Macromolecular Rapid Communications
Communication
An Efficient N-Heterocyclic CarbeneRuthenium Complex: Application Towards the Synthesis of Polyesters and Polyamides Jagadeesh Malineni, Helmut Keul,* Martin Möller*
The ruthenium benzimidazolylidene-based N-heterocyclic carbene (NHC) complex 4 catalyzes the direct dehydrogenative condensation of primary alcohols into esters and primary alcohols in the presence of amines to the corresponding amides in high yields. This efficient new catalytic system shows a high selectivity towards the conversion of diols to polyesters and of a mixture of diols and diamines to polyamides. The only side product formed in this reaction is molecular hydrogen. Remarkable is the conversion of hydroxytelechelic polytetrahydrofuran (Mn = 1000 g mol−1)—a polydispers starting material—into a hydrolytically degradable polyether with ester linkages ( Mn = 32 600 g mol−1) and, in the presence of aliphatic diamines, into a polyether with amide linkages in the back bone ( Mn = 16 000 g mol−1).
1. Introduction The design of new synthetic strategies for polymers used in daily life is a challenging task from academic point of view and is of technical and economic interest. Polyesters, polyamides, and poly(ester amide)s are important classes of polymers, which can be produced from renewable recourses and are biodegradable materials of high importance for biomedical application.[1,2] The general method used for the preparation of polyesters and polyamides is step growth reaction—polycondensation of alcohols or of amines with carboxylic acids, respectively, activated carboxylic acid derivatives[3] in the presence of suitable catalysts or strong coupling reagents.[4] Alternatively polyester and polyamides can be prepared by
Dr. J. Malineni, Dr. H. Keul, Prof. M. Möller DWI – Leibniz Institute for Interactive Materials and Institute of Technical and Macromolecular Chemistry, RWTH Aachen University, Forckenbeckstraße 50, D-52074 Aachen, Germany E-mail: [email protected]; [email protected]
chain growth reaction—ring opening polymerization of lactones or lactams.[5] All these syntheses require the use of activated carboxylic acids, or harsh reaction conditions and in some cases produce stoichiometric amounts of toxic wastes. Due to these limitations in the synthesis of polyesters and polyamides for both technical and biomedical applications an atom economic synthesis that avoids toxic reagents, toxic waste generation as well as harsh reaction conditions is a highly challenging goal. Recently, Milstein and co-workers[6] extensively reported on a remarkable electron-rich hemilabile rutheniumbased PNN pincer complex 1, which readily catalyzes the dehydrogenative coupling of primary alcohols to esters and of primary alcohols and amines to amides liberating molecular hydrogen as byproduct without use of any base or acid activators. This highly efficient catalyst was further applied for the hydrogenation of esters to alcohols, and the dehydrogenation of secondary alcohols to ketones.[7] Based on these promising results of the Milstein group recent developments in several other groups showed that direct synthesis of esters from alcohols as well as of amides from alcohols and amines is possible
Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201400699
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Scheme 1. NNP and NHC ruthenium complexes.
by using various Ru, Rh, and Ag-based catalyst systems.[8] More recently, Zeng and Guan[9] and Milstein and coworkers[10] reported novel protocols for the direct synthesis of high-molecular-weight polyamides from diols and diamines by catalytic dehydrogenation using the ruthenium-based catalysts 1 and 2 (see Scheme 1). The Madson[11] group further applied the Milstein catalyst for the direct synthesis of high-molecular-weight polyesters from diols through polycondensation under reduced pressure. The main draw back of these PNN ruthenium-pincer catalysts is their high sensitivity towards oxygen; consequently, these catalysts are handled in the glove box. Currently, metal complexes of N-heterocyclic carbenes (NHC)s have proven to be intermediates in the conversion of different substrates: C,C bond formation, oxidation, and reduction.[12,13] The NHC ligands act as strong σ–donors and weak π-acceptors, which increases the electronic density at the metal center, thus improving the catalytic activity.[13] The nature of the carbon-metal bond combines the high stability of the carbene complexes against moisture, heat, and air with a high reactivity of the catalyst at low concentration.[14] On the basis of these exciting properties of the NHC carbene complexes, Hong[15] and several other groups[16] developed the imidazole-based NHC ruthenium complex 3 and applied this complex for the direct synthesis of esters and amides; however, no polyester and polyamide syntheses were reported. Our group has recently reported on a novel benzimidazole-based NHC ruthenium complex 4.[17] Compared to the imidazole-based NHC ruthenium complex 3, the benzimidazole-based NHC ruthenium complex 4 has shown higher reactivity in the conversion of the primary alcohols to esters as well as the conversion of primary alcohols in the presence of primary amines to amides using a low catalyst loading. However, in the synthesis of high-molecularweight polyamides from diols and diamines catalyst 4 was not highly efficient due insufficient electron density at the metal center. To overcome this problem, we introduced an electron-rich phosphine ligand (tricyclohexylphosphine, PCy3) into the reaction medium. We assume that by ligand exchange the electron density at the metal center is increased, leading to a significant improvement of the catalytic activity, which raises the efficiency in the synthesis of high-molecular-weight polyesters and polyamides.
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In this communication, we report an efficient and simple protocol for the direct synthesis of high-molecularweight polyesters from diols and of polyamides from diols and diamines using the benzimidazole-based NHC ruthenium complex 4 in combination with the tricyclohexylphosphine ligand. This is a highly atom economic, and environmentally friendly procedure that avoids the use of additional oxidizing agents, activators, and other additives. The only by product in this conversion is molecular hydrogen.
2. Results and Discussion To examine the influence of tricyclohexylphosphine on the catalytic dehydrogenation, for preliminary studies, hexanol was selected as a model substrate and the reaction was performed in refluxing toluene under inert gas atmosphere using catalyst 4. With 1 mol% of catalyst loading and 3 mol% of KOtBu, in the absence of PCy3 dehydrogenation of hexanol led to hexyl hexanoate in low yield within 24 h (41%). The presence of 2 mol% of the electron-donating phosphine ligand (PCy3) in the reaction medium leads to an increase of the yield up to 96% (Scheme 2). We assumed that the phosphine ligand coordinates to the Ruthenium, thus increasing the electron density at the metal center, generating a highly active species in the reaction medium, which undergoes partial dehydrogenation of hexanol and formation of hexyl hexanoate (Supporting Information). Next, we turned our attention towards the direct synthesis of amides from primary alcohols in combination with primary amines by the catalytic dehydrogenation method. A first experiment was carried out using an equimolar mixture of hexanol and hexylamine in the presence of complex 4 (1 mol%), PCy3 (2 mol%), and KOtBu (3 mol%), in toluene at reflux for 24 h, leading to hexyl hexanamide in 92% yield (Supporting Information). This result is indicating that the electron-rich phosphine ligand shows a significant effect on the amide synthesis. Under similar conditions, a range of different alcohols were reacted with amines to afford the corresponding amides in 42%–98% isolated yield (Table 1). Aliphatic alcohols with aliphatic amines or even benzyl amine (Table 1, entry 1–3) as well as benzyl alcohol, 2-phenyl ethanol, and 3-phenyl propanol with benzyl amine or hexylamine (Table 1, entry 4–6) result in high yields of the corresponding amides. Reaction of
Scheme 2. Synthesis of hexyl hexanoate from hexanol.
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Table 1. Direct synthesis of amides from primary alcohols and amines.
Entrya)
Alcohol
Amine
Amide
Yieldb)
1
92
2
96
3
89
4
84
5
91
6
98 76
7 —
8
68
9
85
10
42
a)Reaction conditions: 1 mmol of amine, 1 mmol of alcohol, 1 mol% catalyst 4, 2 mol% PCy3, 3 mol% of base, toluene (1 mL), reflux, 24 h; b)Isolated yield.
hex-5-en-1-ol with benzyl amine yields exclusively the N-benzyl hexanamide, in which the C,C double bond was hydrogenated by the molecular hydrogen liberated during the dehydrogenation of the primary alcohol (Table 1, entry 7). Formation of a five-membered lactam was observed by intramolecular dehydrogenative amidation of 5-aminopentan-1-ol (Table 1, entry 8). In case of the less nucleophilic aniline, reaction with 2-phenyl ethanol results in the corresponding amide in low yield (42%) (Table 1, entry 10). In order to investigate the potential of the novel catalytic system, we shifted our attention towards the direct synthesis of high-molecular-weight polyesters. We first examined the conversion of 1,10-decanediol by running a model reaction with 1 mol% catalyst 4, 2 mol% PCy3 ligand, and 3 mol% of KOtBu in refluxing toluene under inert gas atmosphere for 48 h. NMR and size-exclusion
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Table 2. Synthesis of polyesters by catalytic dehydrogenation of diols.
M n b) (Pn)
Ð
Yieldc)
1
n.d.
n.d.
n.d
2
n.d.
n.d.
90d)
3
4500 (39)
1.81
67
4
9800 (58)
1.28
81
5
12 100 (61)
1.45
72
6
32 600 (32)
1.44
84
Entrya)
Diols
a)Reaction conditions: 1 mmol of diol, 5 mol% catalyst 4, 10 mol% PCy3, 15 mol% of base, toluene (1 mL), reflux, 48 h; b)Determined by size-exclusion chromatography (SEC); c)Isolated yield; d)Lactone formation. Degree of polymerization (Pn) and Mn were not determined (n.d).
chromatography (SEC) analysis revealed the formation of an oligomeric mixture of polyesters (Mn = 800 g mol−1). An attempt to improve the result for the polymerization of 1,10-decanediol by increasing the catalyst loading to 5 mol% 4, 10 mol% PCy3 ligand, and 15 mol% of KOtBu leads to the formation of polyester (81%) with a molecular weight Mn = 9800 g mol−1, a dispersity Ð = 1.28, and a degree of polymerization of Pn = 58 (Table 2, Entry 4). To test the limits of this method, various diols with linear aliphatic spacer were subjected to the reaction conditions mentioned before (Table 2). By increasing the number of carbon atoms in the spacer from 6 to 10 and 12, the molecular weight and the degree of polymerization of the corresponding polyesters increase (Table 2, entry 3, 4, and 5). Best results were obtained in the polymerization of 1,12-dodecanediol with an Mn approaching 12 100 g mol−1, with Ð = 1.45 and Pn = 61 (Table 2, entry 5). In contrast, the dehydrogenation of 1,4-butanediol favors cyclization to yield γ-butyrolactone in 90% isolated yields (Table 2, entry 2). Furthermore, the reactivity of the catalyst toward 1,3-propanediol was greatly reduced, only a trace amount of β-propiolactone and oligomers were observed by 1H NMR analysis (Table 2, entry 1). These results show that a minimum spacer length of five carbon atoms between the two hydroxyl groups is necessary for an efficient conversion to polyester. Is the number of carbon atoms smaller than 5, no polymer is formed. One reason could be chelation of the two hydroxyl groups to the active catalytic center with formation of metalla-cycles.[9,18] Another reason could be the thermodynamic stability of the five-membered ring lactone
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Scheme 3. Hydrolysis of polyesters and esterification.
compared to corresponding linear chains. Surprisingly, the novel catalytic system shows a high reactivity towards the conversion of hydroxyl-telechelic polytetrahydrofuran (Mn = 1000); high yields (84%) of the corresponding polyester with Mn approaching 32600 g mol−1, Ð = 1.44 and Pn = 32 (Table 2, entry 6) were obtained. This result is remarkable since a polydispers starting material—a polyether—is used and the result is a degradable polyether with ester bonds in the back bone (see Supporting Information). For the polyester derived from 1,12-dodecanediol, we analyzed the components of the resulting polyester. Alkaline hydrolysis of the corresponding polyester, followed by esterification of the product mixture yielded 1,12-dodecanediol and an equimolar mixture of dimethyl dodecanoate and methyl 12-hydroxydodecanoate. This result can be generalized and it can be concluded that for the examples studied the polyester backbone is composed of different
building blocks, dicarboxylic acids, and diols on one hand and hydroxyl-acids on the other hand (Scheme 3). The established reaction conditions were further applied for the direct synthesis of polyamides starting with different diols and diamines. An equimolar mixture of 1,6-hexanediol and 1,6-hexanediamine was subjected to the dehydrogenation with 5 mol% catalyst 4, 10 mol% PCy3 ligand, and 15 mol% of KOtBu in refluxing toluene under inert gas atmosphere for 48 h, which resulted in low-molecular-weight polyamides (Mn = 2.1 kDa). The most plausible reason is that, because of the extensive hydrogen bonding between the amide linkages and poor solubility in toluene, the step growth polymerization was prematurely hampered and leads to precipitation of oligomers. To overcome this problem, we introduced a polar solvent (anisole) into the reaction medium. Due to the polar effect, the intermolecular H-bonding between the amide linkages is broken and the solubility of the low-molecular-weight polyamides in the reaction medium increases, thus allowing the polymerization to proceed. Exploring the limits of the polyamidation reaction, a series of different diols and diamines was reacted yielding polyamides 5a–g (Table 3). By using the optimized reaction
Table 3. Catalytic dehydrogenative polycondensation of diols and diamines.
Entrya)
Product
M n b) (Pn)
Ð
Yieldc)
1
5a
12300 (54)
1.97
72
2
5b
—d)
—
82
3
5c
—d)
—
81
4
5d
16 000 (14)
1.47
84
5
5e
12 000 (10)
2.89
91
6
5f
12 400 (31)
1.56
78
7
5g
7250 (7)
3.5
56
Dialcohols
Diamines
a)Reaction conditions: 1 mmol of diols, 1 mmol of diamine, 5 mol% catalyst 4, 10 mol% PCy , 15 mol% of base, toluene (1 mL), anisole 3 (1 mL), reflux, 48 h; b)Determined by size-exclusion chromatography (SEC); c)Isolated yield; d)SEC analysis was not performed due to insolubility in DMF or THF.
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An Efficient N -Heterocyclic Carbene–Ruthenium Complex: Application Towards the Synthesis . . .
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conditions, a mixture of 1,6-hexanediol and 1,6-hexanediamine in a solution of toluene/anisole (1/1, vol/vol) was refluxed under inert gas atmosphere for 48 h, resulting in polymer 5a with Mn = 12.3 kDa with 72% yield (Table 3, entry 1). The obtained polyamide was characterized by 1H NMR and IR spectroscopy (see Supporting Information). For diamines with longer alkyl chains, the resulting polymer is less polar and consequently shows a higher solubility Figure 1. Thermal studies of polyamide 5e a) DSC b) TGA. in the reaction medium; the consequence is a higher monomer conversion. Reaction of 1,12-dodecanediol with 1,12-dodecandiamin (Figure 1a). Thermogravimetric analysis (TGA) of the under similar conditions yielded polyamide 5b in 82% polyamide 5e exhibits high stability up to 350 °C when yield (Table 3, entry 2). The dehydrogenative condensadecomposition starts; 90% weight loss is observed at tion of aromatic alcohols with aliphatic diamines leads to 462 °C (Figure 1b). corresponding polyamides having aromatic and aliphatic segments. The reaction of 1,4-phenylenedimethanol with 1,12-dodecanediamine leads to polyamides 5c (Table 3, 3. Conclusion entry 3). Under similar condition, in order to increase the solubility of the polyamides, glycol-substituted aromatic diols were used. Reaction of hydroquinone bis(2We have developed an efficient protocol for the direct synhydroxyethyl)ether and 1,12-dodecanediamine gave 78% thesis of polyesters from diols and of polyamides from diols of polyamide 5f with Mn = 12.4 kDa and Ð = 1.56 (Table 3, and diamines by catalytic dehydrogenation polycondensation, hydrogen being the only side product. In the presence entry 6) (see Supporting Information). of an electron-rich phosphine ligand, the NHC–ruthenium The efficiency of the catalytic system was proven complex 4 showed high efficiency for the conversion of by reaction of hydroxytelechelic polytetrahydrofuran aliphatic diols and polyether-based diols to corresponding with different aliphatic diamines resulting in the corpolyesters; high yields and molecular weights of 4.5 < responding polyamides in high yields. The polyamidation reaction of polytetrahydrofuran and 1,6-hexanediMn < 32.5 kDa with dispersities in the range of 1.28 < Ð < amine gave 84% of polyamide 5d with Mn = 16.0 kDa 1.81 were obtained. At the time, we started studies on the interaction of PCy3 with the NHC-based Ruthenium cataand Ð = 1.47 (Table 3, entry 4). Under similar condition, the reaction of polytetrahydrofuran with a long-chain lyst. The working hypothesis for this catalytic system comaliphatic diamine (1,12-dodecandiamin) leads to polyamprising the benzimidazolylidene-based NHC ruthenium ides 5e in 91% yield and SEC analysis of the polyamide catalyst and PCy3 is the following: i) the p-cymene ligand 5e exhibits Mn = 12.0 kDa with Ð = 2.89 (Table 3, entry in complex 4 is replaced by the PCy3 ligand; ii) the two 5). Conversion of polytetrahydrofuran with O,O′-bis(2chlorine atoms are then replaced with hydride through aminoethyl)-hexa-(ethylene glycol) afforded a low-molecalkoxide substitution and β-hydride elimination, which ular-weight polyamide 5g in 56% yield. The SEC analysis leads to an electronically rich active ruthenium complex. of 5g showed a molecular weight Mn = 7.2 kDa correThis highly catalytically active complex is suited also for an efficient synthesis of polyamide building blocks. Howsponding to a degree of polymerization Pn = 7 (Table 3, ever, the polarity of the solvent used strongly influences entry 7) with Ð = 3.5. the course of the reaction. There are two oppositely acting For the most interesting polyamide 5e composed of effects induced by the polarity of the solvent: on one hand, polytetrahydrofuran and 1,12-dodecanediamine, we higher solvent polarity leads to higher solubility of the poldetermined the thermal properties. Differential scanyamide formed. On the other hand, higher polar solvents ning calorimetry (DSC) analysis of polyamide 5e shows interact with Ruthenium and reduce its catalytic activity. on first heating a melting point with maximum at We studied a variety of solvents and solvent mixtures; 74.7 °C (ΔHm1 = 14.1 J g−1) and on second heating a melting however, the best result is obtained in refluxing toluene. point with maximum at 77.3 °C, ΔHm2 = 13.0 J g−1); The molecular weights of the polyamides prepared were in on cooling crystallization is observed with a maximum at the range of 7.2 < Mn < 16.0 kDa with dispersities in the 58.3 °C (ΔHcr = 13.1 J g−1). This result proves that a few amide groups in a polymer chain can induce crystallization range of 1.47 < Ð < 3.5.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: This study was performed within the Interreg Euregio Meuse-Rhine IV-A consortium “BioMiMedics” (2011–2014) financed through generous contributions of the European Union (through Interreg IV-A) and the government of North Rhine-Westphalia (Germany).
[8]
[9] [10] [11]
Received: December 6, 2014; Revised: December 27, 2014; Published online: ; DOI: 10.1002/marc.201400699
[12]
Keywords: dehydrogenation of alcohols; polyamides; polyesters; ruthenium benzimidazolylidene N-heterocyclic carbene complex [13] [1] a) G. Odian, Principles of Polymerization, 4th ed., Wiley, Hoboken, NJ 2004; b) J. M. Garcia, F. C. Garcia, F. Serna, J. L. de la Pena, Prog. Polym. Sci. 2010, 35, 623; c) R. Langer, D. A. Tirrell, Nature 2004, 428, 487; d) M. A. Mintzer, E. E. Simanek, Chem. Rev. 2009, 109, 259; e) E. S. Place, N. D. Evans, M. M. Stevens, Nat. Mater. 2009, 8, 457. [2] a) K. Pang, R. Kotek, A. Tonelli, Prog. Polym. Sci. 2006, 31, 1009; b) M. Okada, Prog. Polym. Sci. 2002, 27, 87; c) M. J. L. Tschan, E. Brulé, P. Haquette, C. M. Thomas, Polym. Chem. 2012, 3, 836. [3] R. C. Jeske, A. M. DiCiccio, G. W. Coates, J. Am. Chem. Soc. 2007, 129, 11330. [4] a) M. Ueda, M. Kakuta, T. Morosumi, R. Sato, Polym. J. 1991, 23, 167; b) W. C. Chan, P. D. White, Fmoc Solid Phase Peptide Synthesis: A Practical Approach, Oxford University Press, New York 2000. [5] a) J. H. Zhang, D. A. Kissounko, S. E. Lee, S. H. Gellman, S. S. Stahl, J. Am. Chem. Soc. 2009, 131, 1589; b) T. J. Deming, Adv. Polym. Sci. 2006, 202, 1. [6] C. Gunanathan, Y. Ben-David, D. Milstein, Science 2007, 317, 790. [7] a) J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, J. Am. Chem. Soc. 2005, 127, 10840; b) J. Zhang, M. Gandelman, L. J. W. Shimon, D. Milstein, Dalton Trans. 2007, 107; c) J. Zhang, M. Gandelman, L. J. W. Shimon, D. Milstein, Organometallics 2004, 23, 4026; d) J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, Angew. Chem. 2006, 118, 1131; Angew. Chem. Int. Ed. 2006, 45, 1113; e) D. Milstein, Top. Catal. 2010, 53, 915; f) C. Gunanathan, D. Milstein, Acc. Chem. Res. 2011, 44, 588; g) J. R. Khusnutdinova, Y. Ben-David, D. Milstein, J. Am.
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Communication
An Efficient N-Heterocyclic CarbeneRuthenium Complex: Application Towards the Synthesis of Polyesters and Polyamides Jagadeesh Malineni, Helmut Keul,* Martin Möller*
The ruthenium benzimidazolylidene-based N-heterocyclic carbene (NHC) complex 4 catalyzes the direct dehydrogenative condensation of primary alcohols into esters and primary alcohols in the presence of amines to the corresponding amides in high yields. This efficient new catalytic system shows a high selectivity towards the conversion of diols to polyesters and of a mixture of diols and diamines to polyamides. The only side product formed in this reaction is molecular hydrogen. Remarkable is the conversion of hydroxytelechelic polytetrahydrofuran (Mn = 1000 g mol−1)—a polydispers starting material—into a hydrolytically degradable polyether with ester linkages ( Mn = 32 600 g mol−1) and, in the presence of aliphatic diamines, into a polyether with amide linkages in the back bone ( Mn = 16 000 g mol−1).
1. Introduction The design of new synthetic strategies for polymers used in daily life is a challenging task from academic point of view and is of technical and economic interest. Polyesters, polyamides, and poly(ester amide)s are important classes of polymers, which can be produced from renewable recourses and are biodegradable materials of high importance for biomedical application.[1,2] The general method used for the preparation of polyesters and polyamides is step growth reaction—polycondensation of alcohols or of amines with carboxylic acids, respectively, activated carboxylic acid derivatives[3] in the presence of suitable catalysts or strong coupling reagents.[4] Alternatively polyester and polyamides can be prepared by
Dr. J. Malineni, Dr. H. Keul, Prof. M. Möller DWI – Leibniz Institute for Interactive Materials and Institute of Technical and Macromolecular Chemistry, RWTH Aachen University, Forckenbeckstraße 50, D-52074 Aachen, Germany E-mail: [email protected]; [email protected]
chain growth reaction—ring opening polymerization of lactones or lactams.[5] All these syntheses require the use of activated carboxylic acids, or harsh reaction conditions and in some cases produce stoichiometric amounts of toxic wastes. Due to these limitations in the synthesis of polyesters and polyamides for both technical and biomedical applications an atom economic synthesis that avoids toxic reagents, toxic waste generation as well as harsh reaction conditions is a highly challenging goal. Recently, Milstein and co-workers[6] extensively reported on a remarkable electron-rich hemilabile rutheniumbased PNN pincer complex 1, which readily catalyzes the dehydrogenative coupling of primary alcohols to esters and of primary alcohols and amines to amides liberating molecular hydrogen as byproduct without use of any base or acid activators. This highly efficient catalyst was further applied for the hydrogenation of esters to alcohols, and the dehydrogenation of secondary alcohols to ketones.[7] Based on these promising results of the Milstein group recent developments in several other groups showed that direct synthesis of esters from alcohols as well as of amides from alcohols and amines is possible
Macromol. Rapid Commun. 2015, DOI: 10.1002/marc.201400699
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Scheme 1. NNP and NHC ruthenium complexes.
by using various Ru, Rh, and Ag-based catalyst systems.[8] More recently, Zeng and Guan[9] and Milstein and coworkers[10] reported novel protocols for the direct synthesis of high-molecular-weight polyamides from diols and diamines by catalytic dehydrogenation using the ruthenium-based catalysts 1 and 2 (see Scheme 1). The Madson[11] group further applied the Milstein catalyst for the direct synthesis of high-molecular-weight polyesters from diols through polycondensation under reduced pressure. The main draw back of these PNN ruthenium-pincer catalysts is their high sensitivity towards oxygen; consequently, these catalysts are handled in the glove box. Currently, metal complexes of N-heterocyclic carbenes (NHC)s have proven to be intermediates in the conversion of different substrates: C,C bond formation, oxidation, and reduction.[12,13] The NHC ligands act as strong σ–donors and weak π-acceptors, which increases the electronic density at the metal center, thus improving the catalytic activity.[13] The nature of the carbon-metal bond combines the high stability of the carbene complexes against moisture, heat, and air with a high reactivity of the catalyst at low concentration.[14] On the basis of these exciting properties of the NHC carbene complexes, Hong[15] and several other groups[16] developed the imidazole-based NHC ruthenium complex 3 and applied this complex for the direct synthesis of esters and amides; however, no polyester and polyamide syntheses were reported. Our group has recently reported on a novel benzimidazole-based NHC ruthenium complex 4.[17] Compared to the imidazole-based NHC ruthenium complex 3, the benzimidazole-based NHC ruthenium complex 4 has shown higher reactivity in the conversion of the primary alcohols to esters as well as the conversion of primary alcohols in the presence of primary amines to amides using a low catalyst loading. However, in the synthesis of high-molecularweight polyamides from diols and diamines catalyst 4 was not highly efficient due insufficient electron density at the metal center. To overcome this problem, we introduced an electron-rich phosphine ligand (tricyclohexylphosphine, PCy3) into the reaction medium. We assume that by ligand exchange the electron density at the metal center is increased, leading to a significant improvement of the catalytic activity, which raises the efficiency in the synthesis of high-molecular-weight polyesters and polyamides.
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In this communication, we report an efficient and simple protocol for the direct synthesis of high-molecularweight polyesters from diols and of polyamides from diols and diamines using the benzimidazole-based NHC ruthenium complex 4 in combination with the tricyclohexylphosphine ligand. This is a highly atom economic, and environmentally friendly procedure that avoids the use of additional oxidizing agents, activators, and other additives. The only by product in this conversion is molecular hydrogen.
2. Results and Discussion To examine the influence of tricyclohexylphosphine on the catalytic dehydrogenation, for preliminary studies, hexanol was selected as a model substrate and the reaction was performed in refluxing toluene under inert gas atmosphere using catalyst 4. With 1 mol% of catalyst loading and 3 mol% of KOtBu, in the absence of PCy3 dehydrogenation of hexanol led to hexyl hexanoate in low yield within 24 h (41%). The presence of 2 mol% of the electron-donating phosphine ligand (PCy3) in the reaction medium leads to an increase of the yield up to 96% (Scheme 2). We assumed that the phosphine ligand coordinates to the Ruthenium, thus increasing the electron density at the metal center, generating a highly active species in the reaction medium, which undergoes partial dehydrogenation of hexanol and formation of hexyl hexanoate (Supporting Information). Next, we turned our attention towards the direct synthesis of amides from primary alcohols in combination with primary amines by the catalytic dehydrogenation method. A first experiment was carried out using an equimolar mixture of hexanol and hexylamine in the presence of complex 4 (1 mol%), PCy3 (2 mol%), and KOtBu (3 mol%), in toluene at reflux for 24 h, leading to hexyl hexanamide in 92% yield (Supporting Information). This result is indicating that the electron-rich phosphine ligand shows a significant effect on the amide synthesis. Under similar conditions, a range of different alcohols were reacted with amines to afford the corresponding amides in 42%–98% isolated yield (Table 1). Aliphatic alcohols with aliphatic amines or even benzyl amine (Table 1, entry 1–3) as well as benzyl alcohol, 2-phenyl ethanol, and 3-phenyl propanol with benzyl amine or hexylamine (Table 1, entry 4–6) result in high yields of the corresponding amides. Reaction of
Scheme 2. Synthesis of hexyl hexanoate from hexanol.
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Table 1. Direct synthesis of amides from primary alcohols and amines.
Entrya)
Alcohol
Amine
Amide
Yieldb)
1
92
2
96
3
89
4
84
5
91
6
98 76
7 —
8
68
9
85
10
42
a)Reaction conditions: 1 mmol of amine, 1 mmol of alcohol, 1 mol% catalyst 4, 2 mol% PCy3, 3 mol% of base, toluene (1 mL), reflux, 24 h; b)Isolated yield.
hex-5-en-1-ol with benzyl amine yields exclusively the N-benzyl hexanamide, in which the C,C double bond was hydrogenated by the molecular hydrogen liberated during the dehydrogenation of the primary alcohol (Table 1, entry 7). Formation of a five-membered lactam was observed by intramolecular dehydrogenative amidation of 5-aminopentan-1-ol (Table 1, entry 8). In case of the less nucleophilic aniline, reaction with 2-phenyl ethanol results in the corresponding amide in low yield (42%) (Table 1, entry 10). In order to investigate the potential of the novel catalytic system, we shifted our attention towards the direct synthesis of high-molecular-weight polyesters. We first examined the conversion of 1,10-decanediol by running a model reaction with 1 mol% catalyst 4, 2 mol% PCy3 ligand, and 3 mol% of KOtBu in refluxing toluene under inert gas atmosphere for 48 h. NMR and size-exclusion
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Table 2. Synthesis of polyesters by catalytic dehydrogenation of diols.
M n b) (Pn)
Ð
Yieldc)
1
n.d.
n.d.
n.d
2
n.d.
n.d.
90d)
3
4500 (39)
1.81
67
4
9800 (58)
1.28
81
5
12 100 (61)
1.45
72
6
32 600 (32)
1.44
84
Entrya)
Diols
a)Reaction conditions: 1 mmol of diol, 5 mol% catalyst 4, 10 mol% PCy3, 15 mol% of base, toluene (1 mL), reflux, 48 h; b)Determined by size-exclusion chromatography (SEC); c)Isolated yield; d)Lactone formation. Degree of polymerization (Pn) and Mn were not determined (n.d).
chromatography (SEC) analysis revealed the formation of an oligomeric mixture of polyesters (Mn = 800 g mol−1). An attempt to improve the result for the polymerization of 1,10-decanediol by increasing the catalyst loading to 5 mol% 4, 10 mol% PCy3 ligand, and 15 mol% of KOtBu leads to the formation of polyester (81%) with a molecular weight Mn = 9800 g mol−1, a dispersity Ð = 1.28, and a degree of polymerization of Pn = 58 (Table 2, Entry 4). To test the limits of this method, various diols with linear aliphatic spacer were subjected to the reaction conditions mentioned before (Table 2). By increasing the number of carbon atoms in the spacer from 6 to 10 and 12, the molecular weight and the degree of polymerization of the corresponding polyesters increase (Table 2, entry 3, 4, and 5). Best results were obtained in the polymerization of 1,12-dodecanediol with an Mn approaching 12 100 g mol−1, with Ð = 1.45 and Pn = 61 (Table 2, entry 5). In contrast, the dehydrogenation of 1,4-butanediol favors cyclization to yield γ-butyrolactone in 90% isolated yields (Table 2, entry 2). Furthermore, the reactivity of the catalyst toward 1,3-propanediol was greatly reduced, only a trace amount of β-propiolactone and oligomers were observed by 1H NMR analysis (Table 2, entry 1). These results show that a minimum spacer length of five carbon atoms between the two hydroxyl groups is necessary for an efficient conversion to polyester. Is the number of carbon atoms smaller than 5, no polymer is formed. One reason could be chelation of the two hydroxyl groups to the active catalytic center with formation of metalla-cycles.[9,18] Another reason could be the thermodynamic stability of the five-membered ring lactone
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Scheme 3. Hydrolysis of polyesters and esterification.
compared to corresponding linear chains. Surprisingly, the novel catalytic system shows a high reactivity towards the conversion of hydroxyl-telechelic polytetrahydrofuran (Mn = 1000); high yields (84%) of the corresponding polyester with Mn approaching 32600 g mol−1, Ð = 1.44 and Pn = 32 (Table 2, entry 6) were obtained. This result is remarkable since a polydispers starting material—a polyether—is used and the result is a degradable polyether with ester bonds in the back bone (see Supporting Information). For the polyester derived from 1,12-dodecanediol, we analyzed the components of the resulting polyester. Alkaline hydrolysis of the corresponding polyester, followed by esterification of the product mixture yielded 1,12-dodecanediol and an equimolar mixture of dimethyl dodecanoate and methyl 12-hydroxydodecanoate. This result can be generalized and it can be concluded that for the examples studied the polyester backbone is composed of different
building blocks, dicarboxylic acids, and diols on one hand and hydroxyl-acids on the other hand (Scheme 3). The established reaction conditions were further applied for the direct synthesis of polyamides starting with different diols and diamines. An equimolar mixture of 1,6-hexanediol and 1,6-hexanediamine was subjected to the dehydrogenation with 5 mol% catalyst 4, 10 mol% PCy3 ligand, and 15 mol% of KOtBu in refluxing toluene under inert gas atmosphere for 48 h, which resulted in low-molecular-weight polyamides (Mn = 2.1 kDa). The most plausible reason is that, because of the extensive hydrogen bonding between the amide linkages and poor solubility in toluene, the step growth polymerization was prematurely hampered and leads to precipitation of oligomers. To overcome this problem, we introduced a polar solvent (anisole) into the reaction medium. Due to the polar effect, the intermolecular H-bonding between the amide linkages is broken and the solubility of the low-molecular-weight polyamides in the reaction medium increases, thus allowing the polymerization to proceed. Exploring the limits of the polyamidation reaction, a series of different diols and diamines was reacted yielding polyamides 5a–g (Table 3). By using the optimized reaction
Table 3. Catalytic dehydrogenative polycondensation of diols and diamines.
Entrya)
Product
M n b) (Pn)
Ð
Yieldc)
1
5a
12300 (54)
1.97
72
2
5b
—d)
—
82
3
5c
—d)
—
81
4
5d
16 000 (14)
1.47
84
5
5e
12 000 (10)
2.89
91
6
5f
12 400 (31)
1.56
78
7
5g
7250 (7)
3.5
56
Dialcohols
Diamines
a)Reaction conditions: 1 mmol of diols, 1 mmol of diamine, 5 mol% catalyst 4, 10 mol% PCy , 15 mol% of base, toluene (1 mL), anisole 3 (1 mL), reflux, 48 h; b)Determined by size-exclusion chromatography (SEC); c)Isolated yield; d)SEC analysis was not performed due to insolubility in DMF or THF.
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conditions, a mixture of 1,6-hexanediol and 1,6-hexanediamine in a solution of toluene/anisole (1/1, vol/vol) was refluxed under inert gas atmosphere for 48 h, resulting in polymer 5a with Mn = 12.3 kDa with 72% yield (Table 3, entry 1). The obtained polyamide was characterized by 1H NMR and IR spectroscopy (see Supporting Information). For diamines with longer alkyl chains, the resulting polymer is less polar and consequently shows a higher solubility Figure 1. Thermal studies of polyamide 5e a) DSC b) TGA. in the reaction medium; the consequence is a higher monomer conversion. Reaction of 1,12-dodecanediol with 1,12-dodecandiamin (Figure 1a). Thermogravimetric analysis (TGA) of the under similar conditions yielded polyamide 5b in 82% polyamide 5e exhibits high stability up to 350 °C when yield (Table 3, entry 2). The dehydrogenative condensadecomposition starts; 90% weight loss is observed at tion of aromatic alcohols with aliphatic diamines leads to 462 °C (Figure 1b). corresponding polyamides having aromatic and aliphatic segments. The reaction of 1,4-phenylenedimethanol with 1,12-dodecanediamine leads to polyamides 5c (Table 3, 3. Conclusion entry 3). Under similar condition, in order to increase the solubility of the polyamides, glycol-substituted aromatic diols were used. Reaction of hydroquinone bis(2We have developed an efficient protocol for the direct synhydroxyethyl)ether and 1,12-dodecanediamine gave 78% thesis of polyesters from diols and of polyamides from diols of polyamide 5f with Mn = 12.4 kDa and Ð = 1.56 (Table 3, and diamines by catalytic dehydrogenation polycondensation, hydrogen being the only side product. In the presence entry 6) (see Supporting Information). of an electron-rich phosphine ligand, the NHC–ruthenium The efficiency of the catalytic system was proven complex 4 showed high efficiency for the conversion of by reaction of hydroxytelechelic polytetrahydrofuran aliphatic diols and polyether-based diols to corresponding with different aliphatic diamines resulting in the corpolyesters; high yields and molecular weights of 4.5 < responding polyamides in high yields. The polyamidation reaction of polytetrahydrofuran and 1,6-hexanediMn < 32.5 kDa with dispersities in the range of 1.28 < Ð < amine gave 84% of polyamide 5d with Mn = 16.0 kDa 1.81 were obtained. At the time, we started studies on the interaction of PCy3 with the NHC-based Ruthenium cataand Ð = 1.47 (Table 3, entry 4). Under similar condition, the reaction of polytetrahydrofuran with a long-chain lyst. The working hypothesis for this catalytic system comaliphatic diamine (1,12-dodecandiamin) leads to polyamprising the benzimidazolylidene-based NHC ruthenium ides 5e in 91% yield and SEC analysis of the polyamide catalyst and PCy3 is the following: i) the p-cymene ligand 5e exhibits Mn = 12.0 kDa with Ð = 2.89 (Table 3, entry in complex 4 is replaced by the PCy3 ligand; ii) the two 5). Conversion of polytetrahydrofuran with O,O′-bis(2chlorine atoms are then replaced with hydride through aminoethyl)-hexa-(ethylene glycol) afforded a low-molecalkoxide substitution and β-hydride elimination, which ular-weight polyamide 5g in 56% yield. The SEC analysis leads to an electronically rich active ruthenium complex. of 5g showed a molecular weight Mn = 7.2 kDa correThis highly catalytically active complex is suited also for an efficient synthesis of polyamide building blocks. Howsponding to a degree of polymerization Pn = 7 (Table 3, ever, the polarity of the solvent used strongly influences entry 7) with Ð = 3.5. the course of the reaction. There are two oppositely acting For the most interesting polyamide 5e composed of effects induced by the polarity of the solvent: on one hand, polytetrahydrofuran and 1,12-dodecanediamine, we higher solvent polarity leads to higher solubility of the poldetermined the thermal properties. Differential scanyamide formed. On the other hand, higher polar solvents ning calorimetry (DSC) analysis of polyamide 5e shows interact with Ruthenium and reduce its catalytic activity. on first heating a melting point with maximum at We studied a variety of solvents and solvent mixtures; 74.7 °C (ΔHm1 = 14.1 J g−1) and on second heating a melting however, the best result is obtained in refluxing toluene. point with maximum at 77.3 °C, ΔHm2 = 13.0 J g−1); The molecular weights of the polyamides prepared were in on cooling crystallization is observed with a maximum at the range of 7.2 < Mn < 16.0 kDa with dispersities in the 58.3 °C (ΔHcr = 13.1 J g−1). This result proves that a few amide groups in a polymer chain can induce crystallization range of 1.47 < Ð < 3.5.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements: This study was performed within the Interreg Euregio Meuse-Rhine IV-A consortium “BioMiMedics” (2011–2014) financed through generous contributions of the European Union (through Interreg IV-A) and the government of North Rhine-Westphalia (Germany).
[8]
[9] [10] [11]
Received: December 6, 2014; Revised: December 27, 2014; Published online: ; DOI: 10.1002/marc.201400699
[12]
Keywords: dehydrogenation of alcohols; polyamides; polyesters; ruthenium benzimidazolylidene N-heterocyclic carbene complex [13] [1] a) G. Odian, Principles of Polymerization, 4th ed., Wiley, Hoboken, NJ 2004; b) J. M. Garcia, F. C. Garcia, F. Serna, J. L. de la Pena, Prog. Polym. Sci. 2010, 35, 623; c) R. Langer, D. A. Tirrell, Nature 2004, 428, 487; d) M. A. Mintzer, E. E. Simanek, Chem. Rev. 2009, 109, 259; e) E. S. Place, N. D. Evans, M. M. Stevens, Nat. Mater. 2009, 8, 457. [2] a) K. Pang, R. Kotek, A. Tonelli, Prog. Polym. Sci. 2006, 31, 1009; b) M. Okada, Prog. Polym. Sci. 2002, 27, 87; c) M. J. L. Tschan, E. Brulé, P. Haquette, C. M. Thomas, Polym. Chem. 2012, 3, 836. [3] R. C. Jeske, A. M. DiCiccio, G. W. Coates, J. Am. Chem. Soc. 2007, 129, 11330. [4] a) M. Ueda, M. Kakuta, T. Morosumi, R. Sato, Polym. J. 1991, 23, 167; b) W. C. Chan, P. D. White, Fmoc Solid Phase Peptide Synthesis: A Practical Approach, Oxford University Press, New York 2000. [5] a) J. H. Zhang, D. A. Kissounko, S. E. Lee, S. H. Gellman, S. S. Stahl, J. Am. Chem. Soc. 2009, 131, 1589; b) T. J. Deming, Adv. Polym. Sci. 2006, 202, 1. [6] C. Gunanathan, Y. Ben-David, D. Milstein, Science 2007, 317, 790. [7] a) J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, J. Am. Chem. Soc. 2005, 127, 10840; b) J. Zhang, M. Gandelman, L. J. W. Shimon, D. Milstein, Dalton Trans. 2007, 107; c) J. Zhang, M. Gandelman, L. J. W. Shimon, D. Milstein, Organometallics 2004, 23, 4026; d) J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, Angew. Chem. 2006, 118, 1131; Angew. Chem. Int. Ed. 2006, 45, 1113; e) D. Milstein, Top. Catal. 2010, 53, 915; f) C. Gunanathan, D. Milstein, Acc. Chem. Res. 2011, 44, 588; g) J. R. Khusnutdinova, Y. Ben-David, D. Milstein, J. Am.
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