Fries rearrangement ortho/para selectivity

Dealumination of the ZSM-5 zeolite shows a great effect on ortho/ para selectivity in the acylation of phenol by AAC. Thus, for a phenol conversion of 20%, ortho/para selectivity is 7.0 when ZSM-5(41.8) is utilized, and becomes 13.0 in the presence of ZSM-5(42.4). This unexpected increase in the ortho/para ratio can only be explained by postulating that ortho-HAP and para-HAP result from different pathways. The ortho isomer is mainly produced in the pores, whereas para isomer production occurs only on the external acid sites. The ortho isomer can be formed by direct C-acylation of phenol with AAC this selective reaction can be related to the general mechanism reported in Scheme 5.1. ortho-HAP can also be obtained by the Fries rearrangement. On the contrary, the para-isomer is a secondary product and, therefore, it results from the acylation of phenol by PA according to Scheme 5.6. 

Prior to solving the structure for SSZ-31, the catalytic conversion of hydrocarbons provided information about the pore structure such as the constraint index that was determined to be between 0.9 and 1.0 (45, 46). Additionally, the conversion of m-xylene over SSZ-31 resulted in a para/ortho selectivity of <1 consistent with a ID channel-type zeolite (47). The acidic NCL-1 has also been found to catalyze the Fries rearrangement of phenyl acetate (48). The nature of the acid sites has recently been evaluated using pyridine and ammonia adsorption (49). Both Br0nsted and Lewis acid sites are observed where Fourier transform-infrared (FT IR) spectra show the hydroxyl groups associated with the Brpnsted acid sites are at 3628 and 3598 cm-1. The SSZ-31 structure has also been modified with platinum metal and found to be a good reforming catalyst. 

Ramamurthy and coworkers studied the photo-Fries rearrangement of phenyl acetate and phenyl benzoate and photo-Claisen rearrangement of allyl phenyl ether (Fig. 34) included in two types of zeolite (faujasites X and Y and pen-tasils ZSM-5 andZSM-11) [192], The photolysis was performed with the zeolite slurry in either hexane or iso-octane. One of the most remarkable observations is that the product distribution is altered within zeolites from that in isotropic solvent. Furthermore, while in solution, nearly a 1 1 mixture of ortho and para isomers 40 and 41 (Fig. 34) was obtained, within zeolites one is able to direct the photoreaction selectively toward either the ortho or the para products by conducting the reaction either within faujasites or pentasils, respectively (Fig. 34). 

Another example of the effect of confined medium is found during photo-Fries rearrangement of naphthyl esters in zeolites [103,104]. Upon photolysis in isotropic solution 1-naphthyl benzoate undergoes the photo-Fries rearrangement to yield both ortho (2-) and para (4-) phenolic ketones (Sch. 4). When this ester is included in NaY zeolite and irradiated the main product (96%) is the ortho isomer. This remarkable ortho-selectivity within zeolites has been rationalized on the basis of interactions of the reactant 1-naphthyl benzoate and intermediate radicals with the sodium ion. Due to restrictions imposed by the medium the benzoyl radical, once formed, is compelled to react only with the accessible ortho position. 

In the Fries liquid-phase rearrangement of PA carried out in the presence of ZSM-5(41) zeolite at 170°C, a good shape selectivity (ortho/para = 0.17) can be reached after 24 h by keeping modest the conversion of the starting ester (- 20%). When the reaction is repeated over Nu-10 zeolite, a 1 1 mixture of ortho- and para-products is isolated this different behavior can be ascribed to the smaller pore dimension of Nu-10 zeolite, which hampers the entrance of the reagents producing both isomers. 

Silica composite materials constituted by Nafion entrapped in a highly porous silica matrix (13% and 40% Nafion) are utilized as catalysts for the Fries rearrangement of PA. The conversions of the reactions performed in cumene are 10% and 16%, respectively, and the selectivity with respect to para- and ortho-HAF ranges from 20% up to 56%, phenol being the main side product accompanied by deposition of carbonaceous materials these... 

A continuous gas-phase catalytic acetylation of phenol with acetic anhydride to phenyl acetate followed by simultaneous Fries rearrangement to yield ortho- and para-hydroxyacetophenones over a silica-supported heteropoly acid has been described. A complete conversion of phenol to phenyl acetate is achieved at 140°C. Upon increasing the temperature to 200°C, the phenyl acetate formed rearranges into hydroxyacetophenones with 10% yield and 90% selectivity to the para isomer. 

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