Stereo- and Regioselective Addition of Arene to Alkyne Using

Nov 14, 2017 - Hydroarylation of alkyne was reported by Fujiwara nearly two decades back. Interestingly, this reaction does not proceed in the absence...
0 downloads 10 Views 3MB Size
Download PDF
Article Cite This: Organometallics XXXX, XXX, XXX-XXX

pubs.acs.org/Organometallics

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 CO2 H), 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.

■

INTRODUCTION 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 functionalization method has several benefits in comparison to conventional synthetic routes. This protocol does not require prefunctionalization (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 © XXXX American Chemical Society

ion in situ via weakly coordinating trifluoroacetate anion, CF3CO2− (Scheme 1).1,2 According to the Fujiwara process, 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

A

DOI: 10.1021/acs.organomet.7b00649 Organometallics XXXX, XXX, XXX−XXX

Organometallics

■

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-diisopropylphenyl)-2,4-diphenylimidazolium salt7 as an impressive ligand for convenient metal-free catalysts8 and organometallic catalysts9−11 for a variety of organic conversions. For instance, we recently reported the synthesis of aNHC-supported halobridged 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.

Article

RESULTS AND DISCUSSION

In the present work, 1,3-bis(2,6-dimethylphenyl)-2,4-diphenylimidazolium 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 2bromoacetophenone 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 singlet at δ 8.87 ppm in 1H 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 cyclopalladated 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

Article

Organometallics 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 cisarylalkenes 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 3arylacrylate (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 mesitylene 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). 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 (anticoagulants, 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

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).

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

Article

Organometallics Table 1. Pd(II)-Catalyzed Addition of Pentamethylbenzene to Methyl Propiolatea

entry

Pd(II) (mol %)

acid (mL)

time (h)

yield (%)b 3a/4a

1 2 3 4 5 6 7 8 9 10

PdCl2 (0.5) Pd(OAc)2 (0.5) 1 (0.5) 2 (0.5) 1 (0.5) 2 (0.5) 2 (0.25) 2 (0.1) 2 (0.5) none

TFA TFA TFA TFA AcOH AcOH TFA TFA none TFA

12 3 3 3 12 12 12 24 12 24

21/5 52/12 81/8 83/9 0 0 48/8 32/13 0 0

a

All reactions were performed with pentamethylbenzene (1 mmol), methyl propiolate (1 mmol), and TFA (1 mL) at room temperature. bIsolated yields.

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 1 H and 13 C 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 Xray studies. The single-crystal X-ray analysis of 1a and 2a confirmed incorporation of trifluoroacetate groups by replacement 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 TFAbridged 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

Scheme 3. Hydroarylation of Alkynes Using Catalytic Amount of 2

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

Article

Organometallics Table 2. Catalytic Addition of Arenes to Alkynes Using 2a

Table 3. Catalytic Reactions Leading to the Synthesis of Coumarin Derivativesa

a

All reactions were conducted with arene (1 mmol), alkyne (1 mmol), 2 (0.5 mol %), and TFA (1 mL) at room temperature. bIsolated 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 in the 1H NMR spectrum. The 1H 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. It may be recapitulated that this upfield peak in 1H 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), 2 (0.5 mol %), and TFA (1 mL) at room temperature. bIsolated 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

Article

Organometallics

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.

Table 4. Hydroarylation with Trifluoroacetate-Bridged Dinuclear Palladium Complexes 1a and 2aa

a b

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.

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

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 F

DOI: 10.1021/acs.organomet.7b00649 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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 σaryl−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 temperature. 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 proposed on the basis of the observation from the 1H NMR 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

■

EXPERIMENTAL SECTION

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 G

DOI: 10.1021/acs.organomet.7b00649 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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. mmol, which was prepared according to literature procedure),12 2bromoacetophenone (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 desired salt, L1HCl·Cl: yield 76%; mp 113−115 °C. 1H 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.

(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 measured in a sealed glass tube on a Büchi B-540 melting point apparatus and are uncorrected. Synthesis and Characterization Data of 1,3-Bis(2,6-dimethylphenyl)-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

Article

Organometallics 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. 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 (calculated for C48H34F6N4O4Pd2, 1056.0554 [M]+). 1H 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. 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 singlecrystal 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).

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 or 1H 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. 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.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00649. X-ray crystallography details 1H, 13C, and 19F NMR data along with spectra (PDF) Cartesian coordinates of the calculated structures (XYZ) Accession Codes

CCDC 1548600−1548602 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], 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

Article

Organometallics

■

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. (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.

AUTHOR INFORMATION

Corresponding Author

*S.K.M.: e-mail, [email protected]; 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 SigmaAldrich 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. J

DOI: 10.1021/acs.organomet.7b00649 Organometallics XXXX, XXX, XXX−XXX