CHAPTER 5 ACYLATION OF PHENOL AND SALICYLALDEHYDE WITH ACETIC ANHYDRIDE

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1 128 CHAPTER 5 ACYLATION OF PHENOL AND SALICYLALDEHYDE WITH ACETIC ANHYDRIDE 5.1 ACYLATION OF PHENOL In recent years there has been a tremendous upsurge of interest in various chemical transformations performed under heterogeneous catalysts. Acylation reactions are largely employed in the fine chemical industry producing a variety of synthetic fragrances and pharmaceuticals (Sorbinho et al 1998). Acylation of alcohols and phenols are usually carried out by acid anhydrides or acid chlorides in the presence of tertiary amines (Zhdanor and Zhenodarova 1975). Vedegs et al (1993) reported the use of tributylphospine as a catalyst for the acylation of base sensitive alcohols. In addition, a number of Lewis acids are also known to catalyse the acylation of alcohols with acid anhydride. More recently montmorillonite K-10 and KSF were applied as solid catalyst for acylation of alcohols and phenols to obtain relatively better results (Li et al 1997 and Li and Li 1998). Hydroxyacetophenone, especially the ortho and para isomers are important compounds in the pharmaceutical industry (Pouilloux et al 1987 and Neves et al 1994). Previous works claimed that phenylacetate and o-hydroxyacetophenone could be primary products, while p-hydroxyacetophenone is a secondary one. The Fries rearrangement brings about phenylacetate transformation into o-hydroxyacetophenone and

2 129 p-hydroxyacetophenone via intra and inter molecular mechanism, respectively (Pouilloux et al 1987, Neves et al 1994a). Furthermore, it has been showed that the formation of phenylacetate, o-and p-hydroxyacetophenone during phenol acylation with acetic acid over HZSM-5 occurs through different reaction (Figure 5.1). OCOCH 3 COCH 3 (P-HAP) Fig. 5.1 Reaction scheme of phenol acylation with acetic acid (Neves et al 1994) (P) phenol, (PA) phenylacetate, (o-hap) o-hydroxyacetophenone, (p-hap) p-hydroxyacetophenone, (p-aap) p-acetoxyacetophenone Several authors have studied this reaction in liquid phase or vapour phase. However, the reports on the use of basic catalyst for the acylation reactions are scarce. Recently Ga-Mg hydrotalcites showed high activity both in benzoylation and acylation of toluene (Choudhary et al 2001). Hydrotalcites are known to be highly basic solids. In contrast the surface Lewis acidic sites are

3 130 provided by both Mg2+ and Al3+ cations (Di Cosimo et al 1998). The use of these anionic clays after their thermal decomposition to mixed metal oxides has been reported for a number of catalytic reactions (Cavani et al 1991). No information is available in the use of hydrotalcites or also basic catalysts derived from them for the phenol acylation reaction with acetic anhydride. Since acylation of phenol with acetic acid would produce water as the byproduct. Hence, in the present investigation, the acylation of phenol has been carried out with acetic anhydride, as the byproduct produced is only acetic acid. Though acetic acid thus produced could acylate, acetic anhydride is far more reactive than acetic acid. The main reaction products is phenylacetate resulting from phenol O-acylation, o-hydroxyacetophenone results from phenol C-acylation with PA (formed by a rapid O-acylation of phenol) of from Fries rearrangement of phenylacetate. 5.2 DISCUSSION Acylation of Phenol over MgAI-CHT The results of acylation of phenol with acetic anhydride obtained over MgAI-CHT are presented in the Table 5.1. The selectivity towards p-hydroxyacetophenone and p-acetoxyacetophenone are very small regardless of the feed composition. Therefore, we consider the data on phenylacetate and o-hydroxyacetophenone. The reactant ratio, phenol and acetic anhydride was set at 1:1. Phenol conversion increased with increase in temperature. The reaction required formation of acyl cation and this cation is to be generated via., (1) co-ordination of acetic anhydride to the Lewis acid sites and (2) Bronsted acid adsorption of acetic anhydride generated through dissociative adsorption of phenol on the Lewis acid sites. The acyl cation formed by the former route could more favour the formation of phenylacetate and that the latter route

4 131 Table 5.1 Acylation of phenol over Mg/AI-CHT Feed ratio Temperature < C) Phenol Conversion Product Selectivity Phenyl acetate o-hydroxy acetophenone 1: : : Catalyst weight : 0.5 g

5 132 favours the formation of o-hydroxyacetophenone. The selectivity of phenylacetate increased with increase in temperature, hence formation of acyl cation by the route (1) is high activation energy demanding. The selectivity to o-hydroxyacetophenone decreased with increase in temperature suggesting less dissociative adsorption of phenol on the Lewis acid site at higher temperature. As acyl cations could be generated through adsorption of acetic anhydride on the Lewis acid sites, the reaction was carried out with the high feed ratio 1:2 and 1:3 in order to examine its influence on the phenol conversion and product selectivity. Phenol conversion registered an increase with increase in temperature. Similarly, the selectivity to phenylacetate also increased with increase in temperature. Though the selectivity to o-hydroxyacetophenone decreased with increase in temperature as with 1:1 feed ratio, the selectivity was not less with increase in feed ratio suggesting dissociative adsorption, as and above alone is not the route for the formation of o-hydroxyacetophenone Acylation of Phenol over Ni/Al, Cu/Al, Co/Al and Zn/Al CHTs The reaction was also studied over Ni11, Cu11, Co11 and Zn11 hydrotalcite like compounds. The results are presented in Table 5.2. The data indicate similarity in variation of product selectivity as with Mg11 hydrotalcite. Comparison of the activity of Mg11 and transition metal hydrotalcites indicates the following order MgAl > NiAl> CuAl > CoAl > ZnAl. The selectivity of o-hydroxyacetophenone over MgAl and transition metal hydrotalcites is compared, it is more over the latter than the former. Hence, dissociative adsorption of phenol on the transition metal Lewis acid sites to favour o-hydroxyacetophenone becomes more important over these

6 133 Table 5.2 Acylation of phenol over M(I1)/AI - CHTs Catalyst Temperature ( C) Phenol Conversion Product Selectivity Phenyl acetate o-hydroxy acetophenone Ni/Al-CHT Cu/Al-CHT Co/Al-CHT Zn/Al-CHT Phenol: Acetic anhydride =1:3 Catalyst weight : 0.5g

7 134 catalysts. Selectivity to phenylacetate is higher at 150 C over all the catalysts and at all temperatures irrespective of the feed ratio. Hence the electrophilic acylation at the ring carbons by destroying and restoring aromaticity is not as advantageous as compared to substitution at the phenolic oxygen. 5.3 ACYLATION OF SALICYLALDEHYDE Introduction The acylation of salicylaldehyde is considered to be one of the most important reaction as one of its product is coumarin which has tremendous applications in various fields as given in its scope. So far, many scientists have worked on this area using salicylaldehyde as the starting material. William H. Perkin in 1968, synthesised coumarin by heating the sodium salt of salicylaldehyde with acetic anhydride. Sodium acetate could serve as the base catalyst (Fig 5.2). Modification of Perkin s reaction was done by Knoevenagal where acetic anhydride was replaced by acetic acid, primary and secondary amines, and ammonium salts. Coumarin from salicylaldehyde with acetic anhydride using various homogeneous catalysts has been reported (Nankee et al 1977). Coumarin from salicylaldehyde condensed with malonic ester has also been reported (Voronin and Bratus 1960). Of the various alternate routes available, the salicylaldehyde route appear to be ideal for Indian conditions. There is no technology for its synthesis using heterogeneous catalysts, which has higher advantages over homogeneous catalysts. This kind of reaction was not reported using hydrotalcites.

8 135 oh ococy Coumarin Fig. 5.2 Perkin s reaction : Coumarin synthesis from salicylaldehyde with acetic anhydride Acylation of salicylaldehyde in the liquid phase over Mg/Al-CHT was carried out at 80, 100 and 150 C with salicylaldehyde and acetic anhydride ratio 1:1, 1:2 and 1:3. The reaction was carried out in the nitrogen atmosphere in order to prevent air oxidation of salicylaldehyde at higher temperatures. The results are presented in the Table 5.3. The major products are 2-formylphenyl acetate and 2-acetyl-6-formyl phenol and coumarin. Conversion increases with temperature in each feed ratio. As this reaction requires formation of acyl cation, Lewis acid adsorption of phenol leading to its dissociation becomes necessary and the released proton is to rest on the adjacent basic site. Acetic anhydride can be adsorbed on the Bronsted acid site to produce acyl cation. This acyl cation can make electrophilic attack on the ortho position of salicylaldehyde to yield 2-acetyl-6-formyl phenol. Free salicylaldehyde can also react with acyl cation to form 2-formylphenyl acetate. In addition, acetic anhydride can be directly adsorbed on Lewis acid site with the formation of acyl cation which can react with free salicylaldehyde to form the above said products. If the formation of 2-acetyl 6-formylphenol requires dissociative adsorption of phenol on the Lewis acid sites, than with the feed ratio 1:2, there must be reduction in its formation. As expected, there is a slight decrease in the

9 136 Table 5.3 Acylation of salicylaldehyde over MgAl-CHT Feed ratio Temperature ( C) Salicylaldehyde Conversion Product Selectivity : : : Catalyst weight : 0.5 g 1 : 2-formylphenylacetate 2 : 2-acetyl-6-formylphenol 3 : Coumarin 4 : Cinnamic acid

10 137 selectivity of 2-acetyl-6-formylphenol compared to 1:1 at each temperature. Again at this feed ratio, since acetic anhydride is to be preferentially adsorbed on the Lewis acid site, the selectivity to 2-formyl-phenylacetate should increase. This is also observed in this study. But, when the feed ratio is increased to 1:3 there is no increase either in salicylaldehyde conversion or the selectivity to 2-formylphenyl acetate. Hence little dilution of salicylaldehyde by acetic anhydride becomes important with high content of the latter in the feed. Selectivity to coumarin, which is to be formed by intramolecular conversion of 2-formylphenyl acetate over the basic site is also formed in more amount with the feed ratio 1:2, as at this ratio 2-formylphenyl acetate is also formed in more amount. The reaction was also studied over Ni/Al-CHT at 80, 100 and 150 C with the feed ratio 1:2 (Table 5.4). There is also increase in conversion as observed over Mg/Al-CHT. The selectivity patterns of the products also follow similar trend. Competition of conversion over this catalyst with Mg/Al-CHT indicates that the latter is more active than the former which may be due to its more Lewis acidity. Again as Ni in Ni/Al-CHT is more Lewis acidic than Mg in Mg/Al- CHT, it could favour dissociative adsorption of phenol and so also facilitating formation and the selectivity of 2-acetyl-6-formyl phenol. It is actually observed in this study when the reaction was studied over Cu/Al-CHT, the conversion and products selectivity follows the similar trend as Ni/Al-CHT (Table 5.4). The conversion is less at each temperature compared to Mg/Al- CHT. The selectivity to 2-acetyl-6-formyl phenol is also high compared to Mg/Al-CHT. Based on this study the order of the activity of the catalysts follows the order Mg/Al-CHT > Ni/Al-CHT > Cu/Al-CHT.

11 138 Table 5.4 Acylation of salicylaldehyde over M(II)/Al-CHTs Catalysts Temperature ( C) Salicylaldehyde Conversion Product selectivity NiAl CuAl Catalyst weight: 0.5 g Salicylaldehyde : acetic anhydride =1:2 1 : 2-formylphenylacetate 2 : 2-acetyl-6-formylphenol 3 : Coumarin 4 : Cinnamic acid

CHAPTER 7. ACYLATION OF ANISOLE WITH ACETIC ANHYDRIDE OVER MnAPO-5 AND LEWIS ACID METAL ION-EXCHANGED MnAPO-5

CHAPTER 7. ACYLATION OF ANISOLE WITH ACETIC ANHYDRIDE OVER MnAPO-5 AND LEWIS ACID METAL ION-EXCHANGED MnAPO-5 103 CHAPTER 7 ACYLATIN F ANISLE WITH ACETIC ANHYDRIDE VER MnAP-5 AND LEWIS ACID METAL IN-EXCHANGED MnAP-5 7.1 INTRDUCTIN Friedel-Crafts acylation is one of the most important methods for the synthesis