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Results and discussion On the basis of the information obtained from the studies on RHA/TiO2 nanocomposites, it 6-Bnz-cAMP sodium salt is expected that this reagent can be used as a catalyst for the promotion of the organic reactions. So this reagent was used in the promotion of the conversion of alcohols, phenols and amines to their corresponding acetates and/or amides with acetic anhydride (Scheme 1). In the first step we focused our attention toward the optimization of the ratio of RHA to TiO2 for obtaining the highest catalytic activity. For this purpose, acetylation of 4-chlorobenzyl alcohol was studied and the best results were obtained using RHA/TiO2(30%) (Table 2). Further increase in this ratio did not improve the activity of the prepared nanocomposite. Then for obtaining the optimum reaction conditions, we studied the influence of the following factors on the acetylation of 4-chlorobenzyl alcohol with acetic anhydride: i) the amounts of the catalyst; ii) solvent or solvent-less media and iii) temperature. The obtained results clarified that the best conditions are the ones which are shown in Scheme 2. After optimization of the reaction conditions, different types of alcohols were subjected to the acetylation using this method (Table 3). Acetylation of various benzylic alcohols containing electron-withdrawing and electron-donating substituents proceeded efficiently with high isolated yields (Table 3, Entries 1–7). Primary, secondary and tertiary aliphatic alcohols were also efficiently converted to their corresponding acetates in almost quantitative yields at room temperature (Table 3, Entries 8–12). No elimination and rearrangement by-products were observed at all. Phenol and its derivatives also undergo acetylation easily using this method and their corresponding acetates can be isolated in excellent yields (Table 3, Entries 13–16). In the case of pyrogallol, higher amounts of the catalyst are needed due to high steric hindrance (Table 3, Entry 16). This method is also very useful for the acetylation of amines with acetic anhydride. All reactions are performed under mild reaction conditions in very short reaction times with high yields (Table 3, Entries 17–20). To check the reusability of the catalyst, the reaction of 4-chlorobenzyl alcohol and acetic anhydride under the optimized reaction conditions was studied. When the reaction was completed, dichloromethane was added and the catalyst was separated by filtration. The recovered catalyst was washed with dichloromethane, dried and reused for the same reaction. This process was carried out over five runs and all reactions led to the desired products with high efficiency (Table 3, Entry 2). The possible mechanism for the acetylation of alcohols, phenols and amines in the presence of RHA/TiO2(30%) as a promoter is shown in Scheme 3. On the basis of this mechanism, RHA/TiO2(30%) catalyzes the reaction by the electrophilic activation of Ac2O to form a zwitterionic species, making the carbonyl group susceptible to nucleophilic attack by the substrate. Successive elimination of CH3CO2H results in the formation of the requested product and regenerates RHA-SO3H in the reaction mixture. To illustrate the efficiency of the proposed method, Table 4 compares our results in the acetylation of 4-chlorobenzyl alcohol with those reported by using the relevant reagents in the literature. It is clear that the present method is superior in terms of the reaction time and catalyst amount, especially compared with rice husk (RiH), rice husk ash (RHA) and TiO2 (Table 4, Entries 5–7).
Conclusions In conclusion, rice husk ash was used as a silica support for the synthesis of anatase-phase titania nanoparticles and a RHA/TiO2 nanocomposite was obtained. Then, a simple and efficient protocol for the acetylation of alcohols, phenols and amines was developed using this nanocomposite. The methodology has several advantages such as: (i) high reaction rates and excellent yields, (ii) no side reactions, (iii) ease of preparation and handling of the catalyst, (iv) cost efficiency and effective reusability of the catalyst, (v) use of an inexpensive catalyst with lower loading and (vi) a simple experimental procedure and solvent free conditions. Further work to explore this catalyst in other organic transformations is in progress.