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Synthesis of Ru(II) pyridoxal thiosemicarbazone complex and its catalytic application to one-pot conversion of aldehydes to primary amides
Inorganic Chemistry Communications 56 (2015) 116–119
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Synthesis of Ru(II) pyridoxal thiosemicarbazone complex and its catalytic application to one-pot conversion of aldehydes to primary amides Appukutti Kanchanadevi a, Rengan Ramesh a,⁎, David Semeril b a b
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India Laboratoire de Chime, Inorganique Moleculaire, Institut de Strasbourg, UMR 7177 CNRS, France
a r t i c l e
i n f o
Article history: Received 18 March 2015 Received in revised form 30 March 2015 Accepted 7 April 2015 Available online 09 April 2015 Keywords: Ruthenium(II) TSC complex Characterization Crystal structure One-pot synthesis of primary amide
a b s t r a c t A convenient method for the synthesis of Ru(II) pyridoxal thiosemicarbazone complex has been described. Elemental analysis, spectral methods and single crystal X-ray diffraction analysis were used to confirm the composition of the complex. The synthesized complex could act as an efficient, reusable homogeneous catalyst for transformation of aldehydes to the corresponding primary amides in the presence of NH2OH·HCl. The effect of solvent, base, temperature, time, catalyst loading and recyclability was also investigated. © 2015 Elsevier B.V. All rights reserved.
Amides are a very important class of compounds in chemistry as well as biology that have widely been utilized as intermediate in peptide and protein syntheses, intensifiers of perfume, drugs, fine chemicals, anti-block reagents, color pigments for ink detergents and lubricants [1]. The classical method for amide synthesis is the acylation of amines with acid chlorides, acid anhydrides, and active esters [2]. Several alternative strategies such as the Schmidt reaction [3], the Beckmann rearrangement [4], the direct amide formation from carboxylic acids with amines [5] and the oxidative amidation of aldehydes [6] have been developed. The Beckmann rearrangement is commonly used to transform ketoximes into the corresponding secondary amides even though it generally requires the use of strong acids under severe reaction conditions. However, all these methods require stoichiometric amount of various reagents and lead to equimolar amount of byproducts. This situation is a severe drawback for scaling up and industrial applications. The one-pot synthesis of amide from aldehyde with amines can be a potentially elegant alternative pathway. It has attracted much attention because it eliminates isolation of unstable intermediates, reduces hazardous wastage, and is more efficient and selective and no by-product formation is observed [7]. Significant efforts have been developed in recent years to the development of one-pot process enabling the direct formation of primary amides from aldehydes and hydroxylamine derivatives via rearrangement of the in situ formed aldoximes (Scheme 1).
⁎ Corresponding author. E-mail address: [email protected] (R. Ramesh).
http://dx.doi.org/10.1016/j.inoche.2015.04.006 1387-7003/© 2015 Elsevier B.V. All rights reserved.
Rh(OH)x/Al2O3 was reported as an effective heterogeneous catalyst for the one-pot synthesis of primary amides from various aldehydes and NH2OH·H2SO4 in water at elevated temperatures with the formation of unwanted nitriles, aldoximes and carboxylic acids as by-products [8]. Crabtree et al., have reported terpyridine ruthenium(II) complexes [9] (a) and [Ru(DMSO)4Cl2] [10] (b) in the presence of NH2OH·HCl and a base as efficient catalysts for synthesis of primary amides. In addition, inexpensive zinc salts were found to be good homogeneous catalyst for the conversion of aldehydes into primary amides using NH2OH·HCl and NaHCO3 in moderate conversions under high catalyst loading have been reported by Williams and nitriles are formed as a by-product [11]. Further, a direct synthetic route for the transformation of aldehydes to primary amides using FeCl3 in water has been described [12]. Recently, Singh and co-workers have reported phosphine free ruthenium arene complexes (c) as catalyst for one-pot conversion of aldehydes to primary amides in water medium [13]. It has been observed from the literature that only a few reports are available on the one-pot conversion of aldehydes into primary amides. Generally thiosemicarbazone is a class of Schiff base ligands that exhibit a broad range of stereochemistry in complexation with transition metal ions. These are versatile ligands which can coordinate to metals as neutral or in their tautomeric form and they usually bind to a metal ion as bidentate N,S-donor ligands via dissociation of the hydrazinic proton, forming a five-membered chelate ring. When an additional donor site D is incorporated in such ligands, linked to the carbonylic carbon via one or two intervening atoms, D,N,S coordination usually takes place in a tridentate manner. Though there are a number of reports on the synthesis, characterization, and catalytic and biological applications
A. Kanchanadevi et al. / Inorganic Chemistry Communications 56 (2015) 116–119
O R
Base NH2OH.HCl
H
HO R
[M-catalyst]
N
O NH2
R
H
117
Scheme 1. Synthesis of primary amides from aldehydes.
of ruthenium(II) thiosemicarbazone complexes [14], the use of this complex as catalysts in the synthesis of amides from aldehydes reaction is not explored in the literature.
In continuation of our investigations on the synthesis and catalytic application of transition metal complexes for various organic transformations [15], in the present report, we wish to describe the synthesis and characterization of a new mononuclear Ru(II) complex containing the tridentate pyridoxal thiosemicarbazone along with triphenylphosphine and carbonyl as ancillary ligands (1). The synthesized complex was characterized by various analytical and spectral methods. The molecular structure of the complex 1 has been probed with the help of single crystal X-ray diffraction analysis. Further, the catalytic efficiency of the complex has been explored for the one-pot synthesis of primary amides from a wide range of aryl and heterocyclic aldehydes in the presence of NH2OH·HCl and a base. The pyridoxal thiosemicarbazone ligand (L) was prepared by condensation of pyridoxal hydrochloride with thiosemicarbazide in ethanol medium [16]. The ligand was allowed to react with the ruthenium(II) precursor, [RuHCl(CO)(PPh3)3] in equimolar ratio in methanol under reflux for 5 h in the presence of triethylamine as a base and the new octahedral Ru(II) complex, [Ru(L)(CO)(PPh3)2] (1) was obtained in 86% yield (Scheme 2). The complex is air stable in both the solid and the solution states and is non-hygroscopic in nature. The complex is readily soluble in solvents such as chloroform, dichloromethane, acetonitrile, dimethyl formamide (DMF) and dimethyl sulphoxide (DMSO) etc., producing intense orange-colored solutions. The elemental analysis of complex 1 (Calcd: C, 64.52; H, 4.58; N, 5.79; S, 3.31 Found: C, 64. 58, H, 4.63, N, 5. 74, S, 3.37) is in good agreement with the molecular structure proposed. The νC = S and νN–H bands of the ligand were disappeared upon complexation with ruthenium and the appearance of a new band in the region 840 cm− 1 is assigned to the ν(C–S) mode. In addition, the complex shows new bands near 1644 cm−1 and 1330 cm−1 confirm the coordination of imine nitrogen ν(C_N) and phenolic oxygen ν(C–O) to the ruthenium, respectively. The electronic spectrum of the ruthenium(II) pyridoxal thiosemicarbazone displays an intense ligand-centered (LC) π → π* and n → π* transitions at 241 and 255 nm respectively, and moderately intense metal to ligand charge-transfer (MLCT) transition in the visible region 416 nm. 1H NMR spectrum of the complex illustrates a multiplet in the range 7.3–7.8 ppm assigned to aromatic protons of the phenyl group of triphenylphosphine and the ligand aromatic protons. The methylene and methyl protons of the pyridoxal moiety of the ligand appear at δ 4.1 and δ 3.0 ppm, respectively. The peaks HO
HO
H N
N
OH CH3
H N
H N S
H
[RuHCl(CO)(PPh3)3] MeOH/TEA
N N
N
H N
S O Ru PPh3 CH3 Ph3P CO
Scheme 2. Synthesis of Ru(II) TSC complex.
Fig. 1. Molecular structure of (1) showing the 30% probability level and the solvent molecule (CHCl3) is not shown for clarity. All hydrogen atoms are omitted for clarity.
corresponding to the NH and OH protons appeared in the regions of 2.4 and 4.4 ppm respectively. Further, the molecular structure of complex 1 was resolved by a single crystal X-ray crystallography (Fig. 1) to confirm the coordination mode of the pyridoxal thiosemicarbazone. Single crystals of the complex 1.CHCl3, as solvate were obtained by the slow evaporation of CHCl3–MeOH solution of the complex at room temperature. The ligand coordinates to the Ru(II) ion via the phenolate oxygen, azomethine nitrogen, and thiolate sulfur forming one five-membered and one sixmembered chelate rings. Carbonyl group trans to the azomethine nitrogen is coordinated to the Ru(II) ion to form an ONSC square-plane and the triphenylphosphine ligands occupy the two axial sites. Ruthenium is, therefore, sitting in an ONSCP2 coordination environment, which gives rise to a distorted octahedral geometry as reflected in all the bond parameters around ruthenium(II) ion. The bite angles around the Ru(II) ion are C(16)–Ru(1)–O(1) = 98.5(1)°, P(1)–Ru(1)– C(16) = 90.3(1)°, N(2)–Ru(1)–O(1) = 89.08(9)°, P(1)–Ru(1)– S(1) = 91.42(3)°and N(2)-Ru(1)-P(1) = 90.25(7) and bond lengths of 1.862(4)A° Ru(1)–C(16), 2.094(2)A° Ru(1)–O(1), 2.080(3)A° Ru(1)–N(2) and 2.312A° Ru(1)–S(1). The catalytic activity of the synthesized ruthenium complex was explored for the one-pot synthesis of amides from various aldehydes with hydroxylamine hydrochloride. For the entire optimization, 4-nitrobenzaldehyde was taken as a test substrate for different conditions. To study the influence of solvents in our catalytic system, we have chosen the reaction between 4-nitrobenzaldehyde (1 mmol), NH2OH·HCl (1 mmol), complex 1 (1 mol%) as the catalyst precursor in the presence of various solvents and NaHCO3 (1 mmol) as the base. Benzene, toluene, xylene, acetonitrile, dichloromethane and chloroform are taken for our solvent variation study. The extent of conversion is solvent-dependent and low conversions were observed in benzene, xylene and toluene as solvent even at a higher temperature. Acetonitrile was found to be the solvent of choice with excellent isolated yield of amide (98%) at a lower temperature. The choice of the base was chosen, as a next step for the optimization. No conversion was observed in the absence of the catalyst or base (Table 1). It has been observed that in acetonitrile solvent, NaHCO3 or KHCO3 gave excellent isolated yields of 98% and 91% respectively, when compared to a much weaker base like CH3COONa or Et3N. Thus, it was concluded that NaHCO3 as a base in acetonitrile solvent at 78 °C is the optimized condition for this conversion.
118
A. Kanchanadevi et al. / Inorganic Chemistry Communications 56 (2015) 116–119
Table 1 Optimization of solvent, temperature and basea.
Table 3 One-pot synthesis of amides from aldehydes by using complex (1)a.
Yieldb (%)
TONc
1
88
880
2
82
820
3
77
770
4
71
710
5
91
910
6
90
900
7
89
890
8
83
830
9
69
690
10
73
730
11
79
790
12
66
660
13
70
700
Entry Entry
Solvent
Base
Temp (°C)
Time (h)
Yieldb (%)
1 2 3 4 5 6 7 8 9
Xylene Toluene CH3CN Benzene CH3CN CH3CN CH3CN CH3CN CH3CN
NaHCO3 NaHCO3 NaHCO3 NaHCO3 KHCO3 CH3COONa Et3N NaHCO3 –
140 110 78 80 78 78 78 78 78
24 24 5 24 5 24 24 24 24
28 70 98 23 91 b20 b20 –c –
a Conditions: Catalyst (1 mol%) 4-nitro benzaldehyde (1 mmol), NH2OH·HCl (1 mmol), base (1 mmol) and 3 ml solvent. b Isolated yield after column chromatography (average of two runs). c Reaction carried out in the absence of catalyst.
In order to optimize the effect of catalyst loading, different molar ratios were tested in the reaction of 4-nitrobenzaldehyde (1 mmol) using catalyst 1 in the presence of NH2OH·HCl, in CH3CN solvent and NaHCO3 (Table 2). The reaction proceeds with high conversion of 4-nitrobenzaldehyde to 4-nitrobenzamide when C: S ratio of 1:1000 (0.1 mol%) is used. When increasing the molar ratio to 0.05 mol%, 0.03 mol%, 0.025 mol% the reaction proceeds with good yields. Further, the catalyst works well with low catalyst loading of 0.02 mol% and 0.01 mol% and shows a yield of 68% and 60%, respectively. Since the isolated yields are good with appreciable turnover numbers when catalyst loading is 0.1 mol%, the C:S ratio of 1:1000 is the best compromise for the optimum reaction rates. The progress of formation of 4-nitrobenzamide as a function of time using the above optimized conditions (supporting information). The results indicate the formation of 4-nitrobenzamide initially increased with the progress of the reaction, reached a maximum and then remain unchanged. A high yield was observed for the formation of amide at the optimum reaction time of 5 h. No noticeable improvement was observed even after extending the reaction time to 7 h. To explore the scope of the new catalyst, a range of other substituted aromatic and heterocyclic aldehydes were converted to primary amides using catalyst 1 under the optimized condition. All the reactions were carried out under identical reaction conditions to allow comparison of results. A broad range of amides were successfully synthesized with good to high isolated yields using the above optimized protocol. The results collected from the catalytic reactions are listed in Table 3. The electron donating groups like –CH3, –OCH3 and–OH (entries 2, 3 and Table 2 Effect of catalyst loadinga.
Entry
Ru mol %
Time
Yieldb(%)
TONc
1 2 3 4 5 6 7 8
1 0.2 0.1 0.05 0.03 0.025 0.02 0.01
5 5 5 5 5 5 5 5
98 97 91 82 77 74 68 60
98 485 910 1640 2310 2960 3400 6000
a
Conditions: Catalyst (1–0.01 mol) 4-nitrobenzaldehyde (1 mmol), NH2OH·HCl (1 mmol), NaHCO3 (1 mmol) and 3 ml CH3CN. b Isolated yield after column chromatography (average of two runs). c Turnover number = ratio of moles of product formed to moles of catalyst used.
Aldehydes
Amides
a Conditions: Catalyst (0.1 mol%) 4-nitro benzaldehyde (1 mmol), NH2OH·HCl (1 mmol), NaHCO 3 (1 mmol) and 3 ml CH3CN. b Isolated yield after column chromatography (average of two runs). c Turnover number = ratio of moles of product formed to moles of catalyst used.
A. Kanchanadevi et al. / Inorganic Chemistry Communications 56 (2015) 116–119
4) on benzaldehyde alters the reactions and the corresponding amides were obtained in good yields of 82%, 77% and 75% respectively and gave slightly lower yields compared with benzaldehyde. On the other hand, electron withdrawing substituents, such as the –NO2, –Cl and –Br substituents (entries 5, 6 and 7) offering excellent yields (91%, 90% and 89%) when compared to substrate containing electron donating group. The introduction of electron-withdrawing substituents to the para position of the aryl ring of the aldehyde decreased the electron density on the C_O bond so that the activity was improved giving rise to easier amidation reaction. Heterocyclic amides are the basic skeleton in pharmaceutical drugs such as anticonvulsant, antidepressant, anti-inflammatory and anti-glycation activities [17]. Hence, the synthesis of heterocyclic amides from the respective aldehydes using catalyst 1 has been performed. It has been found that the reaction proceeded smoothly even in the presence of heteroatoms such as S and N in the substrates (entries 8–13) and a range of heterocyclic aromatic amides were obtained in good isolated yields. It is worth noting that by-products such as nitriles or carboxylic acids are not observed in this protocol. In addition, ketones such as acetophenone and 2-acetylpyridine were inert to this present experimental conditions indicating that this reaction differs from the Beckmann rearrangement. An important goal of this work is to study the recyclability of our ruthenium catalyst. For practical application of any catalyst systems, the life time of the catalyst and its level of reusability are very important factors. Thus, recycling experiments were conducted by using catalyst 1 in the reaction of 4-nitrobenzaldehyde to 4-nitrobenzamide. The results are presented in the supporting information show that catalyst could be used four times without much activity loss. As a result of its high polarity, good thermal stability and insensitivity to moisture and oxygen, catalyst proved to be an efficient and recyclable catalyst for the one-pot synthesis of aldehyde to amide reaction. Furthermore, the ease by which these catalysts are prepared offers another important advantage. The work-up process is very simple for this catalytic system, as the catalyst is stable in all organic solvents. Further, we believe that the catalytic transformation proceeds via the oxidative addition of the aldoxime N–OH bond to Ru(II), followed by nucleophilic attack on the coordinated imine, then β-elimination of cyclometalated, and finally reductive elimination to give the amide according to the proposed mechanism by Crabtree [10]. In this reaction water cannot eliminate as a by-product and indicates the absence of nitrile products. The present Ru(II) catalyst is more efficient in amidation reaction than the reported ruthenium(II) complexes in terms of reaction time, catalyst loading and isolated yields [9,13,18]. In summary, a new air stable Ru(II) complex bearing pyridoxal `ligand was synthesized and characterized. The single crystal X-ray diffraction study evidenced the O, N and S co-ordination mode of the ligand and the presence of distorted octahedral geometry around Ru(II) ion. The utility of the new complex as excellent catalyst for one-pot synthesis of primary amide has been highlighted by the reaction of aryl- and heteroaryl aldehydes with NH2OH·HCl and NaHCO3 as an additive. The reported recyclable ruthenium(II) catalyst (0.1 mol%) efficiently catalyzed the conversion of aldehydes to primary amides with yields up to 91%.
119
Acknowledgments One of the authors, A.K., thanks UGC, for RFSMS fellowship. CEFIPRA is gratefully acknowledged for research collaboration (Project No. 5005-1). We thank DST-FIST, India for the NMR instrumental facility at the School of Chemistry, Bharathidasan University.
Appendix A. Supplementary material Experimental procedure, 1H NMR spectrum for the complex 1, X-ray data, selected bond lengths and bond angles for complex 1. Crystallographic data for the structural analysis have been deposited with Cambridge crystallographic center, CCDC No. 1034924. 1H-NMR spectra for all the amides. Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2015.04.006.
References [1] R.W. Dugger, J.A. Ragan, D.H.B. Ripin, Org. Process Res. Dev. 9 (2005) 253–258. [2] E. Valuer, M. Bradley, Chem. Soc. Rev. 38 (2009) 606–631. [3] a E. Saxo, C.Z. Bertozzi, Science 287 (2000) 2007–2010; b F. Damkachi, P. De Shong, J. Am. Chem. Soc. 125 (2003) 4408–4409. [4] (a) T. Ribelin, C.E. Katz, D.G. English, S. Smith, A.K. Manukyan, V.W. Day, B. Neuenswander, J.L. Poutsma, J. Aube, Angew. Chem. Int. Ed. 47 (2008) 6233–6235; b S. Lang, J.A. Murphy, Chem. Soc. Rev. 35 (2006) 146–156. [5] (a) N.A. Owston, A.J. Parker, J.M.J. Williams, Org. Lett. 9 (2007) 3599–3601; b M. Hashimoto, Y. Obora, S. Sakaguchi, Y. Ishii, J. Org. Chem. 73 (2008) 2894–2897. [6] (a) L. Perreux, A. Loupy, F. Voltron, Tetrahedron 58 (2002) 1215; (b) C.L. Allen, A.R. Chhatwal, J.M.J. Williams, Chem. Commun. 48 (2012) 666–668. [7] (a) D. Gnanamgari, R.H. Crabtree, Organometallics 28 (2009) 922–924; (b) S.T. Chill, R.H. Mebane, Synth. Commun. 40 (2010) 2014–2017. [8] H. Fujiwara, Y. Ogasawara, K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 119 (2007) 5294–5297. [9] D. Gnanamgari, R.H. Crabtree, Organometallics 28 (2009) 922–924. [10] J.F. Hull, S.T. Hilton, R.H. Crabtree, Inorg. Chim. Acta 363 (2010) 1243–1245. [11] C.L. Allen, C. Burel, J.M.J. Williams, Tetrahedron Lett. 51 (2010) 2724–2726. [12] R.R. Gowda, D. Chakraborty, Eur. J. Org. Chem. (2011) 2226–2229. [13] D. Tyagi, R.K. Rai, A.D. Dwivedi, S.M. Mobin, S.K. Singh, Inorg. Chem. Front. 2 (2015) 116–124. [14] a S. Datta, D.K. Seth, R.J. Butcher, S. Bhattacharya, Inorg. Chim. Acta 377 (2011) 120–128; b K. Alomar, M.A. Khan, M. Allain, G. Bouet, Polyhedron 28 (2009) 1273–1280; c T.S. Lobana, P. Kumari, M. Zeller, R.J. Butcher, Inorg. Chem. Commun. 11 (2008) 72–74; d Z. Afrasiabi, E. Sinn, W. Lin, Y. Ma, C. Campana, S. Padhye, J. Inorg. Biochem. 99 (2005) 1526–1531. [15] E. Sindhuja, R. Ramesh, S. Balaji, Y. Liu, Organometallics 33 (2014) 4269–4278; b E. Sindhuja, R. Ramesh, Y. Liu, Dalton Trans. 41 (2012) 5351–5361; c R.N. Prabhu, R. Ramesh, RSC Adv. 2 (2012) 4515–4524. [16] E.W. Ainscough, A.M. Brodie, A. Dobbs, J.D. Ranford, J.M. Waters, Inorg. Chim. Acta 236 (1995) 83–88; b S.C. Chan, L.L. Koh, P.H. Leung, J.D. Ranford, K.Y. Sim, Inorg. Chim. Acta 236 (1995) 101–108; c S.C. Chan, L.L. Koh, P.H. Leung, J.D. Ranford, K.Y. Sim, Inorg. Chim. Acta 236 (1995) 101–108. [17] A.A. Shaikh, M.G. Raghuwanshi, K.I. Molvi, S. Nazim, A. Ahmed, J. Chem. Pharm. Res. 5 (2013) 14–25. [18] S.T. Chill, R.C. Mebane, Synth. Commun. 40 (2010) 2014–2017.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Synthesis of Ru(II) pyridoxal thiosemicarbazone complex and its catalytic application to one-pot conversion of aldehydes to primary amides Appukutti Kanchanadevi a, Rengan Ramesh a,⁎, David Semeril b a b
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India Laboratoire de Chime, Inorganique Moleculaire, Institut de Strasbourg, UMR 7177 CNRS, France
a r t i c l e
i n f o
Article history: Received 18 March 2015 Received in revised form 30 March 2015 Accepted 7 April 2015 Available online 09 April 2015 Keywords: Ruthenium(II) TSC complex Characterization Crystal structure One-pot synthesis of primary amide
a b s t r a c t A convenient method for the synthesis of Ru(II) pyridoxal thiosemicarbazone complex has been described. Elemental analysis, spectral methods and single crystal X-ray diffraction analysis were used to confirm the composition of the complex. The synthesized complex could act as an efficient, reusable homogeneous catalyst for transformation of aldehydes to the corresponding primary amides in the presence of NH2OH·HCl. The effect of solvent, base, temperature, time, catalyst loading and recyclability was also investigated. © 2015 Elsevier B.V. All rights reserved.
Amides are a very important class of compounds in chemistry as well as biology that have widely been utilized as intermediate in peptide and protein syntheses, intensifiers of perfume, drugs, fine chemicals, anti-block reagents, color pigments for ink detergents and lubricants [1]. The classical method for amide synthesis is the acylation of amines with acid chlorides, acid anhydrides, and active esters [2]. Several alternative strategies such as the Schmidt reaction [3], the Beckmann rearrangement [4], the direct amide formation from carboxylic acids with amines [5] and the oxidative amidation of aldehydes [6] have been developed. The Beckmann rearrangement is commonly used to transform ketoximes into the corresponding secondary amides even though it generally requires the use of strong acids under severe reaction conditions. However, all these methods require stoichiometric amount of various reagents and lead to equimolar amount of byproducts. This situation is a severe drawback for scaling up and industrial applications. The one-pot synthesis of amide from aldehyde with amines can be a potentially elegant alternative pathway. It has attracted much attention because it eliminates isolation of unstable intermediates, reduces hazardous wastage, and is more efficient and selective and no by-product formation is observed [7]. Significant efforts have been developed in recent years to the development of one-pot process enabling the direct formation of primary amides from aldehydes and hydroxylamine derivatives via rearrangement of the in situ formed aldoximes (Scheme 1).
⁎ Corresponding author. E-mail address: [email protected] (R. Ramesh).
http://dx.doi.org/10.1016/j.inoche.2015.04.006 1387-7003/© 2015 Elsevier B.V. All rights reserved.
Rh(OH)x/Al2O3 was reported as an effective heterogeneous catalyst for the one-pot synthesis of primary amides from various aldehydes and NH2OH·H2SO4 in water at elevated temperatures with the formation of unwanted nitriles, aldoximes and carboxylic acids as by-products [8]. Crabtree et al., have reported terpyridine ruthenium(II) complexes [9] (a) and [Ru(DMSO)4Cl2] [10] (b) in the presence of NH2OH·HCl and a base as efficient catalysts for synthesis of primary amides. In addition, inexpensive zinc salts were found to be good homogeneous catalyst for the conversion of aldehydes into primary amides using NH2OH·HCl and NaHCO3 in moderate conversions under high catalyst loading have been reported by Williams and nitriles are formed as a by-product [11]. Further, a direct synthetic route for the transformation of aldehydes to primary amides using FeCl3 in water has been described [12]. Recently, Singh and co-workers have reported phosphine free ruthenium arene complexes (c) as catalyst for one-pot conversion of aldehydes to primary amides in water medium [13]. It has been observed from the literature that only a few reports are available on the one-pot conversion of aldehydes into primary amides. Generally thiosemicarbazone is a class of Schiff base ligands that exhibit a broad range of stereochemistry in complexation with transition metal ions. These are versatile ligands which can coordinate to metals as neutral or in their tautomeric form and they usually bind to a metal ion as bidentate N,S-donor ligands via dissociation of the hydrazinic proton, forming a five-membered chelate ring. When an additional donor site D is incorporated in such ligands, linked to the carbonylic carbon via one or two intervening atoms, D,N,S coordination usually takes place in a tridentate manner. Though there are a number of reports on the synthesis, characterization, and catalytic and biological applications
A. Kanchanadevi et al. / Inorganic Chemistry Communications 56 (2015) 116–119
O R
Base NH2OH.HCl
H
HO R
[M-catalyst]
N
O NH2
R
H
117
Scheme 1. Synthesis of primary amides from aldehydes.
of ruthenium(II) thiosemicarbazone complexes [14], the use of this complex as catalysts in the synthesis of amides from aldehydes reaction is not explored in the literature.
In continuation of our investigations on the synthesis and catalytic application of transition metal complexes for various organic transformations [15], in the present report, we wish to describe the synthesis and characterization of a new mononuclear Ru(II) complex containing the tridentate pyridoxal thiosemicarbazone along with triphenylphosphine and carbonyl as ancillary ligands (1). The synthesized complex was characterized by various analytical and spectral methods. The molecular structure of the complex 1 has been probed with the help of single crystal X-ray diffraction analysis. Further, the catalytic efficiency of the complex has been explored for the one-pot synthesis of primary amides from a wide range of aryl and heterocyclic aldehydes in the presence of NH2OH·HCl and a base. The pyridoxal thiosemicarbazone ligand (L) was prepared by condensation of pyridoxal hydrochloride with thiosemicarbazide in ethanol medium [16]. The ligand was allowed to react with the ruthenium(II) precursor, [RuHCl(CO)(PPh3)3] in equimolar ratio in methanol under reflux for 5 h in the presence of triethylamine as a base and the new octahedral Ru(II) complex, [Ru(L)(CO)(PPh3)2] (1) was obtained in 86% yield (Scheme 2). The complex is air stable in both the solid and the solution states and is non-hygroscopic in nature. The complex is readily soluble in solvents such as chloroform, dichloromethane, acetonitrile, dimethyl formamide (DMF) and dimethyl sulphoxide (DMSO) etc., producing intense orange-colored solutions. The elemental analysis of complex 1 (Calcd: C, 64.52; H, 4.58; N, 5.79; S, 3.31 Found: C, 64. 58, H, 4.63, N, 5. 74, S, 3.37) is in good agreement with the molecular structure proposed. The νC = S and νN–H bands of the ligand were disappeared upon complexation with ruthenium and the appearance of a new band in the region 840 cm− 1 is assigned to the ν(C–S) mode. In addition, the complex shows new bands near 1644 cm−1 and 1330 cm−1 confirm the coordination of imine nitrogen ν(C_N) and phenolic oxygen ν(C–O) to the ruthenium, respectively. The electronic spectrum of the ruthenium(II) pyridoxal thiosemicarbazone displays an intense ligand-centered (LC) π → π* and n → π* transitions at 241 and 255 nm respectively, and moderately intense metal to ligand charge-transfer (MLCT) transition in the visible region 416 nm. 1H NMR spectrum of the complex illustrates a multiplet in the range 7.3–7.8 ppm assigned to aromatic protons of the phenyl group of triphenylphosphine and the ligand aromatic protons. The methylene and methyl protons of the pyridoxal moiety of the ligand appear at δ 4.1 and δ 3.0 ppm, respectively. The peaks HO
HO
H N
N
OH CH3
H N
H N S
H
[RuHCl(CO)(PPh3)3] MeOH/TEA
N N
N
H N
S O Ru PPh3 CH3 Ph3P CO
Scheme 2. Synthesis of Ru(II) TSC complex.
Fig. 1. Molecular structure of (1) showing the 30% probability level and the solvent molecule (CHCl3) is not shown for clarity. All hydrogen atoms are omitted for clarity.
corresponding to the NH and OH protons appeared in the regions of 2.4 and 4.4 ppm respectively. Further, the molecular structure of complex 1 was resolved by a single crystal X-ray crystallography (Fig. 1) to confirm the coordination mode of the pyridoxal thiosemicarbazone. Single crystals of the complex 1.CHCl3, as solvate were obtained by the slow evaporation of CHCl3–MeOH solution of the complex at room temperature. The ligand coordinates to the Ru(II) ion via the phenolate oxygen, azomethine nitrogen, and thiolate sulfur forming one five-membered and one sixmembered chelate rings. Carbonyl group trans to the azomethine nitrogen is coordinated to the Ru(II) ion to form an ONSC square-plane and the triphenylphosphine ligands occupy the two axial sites. Ruthenium is, therefore, sitting in an ONSCP2 coordination environment, which gives rise to a distorted octahedral geometry as reflected in all the bond parameters around ruthenium(II) ion. The bite angles around the Ru(II) ion are C(16)–Ru(1)–O(1) = 98.5(1)°, P(1)–Ru(1)– C(16) = 90.3(1)°, N(2)–Ru(1)–O(1) = 89.08(9)°, P(1)–Ru(1)– S(1) = 91.42(3)°and N(2)-Ru(1)-P(1) = 90.25(7) and bond lengths of 1.862(4)A° Ru(1)–C(16), 2.094(2)A° Ru(1)–O(1), 2.080(3)A° Ru(1)–N(2) and 2.312A° Ru(1)–S(1). The catalytic activity of the synthesized ruthenium complex was explored for the one-pot synthesis of amides from various aldehydes with hydroxylamine hydrochloride. For the entire optimization, 4-nitrobenzaldehyde was taken as a test substrate for different conditions. To study the influence of solvents in our catalytic system, we have chosen the reaction between 4-nitrobenzaldehyde (1 mmol), NH2OH·HCl (1 mmol), complex 1 (1 mol%) as the catalyst precursor in the presence of various solvents and NaHCO3 (1 mmol) as the base. Benzene, toluene, xylene, acetonitrile, dichloromethane and chloroform are taken for our solvent variation study. The extent of conversion is solvent-dependent and low conversions were observed in benzene, xylene and toluene as solvent even at a higher temperature. Acetonitrile was found to be the solvent of choice with excellent isolated yield of amide (98%) at a lower temperature. The choice of the base was chosen, as a next step for the optimization. No conversion was observed in the absence of the catalyst or base (Table 1). It has been observed that in acetonitrile solvent, NaHCO3 or KHCO3 gave excellent isolated yields of 98% and 91% respectively, when compared to a much weaker base like CH3COONa or Et3N. Thus, it was concluded that NaHCO3 as a base in acetonitrile solvent at 78 °C is the optimized condition for this conversion.
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Table 1 Optimization of solvent, temperature and basea.
Table 3 One-pot synthesis of amides from aldehydes by using complex (1)a.
Yieldb (%)
TONc
1
88
880
2
82
820
3
77
770
4
71
710
5
91
910
6
90
900
7
89
890
8
83
830
9
69
690
10
73
730
11
79
790
12
66
660
13
70
700
Entry Entry
Solvent
Base
Temp (°C)
Time (h)
Yieldb (%)
1 2 3 4 5 6 7 8 9
Xylene Toluene CH3CN Benzene CH3CN CH3CN CH3CN CH3CN CH3CN
NaHCO3 NaHCO3 NaHCO3 NaHCO3 KHCO3 CH3COONa Et3N NaHCO3 –
140 110 78 80 78 78 78 78 78
24 24 5 24 5 24 24 24 24
28 70 98 23 91 b20 b20 –c –
a Conditions: Catalyst (1 mol%) 4-nitro benzaldehyde (1 mmol), NH2OH·HCl (1 mmol), base (1 mmol) and 3 ml solvent. b Isolated yield after column chromatography (average of two runs). c Reaction carried out in the absence of catalyst.
In order to optimize the effect of catalyst loading, different molar ratios were tested in the reaction of 4-nitrobenzaldehyde (1 mmol) using catalyst 1 in the presence of NH2OH·HCl, in CH3CN solvent and NaHCO3 (Table 2). The reaction proceeds with high conversion of 4-nitrobenzaldehyde to 4-nitrobenzamide when C: S ratio of 1:1000 (0.1 mol%) is used. When increasing the molar ratio to 0.05 mol%, 0.03 mol%, 0.025 mol% the reaction proceeds with good yields. Further, the catalyst works well with low catalyst loading of 0.02 mol% and 0.01 mol% and shows a yield of 68% and 60%, respectively. Since the isolated yields are good with appreciable turnover numbers when catalyst loading is 0.1 mol%, the C:S ratio of 1:1000 is the best compromise for the optimum reaction rates. The progress of formation of 4-nitrobenzamide as a function of time using the above optimized conditions (supporting information). The results indicate the formation of 4-nitrobenzamide initially increased with the progress of the reaction, reached a maximum and then remain unchanged. A high yield was observed for the formation of amide at the optimum reaction time of 5 h. No noticeable improvement was observed even after extending the reaction time to 7 h. To explore the scope of the new catalyst, a range of other substituted aromatic and heterocyclic aldehydes were converted to primary amides using catalyst 1 under the optimized condition. All the reactions were carried out under identical reaction conditions to allow comparison of results. A broad range of amides were successfully synthesized with good to high isolated yields using the above optimized protocol. The results collected from the catalytic reactions are listed in Table 3. The electron donating groups like –CH3, –OCH3 and–OH (entries 2, 3 and Table 2 Effect of catalyst loadinga.
Entry
Ru mol %
Time
Yieldb(%)
TONc
1 2 3 4 5 6 7 8
1 0.2 0.1 0.05 0.03 0.025 0.02 0.01
5 5 5 5 5 5 5 5
98 97 91 82 77 74 68 60
98 485 910 1640 2310 2960 3400 6000
a
Conditions: Catalyst (1–0.01 mol) 4-nitrobenzaldehyde (1 mmol), NH2OH·HCl (1 mmol), NaHCO3 (1 mmol) and 3 ml CH3CN. b Isolated yield after column chromatography (average of two runs). c Turnover number = ratio of moles of product formed to moles of catalyst used.
Aldehydes
Amides
a Conditions: Catalyst (0.1 mol%) 4-nitro benzaldehyde (1 mmol), NH2OH·HCl (1 mmol), NaHCO 3 (1 mmol) and 3 ml CH3CN. b Isolated yield after column chromatography (average of two runs). c Turnover number = ratio of moles of product formed to moles of catalyst used.
A. Kanchanadevi et al. / Inorganic Chemistry Communications 56 (2015) 116–119
4) on benzaldehyde alters the reactions and the corresponding amides were obtained in good yields of 82%, 77% and 75% respectively and gave slightly lower yields compared with benzaldehyde. On the other hand, electron withdrawing substituents, such as the –NO2, –Cl and –Br substituents (entries 5, 6 and 7) offering excellent yields (91%, 90% and 89%) when compared to substrate containing electron donating group. The introduction of electron-withdrawing substituents to the para position of the aryl ring of the aldehyde decreased the electron density on the C_O bond so that the activity was improved giving rise to easier amidation reaction. Heterocyclic amides are the basic skeleton in pharmaceutical drugs such as anticonvulsant, antidepressant, anti-inflammatory and anti-glycation activities [17]. Hence, the synthesis of heterocyclic amides from the respective aldehydes using catalyst 1 has been performed. It has been found that the reaction proceeded smoothly even in the presence of heteroatoms such as S and N in the substrates (entries 8–13) and a range of heterocyclic aromatic amides were obtained in good isolated yields. It is worth noting that by-products such as nitriles or carboxylic acids are not observed in this protocol. In addition, ketones such as acetophenone and 2-acetylpyridine were inert to this present experimental conditions indicating that this reaction differs from the Beckmann rearrangement. An important goal of this work is to study the recyclability of our ruthenium catalyst. For practical application of any catalyst systems, the life time of the catalyst and its level of reusability are very important factors. Thus, recycling experiments were conducted by using catalyst 1 in the reaction of 4-nitrobenzaldehyde to 4-nitrobenzamide. The results are presented in the supporting information show that catalyst could be used four times without much activity loss. As a result of its high polarity, good thermal stability and insensitivity to moisture and oxygen, catalyst proved to be an efficient and recyclable catalyst for the one-pot synthesis of aldehyde to amide reaction. Furthermore, the ease by which these catalysts are prepared offers another important advantage. The work-up process is very simple for this catalytic system, as the catalyst is stable in all organic solvents. Further, we believe that the catalytic transformation proceeds via the oxidative addition of the aldoxime N–OH bond to Ru(II), followed by nucleophilic attack on the coordinated imine, then β-elimination of cyclometalated, and finally reductive elimination to give the amide according to the proposed mechanism by Crabtree [10]. In this reaction water cannot eliminate as a by-product and indicates the absence of nitrile products. The present Ru(II) catalyst is more efficient in amidation reaction than the reported ruthenium(II) complexes in terms of reaction time, catalyst loading and isolated yields [9,13,18]. In summary, a new air stable Ru(II) complex bearing pyridoxal `ligand was synthesized and characterized. The single crystal X-ray diffraction study evidenced the O, N and S co-ordination mode of the ligand and the presence of distorted octahedral geometry around Ru(II) ion. The utility of the new complex as excellent catalyst for one-pot synthesis of primary amide has been highlighted by the reaction of aryl- and heteroaryl aldehydes with NH2OH·HCl and NaHCO3 as an additive. The reported recyclable ruthenium(II) catalyst (0.1 mol%) efficiently catalyzed the conversion of aldehydes to primary amides with yields up to 91%.
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Acknowledgments One of the authors, A.K., thanks UGC, for RFSMS fellowship. CEFIPRA is gratefully acknowledged for research collaboration (Project No. 5005-1). We thank DST-FIST, India for the NMR instrumental facility at the School of Chemistry, Bharathidasan University.
Appendix A. Supplementary material Experimental procedure, 1H NMR spectrum for the complex 1, X-ray data, selected bond lengths and bond angles for complex 1. Crystallographic data for the structural analysis have been deposited with Cambridge crystallographic center, CCDC No. 1034924. 1H-NMR spectra for all the amides. Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2015.04.006.
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