Solvent-free synthesis of cyclic carbonates from CO2 and epoxides catalyzed by reusable alumina-supported zinc dichloride
Graphical abstract
Introduction
Carbon dioxide is, among others (CO, CO2, CH4), the most appealing C1 carbon source because of being renewable, nontoxic, non-flammable, readily available and relatively inexpensive. It can be converted into fuels, materials and value-added chemicals by replacing other environmentally less benign substances with a lower E-factor impact [1]. It is a crucial building block for the synthesis of useful organic compounds [[2], [3], [4], [5]], being its reaction with epoxides [6,7] of paramount importance for the industrial preparation of carbonates (e.g., dimethyl carbonate, diphenyl carbonate, ethylene carbonate, propylene carbonate and glycerol carbonate) [8,9] and polycarbonates [10]. Cyclic carbonates, apart from being present in natural products and potential pharmaceuticals, have found manifold applications in different disciplines; for instance, as electrolytes in lithium-ion batteries, as polar aprotic solvents and as synthetic intermediates [[11], [12], [13], [14]].
Besides being a 100% atom-economical process, the high thermodynamic stability of the CO2 molecule [15] makes the intervention of catalysts imperative for activating the implicated reagents [16]. Homogeneous catalysis and, to some extent, organocatalysis, dominate over heterogeneous catalysis, typically, in the presence of an ammonium salt [6,[17], [18], [19]]. A great advance in the field was made by North et al., who introduced the binuclear Al-salen [20,21] and salphen [22,23] complexes, together with a polystyrene-immobilized counterpart [24], as highly efficient catalysts for the carboxylation of terminal epoxides at room temperature and atmospheric pressure. Notwithstanding this research merit, the catalysts preparation requires multistep synthesis and the immobilized catalyst suffers from deactivation. Mononuclear Al-salen complexes [25] were shown to be also effective in the carboxylation of terminal epoxides, though higher temperature and longer reaction time were mandatory.
Lately, new catalytic systems have emerged with the aim to upgrade the performance of the epoxide−CO2 cycloaddition, namely: CaI2-(PEG DME 500) [26], CaI2-crown ether [27,28], tri-Co complex containing amine-bis(benzotriazole phenolate) ligands (in combination with TBAC) [29], highly active Al-aminotriphenolate [30] and Al-porphyrin based catalysts [31], Mn(II)-pyridin-4-yl-phosphonate MOF-TBAB [32], Ln(III)-coordinated polymers-TBAB [33] and La-heteroscorpionate-TBAB [34]. Notably, the formation of the cyclic carbonates could be accomplished not only in the presence of rather sophisticated complexes but also with simple catalysts such as MgCl2 (DMF, 100 °C) [35].
The combination of a ZnX2 Lewis acid catalyst with an ammonium salt has been also successfully exploited in the epoxide carboxylation [36]. For instance, TBAI with Zn-salphen [37,38], bis-(Zn-salphen) [39], Zn-azatrane [40], Zn-pyrrolidine [41] complex or Zn-MOF [42,43] allowed to carry out the reaction under mild conditions in good yields. The ammonium salt could be replaced with an ionic liquid [44] to furnish good yields with the assistance of a ligand-free zinc salt at 30 °C and 1 atm CO2 [45]. Ema et al. inserted an ammonium-fragment linker into zinc porphyrinate [[46], [47], [48]], with the resulting catalyst showing high TONs and TOFs, albeit under harsh reaction conditions (120 °C, 17 atm). A system based on the Zn cluster Zn4(TFA)6O-TBAI, insensitive to moisture and gaseous impurities, was effective under mild conditions (25 °C, 1 atm) but relatively longer reaction time was needed (up to 20 h) [49]. As an alternative to the activation of the epoxide, CO2 can be activated by N-heterocyclic carbenes (NHC) through the formation of NHC−CO2 adducts [50]; in this sense, the NHC-ZnBr2 system catalyzed the cycloaddition of CO2 to epoxides at atmospheric pressure (80 °C, DMSO) [51].
Metal-free organocatalytic systems look very attractive and green [18]. As recent examples, highly active cavitand-based polyphenol catalyst allowed to get good-to-excellent yields of both mono- and disubstituted carbonates in 18 h at 50 °C and 10 bar of CO2 in the presence of TBAI [52]; using squaramide derivatives, terminal carbonates were obtained in shorter time at higher temperature [53], whereas harsher conditions were necessary for internal epoxides. Alkyl ammonium and phosphonium salts themselves are active organocatalysts but they are difficult to recover and are deployed in quasi-equimolar amounts with respect to the substrate.
In spite of the fact of the good catalytic behavior manifested by the aforementioned catalysts, their homogeneous nature precludes reutilization and limits their practical application, especially of those involving tedious preparation procedures. In this vein, heterogeneous catalysis offers the possibility of catalyst recovery and reuse, hence making the whole process more sustainable [54]. Even though heterogeneous catalysts are characterized by a longer life than the homogeneous counterparts, the former have been much less investigated than the latter in the epoxide carboxylation, in part, because they are considered less active and require more severe conditions to reach a satisfactory yield. For instance, the silica/TBAB-catalyzed CO2-epoxide cycloaddition needed high catalyst loading, pressure (40 atm) and temperature (90 °C) [55]. Likewise, elevated temperature (100–140 °C) and pressure (10–45 atm) or prolonged reaction time (up to 48 h) had to be applied for ionic liquids [56] grafted onto different supports [[57], [58], [59], [60], [61]]. A CuII metal-organic hydrogel [62] and indium-based metal organic framework [63] worked under milder conditions but with modest-to-good conversions of the terminal epoxides (32–80%). Recently investigated, the heterogeneous catalysts based on three-dimensional copper-phosphate grid [64], APTES-modified zirconium oxide on MCM-41 [65], and cobalt-coordinated conjugated microporous polymer (Co-CMP-2) [66] allowed to attain high TONs and TOFs, but excellent yields were achieved only under relatively high pressure and temperature (80–100 °C, 10–30 at m). Several zinc-containing heterogeneous systems, such as a zinc complex with mesoporous o-hydroxybenzene polymers [67], a zeolitic imidazolate framework (ZIF-95) containing Zn atoms [68], a binuclear supramolecular zinc complex with hexadentate ligands [69] or Zn-Mg-Al composite oxides [70] have been reported; although all the catalysts were reused, at least, five times without significant loss of their activity, the reaction conditions were rather harsh (80–140 °C, 12–50 atm).
Concluding this introduction, we can say that there is a general upsurge of interest in developing sustainable catalysts [71,72] and reaction media [73] to promote the synthesis of cyclic carbonates from CO2 and epoxides under mild conditions [74]. In an ideal scenario, the catalytic systems should be simple, reusable for several cycles, without toxic reagents, and producing the cyclic carbonates in high yield and selectivity with a minimum generation of waste. In this context, we present herein our endeavor to adhere to these premises and introduce the heterogeneous catalytic system composed of ZnCl2 supported on alumina which, together with a catalytic amount of TBAI, catalyzes the epoxide carboxylation and thiocarboxylation in an efficient manner.
Section snippets
General
All starting materials were purchased from Sigma Aldrich and P&M-Invest (http://en.fluorine1.ru/) and were used without any further purification; solvents were dried and deoxygenated using standard procedures. Carbon dioxide gas of 99.99% purity was used. The carboxylation reactions were performed in a 15 mL-capacity glass low-pressure reactor RLP25ML (http://www.openscience.ru) equipped with a gas feeding system, magnetic stirrer and manometer. The NMR spectra of all products were recorded on
Results and discussion
The reaction of styrene oxide 1a with CO2 was used as a model reaction to gauge the activity of different Lewis acids (LA) under the following conditions: 0.6 mol% LA, 1.6 mol% TBAI as nucleophilic additive, 60 °C, 4 atm, neat (Fig. 1). The highest yields were recorded using zinc(II) bromide and chloride. It is known that NbCl5 can perform well in this reaction [75,76]; however, its catalytic activity (FeCl3 behaved similarly) was found to be inferior to that of the zinc salts under the above
Conclusions
The results of this study suggest a new avenue for research on epoxide carboxylation based on heterogeneous catalysis: the first heterogeneous ZnCl2-catalyzed carboxylation of epoxides in the presence of a tetrabutylammonium halide has been presented. A thorough comparison of the activity of different Lewis acids and their supports led to the conclusion that ZnCl2/Al2O3 was the catalyst of choice for the title reaction because of being efficient, cheap and easily prepared and applied. When
Acknowledgments
This study was generously supported by the Russian Science Foundation [project no. 14-23-00186 P], the M. V. Lomonosov Moscow State University Program of Development, the Spanish Ministerio de Ciencia, Innovación y Universidades [MICIU; grant no. CTQ2017-88171-P] and the Generalitat Valenciana [GV; grant no. AICO/2017/007].
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