Glycerol-Based Carbon-SO 3 H Catalyzed Benign Synthetic Protocol for the Acetylation of Alcohols , Phenols and Amines under Solvent-Free Conditions

A simple and efficient solvent-free method was developed for the acetylation of alcohols, phenols and amines in excellent yields employing glycerol-based sulfonic acid (SO3H) functionalized carbon catalyst under environmentally benign reaction conditions. The salient features of this protocol are the short reaction time, ease of product isolation and reusability of the carbon catalyst.

Carbon-based solid acid catalysts [45][46][47][48] have gained importance due to their significant advantages over homogeneous liquid phase mineral acids, such as increased activity and selectivity, longer catalyst life, negligible equipment corrosion, ease of product separation, and reusability.We, recently developed a simple and fast method for the preparation of a similar sulphonic acid functionalized polycyclic aromatic carbon catalyst from bioglycerol (biodiesel by-product) and also from glycerol-pitch (waste from fat splitting industry) by in situ partial carbonization and sulfonation [49,50].Such catalysts have been shown to be inexpensive, highly stable, robust, recyclable, and easily produced from naturally available bioglycerol, and are demonstrated to be effective for the esterification of fatty acids to its methyl esters [49], THP protection and deprotection of alcohols and phenols [50], and also for the synthesis of highly substituted imidazoles [51], 3,4-dihydropyrimidin-2-(1H)ones [52], amides from aldehydes [53], and spirooxindole derivatives [54].In continuation of our efforts towards exploring the applications of the glycerol-based carbon catalyst, here we report a simple and an efficient methodology for the acetylation of alcohols, phenols and amines with acetic anhydride at 65  C under solvent-free conditions in excellent yields (Scheme 1).

Experimental Details
All chemicals and reagents were procured from suppliers and used without further purification.The isolated products were characterized by chromatographic and spectral studies (GC, GC-MS, FT-IR and 1 H NMR). The spectra were compared with those of standard esters.The NMR spectrums of product were obtained using Bruker AC-300 MHz spectrometer with TMS as the internal standard.

General Experimental Procedure for the Preparation of Glycerol-Based Carbon-SO 3 H Catalyst
Carbon-SO 3 H catalyst was prepared as reported [49] by heating a mixture of glycerol (10 g) and concentrated sulfuric acid (30 g) from ambient temperature to 210  C -220  C for 20 min, to facilitate in situ partial carbonization and sulfonation.The reaction mixture was allowed to remain at that temperature for about 5 min (until foaming ceased) to obtain the carbon material.The solid material was cooled to ambient temperature and washed with hot water under agitation until the wash water showed a neutral pH value.The partially crystalline product was filtered and dried in an oven at 120 º C for 2 h until it was moisture free to obtain the glycerol-based carbon acid catalyst (4.67 g).

General Experimental Procedure for the Catalytic Acetylation of Alcohols, Phenols and Amines
To a stirred mixture of the alcohol/amine ( min.The progress of the reaction was monitored by TLC. After completion of the reaction, ethyl acetate (3 × 5 mL) was added to the reaction mixture and the catalyst was separated by filtration.The organic phase was washed with saturated NaHCO 3 solution (15 mL), dried over anhydrous Na 2 SO 4 and concentrated under reduced pressure in a rotary evaporator to afford the crude product.

Results & Discussion
Acetylation of 1-octanol (1 mmol) with acetic anhydride (2 mmol) in presence of 15 wt% of carbon-SO 3 H catalyst was studied at 65 °C to afford 1-octyl acetate in 96% yield in 30 min.However, the acetylation of 1-octanol with acetic anhydride at room temperature resulted <50% acetylated product even after 4 h.After the successful acetylation of 1-octanol with excellent yield to octyl acetate, the effect of catalyst loading for the acetylation of 1-octanol with acetic anhydride (1:2 mmol) to octyl acetate was studied by varying the catalyst dosage from 2 to 20wt% of alcohol (Table 3).The reaction was found to be slow at room temperature and hence the reaction temperature was increased to 65  C. At this temperature, with the increase of the catalyst loading from 2 to 15 wt% of alcohol decreased the reaction time substantially from 120 min to 30 min with conversions ranging from 65% to 96% (entries 1 -4, Table 3).However, further increase of the catalyst loading to 20 wt% decreased the reaction time marginally from 30 min to 25 min with 98% conversion.The reactions were monitored by GC    using HP-1 capillary column.Based on this study, 15 wt% of the carbon catalyst was found to be optimum for the acetylation of 1-octanol with 96% conversion at 65˚C in 30 min.
In order to establish the effectiveness and the acceptability of the method in a wider context of synthetic organic chemistry, acetylation of various alcohols and phenols with electron donating and electron withdrawing groups were studied in presence of acetic anhydride under optimum conditions and the results are given in Table 1.The study revealed that primary and secondary alcohols (entries 1-5, Table 1) are acetylated with similar yields (95% -96%) within 30 min, where as acetylation of ethoxy alcohol (entry 6, Table 1) and phytosterols (entry 7, Table 1) resulted the corresponding acetylated products in 95% yields in 45 and 60 min respectively.Acetylation of phenols was found to be slow when compared to aliphatic alcohols.Phenols with electron donating and electron withdrawing groups (entries 9 -12 and 14, Table 1) were acetylated in 91% -93% yields within 45 -60 min and similar observation was made in case of benzyl alcohol also (entry 8, Table 1).However acetylation of 2-chlorophenol resulted 2-chlorophenyl acetate (75%, entry 13, Table 1) in less yields when compared to 4-chlorophenyl acetate (92% yield, entry 12, Table 1) due to steric effect.
In a similar fashion acetylation of aromatic amines also executed, and resulted the corresponding acylated products in good yields (92% -97%, entries 1 -6, Table 2) in 30 min.To examine the chemo selectivity of the present method, bi-functional substrates containing -NH 2 and -OH groups were studied (entries 3-5, Table 2).Selective acetylation of the -NH 2 group in the presence of the -OH group was observed at room temperature with 2 equivalent of acetic anhydride to give corresponding Nacetate product, and no O-acetate product was observed under these conditions.This might be due to more nucleophilicity of amines than phenols.The present protocol is excellent for the acetylation of alcohols (primary and secondary), phenols, amines, and bi-functional compounds containing -NH 2 and -OH groups.
To check the reusability of the carbon catalyst, benzyl alcohol was employed for the acetylation for five cycles under the optimum conditions (Figure 1).After each cycle the catalyst was recovered by filtration, washed with methanol, dried in oven at 120  C for 1h and reused and the yields were found to be reduced marginally from 92 to 89% after 5 th cycle.

Conclusion
In conclusion, we have demonstrated a simple, efficient and eco-friendly protocol for the solvent-free acetylation of alcohols, phenols and amines with acetic anhydride employing a novel glycerol-based -SO 3 H functionalized carbon as a solid acid catalyst.The low cost and simple preparation of the catalyst, and the easy procedure and work-up indicate that this catalyst is very attractive for this type of reactions.The yields are very good, and in addition, the carbon-based solid acid catalyst can be recovered by simple filtration for reuse without any pretreatment.The carbon-based solid acid can be used in place of sulfuric acid in the synthesis of organic compounds.

Figure 1 .
Figure 1.Recycling study of the carbon-SO 3 H catalyst for the acetylation of benzyl alcohol with acetic anhydride.