Fries rearrangements aluminum chloride

Acid-catalyzed reactions of the phenol esters resulting in the formation of phenolic ketones are known as Fries rearrangements. Aluminum chloride was the first catalyst used but other Lewis acids are also effective. Examples from the recent literature are shown in equations (44) to (48). A nitrogen analog, that is the rearrangement of an acyl derivative of an arylamine, is shown in equation (49). Photo-Fries rearrangement reactions have also been studied in detail and are exemplified in equations (50) to (55). 

Fries rearrangement Aluminum chloride-promoted rearrangement of an aryl ester to a ring-acylated derivative of phenol. 

The preference for O acylation of phenols arises because these reactions are kmetically controlled O acylation is faster than C acylation The C acyl isomers are more stable how ever and it is known that aluminum chloride is a very effective catalyst for the conversion of aryl esters to aryl ketones This isomerization is called the Fries rearrangement... 

The title compounds also undergo the Claisen rarrangement (5-allyloxypyrazoles 4-allyl-5-pyrazolones) and are readily transformed into 5-chloropyrazoles by means of phosphorus oxychloride (8OCHE1). In the presence of aluminum chloride 5-acyloxypyrazoles (481) undergo the Fries rearrangement affording 4-acyl-5-hydroxypyrazoles (482). 

As catalysts for the Fries rearrangement reaction are for example used aluminum halides, zinc chloride, titanium tetrachloride, boron trifluoride and trifluoromethanesulfonic acid7... 

Fries rearrangement org chem The conversion of a phenolic ester into the corresponding 0- and p-hydroxyketone by treatment with catalysts of the type of aluminum chloride. frez re a ranj-mant)... 

Schotten-Baumann benzoylation or use of an acid chloride in pyridine rapidly causes O-acylalion giving 4-acyloxythiocoumarins (37) in high yield. These derivatives undergo the Fries rearrangement to give the 3-acyl derivatives (38), either by the action of hot pyridine (quantitatively), aluminum chloride, polyphosphoric acid, or even partially by sublimation. Cross-over experiments indicate that a mixture of intra- and intermolecular mechanisms is involved, the products 38 also being available directly from 4-hydroxythiocoumarin. 

The Fries rearrangement of phenol esters gives a mixture of 2- and 4-acylphenols (56). The reaction is catalyzed by Lewis acids such as aluminum chloride or by Brmnsted acids like hydrogen fluoride. This reaction is used in the production of 4-hydroxyacetophenone [99-93-4], a raw material for... 

The exact mechanism has still not been completely worked out. " Opinions have been expressed that it is completely intermolecular, ° completely intramolecular, and partially inter- and intramolecular. One way to decide between inter- and intramolecular processes is to run the reaction of the phenolic ester in the presence of another aromatic compound, say, toluene. If some of the toluene is acylated, the reaction must be, at least in part, intermolecular. If the toluene is not acylated, the presumption is that the reaction is intramolecular, though this is not certain, for it may be that the toluene is not attacked because it is less active than the other. A number of such experiments (called crossover experiments) have been carried out sometimes crossover products have been found and sometimes not. As in 11-17, an initial complex (68) is formed between the substrate and the catalyst, so that a catalyst/substrate molar ratio of at least 1 1 is required. In the presence of aluminum chloride, the Fries rearrangement can be induced with micro-wave irradiationSimply heating phenyl acetate with microwave irradiation gives the Fries rearrangement. " The Fries rearrangement has been carried out in ionic melts. 

Results of the Fries rearrangement of 1-naphthyl esters promoted by scandium triflate (5% mol) are reported in Table 5.6. The reaction proceeds smoothly in toluene at 100°C for 6 h to produce l-hydroxy-2-naphthyl ketones in high selectivity. The yields obtained are much higher than those achieved in the stoichiometric aluminum-chloride-promoted reactions. ... 

A Fries-like rearrangement is employed in the preparation of 6-acyl-2(3H)-benzoxazolones (9, [Table 5.8], X = O) and 6-acyl-2(3H)-ben-zothiazolones (9, [Table 5.8], X = S). The rearrangement is performed by heating at 165°C a mixture of the easily S5mthesizable N-acylated derivative 8 with aluminum chloride for 3 h. Products 9 with a variety of substituents on the acyl moiety can be prepared in high yield (Table 5.8). 

Table 5.8 Aluminum-chloride-promoted Fries rearrangement of 6-acyl-2(3H)-benzoxazolones and 6-acyl-2(3H)-benzothiazolones...

Microwave-induced Fries rearrangement can also be efficiently performed by using an aluminum chloride-zinc chloride mixture supported on silica under solventless conditions. The reaction can be applied to different phenyl and naphthyl acetates. ortho-HAP is isolated in 95% yield... 

Table 5.19 Microwave-induced Fries rearrangement of phenyl cinnamates promoted by aluminum chloride-zinc chloride mixture supported on silica...

Experimentally this is a very important reaction, for on it are based many syntheses of hydroxyaryl ketones of great practical importance. The reason for this, as Fries himself remarked, is that direct introduction of an acetyl group — and particularly of a substituted acetyl group — is by no means as smooth a route as the by-way through the aryl ester since reaction of phenols with aluminum chloride sometimes leads to most unsatisfactory products. Moreover, the technique used for a Fries rearrangement requires very little modification to be suitable for a large number of esters. 

---