoroacetate-bridged dinuclear palladium complexes by stoichiometric reaction of 1 or 2 with TFA. (b) Perspective
ORTEP views of the molecular structures of 1a and 2a. Thermal ellipsoids are drawn with 50% probability. Hydrogen atoms have been omitted for
the sake of clarity.
a
Table 4. Hydroarylation with Trifluoroacetate-Bridged Dinuclear Palladium Complexes 1a and 2a
a
All reactions were conducted with arene (1 mmol), alkyne (1 mmol), catalyst 1a or 2a (0.5 mol %), and TFA (1 mL) at room temperature.
Isolated yields are given in parentheses.
b
2a, which can lose one trifluoroacetate anion to generate a
highly reactive cationic monomeric intermediate responsible for
the catalytic reaction.1,2 Earlier studies have also considered
dissociation of palladium dimer to monomer in solution.18 In
addition, we performed a Hg-drop test, which discarded the
possibility of heterogeneous Pd nanoparticle based catalysis. On
the basis of all these observations, a catalytic cycle for
hydroarylation of an alkyne with 2a as a catalyst has been
proposed in Scheme 4 and DFT calculations were performed to
support the feasibility of the proposed steps in this mechanistic
F
DOI: 10.1021/acs.organomet.7b00649
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
via TS2 (Figure 4), which is the first step in the catalytic cycle.
Next, the alkyne coordinates in an η2 fashion to palladium ion,
forming 2e. A trans insertion of C−C triple bonds into the σ-
aryl−Pd bond19 results in the formation of 2f via TS4 (Figure
4), which can release the desired product upon protonation of
2f by TFA via TS5 (Figure 4) and regenerate the catalytically
active intermediate 2c. Despite our best efforts, we could not
locate the transition state for this step. Figure 4 shows the free
energy landscape for the hydroarylation reaction starting from
compound 2b. To reduce the computational cost, we have
modified the substitutions on the imidazole ring of 2b. Benzene
and methyl propiolate were considered as the model arene and
alkyne for the calculations. Protonation of 2b in excess
trifluoroacetic acid leads to the formation of cationic palladium
complex 2c via TS1. This process was found to be relatively
energy demanding with an activation barrier of 20.95 kcal/mol
in comparison to other steps in the catalytic cycle. Formation of
compound 2d by the electrophilic attack on the aromatic C−H
bond by 2c proceeds via TS2 with an activation barrier of 2.8
kcal/mol, and this step is exergonic with ΔG = −46.7 kcal/mol.
Coordination of alkyne to compound 2d yields compound 2e
via TS3. The activation barrier for this process was found to be
very low (0.1 kcal/mol), and the step is exergonic (ΔG = −1.6
kcal/mol). The palladium and alkynic carbon bond lengths in
TS3 were found to be 2.31 and 2.23 Å, respectively. The C−C
bond length of alkyne increased by 0.03 Å after coordination
with the palladium center in comparison to free alkyne
molecule. In the next step, arene undergoes insertion to alkyne
to form 2f via TS4 exergonically (activation barrier 3.6 kcal/
mol, ΔG = −43.7 kcal/mol).
Scheme 4. Proposed Mechanism for Addition of Arenes to
Alkynes
CONCLUSIONS
■
In conclusion, we have shown a stereo- and regioselective
addition of simple arenes to the activated and unactivated
alkyne with isolated Pd(II) trifluoroacetate intermediate at
room temperature. The addition reaction covers a variety of
alkynes bearing functional groups such as CO2Me, CO2Et, and
CO2H. This reaction affords trans addition of aryl and
hydrogen across C−C triple bonds, giving predominantly the
thermodynamically unfavorable cis adduct. Here we have
developed a simple method for intermolecular coumarin
synthesis. This method is also active for coumarin synthesis
using methyl propiolate and ethyl propiolate as alkyne partners
with selective aromatic C−H bond cleavage at room temper-
ature. To elucidate the exact role of TFA in the hydroarylation
reaction, we have isolated the TFA-bridged active catalysts 1a
and 2a from precatalysts 1 and 2. 1a and 2a carried out the
addition reaction efficiently under ambient conditions. A series
of stoichiometric reactions, isolation of TFA-bridged complexes
and their structural characterization, along with detailed DFT
calculations enabled us to delineate the mechanistic cycle for
this fascinating addition reaction.
cycle. At first, 1 or 2 converts into compound 1a or 2a in the
presence of TFA, which was crystallographically characterized.
In the presence of excess TFA present in the medium, 2a is
converted into the mononuclear Pd complex 2c, which may be
EXPERIMENTAL SECTION
1
■
proposed on the basis of the observation from the H NMR
Experimental Materials and Instrumentation. Unless stated
otherwise, all reactions were performed in oven-dried glassware under
an oxygen-free atmosphere (nitrogen) using standard Schlenk
techniques or inside a MBraun glovebox maintained below 0.1 ppm
of O2 and H2O level. THF, toluene, pentane, hexane, and benzene
were dried over a sodium/benzophenone mixture prior to use.
CH3CN, DMSO, and CH2Cl2 were dried over CaH2 and distilled
before use. All chemicals were purchased from Sigma-Aldrich, Merck,
or Spectrochem and used as received. Thin-layer chromatography
spectrum as discussed above. The formation of 2c from 2a may
be considered by assuming intermediacy of the monomeric
complex 2b, which is a square-planar cyclometalated palladium
complex with bidentate triflate anion and abnormal carbene
ligand. The presence of excess trifluoroacetic acid leads to the
opening of the palladacycle ring in 2b via TS1 (Figure 4).
Subsequently, an electrophilic attack of the aromatic C−H
bond by compound 2c can result in the σ-aryl-Pd complex 2d
G
DOI: 10.1021/acs.organomet.7b00649
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 4. (a) Gibbs free energy profile for hydroarylation reaction with 2c. The relative Gibbs free energy was calculated with respect to separate
reactants 2c, i, ii, and iv. (b) Optimized structures of transition states in hydroarylation reaction. Important bond distances (in Å) are shown. Only
relevant H atoms are shown.
(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 (100−200 mesh). 1H, 13C, and 19F NMR spectra were
recorded on a JEOL ECS 400 MHz spectrometer and on a Bruker
Avance III 500 MHz spectrometer. All chemical shifts are reported in
ppm using tetramethylsilane as a reference. Chemical shifts (δ)
downfield from the reference standard were assigned positive values.
The ESI-MS data were obtained using a Finnigan MAT 8230
instrument. Elemental analyses were carried out using a PerkinElmer
series 2 2400 CHN analyzer, and samples were prepared by keeping
under reduced pressure (10−2 mbar) overnight. Melting points were
mmol, which was prepared according to literature procedure),12 2-
bromoacetophenone (10 mmol), KHCO3 (15 mmol), and 2-propanol
were refluxed at 120 °C for 12 h. Then the aqueous layer was extracted
with CH2Cl2 and the organic extracts were dried over Na2SO4 to give
the product L1B. The product L1B (10 mmol), acetic anhydride (30
mL), and HCl (10 mL) were mixed under ice-cold conditions, and the
reaction mixture was stirred for 12 h at room temperature to give the
1
desired salt, L1HCl·Cl: yield 76%; mp 113−115 °C. H NMR (400
MHz, CDCl3, 25 °C, TMS): δ 8.87 (s, 1H), 7.46−7.35 (m, 8H),
7.28−7.18 (m, 6H), 6.94 (d, J = 8.4 Hz, 2H), 2.24 (s, 6H), 2.04 (s,
6H). 13C NMR (100 MHz, CDCl3, 25 °C, TMS): δ 143.9, 135.9,
134.8, 134.1, 132.9, 132.5, 131.6, 131.3, 130.9, 130.6, 129.7, 129.5,
129.3, 129.2, 128.5, 127.9, 124.1, 122.4, 120.3, 17.9, 17.8. ESI-MS: m/z
(%) found 429.2311 (calculated for C31H30Cl2N2 429.2331 [M]+).
Anal. Calcd for C31H30Cl2N2: C, 74.25; H, 6.03; N, 5.59. Found: C,
74.21; H, 6.05; N, 5.54.
measured in a sealed glass tube on a Buchi B-540 melting point
̈
apparatus and are uncorrected.
Synthesis and Characterization Data of 1,3-Bis(2,6-dime-
thylphenyl)-2,4-diphenylimidazolium Salt. We have synthesized
the title compound following Bertrand’s method, and its formulation
was made following the earlier report.7 The solid product L1A (10
H
DOI: 10.1021/acs.organomet.7b00649
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Synthesis and Characterization Data of 2. Under a nitrogen
atmosphere, a 50 mL Schlenk flask was charged with Pd(OAc)2 (1
mmol) and L1HCl·Cl (1 mmol) in 1,4-dioxane (15 mL). The reaction
mixture was heated to 80 °C for 10 h. After the mixture was cooled to
room temperature, all volatiles were removed under reduced pressure
from the reaction mixture. Complex 2 was obtained by recrystallization
from a hexanes/CH2Cl2 mixture at room temperature as pale yellow
crystals: yield 68%; mp 295−298 °C. 1H NMR (400 MHz, CDCl3, 25
°C, TMS): δ 7.33−7.29 (m, 2H), 7.24−7.15 (m, 4H), 7.08 (t, J = 6.9
Hz, 4H), 6.90 (d, J = 8.4 Hz, 2H), 6.67 (t, J = 6.8 Hz, 1H), 6.57 (t, J =
7.6 Hz, 1H), 5.65 (d, J = 6.8 Hz, 1H), 2.22 (s, 6H), 2.08 (s, 6H). 13C
NMR (125 MHz, CDCl3, 25 °C, TMS): δ 155.4, 151.2, 142.8, 139.9,
137.3, 136.3, 135.5, 135.4, 135.2, 133.4, 130.1, 129.9, 129.1, 128.9,
128.6, 128.4, 128.3, 124.5, 123.8, 123.2, 117.3, 18.6, 17.9. ESI-MS: m/z
(%) found 1138.1813 (calculated for C62H54Cl2N4Pd2 1138.1799
[M]+). Anal. Calcd for C62H54Cl2N4Pd2: C, 65.39; H, 4.78; N, 4.92.
Found: C, 65.33; H, 4.76; N, 4.96.
Crystal data for 2a: C68H56F6N4O4Pd2Cl6, Mr = 1532.67,
orthorhombic, space group Pbcn, a = 22.1715(12) Å, b =
10.4970(6) Å, c = 28.4525(15) Å, α = β = γ = 90.00°, V =
6621.9(6)Å3, Z = 4, calculated density 1.537 g cm−3, μ(Mo Kα) =
0.853 mm−1, T = 99 K, θ range for data collection 1.70−27.51°, 5977
reflections measured, R1 = 0.0509 (I > 2σ(I)), wR2 = 0.1381 (all
data).
CCDC 1548600 (2), CCDC 1548601 (1a), and CCDC 1548602
(2a) contain supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
General Procedure for Catalytic Hydroarylation Reaction.
To a cold mixture of the arene (1 mmol), 1 or 2 (0.5 mol %), TFA (1
mL), and alkyne (1 mmol) were added with stirring on an ice−water
bath. After continuous stirring at the same temperature for 5 min, the
mixture was warmed to room temperature. Stirring was continued
until the disappearance of one starting material as monitored by TLC
Synthesis of 1a. Under a nitrogen atmosphere, a screw-cap vial
was charged with 1 (0.25 mmol) and TFA (0.25 mmol) in CDCl3 (1
mL). The reaction mixture was kept at room temperature for 12 h.
The solution became transparent within 15 min. X-ray-quality crystals
were grown from CDCl3 in an NMR tube inside the glovebox at room
temperature. The resulting solid was washed with hexane (3 × 1 mL)
and dried under reduced pressure to afford 1a as pale yellow crystals:
yield 80%; mp 293−295 °C. ESI-MS: m/z (%) found 1056.0631
1
or H NMR spectroscopy. The reaction mixture was poured into a
saturated NaCl aqueous solution and extracted with ether. The
ethereal layer was neutralized with Na2CO3 solution and dried over
anhydrous Na2SO4. The solvent was removed under vacuum, and the
products were separated by column chromatography.
Procedure for Hg-Drop Test. Hg (1.5 mmol) was added to an
ice-cold mixture of pentamethylbenzene (1 mmol), 1 or 2 (0.5 mol
%), TFA (1 mL), and methyl propiolate (1 mmol). After continuous
stirring at the same temperature for 5 min, the mixture was warmed to
room temperature. Stirring was continued for 3 h. The reaction
mixture was poured into a saturated NaCl aqueous solution and
extracted with ether. The ethereal layer was neutralized with Na2CO3
solution and dried over anhydrous Na2SO4. The solvent was removed
under reduced pressure, and the product was separated by column
chromatography.
1
(calculated for C48H34F6N4O4Pd2, 1056.0554 [M]+). H NMR (400
MHz, CDCl3, 25 °C, TMS): δ 7.72 (t, J = 7.6 Hz, 1H), 7.63 (t, J = 6.9
Hz, 1H), 7.52−7.34 (m, 7H), 7.26−7.24 (m, 1H), 7.02 (d, J = 7.6 Hz,
1H), 6.86 (t, J = 7.6 Hz, 1H), 6.68 (t, J = 7.6 Hz, 1H), 5.92 (d, J = 7.6
Hz, 1H), 3.02 (s, 3H). 19F NMR (470 MHz, CDCl3, 25 °C, TMS): δ
−74.18 ppm. IR (KBr, cm−1): 3426, 3054, 1672, 1493, 1201. Anal.
Calcd for C48H34F6N4O4Pd2: C, 54.51; H, 3.24; N, 5.30. Found: C,
54.59; H, 3.28; N, 5.27.
Computational Details. All of the theoretical calculations to
interpret experimental observations were performed using the
Gaussian09 quantum chemistry package.23 All of the density functional
theory (DFT) calculations were performed at the B3LYP level of
theory.24 We used the LANL2DZ25 basis set with the relativistic
effective core potential for the palladium atom and 6-31+g(d) basis for
other elements (H, C, O, and F). The geometries were optimized
without any symmetry constraints. Frequency calculations were
performed to confirm the absence of any imaginary frequency for
minimum energy structures. All transition states were located by
keeping the reaction coordinate fixed at a particular distance while all
other degrees of freedom were relaxed. Further, the default Berny
algorithm in Gaussian09 was used to optimize the transition state.
Transition states were confirmed by the presence of a unique
imaginary frequency corresponding to breaking or forming of bonds
involved in the step. The zero-point vibrational corrections were
determined from the frequency calculations.
Synthesis of 2a. Under a nitrogen atmosphere, a screw-cap vial
was charged with 2 (0.25 mmol) and TFA (0.25 mmol) in CDCl3 (1
mL). The reaction mixture was kept at room temperature for 12 h.
The solution became transparent within 15 min. X-ray-quality crystals
were grown from CDCl3 in an NMR tube inside the glovebox at room
temperature. The resulting solid was washed with hexane (3 × 1 mL)
and dried under reduced pressure to afford 2a as pale yellow crystals:
yield 71%; mp 200−203 °C. NMR and mass spectra could not be
recorded, as this compound is not soluble in any common solvent. IR
(KBr, cm−1): 3426, 1703, 1477, 1199. Anal. Calcd for
C66H54F6N4O4Pd2: C, 61.26; H, 4.21; N, 4.33. Found: C, 61.39; H,
4.27; N, 4.34.
X-ray Crystallographic Details for 2, 1a, and 2a. The single-
crystal X-ray diffraction data of the crystals were collected on a
SuperNova, Dual, Mo at zero, Eos diffractometer at 132 K for 2 and 99
K for 1a and 2a, both using graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å). Atomic coordinates and isotropic and
anisotropic displacement parameters of all the non-hydrogen atoms of
the three compounds were refined using Olex2,20 and the structures
were solved with the Superflip21 structure solution program using
charge flipping and refined with the ShelXL22 refinement package
using least-squares minimization. Structure graphics shown in the
figures were created using the Diamond software package version 2.1.
A summary of the crystal data is given below.
Crystal data for 2: C62H54Cl2N4Pd2, Mr = 1138.80, monoclinic,
space group P21/n, a = 13.8306(10) Å, b = 17.5899(13) Å, c =
22.635(3) Å, α = γ = 90.00°, β = 100.567(8)°, V = 5413.2(9) Å3, Z =
4, calculated density 1.397 g cm−3, μ(Mo Kα) = 0.806 mm−1, T = 132
K, θ range for data collection 1.8−26.4°, 6580 reflections measured,
R1 = 0.0794 (I > 2σ(I)), wR2 = 0.2277 (all data).
Crystal data for 1a: C50H36F6N4O4Pd2Cl6, Mr = 1296.33,
monoclinic, space group P21/n, a = 17.540(3) Å, b = 13.663(2) Å, c
= 20.964(4) Å, α = γ = 90.00°, β = 92.421(11)°, V = 5019.5(15) Å3, Z
= 4, calculated density 1.715 g cm−3, μ(Mo Kα) = 1.108 mm−1, T = 99
K, θ range for data collection 1.5−27.6°, 4658 reflections measured,
R1 = 0.1063 (I > 2σ(I)), wR2 = 0.2810 (all data).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00649.
1
X-ray crystallography details H, 13C, and 19F NMR data
along with spectra (PDF)
Cartesian coordinates of the calculated structures (XYZ)
Accession Codes
CCDC 1548600−1548602 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
I
DOI: 10.1021/acs.organomet.7b00649
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
K.; Vijaykumar, G.; Mandal, S. K. Eur. J. Org. Chem. 2017, 2017,
1004−1011.
(11) Hota, P. K.; Vijaykumar, G.; Pariyar, A.; Sau, S. C.; Sen, T. K.;
Mandal, S. K. Adv. Synth. Catal. 2015, 357, 3162−3170.
(12) Lei, Y.; Chen, F.; Luo, Y.; Xu, P.; Wang, Y.; Zhang, Y. Inorg.
Chim. Acta 2011, 368, 179−186.
(13) (a) Biffis, A.; Tubaro, C.; Buscemi, G.; Basatoa, M. Adv. Synth.
Catal. 2008, 350, 189−196. (b) Choi, D. S.; Kim, J. H.; Shin, U. S.;
Deshmukh, R. R.; Song, C. E. Chem. Commun. 2007, 3482−3484.
(c) Reetz, M. T.; Sommer, K. Eur. J. Org. Chem. 2003, 2003, 3485−
3496. (d) Vicenzi, D.; Sgarbossa, P.; Biffis, A.; Tubaro, C.; Basato, M.;
Michelin, R. A.; Lanza, A.; Nestola, F.; Bogialli, S.; Pastore, P.; Venzo,
A. Organometallics 2013, 32, 7153−7162.
(14) Li, R.; Wang, S. R.; Lu, W. Org. Lett. 2007, 9, 2219−2222.
(15) (a) Sethna, S. M.; Shah, N. M. Chem. Rev. 1945, 36, 1−62. (b)
Coumarins: Biology, Applications, and Mode of Action; O’Kennedy, R.,
Thomas, D., Eds.; Wiley: Chichester, U.K., 1997. (c) Yu, D.; Suzuki,
M.; Xie, L.; Natschke, S. L. M.; Lee, K.-H. Med. Res. Rev. 2003, 23,
322−345. (d) Murray, R. D. H.; Mendez, J.; Brown, S. A. The Natural
Coumarins; Wiley: Chichester, U.K., 1982.
(16) (a) Wagner, B. D. Molecules 2009, 14, 210−231. (b) Dorlars, A.;
Schellhammer, C.-W.; Schroeder, J. Angew. Chem., Int. Ed. Engl. 1975,
14, 665−679.
(17) (a) Trost, B. M.; Toste, F. D. J. Am. Chem. Soc. 1996, 118,
6305−6306. (b) Trost, B. M.; Toste, F. D. J. Am. Chem. Soc. 1998,
120, 9074−9075. (c) Trost, B. M.; Toste, F. D.; Greenman, K. J. Am.
Chem. Soc. 2003, 125, 4518−4526. (d) Cao, J. − L.; Shen, S.-L.; Yang,
P.; Qu, J. Org. Lett. 2013, 15, 3856−3859.
(18) Herrmann, W. A.; Brossmer, C.; Ofele, K.; Reisinger, C.-P.;
Priermeier, T.; Beller, M.; Fischer, H. Angew. Chem., Int. Ed. Engl.
1995, 34, 1844−184.
(19) (a) Van der Zeijden, A. A. H.; Bosch, H. W.; Berke, H.
Organometallics 1992, 11, 563−573. (b) Lu, X.; Zhu, G.; Ma, S.
Tetrahedron Lett. 1992, 33, 7205−7206.
(20) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.;
Puschmann, H. OLEX2: a complete structure solution, refinement and
analysis program. J. Appl. Crystallogr. 2009, 42, 339−341.
(21) Palatinus, L.; Chapuis, G. SUPERFLIP. J. Appl. Crystallogr.
2007, 40, 786−790.
(22) Sheldrick, G. M. SHELXL. Acta Crystallogr., Sect. A: Found.
Crystallogr. 2008, 64, 112−122.
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Rev. D; Gaussian, Inc., Wallingford, CT, 2013.
AUTHOR INFORMATION
■
Corresponding Author
*S.K.M.: e-mail, swadhin.mandal@iiserkol.ac.in; web, http://
swadhin-mandal.weebly.com/.
ORCID
Swadhin K. Mandal: 0000-0003-3471-7053
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the SERB (DST), India (Grant No. SR/S1/IC−25/
2012) for financial support. PKH is thankful to IISER-Kolkata
for a research fellowship. AJ acknowledges DST-INSPIRE for
fellowship. PKH thanks Gonela Vijaykumar for his help in
single crystal X-ray study. We thank the NMR, X-ray and
computational facilities of IISER-Kolkata. We also thank Sigma-
Aldrich for high purity chemicals.
REFERENCES
■
(1) Jia, C.; Piao, D.; Oyamada, J.; Lu, W.; Kitamura, T.; Fujiwara, Y.
Science 2000, 287, 1992−1995.
(2) Jia, C.; Lu, W.; Oyamada, J.; Kitamura, T.; Matsuda, K.; Irie, M.;
Fujiwara, Y. J. Am. Chem. Soc. 2000, 122, 7252−7263.
(3) Jia, C.; Piao, D.; Kitamura, T.; Fujiwara, Y. J. Org. Chem. 2000, 65,
7516−7522.
(4) Viciu, M. S.; Stevens, E. D.; Petersen, J. L.; Nolan, S. P.
Organometallics 2004, 23, 3752−3755.
(5) (a) Saravanakumar, R.; Ramkumar, V.; Sankararaman, S.
Organometallics 2011, 30, 1689−1694. (b) Gonell, S.; Poyatos, M.;
Mata, J. A.; Peris, E. Organometallics 2011, 30, 5985−5990. (c) Pinter,
P.; Biffis, A.; Tubaro, C.; Tenne, M.; Kalinerb, M.; Strassner, T. Dalton
Trans. 2015, 44, 9391−9399. (d) Biffis, A.; Gazzola, L.; Gobbo, P.;
Buscemi, G.; Tubaro, C.; Basato, M. Eur. J. Org. Chem. 2009, 2009,
3189−3198. (e) Boyarskiy, V. P.; Ryabukhin, D. S.; Bokach, N. A.;
Vasilyev, A. V. Chem. Rev. 2016, 116, 5894−5986.
(6) (a) Albrecht, M. Science 2009, 326, 532−533. (b) Heckenroth,
M.; Kluser, E.; Neels, A.; Albrecht, M. Angew. Chem., Int. Ed. 2007, 46,
6293−6296. (c) Prades, A.; Viciano, M.; Sanau, M.; Peris, E.
Organometallics 2008, 27, 4254−4259. (d) Saha, S.; Ghatak, T.;
Saha, B.; Doucet, H.; Bera, J. K. Organometallics 2012, 31, 5500−5505.
(e) Lavallo, V.; Dyker, C. A.; Donnadieu, B.; Bertrand, G. Angew.
Chem., Int. Ed. 2008, 47, 5411−5414. (f) Arnold, P. L.; Pearson, S.
Coord. Chem. Rev. 2007, 251, 596−609. (g) Albrecht, M. Chem.
Commun. 2008, 3601−3610. (h) Daw, P.; Petakamsetty, R.; Sarbajna,
A.; Laha, S.; Ramapanicker, R.; Bera, J. K. J. Am. Chem. Soc. 2014, 136,
13987−13990.
(7) Aldeco-Perez, E.; Rosenthal, A. J.; Donnadieu, B.; Parameswaran,
P.; Frenking, G.; Bertrand, G. Science 2009, 326, 556−559.
(8) (a) Sen, T. K.; Sau, S. C.; Mukherjee, A.; Modak, A.; Mandal, S.
K.; Koley, D. Chem. Commun. 2011, 47, 11972−11974. (b) Sau, S. C.;
Bhattacharjee, R.; Vardhanapu, P. K.; Vijaykumar, G.; Datta, A.;
Mandal, S. K. Angew. Chem., Int. Ed. 2016, 55, 15147−15151.
(9) Sau, S. C.; Santra, S.; Sen, T. K.; Mandal, S. K.; Koley, D. Chem.
Commun. 2012, 48, 555−557.
(10) (a) Sau, S. C.; Raha Roy, S.; Sen, T. K.; Mullangi, D.; Mandal, S.
K. Adv. Synth. Catal. 2013, 355, 2982−2991. (b) Sau, S. C.; Raha Roy,
S.; Mandal, S. K. Chem. - Asian J. 2014, 9, 2806−2813. (c) Roy, S. R.;
Sau, S. C.; Mandal, S. K. J. Org. Chem. 2014, 79, 9150−9160.
(d) Vijaykumar, G.; Mandal, S. K. Dalton Trans. 2016, 45, 7421−7426.
(e) Bhunia, M.; Hota, P. K.; Vijaykumar, G.; Adhikari, D.; Mandal, S.
K. Organometallics 2016, 35, 2930−2937. (f) Bhunia, M.; Sahoo, S. R.;
Vijaykumar, G.; Adhikari, D.; Mandal, S. K. Organometallics 2016, 35,
3775−3780. (g) Thakur, A.; Vardhanapu, P. K.; Vijaykumar, G.; Hota,
P. K.; Mandal, S. K. Eur. J. Inorg. Chem. 2016, 2016, 913−920.
(h) Ahmed, J.; Sau, S. C.; Prasannan, S.; Hota, P. K.; Vardhanapu, P.
(24) Becke, A. D. J. J. Chem. Phys. 1993, 98, 5648−5652.
(25) (a) Hay, P. J.; Wadt, W. R. J. J. Chem. Phys. 1985, 82, 270−283.
(b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284−298.
J
DOI: 10.1021/acs.organomet.7b00649
Organometallics XXXX, XXX, XXX−XXX