Fries rearrangement

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The Fries rearrangement is a rearrangement reaction of a phenyl ester to a hydroxy aryl ketone by catalysis of lewis acids ^[#endnote_1] ^[#endnote_2] ^[#endnote_3].

Contents

1 Mechanism
2 Scope
3 Limits
4 Photo Fries rearrangement
5 See also
6 References

Mechanism

Despite many efforts a definitive reaction mechanism for the Fries rearrangement is not available. Evidence for inter- and intramolecular mechanisms have been obtained by so-called cross-experiments with mixed reactants. Reaction progress is not dependent on solvent or substrate. A widely accepted mechanism involves a carbocation intermediate.

In the first reaction step a lewis acid for instance aluminium chloride AlCl₃ co-ordinates to the carbonyl oxygen atom of the acyl group. This oxygen atom is more electron rich than the phenolic oxygen atom and is the preferred lewis base. This interaction polarizes the bond between the acyl residue and the phenolic oxygen atom and the aluminium chloride group rearranges to the phenolic oxygen atom. This generates a free acylium carbocation which reacts in a classical electrophilic aromatic substitution with the aromat. The abstracted proton is released as hydrochloric acid where the chlorine is derived from aluminium chloride. The orientation of the substitution reaction is temperature dependent. A low reaction temperature favors para substitution and with high temperatures the ortho product prevails.

Scope

Phenols react to esters but do not react to hydroxyarylketones with acylhalogen compounds under Friedel-Crafts acylation reaction conditions and therefore this reaction is of industrial importance for the synthesis of hydroxyarylketones which are important intermediates for several pharmaceutics such as paracetamol and salbutamol. As an alternative to aluminium chloride, other Lewis acids such as boron trifluoride and bismuth triflate or strong protic acids such as hydrogen fluoride and methanesulfonic acid can also be used. In order to avoid the use of these corrosive and environmentally unfriendly catalysts altogether research into alternative heterogeneous catalysts is actively pursued.

Limits

In all instances only esters can be used with stable acyl components that can withstand the harsh conditions of the Fries rearrangement. Is the aromat or the acyl component heavily substituted then the chemical yield will drop due to steric constraints. Deactivating meta-directing groups on the benzene group will also have an adverse effect as can be expected for a Friedel-Crafts acylation.

Photo Fries rearrangement

In addition to the ordinary thermal phenyl ester reaction a so-called photochemical Photo Fries rearrangement ^[#endnote_4] exists that involves a radical reaction mechanism. This reaction is also possible with deactivating substituents on the aromatic group. Because the yields are low this procedure is confined to the laboratory.

References

↑ Fries, K.; Finck, G. Ber. 1908, 41, 2447.
↑ Fries, K.; Pfaffendorf, W. Ber. 1910, 43, 212.
↑ J. March Advanced Organic Chemistry 3rdEd. John Wiley & Sons 1985, S. 499f.
↑ Bellus D. Advances in Photochemistry, Vol. 8, J. Wiley & Sons Inc.,1971, 109-159