AgX@carbon (X = Br and I) as robust and efficient catalysts for the reaction of propargylic alcohols and CO₂ to carbonates under ambient conditions

Abstract

Development of new efficient catalytic systems for chemical transformation of CO₂ is a very attractive topic in green chemistry. In this work, we studied the synthesis of α-alkylidene cyclic carbonates through the coupling reaction between propargylic alcohols and CO₂ with new silver catalysts. It was found that activated carbon supported AgX (X = Br and I) was a simple and efficient catalyst for the carboxylative cyclization of propargyl alcohols with CO₂ at atmospheric pressure and room temperature. Nearly 99% yield of the desired product was obtained, and the product could be simply separated through solvent extraction. Moreover, the catalyst could be easily recovered and reused at least ten times without a decrease in the catalytic activity and selectivity. These findings are useful for the development of an environmentally friendly chemical process for the production of α-alkylidene cyclic carbonates.

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Introduction

As one of the major greenhouse gases, carbon dioxide has caused serious environmental problems such as global warming and climate change. However, carbon dioxide is a cheap, non-toxic, and renewable C1 building block.^1–4 Thus, how to capture and utilize CO₂ is a very important topic. Chemical fixation is regarded as an efficient route for the utilization of carbon dioxide. At present, many useful organic chemicals have been produced by using CO₂ as a feed stock, such as methanol,⁵ urea,⁶ cyclic carbonates,⁷ formamides,⁸ and alkynyl carboxylic acid derivatives.⁹

α-Alkylidene cyclic carbonates are important heterocyclic compounds used as building blocks for different synthetic purposes.¹⁰ The synthesis of these compounds through coupling reaction between propargylic alcohols and CO₂ is one of the most important methods for the utilization of CO₂. In the past few years, many metal-free catalytic systems and transition-metal catalysts had been developed for this reaction and significant results had been reported. The metal-free catalytic systems mainly include tertiary phosphine,¹¹ phosphorus ylides,¹² N-heterocyclic carbene (NHC)/CO₂ ¹³ and N-heterocyclic olefin/CO₂ adducts,¹⁴ guanidines,¹⁵ and functionalized imidazolium betaines,¹⁶ while Pd,¹⁷ Ag,^18–25 W,²⁶ Co,²⁷ Fe,²⁸ Cu^29,30 and Zn³¹ are typical transition-metal catalysts. In general, the reported catalyst systems required high reaction temperature and/or high CO₂ pressure. Therefore, the exploration of an efficient catalyst system which can work at atmospheric pressure and room temperature is still a challenge.

Recently, great progress has been made in this area. Two types of novel catalyst systems have been developed to catalyze the carboxylative cyclization of propargyl alcohols with CO₂ under ambient condition. He et al.³² reported that [(Ph₃P)₂Ag]₂CO₃ catalyst could efficiently catalyze the reaction, but the recovery of catalyst was difficult. Han and co-workers³³ found that Ag nanoparticles were able to catalyze the carboxylative cyclization with high yields, and the target products could be obtained by means of column chromatograph. Despite these significant advances, the example of CO₂ utilization at atmospheric pressure and room temperature is still seldom, and the recycling of catalysts and separation of products also need to be improved. Therefore, development of efficient, simple and recyclable catalysts for the synthesis of α-alkylidene cyclic carbonates using CO₂ under ambient conditions is highly desirable.

Herein, we demonstrate the first example to use silver salt supported on porous carbon material as a novel catalyst for the reaction of propargylic alcohols with CO₂ at room temperature and atmospheric pressure (Scheme 1). It is found that the catalyst is robust and highly efficient, and it is stable to air and moisture, and can be recovered and reused in the reaction. In addition, the product can be easily separated by simple extraction.

Scheme 1 Carboxylative cyclization of propargyl alcohols with CO₂.

Results and discussion

Characterization of the catalysts

Considering the fact that activated carbon (AC) is cheap, highly stable and easy available, this carbon material was used as a support to prepare heterogeneous silver catalysts in the present work. The novel catalysts AgX@C (X = Cl, Br and I) were prepared by in situ sonication-assisted deposition–precipitation³⁴ of AgX nanoparticles on the surface of activated coconut-shell carbon (100–200 mesh). Silver content in AgCl@C, AgBr@C and AgI@C was found to be 19.8, 18.2 and 16.9 wt%, respectively, as determined from inductively coupled plasma mass spectrometry. Meanwhile, other metal components were not detectable by X-ray fluorescence spectrometer in AgX@C samples, which indicates high purity of AgX@C catalysts. IR spectrometry was used to investigate the interactions between AgX and the support. It was shown that no obvious change was observed in wavenumber of the peaks between the pure activated carbon and the activated carbon in the catalyst (Fig. S1, ESI†). This is an indicative that interaction between AgX and the support was mainly physical.

X-ray diffraction (XRD) patterns of the as-prepared AgX/C composites with different AgX contents were shown in Fig. 1. Compared with the pure activated carbon, various diffraction peaks were observed for AgX@C composites, which are consistent with the standard diffraction peaks of AgCl cubic crystal phase,³⁴ AgBr cubic crystal phase,³⁴ and AgI crystal phase,³⁵ respectively. From the XRD pattern shown in Fig. 2, it is clear that the framework of AgI@C catalyst was well maintained after the catalytic reaction, indicating excellent stability of the AgI@C catalyst. Similar results were obtained for AgCl@C and AgBr@C catalysts.

Fig. 1 XRD patterns of (a) AC, (b) AgCl@C, (c) AgBr@C, and (d) AgI@C.

Fig. 2 XRD patterns of AgI@C before (a) and after (b) the catalytic reaction.

For a better understanding of the microscopic morphology of AgX@C samples before and after the reaction, field emission scanning electron microscope (FESEM) photographs were taken for AgI@C as a model catalyst, and the results were shown in Fig. S2 (ESI†). It is evident that the sample surface was quite smooth, and no significant morphology change was observed before and after the reaction. This result suggests that the AgX was successfully loaded on activated carbon.

Optimization of the reaction conditions

With the as-prepared catalysts in hand, we initiated our experiments by using 2-methyl-3-butyn-2-ol as a model substrate, acetonitrile as the solvent and AgI@C as the catalyst at atmospheric pressure of CO₂ and room temperature. As shown in Table 1, the reaction did not occur in the absence of a base or a silver catalyst (entries 1 and 2). However, it was surprising to note that the product was obtained in a quantitative yield in the presence of 3 mol% catalyst and 1 equivalent (equiv.) of DBU (entry 3). Encouraged by this result, we investigated the effect of base types and dosage on the yield of product. For this purpose, different kinds of bases such as diisopropylethylamine (DIEA), trimethylamine (TEA), K₂CO₃ and NaOH were used in the reaction, but the yields were not satisfactory, and in some cases the yields were very low (entries 4–7). This indicates that DBU was a superior base for the direct carboxylic cyclization of propargylic alcohols with CO₂. Interestingly, a quantitative yield was also obtained when the amount of DBU was deceased to 0.2 equiv. (entry 8). Therefore, the amount of the base used in our catalyst system was lower than that reported in the previous studies.^14,16,19,26 Based on this result, we investigated the effect of the catalyst amount on the reaction in the presence of 0.2 equiv. DBU. It was shown that catalyst loading higher than 3 mol% (5%, 10%) did not improve the yield further (entries 9 and 10), whereas the loading lower than 3 mol% (say 2 mol%) decreased the yield from 99% to 72% (entry 11).

Table 1 Carboxylic cyclization of propargylic alcohols with CO₂^a

Entry	Cat.	Cat. loading (mol%)	Base	Yield^b (%)
a Reaction condition: 2-methyl-3-butyn-2-ol (1 mmol), catalyst (x mol% based on Ag), CH3CN (2 mL), CO2 (99.999%, 0.1 MPa), 4 h, r.t.b The yields were determined by GC with naphthalene as internal standard.c Without base.d Without catalyst.
1	AgI@C	3	—^c	0
2	—	—^d	DBU (1.0 equiv.)	0
3	AgI@C	3	DBU (1.0 equiv.)	>99
4	AgI@C	3	DIEA (1.0 equiv.)	0
5	AgI@C	3	TEA (1.0 equiv.)	0
6	AgI@C	3	K2CO3 (1.0 equiv.)	12
7	AgI@C	3	NaOH (1.0 equiv.)	15
8	AgI@C	3	DBU (0.2 equiv.)	>99
9	AgI@C	5	DBU (0.2 equiv.)	>99
10	AgI@C	10	DBU (0.2 equiv.)	>99
11	AgI@C	2	DBU (0.2 equiv.)	72
12	AgI	3	DBU (0.2 equiv.)	88
13	AgNO3	3	DBU (0.2 equiv.)	73
14	Ag2CO3	3	DBU (0.2 equiv.)	14
15	Ag@C	3	DBU (0.2 equiv.)	78

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For the sake of comparison, the Ag@C catalyst prepared by the procedure (See ESI†) described in the literature³⁶ was used to catalyze the reaction, and the result was given in Table 1 (entry 15). It is clear that the yield of 2a was lower than that catalysed by AgI@C, which indicates that the ability of Ag⁰ to activate the C≡C triple bond was weaker than AgI. In addition, it was found that AgI, AgNO₃ and Ag₂CO₃ without the support significantly decreased the conversion efficiency of propargylic alcohols (entries 12–14). This could be understood from the fact that the AgI on the support has excellent dispersibility and stability in the reaction system, which efficiently increases the contacting area of the catalyst with the reactants.

The effect of solvent on the yield of 2a was investigated and the results were shown in Table 2. It can be seen that yield of the product was strongly dependent on the reaction media, for example, no product was obtained when the reaction was performed in THF, EtOH or H₂O (entries 1–3), while the reaction in toluene and DMSO gave a 51% and 65% yield, respectively (entries 4 and 5). Great improvement was achieved when the reaction was conducted in cyclohexane (96%) and dichloromethane (97%) (entries 6 and 7). Especially, an almost quantitative yield (>99%) was observed by using acetonitrile (CH₃CN) as solvent (entry 8). Thus, acetonitrile was identified as the most suitable medium for the formation of 2a.

Table 2 Solvent effect in carboxylic cyclization of 2-methyl-3-butyn-2-ol^a

Entry	Solvent	Yield^b (%)
a Reaction condition: 2-methyl-3-butyn-2-ol (1 mmol), catalyst (3 mol% based on Ag), DBU (0.2 mmol), CO2 (99.999%, 0.1 MPa), 4 h, r.t.b The yields were determined by GC with naphthalene as internal standard.
1	THF	0
2	EtOH	0
3	H2O	0
4	Toluene	51
5	DMSO	65
6	Cyclohexane	96
7	Dichloromethane	97
8	CH3CN	>99

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To study the effect of different supports and their specific surface area on this reaction, other silver catalysts including AgI@C (300 mesh), AgI@C (400 mesh), and AgI@C (carbon black) were prepared under similar conditions. Their specific surface area and catalytic performance for the cyclization reaction were determined, and the results were listed in Table 3. It can be seen that the yield increased with the increase of specific surface area. Among these catalysts, the AgI@C (100–200 mesh) with the maximum specific surface area was found to give the best yield. This indicates that increase of the contacting area of the catalysts with reactants can effectively improve the interaction between the reactants and the well dispersed small particles catalysts.

Table 3 The effect of specific surface area of catalyst on the reaction^a

Entry	Catalyst	SABET/m^2 g^−1	Yield^b
a Reaction condition: 2-methyl-3-butyn-2-ol (1 mmol), catalyst (3 mol% based on Ag), DBU (0.2 mmol), CH3CN (2 mL), CO2 (99.999%, 0.1 MPa), 3 h, r.t.b Isolated yield.
1	AgI@C (100–200 mesh)	973.44	90
2	AgI@C (300 mesh)	701.26	86
3	AgI@C (400 mesh)	562.35	83
4	AgI@C (carbon black)	194.61	79

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Next, we examined the effect of silver source on the reaction under ambient temperature and pressure. It was found from Table 4 that the use of AgBr@C and AgI@C could lead to a quantitative yield of 2a, while AgCl@C only gave inferior result. Also, the effect of reaction time on the yield of 2a was investigated and the results were shown in Fig. 3. It is evident that the maximum yield (97%) was obtained in 4 h of the reaction. Moreover, we also studied the effect of the reaction temperature and pressure on the yield of 2a (see ESI, Table S1†). It was found that the CO₂ pressure had no effect on the reaction within the pressure range investigated. However, the reaction was affected by temperature, and the yield of the product decreased with the increasing temperature, although the structure of the by-product is not clear. Thus, the following optimal reaction conditions were used for further investigations: AgBr@C or AgI@C as the catalyst (3 mol%), DBU as the base (0.2 equiv.), and CH₃CN as the solvent for 4 h reaction.

Table 4 Influence of the AgX@C catalysts on the carboxylic cyclization^a

Entry	Cat. (3 mol%)	Yield^b (%)
a Reaction condition: 2-methyl-3-butyn-2-ol (1 mmol), catalyst (3 mol% based on Ag), DBU (0.2 mmol), CH3CN (2 mL), CO2 (99.999%, 0.1 MPa), 4 h, r.t.b The yields were determined by GC with naphthalene as internal standard.
1	AgCl@C	71
2	AgBr@C	>99
3	AgI@C	>99

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Fig. 3 Effect of reaction time on the product yield over AgI@C. Reaction conditions: 1 mmol of 2-methyl-3-butyn-2-ol, 0.3 mol% of AgI@C (based on the substrate), 0.2 mmol of DBU, 2 mL of CH₃CN, 0.1 MPa CO₂, r.t. Yields were determined by GC using naphthalene as an internal standard. The results were obtained from at least three separate experiments.

Scope of the substrates

The reaction of CO₂ with a range of different substituted propargylic alcohols were performed under the optimized reaction conditions by using AgI@C as a representative catalyst, and the yields of the target products were summarized in Table 5. It was shown that propargyl alcohols with a variety of alkyl substituents at the propargylic position were effective substrates to give the corresponding carbonates in good to excellent yields. The reaction time did not depend on the steric hindrance effect of the substituted R₂ and R₃ groups. However, different results were obtained for the catalysts reported in literatures where the catalytic effect was significantly affected by the steric hindrance.^20,31,32 In addition, although the reaction was effectively catalyzed by our catalysts, the product was only obtained in moderate yield in the case of the internal propargylic alcohol. These results suggest that the catalyst system reported here is more favorable for terminal propargylic alcohols than internal propargylic alcohols.

Table 5 Scope of the propargylic alcohol substrates for the reaction under optimized conditions^a

Entry	Substrate	Product	Yield^b (%)
a Reaction conditions: propargylic alcohols (1.0 mmol), catalyst (3 mol%, based on Ag), DBU (0.2 mmol), CH3CN (2 mL), CO2 (99.999%, balloon), r.t., 4 h.b The yields were determined by GC with naphthalene as internal standard.c 1 MPa of CO2.d Isolated yield.
1			>99
			95^d
2			>99
			96^d
3			>99
			97^d
4			>99
			96^d
5			>99
			98^d
6			>99
			96^d
7			99
			97^d
8			97
			94^d
9^c			45
			38^d

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Separation of the target products and recyclability of the catalysts

Here, the products could be separated by solvent extraction technique. After the reaction was finished, the catalyst was firstly separated from the reaction mixture by centrifugation. Then the target compound was extracted with n-hexane. The combined organic layers were washed with saturated aqueous NaCl solution and then dried with anhydrous Na₂SO₄. The products were easily obtained after removal of the solvent. Compared with the product separation reported for α-alkylidene cyclic carbonates in literatures,^20,32,33 the strategy developed in this work was simple, fast and efficient, which is more favorable for industrial processes.

As a heterogeneous catalyst, AgX@C was not only robust in catalytic activity, but also easily separated from the reaction system only by centrifugation. In a typical experiment, the catalyst was reused 10 times in the reaction of 1a with CO₂. In each cycle, the heterogeneous catalyst was recovered by filtration and reused directly for the next run after drying. As shown in Fig. 4, the catalytic activity of the catalyst remained unchanged even after being reused 10 times. The leaching of Ag during the reaction was also examined by ICP mass analysis of the filtrates. It was found that the amount of Ag present in the filtrate was less than 0.01% of the initial amount. The above results indicate that the catalyst exhibited excellent activity and stability due to the strong interaction of the activated carbon with AgX nanoparticles.

Fig. 4 Catalyst recycling for the carboxylation of 1a with CO₂ (0.1 MPa) at 25 °C for 4 h. Yields were determined by GC using naphthalene as an internal standard. The results were obtained from at least three separate experiments.

Possible reaction mechanism

The mechanism of the carboxylative cyclization of propargyl alcohols with CO₂ catalyzed by Ag(I) and Ag NPs was studied in the presence of base promoters.^21,33 It is reasonable to assume that the reaction catalyzed by AgX@C should have a similar mechanism (Scheme 2). Firstly, the alcohol reacts with CO₂ to generate a carbonate intermediate via base activated process. Then, an intramolecular ring-closing reaction takes place on the alkyne, which can be activated by AgX catalyst supported on the activated carbon. Finally, the corresponding cyclic carbonate is formed with the release of the catalyst.

Scheme 2 The proposed mechanism for the carboxylative cyclization of propargyl alcohols with CO₂.

Density functional theory (DFT) calculations were performed to gain a deeper insight into structures of 2 and 3 (2-AgX) in Scheme 2. These structures were optimized at B3LYP/BSI level, where BSI (basis set LANL2DZ) was performed for Ag and I atoms and basis set 6-31G* for other non-metal main group atoms, and the results were included in Fig. S3 (ESI†). Based on the optimized geometries, it was found that 2 and AgX catalyst could be combined to form 3, which confirms the above purposed mechanism. In addition, comparison was made for the binding energy of 2 with three different catalysts in acetonitrile calculated at B3LYP/BSI level, where basis set 6-311++G** was employed for C, H and O atoms while basis set LANL2DZ was employed for Ag and I atoms (Table 6). It is clearly indicated that the interaction between 2 and AgCl was the strongest. This suggests that in the case of AgCl, more energy was needed to release the catalyst. Therefore, higher yields were observed by the use of AgBr@C and AgI@C than that of AgCl@C catalyst, which explains the reason why the catalytic activity of AgBr@C and AgI@C was much higher than that of AgCl@C.

Table 6 The binding energy between 2 and different catalysts

Catalyst	ΔE (kJ mol^−1)
AgCl	−125.6
AgBr	−120.3
AgI	−114.8

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Conclusions

In summary, we developed an efficient and cost-competitive catalytic system for chemical fixation of CO₂ to produce α-alkylidene cyclic carbonates. In this chemical reaction, the heterogeneous catalysts AgX@C (X = Br and I) exhibited high activity and excellent stability, and they could be easily recovered and reused without loss of activity. A range of propargylic alcohols could undergo the coupling reaction with CO₂ under conditions of atmospheric pressure and room temperature, and the yields of the desired products reached up to 99% under the optimized condition. In addition, the product could be easily separated by simple solvent extraction.

Experimental

Materials

CO₂ was supplied by Beijing Analytical Instrument Factory with a purity of 99.99%. All solvents were obtained commercially and were used as received unless otherwise indicated. The propargylic alcohols (purity >98%) and AgX were provided by J & K Scientific Ltd. Activated carbon and DBU was purchased from Aladdin Company. Other bases were analytical grade and were supplied from Beijing Chemical Reagent Company. The deuterated solvents (CDCl₃ and DMSO-d₆) were obtained from Sigma-Aldrich Company.

Preparation of the supporter

To the aqueous solution of concentrated hydrochloric acid (12 mol L⁻¹, 8 mL), nitric acid (16 mol L⁻¹, 6 mL), water (400 mL) and coconut shell carbon (100–200 mesh, 20 g) were added. The mixture was stirred at room temperature for 24 h and then filtered. Then, the solid was collected and dried at 110 °C for 16 h under flowing nitrogen gas. Thus obtained solid was employed as supporter for the synthesis of catalyst.

Preparation of activated carbon supported AgX catalysts

Typically, 0.7 g of the activated carbon was added into 50 mL of deionized water and sonicated for 30 min. Next, 0.1 g mL⁻¹ of aqueous AgNO₃ solution (3 mL) was added dropwise to the suspension. The mixture was stirred magnetically for 1 h to allow adsorption of Ag⁺ ions on the surface of activated carbon. Then, the respective aqueous solutions of sodium halides NaX (X = Cl, Br and I) were added dropwise into the suspension with an excess amount of 10% to ensure that the amount of halide ions from NaX was enough to precipitate Ag⁺ on the activated carbon surface. Here, the dropwise addition of aqueous NaX was necessary to avoid rapid nucleation of the Ag⁺ and X⁻ on the activated carbon. The resulting mixture was further stirred vigorously at room temperature for 3 h. The product was obtained by washing with ethanol and deionized water for three times and then dried at 70 °C for 12 h.

The specific surface area of AgI@C was determined by N₂ adsorption at 77 K using a Micromeritics ASAP 2010 system after the sample was degassed in vacuum at 130 °C for 20 h.

General procedure for the carboxylation cyclization of propargylic alcohol with CO₂

In a 20 mL Schlenk flask, propargylic alcohol (1.0 mmol), DBU (0.2 mmol), indicated amount of the catalyst, and CH₃CN (3 mL) were added. The flask was capped with a stopper and sealed. Then the freeze–pump–thaw method was employed for gas exchanging process. The reaction mixture was stirred at room temperature for 4 h under CO₂ atmosphere (balloon). After the reaction was finished, the product was extracted with n-hexane. The combined organic layers were washed with saturated aqueous NaCl solution and then dried with anhydrous Na₂SO₄. The organic phase was treated by reduced pressure distillation to give the desired products. All of these compounds were identified by NMR spectroscopy, which are consistent with the previous reported experimental results.^29,30

To test the reusability, the catalyst was separated from the reaction mixture by centrifugation, washed with CH₃CN for three times (3 × 10 mL) and dried under vacuum for 24 h at 30 °C, and then was reused directly for the next run.

Compound 2a. Colourless oil; ¹H NMR (600 MHz, DMSO-d₆) δ (ppm): 4.79 (d, J = 3.6 Hz, 1H), 4.64 (d, J = 4.2 Hz, 1H), 1.59 (s, 6H); ¹³C NMR (150 MHz, DMSO-d₆) δ (ppm): 158.7, 151.3, 85.9, 85.5, 27.4.

Compound 2b. Colourless oil; ¹H NMR (600 MHz, CDCl₃) δ (ppm): 4.82 (d, J = 3.6 Hz, 1H), 4.26 (d, J = 3.6 Hz, 1H), 1.94–1.88 (m, 1H), 1.79–1.73 (m, 1H), 1.59 (s, 3H), 0.99 (t, J = 7.8 Hz, 3H); ¹³C NMR (150 MHz, CDCl₃) δ (ppm): 157.5, 151.6, 87.6, 85.6, 33.4, 26.0, 7.4.

Compound 2c. Colourless oil; ¹H NMR (600 MHz, CDCl₃) δ (ppm): 4.87 (d, J = 3.6 Hz, 1H), 4.22 (d, J = 3.6 Hz, 1H), 1.97–1.91 (m, 4H), 1.75–1.69 (m, 4H), 0.98 (t, J = 7.8 Hz, 6H); ¹³C NMR (150 MHz, CDCl₃) δ (ppm): 155.8, 151.9, 90.8, 85.8, 31.9, 7.1.

Compound 2d. Colourless oil; ¹H NMR (600 MHz, CDCl₃) δ (ppm): 4.79 (d, J = 4.2 Hz, 1H), 4.26 (d, J = 4.2 Hz, 1H), 1.85–1.79 (m, 2H), 1.68–1.64 (m, 1H), 1.58 (s, 3H), 0.98–0.96 (m, 6H); ¹³C NMR (150 MHz, CDCl₃) δ (ppm): 158.4, 151.5, 87.3, 85.6, 48.6, 27.0, 24.3, 24.0, 23.7.

Compound 2e. Colourless oil; ¹H NMR (600 MHz, CDCl₃) δ (ppm): 4.79 (d, J = 4.2 Hz, 1H), 4.26 (d, J = 3.6 Hz, 1H), 1.89–1.83 (m, 1H), 1.72–1.67 (m, 1H), 1.58 (s, 3H), 1.42–1.25 (m, 8H), 0.88 (t, J = 7.2 Hz, 6H); ¹³C NMR (150 MHz, CDCl₃) δ (ppm): 157.8, 151.6, 87.2, 85.4, 40.4, 31.6, 31.5, 28.9, 26.3, 22.9, 22.7, 22.5, 14.21, 14.0.

Compound 2f. Colourless oil; ¹H NMR (600 MHz, CDCl₃) δ (ppm): 4.82 (s, 1H), 4.27 (s, 1H), 1.97–1.92 (m, 1H), 1.58 (s, 3H), 1.04–1.00 (m, 6H); ¹³C NMR (150 MHz, CDCl₃) δ (ppm): 157.2, 151.7, 89.8, 86.2, 37.0, 24.1, 16.4, 16.1.

Compound 2g. Colourless oil; ¹H NMR (600 MHz, CDCl₃) δ (ppm): 4.79 (d, J = 3.6 Hz, 1H), 4.34 (d, J = 3.6 Hz, 1H), 2.26–2.22 (m, 2H), 1.95–1.83 (m, 6H); ¹³C NMR (150 MHz, CDCl₃) δ (ppm): 157.8, 151.5, 94.2, 85.3, 40.7, 24.3.

Compound 2h. Colourless oil; ¹H NMR (600 MHz, CDCl₃) δ (ppm): 4.76 (d, J = 3.6 Hz, 1H), 4.28 (d, J = 4.2 Hz, 1H), 2.01 (d, J = 13.2 Hz, 2H), 1.78–1.59 (m, 8H); ¹³C NMR (150 MHz, CDCl₃) δ (ppm): 158.8, 151.5, 86.4, 85.5, 36.5, 24.4, 21.6.

Compound 2i. Colourless oil; ¹H NMR (600 MHz, CDCl₃) δ (ppm): 7.55–7.54 (m, 2H), 7.37–7.34 (m, 2H), 7.28–7.26 (m, 1H), 5.50 (s, 1H), 1.70 (s, 6H); ¹³C NMR (150 MHz, CDCl₃) δ (ppm): 151.3, 150.7, 132.4, 128.7, 128.5, 127.6, 101.6, 85.5, 27.8.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21403060, 21133009) and Program for Innovative Research Team in Science and Technology in University of Henan Province (16IRTSTHN002).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: FT-IR spectra, FESEM image, NMR spectra of the products and DFT calculations. See DOI: 10.1039/c6ra05224j

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