Efficient conversion of carbon dioxide at atmospheric pressure to 2-oxazolidinones promoted by bifunctional Cu(II)-substituted polyoxometalate-based ionic liquids

Abstract

Copper(II) substituted polyoxometalate-based ionic liquids e.g. [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] were successfully developed as halogen-free bifunctional catalysts for the carboxylative cyclization of propargylic amines with CO₂. Such a CO₂ fixation protocol proceeded smoothly at atmospheric pressure under solvent-free conditions, in an environmentally benign and low energy-input manner. Notably, various propargylic amines could react smoothly to afford 2-oxazolidinones as the target products in high to quantitative yields. Furthermore, the dual activation of both propargylic amine and CO₂ by [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] was studied using NMR techniques and control experiments.

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Introduction

Carbon dioxide chemistry has drawn much attention in the last two decades as CO₂ is an abundant, nontoxic, non-flammable, easily available, and renewable C₁ resource. Various useful chemicals, such as dimethyl carbonate,¹ urethanes,² formic acid,³ methanol,⁴ cyclic carbonates,⁵ polycarbonates,⁶ and others, have been prepared by using CO₂ as a feedstock. However, compared with traditional C₁ sources (e.g., carbon monoxide and phosgene), CO₂ is thermodynamically stable and kinetically inert, which is still the main obstacle in developing practical processes to convert CO₂ into useful chemicals. Therefore, the exploration of efficient catalysts would be crucial to realize CO₂ conversion.

2-Oxazolidinones are important heterocyclic chemicals playing a significant role as chemical intermediates⁷ and chiral auxiliaries⁸ in organic synthesis, and as antibacterial drugs⁹ in pharmaceutical chemistry. Therefore, many efforts have been made to synthesize these useful heterocycles, such as catalytic asymmetric intermolecular aminopalladation¹⁰ and a formal [3 + 2] cycloaddition reaction.¹¹ Recently, the employment of CO₂ as a C₁ building block to construct oxazolidinone motifs has attracted considerable attention.¹² One of the most promising examples of CO₂ fixation to access 2-oxazolidinones is the carboxylative cyclization of propargylic amines with CO₂ (Scheme 1), which represents an important clean and atom economical reaction. As early as in 1964, Dimroth and Pasedach developed a practicable copper-promoted carboxylative cyclization protocol with benzene or tetrahydrofuran as a solvent.¹³ Subsequently, numerous catalytic systems including super bases,¹⁴ silver,¹⁵t-BuOI,¹⁶ and N-heterocyclic carbene–Au complexes¹⁷ have been developed. Very recently, Han and co-workers reported an effective protic ionic liquid as both a non-metal catalyst and solvent for the carboxylative cyclization of propargylic amines with CO₂.^12a They demonstrated that the activation of the amino group is crucial and the intramolecular cyclization is the rate-determining step. Despite the significant advances in the past few decades, the example of CO₂ utilization at atmospheric pressure is limited. Therefore, development of alternative methodologies for the synthesis of 2-oxazolidinones using CO₂ under mild conditions is still highly desirable.

Scheme 1 Carboxylative cyclization of propargylic amines with CO₂.

Polyoxometalate (POM), as a class of large clusters of metal–oxide units, has been widely used in many fields due to their excellent properties.¹⁸ For example, behaving as an oxygen-rich structure, heteropoly acid anions have rich negative charge on the O atom and strong nucleophilicity, leading to being able to activate CO₂ molecules via the formation of the CO₂-adduct.^19,20 Recently, polyoxometalate-based ionic liquids have received considerable attention in epoxidation,²¹ deep desulfurization of fuels,²² esterification,²³ and so on.²⁴ Notably, excellent POM-based catalysis has also been applied in the cycloaddition of epoxides and CO₂,²⁵ direct conversion of styrene into styrene carbonate,²⁶ and reduction of CO₂.¹⁹

On the other hand, copper salts are widely employed as activators of the C≡C bond in organic synthesis.²⁷ We envisioned that the POM anion could activate both the N–H bond of propargylic amines and CO₂, leading to the generation of the carbamate intermediate, the C≡C bond of which is activated by the Cu(II) species, and thus promotes the intramolecular nucleophilic cyclization to afford 2-oxazolidinones. As expected, [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] could serve as a well-defined single-component bifunctional catalyst for simultaneously activating propargylic amines as well as CO₂ molecules (Scheme 2), and thus helped the reaction to efficiently proceed at low CO₂ pressure. Furthermore, the cations and anions of [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] have synergistic effects in catalyzing the reaction.

Scheme 2 Interaction of [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] with CO₂ and propargylic amines.

Results and discussion

A series of transition-metal-substituted polyoxometalate-based ionic liquids [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉M] (M = Cu(II), Co(II), Fe(II), Ni(II), Zn(II), Mn(II)) were synthesized according to the published procedures (see the ESI†).^20a,28 The carboxylative cyclization of N-benzylprop-2-yn-1-amine (1a) was initially chosen as the model reaction under a CO₂ balloon in the absence of any solvent at 60 °C as summarized in Table 1. The reaction without a catalyst was carried out only to recover the starting material quantitatively (entry 1). (ⁿC₇H₁₅)₄NBr and K₈[α-SiW₁₁O₃₉] were found to be ineffective (entries 2 and 3). Interestingly, K₆[α-SiW₁₁O₃₉Cu], CuCl₂, and a mixture of CuCl₂ and (ⁿC₇H₁₅)₄NBr showed moderate activity (entries 4–6), thus indicating the promotion effect of Cu(II) as expected. It was also worth mentioning that K₈[α-SiW₁₁O₃₉] allowed the reaction to produce the carbamate rather than 2-oxazolidinone 2a (entry 3, see ESI, Scheme S1†), also supporting the activation of both propargylic amines and CO₂ by POM anions. These inspired us to investigate the catalysis of [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu]. Gratifyingly, 96% yield of 3-benzyl-5-methyleneoxazolidin-2-one (2a) was obtained under the given conditions (entry 7). In this context, several transition metal-substituted polyoxometalate-based ionic liquids [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉M] [M = Co(II), Fe(II), Ni(II), Zn(II), Mn(II)] were also screened. Among the tested [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉M], [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] showed much higher catalytic activity than Co(II), Fe(II), Ni(II), Zn(II), Mn(II) counterparts under identical reaction conditions (entry 7 vs. 8–12), indicating efficient activation ability of Cu(II) in the [α-SiW₁₁O₃₉Cu]⁶⁻ on the C≡C bond.

Table 1 POM-promoted reaction of CO₂ with propargylic amine 1a^a

Entry	Cat./%	Conv./%	Yield^b/%
a Unless otherwise specified, the reactions were performed with 1a (145.2 mg, 1.0 mmol), catalyst (molar percentage to 1a, 2.5 mol%), a CO2 balloon, 60 °C, 20 h. b Determined by the ^1H NMR technique, using 1,1,2,2-tetrachloroethane as an internal standard. c A mixture of CuCl2 (2.5 mol%) + (^nC7H15)4NBr (15 mol%). d 25 °C.
1	—	0	0
2	(^nC7H15)4NBr	0	0
3	K8[α-SiW11O39]	0	0
4	K6[SiW11O39Cu]	45	43
5	CuCl2	78	67
6^c	CuCl2 + (^nC7H15)4NBr	91	68
7	[(^nC7H15)4N]6[α-SiW11O39Cu]	>99	96
8	[(^nC7H15)4N]6[α-SiW11O39Co]	70	66
9	[(^nC7H15)4N]6[α-SiW11O39Fe]	18	18
10	[(^nC7H15)4N]6[α-SiW11O39Ni]	18	16
11	[(^nC7H15)4N]6[α-SiW11O39Zn]	45	45
12	[(^nC7H15)4N]6[α-SiW11O39Mn]	47	43
13	[(^nC4H9)4N]6[α-SiW11O39Cu]	>99	95
14	[(^nC16H33)(CH3)3N]6[α-SiW11O39Cu]	94	86
15^d	[(^nC7H15)4N]6[α-SiW11O39Cu]	79	73

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On the other hand, the (ⁿC₇H₁₅)₄N⁺ was found to be favorable for this reaction in comparison with potassium cations (entry 7 vs. 4). In addition, other bulky tetraalkylammonium cations such as (ⁿC₄H₉)₄N⁺ and (ⁿC₁₆H₃₃)(CH₃)₃N⁺ also gave good results (entries 13 and 14). This is understandable because the bulky tetraalkylammonium cation can not only stabilize the carbamate intermediate, but enhance its solubility and O-nucleophilicity, and thus promote subsequent intramolecular nucleophilic cyclization resulting in the formation of 2-oxazolidinones.²⁹ In addition, the bulky tetraalkylammonium cation increases the solubility of the polyoxometalate compared with the potassium cation, leading to higher catalytic activity. Moreover, owing to efficient catalytic activity of [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu], the reaction could also proceed well even at ambient temperature (entry 15).

The carboxylative cyclization for a variety of propargylic amines was then undertaken to explore the scope of this well-developed [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] catalytic system. As shown in Table 2, both terminal and internal propargylic amines performed smoothly to give the corresponding 2-oxazolidinones in excellent yields (entries 1–7). In general, substituents at the propargylic position of propargylic amines have a remarkable effect on the reaction outcome, where the geminal dialkyl inhibited the reaction, probably due to steric hindrance (entry 8). Notably, para-substituents at the benzene ring of the terminal position increased the reactivity for the carboxylative cyclization reaction for an obviously shortened reaction time (entries 6, 7 vs. 3).

Table 2 Substrate scope^a

Entry	Substrate	Product	Yield^e/%
a Unless otherwise stated, all the reactions were carried out with 1 mmol of propargylic amine, [(^nC7H15)4N]6[α-SiW11O39Cu] (130.0 mg, 0.025 mmol), CO2 (99.999%, balloon), 60 °C, 20 h. b 0.3 mL EtOH as the solvent. c 2 h. d 10 h. e Determined by the ^1H NMR technique.
1			96
2			92
3			95
4^b			94
5			93
6^b^,^c			92
7^b^,^d			99
8^b			0

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To further verify our initial assumption that both the propargylic amine and CO₂ could be activated by [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu], the interaction of [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] with CO₂ and 1a was monitored respectively by ¹H and ¹³C NMR techniques as depicted in Fig. 1 and 2. In the ¹H NMR spectrum, the N–H signal of 1a appeared at δ = 1.68 ppm (Fig. 1b). However, this signal disappeared when [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] was added to 1a (Fig. 1c), thus illustrating that the propargylic amine is activated by [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] through the formation of N–H⋯O hydrogen bonds.³⁰ Two new signals appeared at δ = 157.9 and 124.7 ppm when CO₂ was bubbled into a dry solution of [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] and CDCl₃ (Fig. 2). The newly-appeared peak at 157.9 ppm assigned to the carbonyl carbon of the monodentate carbonate presumably supports the CO₂ activation by [α-SiW₁₁O₃₉Cu]⁶⁻,³¹ whilst the signal at 124.7 ppm is due to the physical absorption of CO₂.

Fig. 1 ¹H NMR investigation. (a) [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] (50 mg, 0.01 mmol) in 0.3 mL of CDCl₃. (b) 1a (55 mg, 0.4 mmol) in 0.3 mL of CDCl₃; (c) 1a (55 mg, 0.4 mmol), [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] (50 mg, 0.01 mmol) in 0.3 mL of CDCl₃.

Fig. 2 ¹³C NMR study. [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] (containing a small amount of toluene) in CDCl₃ at atmospheric CO₂ (25 °C).

On the basis of experimental and NMR studies, a plausible mechanism for the [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] promoted fixation of CO₂ with propargylic amines is proposed as illustrated in Scheme 3, taking the carboxylative cyclization of 1a as an example. Propargylic amines and CO₂ are activated by [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] in the form of species I. Then, the activated propargylic amine and CO₂ are further converted into the carbamate II under the participation of another molecular propargylic amine which obtains a proton to be converted into the ammonium cation [1a + H]⁺. Compared with [1a + H]⁺, the bulky tetraheptylammonium cation (Q) can not only stabilize the carbamate anion, but also enhance the O-nucleophilicity, so it could exchange with [1a + H]⁺ to form a favorable carbamate III. Subsequently, with the aid of activation of C≡C bonds by Cu(II), intramolecular nucleophilic cyclization of III gives the intermediate IV. Finally, proto-demetallation with the help of a proton carrier [1a + H]⁺ affords the product 2a along with the regeneration of [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu].

Scheme 3 Plausible mechanism for the [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu]-catalyzed carboxylative cyclization of propargylic amines and CO₂.

Conclusions

In summary, we have described a cost-competitive catalytic process promoted by robust bifunctional [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] which can simultaneously activate both CO₂ and propargylic amines for efficient chemical fixation of CO₂ at atmospheric pressure to produce 5-alkylideneoxazolidin-2-ones at room temperature in excellent yields. This protocol also features a simple process with low energy consumption and avoidance of complicated preparation of the catalyst. Such findings may be of great interest to develop cost-effective, halogen-free and efficient bifunctional catalysts for the preparation of the CO₂-based products under mild reaction conditions.

Experimental section

Transition-metal-substituted polyoxometalate-based ionic liquids were synthesized according to previous reports.^20a,28 Taking the synthesis of [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] as an example: 25 mL of 20 mM aqueous K₈[α-SiW₁₁O₃₉] solution and 50 mL of a 10 mM aqueous CuCl₂ solution were mixed and stirred for 15 min. Then the solution was treated with tetraheptylammonium bromide (2.47 g, 3 mmol) dissolved in 60 mL toluene. After the two phases were allowed to settle for 10 minutes, the organic layer was decanted and washed seven times with 20 mL water portions. The wet toluene was removed by heating to 40 °C under vacuum. Then 30 mL dry toluene was added and evaporated under reduced pressure to remove the traces of water. [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] was obtained as a blue viscous liquid. IR (neat, KBr): 2955, 2927, 2856, 1638, 1618, 1465, 1383, 1265, 949, 898, 798, 739 cm⁻¹. Anal. Calcd for [(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] (M_W = 5202): C 38.78, H 6.98, N 1.62, Found: C 38.19, H 6.80, N 1.66.

General procedure for the synthesis of 5-alkylideneoxazolidin-2-ones

[(ⁿC₇H₁₅)₄N]₆[α-SiW₁₁O₃₉Cu] (130.0 mg, 0.025 mmol) and propargylic amine (1 mmol) were added to a 10 mL Schlenk tube equipped with a magnetic stir bar. The flask was capped and sealed. Then a gas-exchanging process was conducted using the “freeze–pump–thaw” method. The reaction mixture was stirred at 60 °C for 20 h under a CO₂ balloon (99.999%). Upon completion, product yield was determined by the ¹H NMR technique using 1,1,2,2-tetrachloroethane as an internal standard. Then the reaction mixture was diluted with dichloromethane (2 mL). The organic phase was collected and then purified by column chromatography on silica gel using petroleum ether/ethyl acetate as an eluent to afford the desired product. All the products were characterized by NMR and MS and compared with the data available in the literature.^12e,32

3-Benzyl-5-methyleneoxazolidin-2-one (2a). White crystalline solid, m.p. 53–54 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.40–7.27 (m, 5H), 4.72 (dd, J = 5.6 Hz, J = 2.8 Hz, 1H), 4.47 (s, 2H), 4.20 (dd, J = 5.2 Hz, J = 2.4 Hz, 1H), 4.02 (t, J = 2.4 Hz, 2H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ 155.6, 148.8, 134.8, 128.98, 128.9, 128.2, 128.1, 86.7, 47.8, 47.1 ppm. GC-MS (EI, 70 eV) m/z (%) = 189 (19), 92 (52), 91 (100), 65 (16).

3-Butyl-5-methyleneoxazolidin-2-one (2b). Pale yellow oil; ¹H NMR (400 MHz, CDCl₃) δ 4.65 (d, J = 2.4 Hz, 1H), 4.24 (d, J = 2.4 Hz, 2H), 4.12 (d, J = 2.4 Hz, 2H), 3.24 (t, J = 7.2 Hz, 2H), 1.52–1.44 (m, 2H), 1.33–1.24 (m, 2H), 0.88 (t, J = 7.2 Hz, 3H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ 155.4, 149.1, 86.1, 47.6, 43.3, 29.1, 19.6, 13.5 ppm. GC-MS (EI, 70 eV) m/z (%) = 155 (90), 113 (27), 112 (100), 98 (11), 84 (37).

(Z)-5-Benzylidene-3-butyl-4-ethyloxazolidin-2-one (2c). Pale yellow oil; ¹H NMR (400 MHz, CDCl₃) δ 7.62 (d, J = 7.6 Hz, 2H), 7.35 (dd, J = 7.2 Hz, J = 12.8 Hz, 2H), 7.22 (dd, J = 6.8 Hz, J = 12.8 Hz, 1H), 5.48 (s, 1H), 4.56 (s, 1H), 3.66–3.56 (m, 1H), 3.06–2.98 (m, 1H), 1.97–1.93 (m, 1H), 1.78–1.75 (m, 1H), 1.62–1.54 (m, 2H), 1.41–1.34 (m, 2H), 0.99–0.87 (m, 6H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ 155.3, 146.7, 133.7, 128.3, 126.7, 102.3, 59.0, 41.1, 29.2, 24.9, 19.9, 13.7, 6.5 ppm. GC-MS (EI, 70 eV) m/z (%) = 259 (14), 230 (100), 174 (51), 118 (26), 90 (22).

(Z)-5-Benzylidene-3-butyl-4-phenyloxazolidin-2-one (2d). White crystalline solid, m.p. 103–104 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.56 (d, J = 7.6 Hz, 2H), 7.46 (d, J = 6.0 Hz, 3H), 7.37–7.29 (m, 4H), 7.21 (m, 1H), 5.41 (s, 1H), 5.28 (s, 1H), 3.58–3.50 (m, 1H), 2.89–2.82 (m, 1H), 1.52–1.45 (m, 2H), 1.37–1.26 (m, 2H), 0.91 (t, J = 7.2 Hz, 6H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ 155.1, 147.8, 137.4, 133.5, 129.46, 129.42, 128.5, 128.4, 127.9, 127.0, 104.6, 63.9, 41.7, 29.0, 19.9, 13.7 ppm. GC-MS (EI, 70 eV) m/z (%) = 333 (22), 208 (56), 207 (70), 180 (100), 179 (44).

(Z)-5-Benzylidene-3-cyclohexyl-4-phenyloxazolidin-2-one (2e). White crystalline solid, m.p. 121–123 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.53 (d, J = 8 Hz, 2H), 7.44–7.38 (m, 5H), 7.30 (t, J = 7.6 Hz, 2H), 7.19 (t, J = 7.6 Hz, 1H), 5.42 (s, 1H), 5.18 (s, 1H), 3.56 (t, J = 7.2 Hz, 1H), 1.81–1.56 (m, 6H), 1.29–0.96 (m, 4H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ 154.7, 148.4, 139.6, 133.6, 129.2, 128.4, 128.3, 127.8, 126.8, 104.2, 63.2, 54.7, 31.2, 30.0, 25.7, 25.1 ppm. GC-MS (EI, 70 eV) m/z (%) = 307 (33), 262 (10), 208 (29), 207 (28), 180 (100), 179 (66).

(Z)-5-(4-Bromobenzylidene)-3-butyl-4-phenyloxazolidin-2-one (2f). White crystalline solid, m.p. 119–120 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.42–7.31 (m, 9H), 5.36 (s, 1H), 5.18 (s, 1H), 3.54–3.47 (m, 1H), 2.85–2.78 (m, 1H), 1.46–1.41 (m, 2H), 1.29–1.24 (m, 2H), 0.87 (t, J = 7.2 Hz, 3H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ 154.9, 148.5, 137.1, 132.5, 131.6, 129.9, 129.6, 129.5, 127.9, 120.7, 103.5, 64.0, 41.7, 29.0, 19.9, 13.7 ppm. GC-MS (EI, 70 eV) m/z (%) = 387 (33), 385 (33), 288 (17), 286 (17), 261 (14), 259 (14), 178 (100), 179 (91).

(Z)-3-Butyl-5-(4-methylbenzylidene)-4-phenyloxazolidin-2-one (2g). White crystalline solid, m.p. 125–126 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.42–7.40 (m, 5H), 7.33 (d, J = 7.2 Hz, 2H), 7.11–7.09 (d, J = 7.2 Hz, 2H), 5.37 (s, 1H), 5.22 (s, 1H), 3.54–3.47 (m, 1H), 2.85–2.78 (m, 1H), 2.31 (s, 1H), 1.51–1.40 (m, 2H), 1.33–1.23 (m, 2H), 0.87 (t, J = 7.2 Hz, 3H) ppm. ¹³C NMR (101 MHz, CDCl₃) δ 155.2, 147.0, 137.5, 136.8, 130.7, 129.4, 129.2, 128.3, 127.9, 104.6, 63.9, 41.7, 29.0, 21.3, 19.9, 13.7 ppm. GC-MS (EI, 70 eV) m/z (%) = 321 (44), 207 (24), 194 (100), 179 (64), 132 (19).

Acknowledgements

We are grateful to the National Natural Science Foundation of China, the Specialized Research Fund for the Doctoral Program of Higher Education (20130031110013), the MOE Innovation Team (IRT13022) of China, and the “111” Project of the Ministry of Education of China (project no. B06005) for financial support.

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Footnote

† Electronic supplementary information (ESI) available: General procedures, spectral data or other electronic format. See DOI: 10.1039/c5gc02311d

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