2.1 Preparation of polystyrene-supported gallium trichloride (PS/GaCl₃)

Anhydrous GaCl₃ (4 g) was added to polystyrene (8% divinylbenzene, mesh size: 20–75 mm, 8 g) in carbon disulfide (30 mL) as the reaction medium. The mixture was stirred using a magnetic stirrer under reflux condition for 1 h, cooled, and then water (50 mL) was cautiously added to hydrolyze the excess GaCl₃. The mixture was stirred until the bright red color disappeared, and the polymer became yellow. The polymer beads were collected by filtration and washed with water (300 mL) and then with ether (30 mL) and chloroform (30 mL). The catalyst was dried in a vacuum oven overnight at 50 °C before use. The chlorine content of PS/AlCl₃ was 4.17% analyzed by the Mohr titration method [50] and the loading capacity of GaCl₃ on the polymeric catalyst or the amount of GaCl₃ complexed with polystyrene was calculated to be 0.391 mmol/g.

2.2 General experimental procedure for acetylation of alcohols and phenols with Ac₂O catalyzed by PS/GaCl₃ complex

To a solution of alcohol or phenol (1 mmol) and Ac₂O (1.1–1.3 mmol) in methylene chloride (5 mL) was added PS/GaCl₃ (0.1 mmol) and the resulting mixture was stirred at room temperature. The progress of the reaction was monitored by TLC and GLC. After completion of the reaction, the catalyst was filtered off and washed with methylene chloride (2 × 10 mL) and the filtrate concentrated on a rotary evaporator under reduced pressure to afford the crude acetate product. Further purification was achieved by column chromatography on silica gel (Merck, 100–200 mesh, hexane-EtOAC, 90/10, v/v). The spent polymeric catalyst from different experiments was combined, washed with ether and dried overnight in a vacuum oven and reused.

2.3 General experimental procedure for benzoylation of alcohols and phenols with (PhCO)₂O catalyzed by PS/GaCl₃ complex

To a solution of alcohol or phenol (1 mmol) and (PhCO)₂O (1.2–1.5 mmol) in methylene chloride (10 mL) was added PS/GaCl₃ (0.1 mmol) and the resulting mixture was stirred under reflux conditions for the appropriate time according to Table 2. The progress of the reaction followed by GLC or TLC. After completion of the reaction, the catalyst was filtered off and washed with methylene chloride (2 × 10 mL) and the filtrate concentrated on a rotary evaporator under reduced pressure to afford the crude benzoate product which was passed through a short pad of silica gel (Merck, 100–200 mesh, hexane-EtOAC, 95/5, v/v) to provide the pure benzoylated product.

2.4 Catalyst recovery and reuse

The reusability of the catalyst was checked in the multiple acetylation of benzyl alcohol. At the end of each reaction, the solvent was evaporated, ether (Et₂O, 10 mL) was added and the catalyst was filtered. The recovered catalyst was used with fresh benzyl alcohol, Ac₂O and CH₂Cl₂.

3.1 Preparation of PS/GaCl₃ complex

First, cross-linked polystyrene (5–7% divinylbenzene, mesh size: 20–75 mm) was prepared via suspension polymerization as reported in the literature [48]. Then, PS/GaCl₃ was prepared by addition of anhydrous gallium chloride to polystyrene (8% divinylbenzene) in carbon disulfide under reflux conditions. The loading capacity of the polymeric catalyst obtained by gravimetric method and checked by atomic absorption technique was 0.391 mmol GaCl₃/g of complex beads catalyst [50]. The data obtained by these two techniques showed, within experimental error, that the catalyzing species are in the form of GaCl₃ supported on the polymeric support. The UV spectrum of the solution of PS-GaCl₃ complex in CS₂ showed a new strong band at 470 nm, which is due to the formation of a stable π→p type coordination complex between the benzene rings in the polystyrene carrier with gallium trichloride. The FT IR spectrum of PS/GaCl₃ showed new absorption peaks due to the C-C stretching vibration and the C-H bending vibration of the benzene ring at 1500 to 1560 and 400 to 800 cm⁻¹, by which complex formation was demonstrated. The structure of the PS-GaCl₃ complex is similar to that of the PS-AlCl₃ complex as suggested by Neckers et al. [51], because the Lewis acid GaCl₃ is complexed with the benzene rings of the polystyrene and the GaCl₃ is stabilized due to the decreased mobility of the benzene rings hindered by the long polystyrene chain. The PS-GaCl₃ complex catalyst is a non-hygroscopic, water-tolerant, and especially stable species. In addition, this polymeric catalyst is easy to prepare, stable in air for a long time (over 2 years) without any change, easily recycled and reused without appreciable loss of its activity.

3.2 Acetylation and benzoylation of alcohols and phenols with Ac₂O and (PhCO)₂O catalyzed by PS/GaCl₃

In our initial evaluations, we used as the model reaction, the reaction of benzyl alcohol (1 equiv.) with acetic anhydride (1.1 equiv.). The choice of the solvent appeared crucial in order to maximize yield. Among the various solvents surveyed, the highest yield was obtained in CH₂Cl₂ (Table 1, entry 7). The highest yield obtained by using CH₂Cl₂ as the solvent can be ascribed to the fruitful swelling of the polymer network of the catalyst in this media, allowing the metal particles located inside the polymer matrix to effect the catalysis. The optimum molar ratio of the polymeric catalyst to hydroxyl compound was found to be 0.1:1. The key role played by the Lewis acidity of the heterogeneous catalyst PS/GaCl₃ was proved by employing the polystyrene beads (Table 1, entry 9) and the GaCl₃-benzene complex as catalysts. In fact, while in the former case no reaction occurred, which indicated that polystyrene itself did not promote the reaction, in the latter, the desired product was isolated in low yield 46% (Table 1, entry 10). Also, PS/GaCl₃ was found to be a more effective catalyst than PS/AlCl₃ in terms of reaction time and the yield of the product for acetylation of benzyl alcohol under identical conditions (Table 1, entry 12). We then turned our attention on whether the same catalyst could be employed for acetylation of alcohols using acetic acid and ethyl acetate as acylating agents. For this purpose, the model reaction was carried out with acetic acid and ethyl acetate under identical conditions, and only 36% and 23% of the corresponding acetates were produced, respectively.

When using a heterogeneous and metal-supported catalyst one of the most important issues is the possibility of leaching of the reactive center into the reaction mixture, particularly for practical applications of the reaction. Our preliminary investigations demonstrated that PS/GaCl₃ catalyst (a stable π complex) is very stable to air and moisture. To rule out the contribution of concurrent homogeneous catalysis in the results shown in Table 2, the catalyst was allowed to react with dry methylene chloride for 15 min under the operated reaction conditions and then filtered off. We observed that the filtrate (the catalyst-free liquid) showed no catalytic activity in acetylation of benzyl alcohol under the operated reaction condition (Table 1, entry 11). On the other hand, after each run, the filtrates were used for determination of catalyst leaching (gallium content), which showed a negligible release of GaCl₃ by atomic absorption spectroscopy or ICP measurement. The capacity of the catalyst after six uses was 0.38 mmol of GaCl₃ per gram. Also, the catalytic behavior of the separated liquid was tested by addition of fresh benzyl alcohol and Ac₂O to the filtrates after each run. Execution of the acetylation reaction under the same reaction conditions, as with catalyst, showed that the obtained results were as same as blank experiments. These findings confirmed that PS/GaCl₃ is stable and no significant leaching of Lewis acid moieties is operating under our reaction conditions and that the observed catalysis is truly heterogeneous in nature.

Further, we carried out the reactions between Ac₂O and various alcohols to explore the reaction scope of PS/GaCl₃-catalyzed acetylation, the results are summarized in Table 2. As can be seen, all reactions proceeded very cleanly (checked by GC) in good to excellent yields. A broad selection of alcohols, including primary (benzylic and linear ones), secondary (including aliphatic and aromatic alcohols), tertiary and allylic alcohols, were converted to their corresponding acetates successfully at room temperature (Table 2). It is noteworthy that in the case of both tertiary and allylic alcohols, which are acid-sensitive alcohols, no elimination product or migration of C = C bonds (rearrangement) was observed when PS/GaCl₃ was used, probably due to its mild catalytic activity and they were also converted to their corresponding acetates in high yields (Table 2, entries 13, 15, 16). Protection of phenolic hydroxyl group was also achieved under the same reaction conditions described for alcohols and the corresponding acetates were produced in high yields (Table 2). As can be seen, a phenol whose benzene ring substituted with a strong electron-donating group, reacted quickly with excellent yield (Table 2, entries 18, 19). We then turned our attention on whether the same catalyst could be useful for acetylation of substrates containing other protecting groups. We observed that various protected alcohols (Table 2, entries 23–26) containing protecting groups such as acetyl, benzyl, benzoyl and tosyl were smoothly converted into the corresponding acetates without affecting the other protecting groups.

In order to extend the scope of the catalyst further, the benzoylation of alcohols and phenols with benzoic anhydride, a less reactive anhydride, was also investigated. The results of this investigation are shown in Table 2. Treatment of a series of alcohols and phenols with benzoic anhydride in the presence catalytic amount of PS/GaCl₃ provided the corresponding benzoates in good yields (Table 2, entries 1–22).

In order to show that whether the presented method is potentially applicable for chemoselective conversion of alcohols and phenols to the corresponding acetates or benzoates, a set of competitive reactions was allowed between a primary alcohols and a secondary or tertiary alcohols, or phenol with Ac₂O and (PhCO)₂O in the presence of PS/GaCl₃ (Table 3). The results indicated that PS/GaCl₃ was able to discriminate between different types of alcohols and or phenols from each other, a transformation that is difficult to accomplish via conventional methods (Table 3, entries 1–9).

The definite mechanism of the reaction is unclear. However, a plausible mechanism for the formation of acetates in the presence of PS/GaCl₃ as a catalyst is shown in Scheme 2. The acetic anhydride is first activated by Ga(III) as a Lewis acid to afford 1. The hydroxyl compound attacks 1 which in turn converts to the final product and simultaneously releases the catalyst for the next catalytic cycle.

In order to show the efficiency and applicability of the present method, the catalytic activity of PS/GaCl₃ was compared with that of some reported catalysts in the literature, the results are summarized in Table 4. The results have been compared with respect to the reaction times, mol-% of the catalyst used, and yields. As demonstrated in Table 4, PS/GaCl₃ is an equally or more efficient catalyst for this acetylation reaction in terms of yield and reaction rate.

3.3 Catalyst reusability

Finally, the reusability of the catalyst was tested in the acetylation reaction of benzyl alcohol with acetic anhydride as a model substrate. For each of the repeated reactions, the catalyst was filtered, washed exhaustively with methylene chloride and diethyl ether, respectively, and dried before using with fresh benzyl alcohol and Ac₂O. The catalyst was consecutively reused six times with negligible loss in its activity or a negligible catalyst leaching, and also there is no need for regeneration (Table 5).