Robust bifunctional aluminium–salen catalysts for the preparation of cyclic carbonates from carbon dioxide and epoxides

Summary Two new one-component aluminium-based catalysts for the reaction between epoxides and carbon dioxide have been prepared. The catalysts are composed of aluminium–salen chloride complexes with trialkylammonium groups directly attached to the aromatic rings of the salen ligand. With terminal epoxides, the catalysts induced the formation of cyclic carbonates under mild reaction conditions (25–35 °C; 1–10 bar carbon dioxide pressure). However, with cyclohexene oxide under the same reaction conditions, the same catalysts induced the formation of polycarbonate. The catalysts could be recovered from the reaction mixture and reused.


Introduction
Carbon dioxide is a renewable and inexpensive carbon source, so great efforts have been directed at developing novel methods for the valorization of this abundant raw material [1]. One way of achieving this goal is to produce cyclic carbonates or polycarbonates from carbon dioxide and the corresponding epoxides (Scheme 1). Cyclic carbonates are an important class of solvents [2] and starting materials in organic synthesis [3][4][5][6].
Although a significant array of catalysts have been developed for the production of cyclic carbonates [7][8][9] and polycarbon- ates [10,11] from carbon dioxide and epoxides, the most developed and privileged set of catalysts are based on Lewis acidic metal-salen complexes. In particular, cobalt(III) and chromium(III) complexes were found to be highly efficient for polycarbonate production [12]. Further modification of the salen moiety by the introduction of basic or ammonium salts through alkyl spacers attached to the salen aromatic rings led to the formation of a family of bifunctional catalysts possessing both Lewis acid and nucleophilic catalysis capability (via the anion in the case of catalysts containing ammonium salts), with a concomitant increase in their activity [12,13]. Recently, more environmentally benign aluminium-based complexes, including salen complexes, have been introduced to catalyse cyclic carbonate production [14]. The performance of these catalysts was also greatly improved by the introduction of bifunctional versions of the catalyst system, combining an electrophilic aluminium centre with an ammonium cation/nucleophilic-counteranion combination within the framework of a single catalytic species as reported by North [15], Liu and Darensbourg [14], and Lu [16] (Figure 1).
Unfortunately, the bifunctional derivatives with an alkyl spacer are not very stable at higher temperatures because of the wellknown ammonium salt decomposition pathways including: Zaitsev and Hoffman type eliminations [17][18][19] and retro-Menschutkin reactions [20][21][22][23]. We reasoned that the direct introduction of ammonium moieties onto the aromatic rings of the salen ligands (as in structures 1 and 2) would greatly stabilize the whole structure by reducing the number of sp 3hybridized carbon atoms attached to the nitrogen atoms of the ammonium salts and increasing the steric hindrance around the ammonium salts. Herein, we report the synthesis of two aluminium-salen complexes incorporating quaternary ammonium salts directly attached to the salen ligand and their catalytic activities for the coupling of epoxides and carbon dioxide under solvent free conditions. Catalyst recycling experiments are also reported and show the robustness of this system.

Results and Discussion
The preparation of salen ligands 8a and 8b was conducted according to Scheme 2, starting from tert-butylphenol, which was formylated and then nitrated to produce 5-nitro-3-(tertbutyl)salicylaldehyde (5) [24][25][26][27]. 5-N,N-Dimethylamino-3-(tert-butyl)salicylaldehyde (6) was prepared directly from 5, using a literature procedure [28,29]. The salen ligand 7 was then obtained in high yield by condensation with (R,R)-cyclohexanediamine, according to a known technique [30]. Ligand 7 was efficiently alkylated under mild reaction conditions using either methyl iodide or benzyl bromide, giving the corresponding positively charged salen ligands 8a and 8b in 95 and 78% yields respectively. The aluminium-salen complexes were prepared by treating 8a and 8b with diethylaluminium chloride (Scheme 3), affording complexes 1 and 2 in 96% and 91% yields respectively. These complexes could be used without any additional purification. Furthermore, the performance of catalyst 1 could be improved by adding one equivalent of water and triethylamine relative to the catalyst loading (Table 1, entries 5-7). Presumably, some of complex 1 was converted into a highly active oxygen-bridged aluminium complex in situ, as shown in Scheme 4. High activities for this type of dinuclear complexes have been reported before [31].
In order to prove the bifunctional nature of our catalysts, aluminium complex 10 was prepared (Scheme 3) using 3,5-di-(tert-butyl)salicylaldehyde as a starting material. It was found that this catalyst was almost inactive in the reaction of styrene oxide with carbon dioxide ( Table 1, entry 14). After addition of tetrabutylammonium iodide (5 mol %) as a cocatalyst, the conversion was increased to 80% (Table 1, entry 15), which was close to the performance of catalyst 2 ( Table 1, entry 12). This supports the hypothesis that complexes 1 and 2 are bifunctional catalysts in which both the aluminium centre and the ammonium halide play important catalytic roles.
After finding the optimal reaction conditions for each catalyst, both complexes 1 and 2 were tested with a range of epoxides. These experiments were carried out without added water to allow direct comparison of the two catalysts and to avoid complicating the reaction system. The results are summarized in Table 2. Both catalysts proved to be efficient for coupling both aromatic and aliphatic substrates. In all cases reported in Table 2, cyclic carbonate, catalyst and unreacted epoxide (for entries 10 and 11) were the only species detected by 1 H NMR spectroscopy of the crude reaction product prior to purification by column chromatography. The moderate yield for propylene oxide ( Table 2, entry 6) can be explained by volatility of the starting material under the reaction conditions.
No cyclic carbonate was detected when cyclohexene oxide was used as substrate (Table 3, entries 1-5) and almost no conversion at all was detected in the reaction promoted by complex 1 ( Table 3, entries 1 and 2). Catalyst 2 was more active and catalysed the synthesis of the corresponding polycarbonate with 64% conversion at 10 bar (  [32] or polycarbonate [33,34] from cyclohexene oxide, depending on the exact structure of the catalyst and cocatalyst. However, this is the first report of a one-component aluminium-salen-based catalyst for polycyclohexene carbonate synthesis. MALDI-TOF mass-spectra data ( Figure 2) showed that the polycarbonate consisted of a mixture of oligomers with a range of monomer units (n from 4 to 10) with the maximum intensity at n = 6. Both ends of the polymer chain are capped with alcohol groups, suggesting that chain-transfer to adventitious moisture occurred during the polymerisation. GPC data ( Figure 3) was consistent with the MALDI-TOF data, showing that most of the polymer has a molecular weight between 300  and 1000 Daltons. This type of low molecular weight polycarbonate-polyol is currently attracting much interest associated with its use in sustainable polyurethanes [35].
To show the stability of our catalytic system, catalyst 1 was reused three times. For this purpose the catalyst was precipitated from the reaction mixture by the addition of ether followed by filtration. The catalyst was then dried in vacuo and then reused. The results are summarized in Table 4. As can be seen, there were no significant losses of catalytic activity observed after three catalytic cycles.

Experimental Materials
Commercial reagents were used as received unless stated otherwise. Column chromatography was performed using Silica Gel Kieselgel 60 (Merck).

H NMR and 13 C NMR spectra were recorded on Bruker
Avance 300 and Bruker Avance III-400 (operating at 300 and 400 MHz for protons, respectively) spectrometers. Optical rotations were measured on a Perkin-Elmer 341 polarimeter in a 5-cm cell. Melting points were determined in open capillary tubes and are uncorrected. Mass spectra were recorded at the University of York Mass Spectrometry Service Unit using ESI and MALDI ionization methods. GPC was carried out using a set (PSS SDV High) of 3 analytical columns (300 × 8 mm, particle diameter 5 µm) of 1000, 10 5 and 10 6 Å pore sizes, plus guard column, supplied by Polymer Standards Service GmbH (PSS) installed in a PSS SECcurity GPCsystem. Elution was with tetrahydrofuran at 1 mL/min with a column temperature of 23 °C and detection by refractive index. 20 µL of a 1 mg/mL sample in THF was injected for each measurement and eluted for 40 minutes. Calibration was carried out in the molecular weight range 400-2 × 10 6 Da using ReadyCal polystyrene standards supplied by Sigma-Aldrich.

Aluminium-salen complex (1)
To a solution of 8a (113 mg, 0.14 mmol) in dry acetonitrile (5 mL) under argon was added diethylaluminum chloride (0.14 mL, 1 M solution in hexane). The reaction mixture was heated at reflux for 3 hours. The solvent was evaporated under reduced pressure to give a dark yellow powder (95 mg, 78%) which was used without any additional purification. 1  Aluminium-salen complex (10) Prepared as described in previous work [29]. 1

Synthesis of cyclic carbonates
All cyclic carbonate formations were carried out in autoclaves or, in case of 1 bar CO 2 reactions, in sample vials with a balloon of CO 2 attached to them. In both cases the reactions were magnetically stirred. After completion of the experiment, the reaction mixture was analysed by 1 H NMR spectroscopy and passed through a pad of silica to separate the catalyst. In the case of a 100% conversion, CH 2 Cl 2 was used as the eluent, if the conversion was incomplete then column chromatography was used to purify the compounds (SiO 2 , EtOAc/hexane, 1:3).

Synthesis of polycyclohexene carbonate
Prepared as reported above for the synthesis of cyclic carbonates at 10-35 bar CO 2 , but without any additional purification of the reaction product. 1