Direct Synthesis of Amides from Coupling of Alcohols and Amines

Aug 13, 2014 - School of Chemistry, Bharathidasan University, Tiruchirappalli-620024, India. ‡. Institute of High Energy Physics, Chinese Academy of...
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Direct Synthesis of Amides from Coupling of Alcohols and Amines Catalyzed by Ruthenium(II) Thiocarboxamide Complexes under Aerobic Conditions Elangovan Sindhuja,† Rengan Ramesh,*,† Sundarraman Balaji,† and Yu Liu‡ †

School of Chemistry, Bharathidasan University, Tiruchirappalli-620024, India Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

‡

S Supporting Information *

ABSTRACT: Four octahedral ruthenium(II) thiocarboxamide complexes of the general formula [RuClCO(AsPh3)2(L)] (L = N-substituted pyridine-2-thiocarboxamide) incorporating carbonyl and triphenylarsine have been synthesized from the reaction of 1 equiv of ruthenium precursor [RuHClCO(AsPh3)3] with 1 equiv of thiocarboxamide ligands in refluxing ethanol in the presence of base. All the new complexes have been fully characterized by means of elemental analysis, IR, UV−vis, and NMR spectral methods. Molecular structures of all the complexes were determined by X-ray crystallography, which confirm the coordination mode of thiocarboxamide and reveal the presence of a distorted octahedral geometry around the Ru ion. All the ruthenium(II) thiocarboxamide complexes were generated as highly efficient catalysts for synthesis of secondary or tertiary amides by coupling of amines and alcohols with low catalyst loading, and the maximum yield was obtained up to 97%. The coupling reaction can be readily carried out under mild aerobic conditions, and release of water is the only byproduct. Further, the effect of substituents of the ligand, solvents, reaction temperature, time, and catalyst loading on the catalytic activity of the complexes has been investigated. A plausible mechanism is proposed for the synthesis of amides via hemiaminal as intermediate through an oxidation of an alcohol to aldehyde.

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INTRODUCTION Amides are an important class of chemicals in organic chemistry and constitute the key functional group in peptides, polymers, natural products, pharmaceuticals, and raw materials such as engineering plastics, detergents, and lubricants.1 The formation of a chiral amide is one of the structural motifs in drug development and discovery that heavily depends on the amine functionality.2 The aim is to synthesize the target amides, which are acquired by the traditional method of the reaction derivatives of carboxylic acids with amines. Besides, it can also prepared from iodonium-promoted α-halo nitroalkane amine coupling,3 the Staudinger reaction,4 oxidative amidation of aldehydes,5 the Beckmann rearrangement,6 the Schmidt reaction,7 aminocarbonylation of aryl halides,8 the amidation of thioacids with azides,9 hydroamination of alkynes,10 transamidation of amides,11 and from esters and amines.12 However, the selectivities for the desired amides are less due to the formation of undesired nitriles, carboxylic acids, and aldehydes. The amidation of alcohol has significant interest in recent years due to the relatively environmentally benign alternative for using alcohols as the greener acylating reagent while producing a minimum of waste,13 in addition, for using oxygen or air as the green oxidant and high atom efficiency achievable by yielding water as the only byproduct (Scheme 1). Various metal-based catalysts have been shown to promote this transformation reported by several groups; quite a new catalyst is still required to rectify the drawbacks such as inert © XXXX American Chemical Society

Scheme 1. Formation of Amide from Alcohol and Amine

atmosphere protection, high temperature, and long reaction time or high base loadings.14 Milstein and co-workers reported a ruthenium pincer complex (0.1 mol %) as catalyst (A) for the preparation of amides under an argon atmosphere at reflux (Scheme 2).15 Similarly, Madsen and co-workers achieved the dehydrogenative amide synthesis with a in situ formation of ruthenium N-heterocyclic carbene (B) from [Ru(COD)Cl2] (5 mol %) and N-heterocyclic carbenes (5 mol %) as ligands and in the presence of KOtBu (20 mol %) and toluene at reflux.16 In addition, a ruthenium arene complex with bis(diphenylphosphino)butane has been used as the catalyst for Scheme 2. Reported Ruthenium Catalysts for Amidation

Received: May 27, 2014

A

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N-acylation of amines with alcohols by using a ketone as oxidant for 24 h.17 Further, Hong and co-workers reported an NHC-based ruthenium catalyst (5 mol %) (C) mediated amide synthesis from sterically hindered alcohols and amines, which is limited to sterically bulky substrates and less basic aniline for 24 h in toluene at reflux.18 Although, Kobayashi and co-workers reported heterogeneous based catalysts involving Au, Au/Pd, Ni, and Co nanoparticles (1 mol %) with molecular oxygen as co-oxidant for 12 h.19 Crabtree and co-workers reported ruthenium diphosphine diamine complexes (D) that catalyze oxidative synthesis of amides and pyrroles via dehydrogenative alcohol oxidation.20 Furthermore, gold mediated amidation under aerobic conditions has been achieved via the corresponding methyl ester in methanol at 65 °C for 24 h.21 Thiocarboxamides containing the NHC(S) unit are versatile ligands, which coordinate in either thione or thiol form. It has been found in the literature that the thiocarboxamide ligands are known to coordinate metal ions usually in a bidentate fashion with an N,S donor forming a five-membered chelate ring. However, sulfur-containing SCS pincers have gained recognition because of the added stability via tridentate coordination imparts to the formation of pincer complex. In terms of sulfur Lewis base donors, thiocarboxamides are a relatively unexplored functional group in designing of ligands. Palladium(II), gold(III), and platinum(II) have been reported for the bidentate coordination.22 Arene ruthenium metalacycles containing anionic chelating thiocarboxamides have been reported.23 Recently, deprotonation-induced structural changes in ruthenium pincer complexes with secondary thioamide groups have been reported.24 Metal complexes containing thiocarboxamide ligands were well studied in biological systems rather than their catalytic activity. We have previously reported the catalytic and biological utility of ruthenium and palladium thiocarboxamide complexes for various organic transformations.25 Our interest is to develop more practical and desirable catalysts for synthesis of amides under aerobic conditions for a coupling of a wide range of amines and alcohols. In view of the excellent properties of thiocarboxamides, we describe the synthesis of the monomeric ruthenium(II) N-substituted pyridine-2-thiocarboxamide complexes incorporating carbonyl and AsPh3 as ancillary ligands. Further, we have first explored the catalytic efficiency of the complexes in coupling of amines and alcohols. The assessment of catalysts and optimization of the reaction for the coupling of a number of substrates including primary aromatic/heterocyclic alcohols and primary/secondary amines, diamines, or chiral amines have been carried out for one-pot synthesis of amides.

Scheme 3. Synthesis of Ruthenium Complexes

a more nucleophilic thiolate sulfur by abstracting proton from the thiolic sulfur, which would rapidly coordinate to the ruthenium(II) ion. All the complexes are air-stable and soluble in all common polar solvents, such as CH2Cl2, CH3OH, DMF, DMSO, and CH3CN. The analytical data of all the ruthenium(II) thiocarboxamide complexes are in good agreement with the molecular formula proposed. Characterization of the Complexes. Selected diagnostic bands of the IR spectra of the complexes 5−8 in a KBr pellet showed useful information about the coordination mode of the ligands and structure of the complexes. The IR spectra of 5−8 did not display the characteristic bands associated with the νS−H and νN−H bands at 2571−2590 and 3015−3098 cm−1 present in the free ligand. All the ligands showed intense bands in the region of 820−899 cm−1 due to the νCS stretch of the thiocarboxamide, which disappeared upon complexation. This supports the lengthening of the C−S bond that is evidenced by the molecular structure. Similarly, weak intensity bands at 1065−1093 cm−1 are assigned to the pyridine nitrogen coordinated to ruthenium.27 Thus, the spectra confirm the coordination of thiol sulfur and pyridine nitrogen to ruthenium and the formation of a five-membered chelate ring. This is further confirmed by the presence of νRu−N and νRu−S bands at 440−464 and 417−418 cm−1 in the far-IR frequency region of the complexes.28 The absorption bands at 1946−1951 cm−1 were attributed to the terminally coordinated carbonyl group and the shift of this band by 26−64 cm−1 to higher frequency than in the precursor complexes.29 In addition, a set of four bands at 520, 681, 753, and 1484 cm−1 in the spectra of all the complexes were attributed to ruthenium bound triphenylarsine, respectively.30 In the 1H NMR spectra of the complexes 5−8, a C2 proton in the range of δ 8.59−8.97 ppm is deshielded more when compared to other aromatic protons of the pyridine ring and confirms the coordination of the ruthenium atom to pyridine nitrogen. However, the significant loss of the thiocarboxamide (CS-NH) proton that had occurred in the spectra of all the ruthenium complexes, which appeared as a singlet at δ 11.68− 11.95 ppm in free ligand, can be assumed due to its coordination in thiol form. The aromatic proton of the thiocarboxamide ligand and AsPh3 showed a multiplet in the region of δ 6.02−7.96 ppm. The deshielded methyl proton signal of the complexes 7 and 8 shifted upfield as that of the free ligand and appeared as a singlet at 2.61 and 1.15, 2.16 ppm, respectively. The 13C NMR spectra clearly showed that the chemical shift of the thioketone carbon moved upfield from δ 182−194 ppm in the ligand to δ 159−162 ppm in the ruthenium(II) complexes, indicating the reduced C−S bond order on coordination. The absorption spectra of all the complexes were recorded in chloroform and showed three bands in the region of 246−451 nm. The high energy bands between 246 and 282 nm

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RESULTS AND DISCUSSION Synthesis of the Complexes. The thiocarboxamide ligands 1−4 were prepared from 2-chloro aniline or 4-bromo aniline or 4-methyl aniline or 2,4,6-trimethyl aniline in the presence of sulfur and Na2S·9H2O in 2-methylpyridine as a solvent.26 Initially, one equivalent of N-substituted pyridine-2thiocarboxamide (1−4) [C5H4-(CS)-NH-R] and 0.1 equivalent of triethylamine are stirred in ethanol medium. To the above solution one equivalent of ruthenium precursor [RuHCl(CO)(AsPh3)3], was added and the reaction mixture was refluxed for 5 h, leads to the formation of the new monomeric complexes 5−8 of general molecular formula [RuClCO(AsPh3)2(L)] (L = N-substituted pyridine-2-thiocarboxamide). All the complexes were isolated as a orange-brown solid in 72− 94% yield (Scheme 3). The role of triethylamine is to generate B

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Table 1. Selected Bond Lengths (Å) and Angles (deg) for the Complexes 5−8 Ru(1)−N(1) Ru(1)−S(1) Ru(1)−C(1) Ru(1)−Cl(1) Ru(1)−As(1) Ru(1)−As(2) S(1)−C(7) N(2)−C(7) C(1)−Ru(1)−N(1) C(1)−Ru(1)−S(1) N(1)−Ru(1)−S(1) C(1)−Ru(1)−Cl(1) N(1)−Ru(1)−Cl(1) S(1)−Ru(1)−Cl(1) C(1)−Ru(1)−As(1) N(1)−Ru(1)−As(1) S(1)−Ru(1)−As(1) Cl(1)−Ru(1)−As(1) C(1)−Ru(1)−As(2) N(1)−Ru(1)−As(2) S(1)−Ru(1)−As(2) Cl(1)−Ru(1)−As(2) As(1)−Ru(1)−As(2)

5

6

7

8

2.164(2) 2.3375(9) 1.840(3) 2.4507(9) 2.4739(6) 2.4763(6) 1.736(3) 1.345(4) 172.78(12) 91.62(11) 81.55(6) 94.83(11) 92.08(7) 173.28(3) 91.18(10) 90.71(6) 86.39(2) 91.57(2) 88.56(10) 89.24(6) 91.01(2) 91.05(2) 177.382(14)

2.197(4) 2.3387(11) 1.880(5) 2.4370(13) 2.4684(5) 2.4856(5) 1.737(5) 1.274(6) 170.13(18) 89.12(16) 81.66(11) 98.47(16) 90.77(11) 172.41(5) 88.84(15) 87.76(9) 90.65(3) 89.63(3) 85.90(15) 97.38(9) 88.93(3) 91.48(3) 174.72(2)

2.174(2) 2.3470(9) 1.853(3) 2.4392(10) 2.4642(6) 2.4716(6) 1.725(3) 1.285(4) 173.26(10) 91.46(9) 81.80(6) 96.41(9) 90.33(6) 172.12(3) 90.93(9) 89.28(6) 90.74(3) 89.48(3) 87.44(9) 92.45(6) 90.20(3) 89.80(3) 178.133(13)

2.161(3) 2.4449(10) 1.842(4) 2.532(6) 2.4434(7) 2.4401(8) 1.738(4) 1.283(5) 178.20(14) 98.01(12) 80.36(8) 91.44(19) 90.22(16) 170.33(14) 91.10(12) 89.70(8) 90.98(4) 86.74(13) 90.58(12) 88.93(8) 99.31(4) 82.59(13) 169.241(17)

corresponding to ligand-centered (LC) transitions of the aromatic ligands have been designated as π−π* and n−π* transitions. The low energy bands around 424−451 nm for all the complexes are assigned as the Ru(dπ)-to-L(π*) metal-toligand charge transfer (MLCT) transitions. This spectral feature that there is typically no variation in the energy of the MLCT band suggests that the energy gap between the metal-dπ and the ligand-π* levels remains constant even with the variation of the substituent in the complexes. The pattern of the electronic spectra of all the complexes indicated the presence of an octahedral environment around the ruthenium(II) ion, similar to that of other octahedral ruthenium(II) complexes.29 X-ray Crystallographic Studies. The solid-state structures of all the ruthenium thiocarboxamide complexes 5−8 were determined by single-crystal X-ray diffraction to confirm the coordination mode of the thiocarboxamide ligand in the complexes and the geometry of the complexes. Crystallographic data and structural refinement parameters are given in Table S1 (Supporting Information), whereas selected bond lengths and bond angles are given in Table 1. The ORTEP views of the molecules with the atom numbering are shown in Figures 1−4, respectively. Crystals of 5−8 grew from slow diffusion of dichloromethane into ethanol solutions and crystallized in the monoclinic space groups. The complex 5 contains four molecules in the unit cell, crystallizes in the “P2(1)/c” space group, and is located in NSCClAs2 coordination sphere. The thiocarboxamide ligand as their deprotonated thiol tautomer binds the metal center at the pyridine nitrogen and thiol sulfur, forming one five-membered chelate ring. The two AsPh3 are trans to each other due to the bulky nature, and the fifth coordination site is occupied by the chloride. The remaining sixth coordination site is filled by the carbon atom of the carbonyl, and the resulting coordination around ruthenium is octahedral. The chlorine is trans to the thiolate sulfur, and the carbon monoxide coordinates to the ruthenium center trans to the pyridine nitrogen. The bite angles between adjacent atoms

Figure 1. Molecular structure of 5; thermal ellipsoids are drawn at the 50% probability level.

in the coordination sphere of ruthenium are close to the expected value of 90° with the most noticeable distortions corresponding to the bite angles of 81.55(6)° N(1)−Ru(1)− S(1), 86.39(2)° S(1)−Ru(1)−As(1), 92.08(7)° N(1)−Ru(1)− Cl(1), 94.83(11)° C(1)−Ru(1)−Cl(1), and 91.57(2)° As(1)− Ru(1)−Cl(1). The bond lengths of Ru(1)−N(1) and Ru(1)− S(1) are 2.164(2) and 2.3375(9) Å, respectively. Similarly, the Ru(1)−As(1) 2.4739(6) Å and Ru(1)−As(2) 2.4763(6) Å bond distances are equal from the metal center. The coordination geometry of the ruthenium atom is of distorted C

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Figure 4. Molecular structure of 8; thermal ellipsoids are drawn at the 50% probability level. Figure 2. Molecular structure of 6; thermal ellipsoids are drawn at the 50% probability level.

Table 2. Optimization of Solvent and Temperaturea

entry

solvent

temp (°C)

yield (%)b

1 2 3 4 5 6 7 8 9

toluene t-BuOH DME dioxane MeCN MeOH t-BuOH t-BuOH t-BuOH

111 82 85 101 82 65 42 52 62

79 97 86 84 47 66 31 73 97

a

Conditions: benzylalcohol (1.0 mmol), benzylamine (1.0 mmol), catalyst 5 (0.1 mol %), 2 mL solvent, 8 h. bIsolated yields.

Figure 3. Molecular structure of 7; thermal ellipsoids are drawn at the 50% probability level.

toluene and dioxane (entries 1 and 4) gave conversions of Nbenzylbenzamide accompanied by considerable amounts of esters. It became apparent that amidation reaction could only be facilitated in t-BuOH and DME (entries 2 and 3), giving the highest yields, 97% and 86%, respectively. In highly polar acetonitrile solvent (entry 5), amide was obtained only in poor yields (47%). Though reaction was more efficient in methanol (66%), a significant amount of ester is formed as a byproduct (entry 6). It is worth noting here that the maximum reaction rate was obtained in t-BuOH as solvent. With the most promising complexes in hand, the influence of time, temperature, and catalyst loading on the model reaction was further evaluated. Development of active systems containing thermally unstable functional groups at low temperatures is of enormous significance, and it can be achieved by using mild reaction conditions. Therefore, the effect of the reaction temperature was investigated on the catalytic activity of catalyst 5 (0.1 mol %) and is summarized in Table 2. When the reaction was carried out at 42 °C (entry 7) in t-BuOH, the yield of product was 31% and the yield is

octahedral geometry and is essentially identical in all the other three complexes 6−8, with slight changes in bond angles and bond distances. The X-ray determination confirms the structure proposed on the basis of spectroscopic data, consistent with metal bivalency and the monoionized nature of the ligand. The organic transformations catalyzed by ruthenium complexes are enlarged due to their unique properties and variable oxidation states. Our study commenced with the coupling of a wide range of primary and secondary amines and primary alcohols using all four Ru(II) thiocarboxamide complexes as catalysts for the synthesis of amides. A survey of the literature shows that there is no set rule that a specific solvent can be used to attain the highest efficiency. To define the scope of the catalyst, we optimize the amidation by employing a variety of solvents (Table 2) for coupling of benzylalcohol and benzylamine as test substrates in the presence of catalyst 5 (0.1 mol %). Apolar solvents such as D

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improved at 52 °C (entry 8, 73%) with traces of ester detected by 1H NMR analysis of the crude reaction mixture. A temperature of 62 °C (entry 9) was required to reach a reasonable reaction speed, and the isolated yield of amide reached to 97%. The progress of the formation of Nbenzylbenzamide as a function of time using t-BuOH is displayed in Figure 5. The results indicate that the formation of

Designing of a new catalyst in a catalytic system is either to enhance the rate of regeneration or to slow down the rate of decomposition of catalyst. In the course of our studies into developing efficient amide bond formation, we became interested to find the varying substituent groups in the aromatic ring of the complexes. To this purpose, we have studied the catalytic activities of all complexes 5−8 in the coupling of amine and alcohol using the catalyst loading (0.05 mol %), which are summarized in Table 3. It was observed that all the complexes displayed more or less similar catalytic activity, suggesting that there is no significant effect on the catalysis despite the change in the substituent on the aromatic fragment of the thiocarboxamide ligand in the complexes. The importance of catalyst stability and longevity has major applications for industrial processes. On the basis of the results obtained, complex 7 shows relatively better catalytic activity among the four complexes. Hence, complex 7 was selected as the model catalyst for the coupling of primary and heterocyclic aromatic alcohols with primary and secondary aromatic amines to afford the corresponding secondary and tertiary amides; the results are summarized in Table 4. When an equimolar solution of benzyl alcohol and benzylamine with 0.05 mol % of 7 in t-BuOH was used, the reaction went smoothly to afford the expected Nbenzylbenzamide (11a) in excellent isolated yield (97%). Benzyl alcohols undergo efficient dehydrogenative coupling with amines containing electron-withdrawing substituents. The reaction of 4-nitrobenzyl amine with benzyl alcohol gave N-(4nitrophenyl)benzamide (11b) in 82% isolated yield. Similarly, the coupling of 4-bromobenzyl amine and benzylalcohol yielded N-(4-bromophenyl)benzamide (11c, 85%). The above results indicated that electron-withdrawing substituents on the aromatic ring amines suppressed the formation of yield. Both electron-donating 4-methoxybenzyl amine and 4-ethoxybenzyl amine were converted into the corresponding amides (11d and 11e) in good isolated yields of 94% and 96% (entries 4 and 5), respectively. Heterocyclic derivatives are pharmaceutically active compounds and a difficult class of substrate and remain an essential research topic in organic synthesis.31 The compatibility of the catalytic system with heterocycles was demonstrated, and very good results were obtained for the coupling of heterocyclic alcohols with amines using the catalyst 7. Thus, reaction of pyridin-2-yl-methanol and thiophen-2-yl-methanol with benzylamine gives the corresponding amides (11f and 11g) and were isolated in 91% and 88% yields, respectively (entries 6 and 7). Further, reaction of pyrrol-2-yl-methanol and indol-3-ylmethanol with benzylamine resulted in the formation of a desired product (11h and 11i), isolated in 82% and 73% yields (entries 8 and 9). The formation of N-(1,3-thiazol-2yl)benzamide (11j) from 2-thiazolyl amine was easily coupled with benzylamine and was isolated in 76% yield (entry 10). The system displays good efficiency for the coupling of primary alcohols with cyclic secondary amines and affords the corresponding tertiary amides (11k and 11l) in isolated yields (87% and 92%): a solution of piperidine and pyrrolidine (entries 11 and 12) could be successfully coupled with benzyl alcohol in the presence of catalyst 7. Similarly, a morpholine and imidazole derivatives (entries 13−15) gave slightly lower yield of tertiary amides (11m, 11n, and 11o), respectively. Further, the reaction was performed with primary alcohol and cyclic primary amine; reaction of benzyl alcohol and cyclohexylamine leads to the formation of N-cyclohexylbenzamide

Figure 5. Influence of reaction time on the formation of Nbenzylbenzamide.

N-benzylbenzamide initially increased with the progress of the reaction, reached a maximum, and then remained unchanged. A reasonably good isolated yield (96%) for the formation of Nbenzylbenzamide was observed at the optimum reaction time of 8 h. No noticeable improvement was observed even after extending the reaction time to 10 h. Low catalyst loading tests were competent to discover the effectiveness of the catalyst. In order to optimize the reaction conditions, the coupling reactions were performed at catalyst loadings in the range of 0.1−0.02 mol % and are summarized in Table 3. The reaction proceeds with good isolated yield (97%) Table 3. Effect of a Low Loading of Catalyst and Catalyst Screeninga

entry

mol % Ru

catalyst

yield (%)b

1 2 3 4 5 6 7 8

0.100 0.050 0.030 0.025 0.020 0.050 0.050 0.050

RuCl(CO)(AsPh3)2(L1) RuCl(CO)(AsPh3)2(L1) RuCl(CO)(AsPh3)2(L1) RuCl(CO)(AsPh3)2(L1) RuCl(CO)(AsPh3)2(L1) RuCl(CO)(AsPh3)2(L2) RuCl(CO)(AsPh3)2(L3) RuCl(CO)(AsPh3)2(L4)

97 95 84 67 55 93 97 89

a

Conditions: benzylalcohol (1.0 mmol), benzylamine (1.0 mmol), catalyst 5−8, t-BuOH (2 mL), 62 °C, 8 h. bIsolated yields.

when the catalyst loading is 0.1 mol %. When decreasing the catalyst loading to 0.05, 0.03, or 0.025 mol %, the reaction still proceeds accompanied by a drop in the isolated yield. However, these reactions could also be conducted with a low loading of catalyst (0.02 mol %) with high turnover numbers (TONs). Thus, it was concluded that the catalyst loading of 0.05 mol % of Ru catalyst is the best compromise between optimal reaction rate in t-BuOH. E

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Table 4. Investigation of Substrate Scopea,b,c,d

amides (11s and 11t) in quantitative yields (78% and 67%), respectively. For practical applications of any catalyst system, the lifetime of the catalyst and its level of reusability are very important factors. Although recycling of homogeneous catalysts is difficult and generally not preferred, we studied the possibility of recycling and reuse of the catalyst. The pronounced thermal stabilities of ruthenium were demonstrated when no deactivation of the catalyst was observed in subsequent catalytic runs for the amidation of benzyl alcohol and benzyl amine in tBuOH. After completion of the reaction, the N-benzylbenzamide was extracted from the reaction mixture and the catalyst was separated and reused for another run by adding fresh substrates to the reaction media. The first reaction afforded the corresponding coupling product in 95% yield; the product yield for the second cycle was nearly the same (91%). However, in the third cycle, it was 85%, and it was reduced to 74% in the fourth cycle. The product was significantly dropped to 63% for the fifth run. Thus, this catalyst system could be used for at least five times, which is fairly good for a homogeneous system than for the reaction catalyzed by ruthenium complexes.32 In contrast to what is observed for most other catalysts, the notable advantages of these catalysts observed in the systems are their insensitivity toward air as the greener oxidant and moisture. This is a very convenient method to monitor the reaction progress by avoiding the rigorous pretreatment of solvents and substrates; thus, reaction mixtures can be loaded into the reaction vessel in the open air. Further, of note is the ease by which these catalysts performed in the reaction in lower temperature without any base. Furthermore, the workup process and recovery of the catalyst are very simple for this catalytic system, which is stable in all organic solvents to produce only water as byproduct. A possible reaction mechanism for the amidation of amine with alcohol is believed to proceed by an initial oxidation step to produce aldehyde, which stays coordinated to ruthenium and is not released into the mixture, followed by the nucleophilic addition of amine to coordinated ruthenium, which affords the hemiaminal (Scheme 4). Further, aerobic dehydrogenation of hemiaminal would lead to the formation of amide. This classical mechanism is proposed by several workers on the studies of metal complexes catalyzed coupling of alcohol and amine to produce amide.15,18 The initial process was confirmed by the oxidation of benzyl alcohol to benzaldehyde in the absence of benzylamine under the present reaction conditions. The reaction is carried out in a nitrogen atmosphere for 16 h, but it failed to afford any noticeable production of amide, whereas, in an open air atmosphere, the catalytic activity had good conversion. This observation is in accord with the fact that oxygen plays a major role in the catalytic cycle.19,21

a

Conditions: benzylalcohol (1.0 mmol), benzylamine (1.0 mmol), catalyst 7 (0.05 mol %), t-BuOH (2 mL), 62 °C, 8 h. bIsolated yields. c Bis-amide (benzylalcohol (2.0 mmol), benzylamine (1.0 mmol), catalyst 7 (0.10 mol %) were used). dChiral amide.

(11p) in 85% isolated yield (entry 16). Like aromatic primary and secondary amines, bis-amidation was done under the standard conditions using an aromatic diamine with alcohol that reacted efficiently to afford high yields of the desired products. When these reactions were carried out with 1,4diamino benzene and 2,6-diamino pyridine (entries 17 and 18) with benzyl alcohol, bis-amides (11q and 11r) were obtained in high yields of 88% and 68%, respectively, and all the amides were characterized by NMR spectroscopy. Chiral amines appeared to be compatible with our catalytic system, and the product showed no sign of racemization. We initially chose chiral amine and alcohol for the design and the synthesis of chiral amide derivatives. Optically pure S-(−)-αmethyl benzylamine and S-(+)-2-phenylglycine (entries 19 and 20) were rapidly coupled to the benzylamine to produce chiral

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CONCLUSION We have synthesized a new series of ruthenium(II) carbonyl complexes featuring thiocarboxamide ligands incorporating carbonyl and triphenylarsine. The characterization of the complexes was accomplished by analytical and spectral (IR, UV−vis, 1H NMR) methods. X-ray diffraction study of all the complexes confirms the N and S coordination mode of ligands and reveals the presence of a distorted octahedral geometry around the ruthenium(II) ion. In addition, the ruthenium(II) thiocarboxamide complexes were developed as efficient catalysts for synthesis of a variety of amides from coupling of primary/heterocyclic alcohols and aromatic primary/secondary F

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thiocarboxamide ligand (27.0 mg, 0.0922 mmol) (1 equiv) with triethylamine (1.2 μL, 0.00922 mmol) in ethanol (25 mL) was stirred under an air atmosphere. Upon dissolution of the pyridine-2thiocarboxamide in ethanol, [RuHCl(CO)(AsPh3)3] (100 mg, 0.0922 mmol) (1 equiv) was added, and the resultant mixture was then heated to reflux temperature for 5 h and then allowed to cool to room temperature. The solution was concentrated to about 3 mL, and light petroleum ether (60−80 °C) (10 mL) was added, whereby the solid that precipitated out was washed with ethanol (5 mL) and dried in vacuo to give an orange powder. Yield: 72% (68.5 mg, 0.0640 mmol). mp 216 °C (with decomposition). Found: C, 55.28; H, 3.57; N, 2.61; S, 3.01. Calc. for C49H38As2BrClN2ORuS: C, 55.04; H, 3.58; N, 2.62; S, 3.00%. NMR (CDCl3): δH (400 MHz) 8.97 (t, 1H, C-2), 7.93−6.86 (m, 37H, Ar, AsPh3) ppm. δC (100 MHz) 160.1, 147.8, 138.4, 135.7, 133.4, 131.8, 131.1, 128.7, 128.3, 127.7, 125.1, 122.8 ppm. FT-IR (KBr cm−1) 1525(m), 1261(s). UV−vis (CHCl3; λ, nm): 426, 280, 246. Synthesis of [Ru(Cl)(CO)(κ 2 -S,N-C 6 H 4 CSN-(4-MePh)(AsPh3)2], (7). A heated solution (50 °C) of N-(4-methylphenyl)pyridine-2-thiocarboxamide ligand (21.0 mg, 0.0922 mmol) (1 equiv) with triethylamine (1.2 μL, 0.00922 mmol) in ethanol (25 mL) was stirred under an air atmosphere. Upon dissolution of the pyridine-2thiocarboxamide in ethanol, [RuHCl(CO)(AsPh3)3] (100 mg, 0.0922 mmol) (1 equiv) was added, and the resultant mixture was then heated to reflux temperature for 5 h and then allowed to cool to room temperature. The solution was concentrated to about 3 mL, and light petroleum ether (60−80 °C) (10 mL) was added, whereby the solid that precipitated out was washed with ethanol (5 mL) and dried in vacuo to give an orange powder. Yield: 94% (89.5 mg, 0.0891 mmol). mp 200 °C (with decomposition). Found: C, 59.63; H, 4.10; N, 2.80; S, 3.19. Calc. for C50H41As2ClN2ORuS: C, 59.80; H, 4.11; N, 2.79; S, 3.19%. NMR (CDCl3): δH (400 MHz) 8.91 (d, 1H, C-2), 7.81−6.18 (m, 37H, Ar, AsPh3), 2.61 (s, 3H, CH3) ppm. δC (100 MHz) 161.3, 147.4 137.9, 133.6, 132.4, 131.3, 131.1, 129.5, 128.5, 128.4, 128.2, 128.1, 126.4, 124.7, 121.4, 20.5 ppm. FT-IR (KBr cm−1) 1562(m), 1259(s). UV−vis (CHCl3; λ, nm): 451, 280, 246. Synthesis of [Ru(Cl)(CO)(κ2-S,N-C6H4CSN-(2,4,6-TriMePh)(AsPh 3 ) 2 ] (8). A heated solution (50 °C) of N-(2,4,6trimethylphenyl)pyridine-2-thiocarboxamide ligand (23.6 mg, 0.0922 mmol) with triethylamine (1.2 μL, 0.00922 mmol) in ethanol (25 mL) was stirred under an air atmosphere. Upon dissolution of the pyridine2-thiocarboxamide in ethanol, [RuHCl(CO)(AsPh3)3] (100 mg, 0.0922 mmol) was added, and the resultant mixture was then heated to reflux temperature for 5 h and then allowed to cool to room temperature. The solution was concentrated to about 3 mL, and light petroleum ether (60−80 °C) (10 mL) was added, whereby the solid that precipitated out was washed with ethanol (5 mL) and dried in vacuo to give an orange powder. Yield: 76% (72.4 mg, 0.0701 mmol). mp 210 °C (with decomposition). Found: C, 60.26; H, 4.39; N, 2.70; S, 3.10. Calc. for C52H45As2ClN2ORuS: C, 60.50; H, 4.39; N, 2.71; S, 3.11%. NMR (CDCl3): δH (400 MHz) 8.59 (d, 1H, C-2), 7.96−6.58 (m, 35H, Ar, AsPh3), 2.16 (s, 6H, CH3), 1.15 (s, 3H, CH3) ppm. δC (100 MHz) 159.6, 138.4, 134.6, 134.9, 133.1, 132.3, 131.9, 131.1, 130.6, 129.4, 127.9, 127.0, 126.7, 126.2, 122.3, 120.8, 20.9, 18.2 ppm. FT-IR (KBr cm−1) 1564(m), 1264(s). UV−vis (CHCl3; λ, nm): 445, 282, 256. Typical Procedure for the Catalytic Dehydrogenative Monoamidation of Primary and Secondary Amines with Primary Alcohols (Table 4). Catalyst (0.5 mg, 0.05 mol %), an alcohol (1.0 mL, 1.0 mmol), an amine (1.1 mL, 1.0 mmol), and tBuOH (2 mL) were placed in a two-neck round-bottom flask fitted with a reflux condenser. The resulting reaction mixture was heated to 62 °C in an open atmosphere for 8 h. After completion of the reaction, the solvent was removed under vacuum and the resulting residue was purified by column chromatography on silica gel using EtOAc/nhexane. The amides were dried under vacuum overnight. Typical Procedure for the Catalytic Dehydrogenative Bisamidation of Diamines with Alcohols (Table 4). Catalyst (1.0 mg, 0.10 mol %), alcohol (2.0 mL, 2.0 mmol), amine (1.1 mL, 1.0 mmol), and t-BuOH (2 mL) were placed in a two-neck round-bottom flask

Scheme 4. Plausible Mechanism for Amidation

diamine/chiral amines. The influence of a substituent effect of the ligand on the catalytic activity was studied. Detailed studies of the mechanism, scope, and generality of the chiral ruthenium catalyst are ongoing in our laboratory.

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EXPERIMENTAL SECTION

General. All experiments were carried out under an atmosphere of air. All solvents were reagent grade or better. Melting points were performed with an electrical instrument and are uncorrected. The microanalyses of carbon, hydrogen, nitrogen, and sulfur were recorded by an analytical function testing CHNS elemental analyzer. IR spectra were recorded in KBr pellets in the range of 4000−400 cm−1. Electronic spectra of the complexes in chloroform solution were recorded with a UV−vis spectrometer using cuvettes of 1 cm path length. Deuterated solvents were used as received. 1H and 13C NMR and DEPT spectra were recorded in CDCl3 solvent with a 400 MHz spectrometer. All chemical shifts are reported in parts per million (ppm) downfield from tetramethylsilane. Abbreviations used in the NMR follow-up experiments: br, broad; s, singlet; d, doublet; t, triplet; m, multiplet. Synthesis of [Ru(Cl)(CO)(κ2-S,N-C6H4CSN-(2-ClPh)(AsPh3)2], (5). A heated solution (50 °C) of N-(2-chlorophenyl)pyridine-2thiocarboxamide ligand (22.9 mg, 0.0922 mmol) (1 equiv) with triethylamine (1.2 μL, 0.00922 mmol) in ethanol (25 mL) was stirred under an air atmosphere. Upon dissolution of the pyridine-2thiocarboxamide in ethanol, [RuHCl(CO)(AsPh3)3] (100 mg, 0.0922 mmol) (1 equiv) was added, and the resultant mixture was then heated to reflux temperature for 5 h and then allowed to cool to room temperature. The solution was concentrated to about 3 mL, and light petroleum ether (60−80 °C) (10 mL) was added, whereby the solid that precipitated out was washed with ethanol (5 mL) and dried in vacuo to give an orange powder. Yield: 85% (80.9 mg, 0.0789 mmol). mp 204 °C (with decomposition). Found: C, 57.69; H, 3.73; N, 2.73; S, 3.14. Calc. for C49H38As2Cl2N2ORuS: C, 57.43; H, 3.74; N, 2.73; S, 3.13%. NMR (CDCl3): δH (400 MHz) 8.91 (q, 1H, C-2), 7.91−6.02 (m, 37H, Ar, AsPh3) ppm. δC (100 MHz) 162.0, 149.6, 137.8, 134.7, 134.6, 132.1, 131.1, 129.5, 128.6, 128.4, 128.2, 128.1, 126.4, 124.2, 121.4 ppm. FT-IR (KBr cm−1) 1602(m), 1262(s). UV− vis (CHCl3; λ, nm): 424, 282, 246. Synthesis of [Ru(Cl)(CO)(κ2-S,N-C6H4CSN-(4-BrPh)(AsPh3)2] (6). A heated solution (50 °C) of N-(4-bromophenyl)pyridine-2G

dx.doi.org/10.1021/om500556b | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

fitted with a reflux condenser. The resulting reaction mixture was heated to 62 °C in an open atmosphere for 8 h. Recycling of catalyst. After completion of the reaction of each cycle, the product mixture was cooled to room temperature; the catalyst was recovered by centrifugation and then washed thoroughly with diethyl ether and finally with hexane. The recovered catalyst was dried under vacuum at 100−120 °C overnight. This used catalyst was re-employed in five successive cycles under identical conditions. The combined organic layers were concentrated in vacuo, and the remaining residue was purified by column chromatography (nhexane/EtOAc: 200/1) to yield a compound. Spectral Data for the Amide Compounds. N-Benzylbenzamide (Entry 1, CAS No.: 1485-70-7). 1H NMR (CDCl3, 400 MHz): 4.65(d, J = 5.6 Hz, CH2, 2H), 6.43 (bs, 1H), 7.25−7.36 (m, 1H), 7.40−7.44 (m, 4H), 7.47−7.49(m, 2H), 7.51 (t, J = 3.6 Hz, 1H), 7.78 (q, J = 3.6 Hz, 2H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 44.2, 127.0, 127.7, 127.9, 128.6, 128.8, 131.6, 134.4, 138.2, 167.4(NCO) ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. N-(4-Nitrophenyl)benzamide (Entry 2). 1H NMR (CDCl3, 400 MHz): 7.51−7.63 (m, 3H), 7.85−7.89 (m, 4H), 8.22 (bs, 1H), 8.27 (d, J = 9 Hz, 2H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 117.8, 121.0, 128.5, 129.3, 130.2, 133.8, 143.1, 166.3 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.33 N-(4-Bromophenyl)benzamide (Entry 3, CAS No.: 7702-38-7). 1H NMR (CDCl3, 400 MHz): 7.48−7.57 (m, 7H), 7.80 (bs, 1H), 7.81− 7.87 (m, 2H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 122.3, 127.0, 127.7, 128.7, 129.0, 131.3, 139.9,165.3 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.34 N-(4-Methoxyphenyl)benzamide (Entry 4). 1H NMR (CDCl3, 400 MHz): 3.74 (s, 3H), 6.74 (d, J = 8.8 Hz, 2H), 7.45−7.54 (m, 5H), 7.80 (bs, 1H), 7.86 (d, J = 7.2 Hz, 2H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 56.0, 114.4, 128.3, 129.6, 130.5, 132.8, 155.0, 166.7 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.34 N-(4-Ethoxyphenyl)benzamide (Entry 5). 1H NMR (CDCl3, 400 MHz): 1.41(t, J = 6.8 Hz, 3H), 4.03(q, J = 7.2 Hz, 2H), 6.90 (d, J = 8.8 Hz, 2H), 7.46−7.54 (m, 5H), 7.74 (bs,1H), 7.86 (d, J = 7.2 Hz, 2H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 14.9, 63.8, 114.9, 122.0, 127.0, 128.8, 130.9, 131.7, 135.0, 156.0, 165.6 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.35 N-Benzylpyridine-2-carboxamide (Entry 6). 1H NMR (CDCl3, 400 MHz): 4.80 (d, J = 8.4 Hz, 2H), 7.17−7.37 (m, 6H), 7.32 (s, 2H), 7.58 (d, J = 7.6 Hz, 1H),7.65−7.69 (m, 1H), 8.38 (bs, 1H), 8.46 (d, J = 4.4 Hz, 1H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 42.1, 121.3, 124.8, 126.8, 127.1, 128.5, 138.7, 142.6, 148.8, 149.3, 166.7 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.36 N-Benzylthiophene-2-carboxamide (Entry 7). 1H NMR (CDCl3, 400 MHz): 4.42 (d, J = 6 Hz, 2H), 6.50 (bs, 1H), 7.15−7.21 (m, 1H), 7.24−7.28 (m, 5H,), 7.35 (d, J = 4 Hz, 1H), 7.37 (d, J = 4.4 Hz, 1H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 45.4, 127.2, 127.7, 128.6, 128.9, 129.3, 130.3, 134.3, 136.4, 162.1 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.33 N-Benzyl-1H-pyrrole-2-carboxamide (Entry 8). 1H NMR (CDCl3, 400 MHz): 4.70 (d, J = 4 Hz, 2H), 6.48 (bs, 1H), 7.06−7.23 (m, 3H), 8.21−8.28 (m, 5H), 9.92 (bs, 1H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 45.9, 107.6, 117.1, 128.3, 129.8, 132.6, 136.9, 143.1, 159.8 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. N-Benzyl-1H-indole-3-carboxamide (Entry 9). 1H NMR (CDCl3, 400 MHz): 4.48 (d, J = 5.6 Hz, 2H), 6.35 (bs, 1H), 7.10−7.24 (m, 6H), 7.47(d, J = 7.6 Hz, 1H), 7.97 (d, J = 2.4 Hz, 1H), 8.37 (d, J = 7.6 Hz, 1H), 8.81(d, J = 8 Hz, 1H), 11.7 (s, 1H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 45.0, 102.6, 114.1, 127.0, 127.2, 128.0, 128.3, 128.5, 128.6, 130.8, 136.1, 139.3, 162.1 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies.

N-(1,3-Thiazol-2-yl)benzamide (Entry 10). 1H NMR (CDCl3, 400 MHz): 6.48 (br, 1H), 6.86−6.89 (m, 1H), 7.07−7.09 (m, 1H), 7.10− 7.19 (m, 2H), 7.21−7.38 (m, 3H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 126.9,127.9, 128.6, 130.8, 138.3, 143.1, 166.0, 172.5 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Phenyl(piperidin-1-yl)methanone (Entry 11, CAS No.: 776-75-0). 1 H NMR (CDCl3, 400 MHz): 1.52 (bs, 2H), 1.67 (bs, 4H), 3.34 (bs, 2H), 3.72 (bs, 2H), 7.39 (s, 5H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 24.5, 25.6, 26.5, 43.3, 48.8, 126.8, 128.3, 129.5, 136.2, 170.6 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Phenyl(pyrrolidin-1-yl)methanone (Entry 12). 1H NMR (CDCl3, 400 MHz): 1.84−1.95 (m, 4H), 3.41(t, J = 6.8 Hz, 2H), 3.64 (t, J = 6.8 Hz, 2H), 7.38−7.39 (m, 3H), 7.49−7.51(m, 2H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 24.4, 26.4, 46.2,49.6, 127.1, 128.2, 129.8, 137.2, 169.8 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.37 Morpholin-4-ylphenyl-methanone (Entry 13). 1H NMR (CDCl3, 400 MHz): 3.29−3.32 (m, 2H), 3.46 (t, J = 6.8 Hz, 2H), 3.78 (t, J = 6.8 Hz, 4H),7.41 (s, 5H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 43.0, 48.4, 66.9, 127.1, 128.5, 129.9, 135.3, 171.1 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.38 Imidazol-1-yl-phenyl-methanone (Entry 14). 1H NMR (CDCl3, 400 MHz): 7.46−7.50 (m, 3H), 7.59−7.63 (m, 2H), 8.11−8.13 (m, 3H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 116.4, 120.0, 128.5, 130.2, 133.6, 141.5, 171.3 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Benzoimidazol-1-yl-phenyl-methanone (Entry 15). 1H NMR (CDCl3, 400 MHz): 7.22−7.24 (m, 2H), 7.42 (t, J = 7.6, 2H), 7.53−7.61 (m, 2H), 8.12−8.22 (m, 3H), 8.72 (s, 1H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 116.4, 119.5, 120.8, 127.0, 128.5, 130.2, 133.6, 141.5, 172.5 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.39 N-Cyclohexylbenzamide (Entry 16). 1H NMR (CDCl3, 400 MHz): 1.28−1.38 (m, 5H), 1.56−1.82 (m, 5H), 4.13 (d, J = 7.2 Hz, 1H), 6.48 (bs, 1H), 7.35−7.68 (m, 5H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 24.9, 25.3, 33.5, 45.4, 128.4, 130.1, 133.7, 167.8 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.37 N,N′-Benzene-1,4-diyldibenzamide (Entry 17). 1H NMR (CDCl3, 400 MHz): 6.77 (bs, 2H), 7.48 (t, J = 7.6 Hz, 7H), 7.60 (d, J = 7.2 Hz, 3H), 8.12(d, J = 7.6 Hz, 4H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 117.1, 121.0, 128.5, 130.2, 133.8, 144.0, 166.0 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. N,N′-Pyridine-2,6-diyldibenzamide (Entry 18). 1H NMR (CDCl3, 400 MHz): 7.41−7.48 (m, 6H), 7.53 (d, J = 8 Hz, 1H), 7.60 (t, J = 7.6 Hz, 1H), 8.04 (t, J = 7.6 Hz, 2H), 8.11 (d, J = 7.6 Hz, 3H), 8.31 (bs, 2H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 112.8, 128.3, 129.5, 130.2, 132.8, 133.7, 155.9, 166.7 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.40 N-[(1S)-1-Phenylethyl]benzamide (Entry 19). 1H NMR (CDCl3, 400 MHz): 1.59 (d, J = 7.2 Hz, 3H), 5.3 (t, J = 7.2 Hz, 1H), 6.57 (bs, 1H), 7.24−7.28 (m, 1H), 7.32−7.39 (m, 6H), 7.41−7.49 (m, 1H), 7.76 (t, J = 7.2 Hz, 2H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 21.0, 49.2, 126.3, 127.0, 127.4, 128.5, 131.5, 134.6, 143.2, 166.7 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. (2S)-(Benzoylamino)(phenyl)ethanoic Acid (Entry 20). 1H NMR (CDCl3, 400 MHz): 5.81(d, 6.8 Hz, 1H), 7.45−7.59 (m, 8H), 7.81 (d, 7.2 Hz, 2H), 8.11 (d, 7.6 Hz, 1H) ppm. 13C{1H}NMR (CDCl3, 100 MHz): 57.1, 114.1, 127.4, 128.4, 128.7, 130.1, 132.2, 133.2, 139.3, 167.0, 172.2 ppm. Assignment of signals was confirmed by DEPT-135 NMR studies. Spectral data are consistent with literature values.41 X-ray Structure Determinations. Crystals 5−8 suitable for single- crystal X-ray diffraction were obtained from vapor diffusion of dichloromethane into an ethanol solution at room temperature. A single crystal of suitable size was covered with Paratone oil, mounted on the top of a glass fiber, and transferred to a Bruker SMART APEX H

dx.doi.org/10.1021/om500556b | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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II single-crystal X-ray diffractometer using monochromated Mo−Kα radiation (kI = 0.71073 Å). Data were collected at 293 K. The structure was refined by full-matrix least-squares method on F2 with SHELXL-97. Non-hydrogen atoms were refined with anisotropy thermal parameters. All hydrogen atoms were geometrically fixed and allowed to refine using a riding model. Frame integration and data reduction were performed using the Bruker SAINT-Plus (Version 7.06a) software. The multiscan absorption corrections were applied to the data using SADABS software. CCDC reference numbers 953733, 953734, 898582, and 956786 contain the supplementary crystallographic data for 5−8. The data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/ retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; Fax: (+44) 1223-336033; or e-mail: [email protected].

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(11) (a) Kolakowski, R. V.; Shangguan, N.; Sauers, R. R.; Williams, L. J. J. Am. Chem. Soc. 2006, 128, 5695. (b) Zhang, X.; Li, F.; Lu, X. W.; Liu, C. F. Bioconjugate Chem. 2009, 20, 197. (12) Gnanaprakasam, B.; Milstein, D. J. Am. Chem. Soc. 2011, 133, 1682. (13) (a) Nixon, T. D.; Whittlesey, M. K.; Williams, J. M. J. Dalton Trans. 2009, 753. (b) Watson, A. J. A.; Williams, J. M. J. Science 2010, 329, 635. (c) Dobereiner, G. E.; Crabtree, R. H. Chem. Rev. 2010, 110, 681. (d) Guillena, G.; Ramonand, D. J.; Yus, M. Chem. Rev. 2010, 110, 1611. (e) Crabtree, R. H. Organometallics 2011, 30, 17. (f) Suzuki, T. Chem. Rev. 2011, 111, 1825. (14) Chen, C.; Hong, S. H. Org. Biomol. Chem. 2011, 9, 20. (15) Gunanathan, C.; David, Y. B.; Milstein, D. Science 2007, 317, 790. (16) (a) Nordstrom, L. U.; Vogt, H.; Madsen, R. J. Am. Chem. Soc. 2008, 130, 17672. (b) Dam, J. H.; Osztrovszky, G.; Nordstrom, L. U.; Madsen, R. Chem.Eur. J. 2010, 16, 6820. (17) Watson, A. J. A.; Maxwell, A. C.; Williams, J. M. J. Org. Lett. 2009, 11, 2667. (18) (a) Zhang, Y.; Chen, C.; Ghosh, S. C.; Li, Y.; Hong, S. H. Organometallics 2010, 29, 1374. (b) Muthaiah, S.; Ghosh, S. C.; Jee, J. E.; Chen, C.; Zhang, J.; Hong, S. H. J. Org. Chem. 2010, 75, 3002. (c) Chen, C.; Zhang, Y.; Hong, S. H. J. Org. Chem. 2011, 76, 10005. (19) Soule, J. F.; Miyamura, H.; Kobayashi, S. J. Am. Chem. Soc. 2011, 133, 18550. (20) Schley, N. D.; Dobereiner, G. E.; Crabtree, R. H. Organometallics 2011, 30, 4174. (21) Kegnaes, S.; Mielby, J.; Mentzel, U. V.; Jensen, T.; Fristrup, P.; Riisager, A. Chem. Commun. 2012, 48, 2427. (22) (a) Green, C. R.; Angelici, R. J. Inorg. Chem. 1972, 9, 2095. (b) Xiong, Z.; Wang, N.; Dai, M.; Li, A.; Chen, J.; Yang, Z. Org. Lett. 2004, 6, 3337. (c) Okamoto, K.; Kanbara, T.; Yamamoto, T.; Wada, A. Organometallics 2006, 25, 4026. (d) Kilpin, K. J.; Henderson, W.; Nicholson, B. K. Inorg. Chim. Acta 2010, 363, 41. (23) Alagoz, C.; Brauer, D. J.; Mohr, F. J. Organomet. Chem. 2009, 694, 1283. (24) Akaiwa, M.; Kanbara, T.; Fukumoto, H.; Yamamoto, T. J. Organomet. Chem. 2005, 690, 4192. (25) (a) Raja, M. U.; Ramesh, R.; Liu, Y. Tetrahedron Lett. 2011, 52, 5427. (b) Sindhuja, E.; Ramesh, R.; Liu, Y. Dalton Trans. 2012, 41, 5351. (c) Raja, M. U.; Ramesh, R. J. Organomet. Chem. 2012, 699, 5. (d) Pandiarajan, D.; Ramesh, R. J. Organomet. Chem. 2013, 723, 26. (e) Sindhuja, E.; Ramesh, R.; Dharmaraj, N.; Liu, Y. Inorg. Chim. Acta 2014, 416, 1. (26) Klingele, M. H.; Brooker, S. Eur. J. Org. Chem. 2004, 3422. (27) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley Interscience: New York, 1971. (28) (a) Kannan, S.; Ramesh, R. Polyhedron 2006, 25, 3095. (b) Qing, Y.; Hua, D. J.; Gang, Z. L.; Qing, Z. X.; Dong, B. H.; Hong, L. J. Mol. Struct. 2006, 794, 71. (29) Kannan, S.; Sivagamasundari, M.; Ramesh, R.; Liu, Y. J. Organomet. Chem. 2008, 693, 2251. (30) Das, A. K.; Peng, S. M.; Bhattacharya, S. J. Chem. Soc., Dalton Trans. 2000, 181. (31) Bonnet, V.; Mongin, F.; TreLcourt, F.; Breton, G.; Marsais, F.; Knochel, P.; QueLguiner, G. Synlett 2002, 1008. (32) (a) Kim, J. W.; Yamaguchi, K.; Mizuno, N. J. Catal. 2009, 263, 205. (b) Yamaguchi, K.; He, J.; Oishi, T.; Mizuno, N. Chem.Eur. J. 2010, 16, 7199. (33) Katritzky, A. R.; Cai, C.; Singh, S. K. J. Org. Chem. 2006, 71, 3375. (34) Kuninobu, Y.; Uesugi, T.; Kawata, A.; Takai, K. Angew. Chem., Int. Ed. 2011, 50, 10406. (35) Ueda, S.; Nagasawa, H. J. Org. Chem. 2009, 74, 4272. (36) Gou, F. R.; Wang, X. C.; Huo, P. F.; Bi, H. P.; Guan, Z. H.; Liang, Y. M. Org. Lett. 2009, 11, 5726. (37) Shimizu, K.; Ohshima, K.; Satsuma, A. Chem.Eur. J. 2009, 15, 9977.

ASSOCIATED CONTENT

S Supporting Information *

X-ray structure and data, and copies of 1H and 13C NMR spectra are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.R.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS One of the authors (E.S) thank UGC, India for BSR research fellowship. We thank DST-India (FIST programme) for the use of the Bruker Avance DPX 400 MHz NMR at the School of Chemistry, Bharathidasan University.

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