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Il '''riarrangiamento di Fries''' o '''reazione di Fries''', che prende il nome dal chimico tedesco [[Karl Theophil Fries]], è una [[reazione di riarrangiamento]] di un estere fenolico a un [[idrossile|idrossi]] [[arile]] [[chetone]] tramite [[catalisi]] di un [[acido di Lewis]].<ref>{{cite journal | title = Über Homologe des Cumaranons und ihre Abkömmlinge | author = [[Karl Theophil Fries|Fries, K.]] ; Finck, G. | journal = [[Chemische Berichte]] | volume = 41 | issue = 3 | pages = 4271–4284 | year = 1908 | url = | doi = 10.1002/cber.190804103146}}</ref><ref>{{cite journal | title = Über ein Kondensationsprodukt des Cumaranons und seine Umwandlung in Oxindirubin |author1=Fries, K. |author2=Pfaffendorf, W. | journal = [[Chemische Berichte]] | volume = 43 | issue = 1 | pages = 212–219 | year = 1910 | url = | doi = 10.1002/cber.19100430131}}</ref><ref>March, J. ''Advanced Organic Chemistry'', 3rd Ed.; John Wiley & Sons: Chichester, 1985; S. 499ff.</ref><ref>Blatt, A. H. [[Organic Reactions|''Org. React.'']] '''1942''', ''1''.</ref>
[[Image:Fries-Umlagerung 1.svg|600px|center|Riarrangiamento di Fries]]
Implica la migrazione di un gruppo [[acile]] di un estere [[fenolo|fenolico]] nell'anello [[arile]]. La reazione è [[Pattern di sostituzione in idrocarburi aromatici|orto e para selettiva]] e uno dei due prodotti si ottiene cambiando le condizioni di reazione, come la [[temperatura]] e il [[solvente]].
== Meccanismo della reazione ==
Despite many efforts, a definitive [[reaction mechanism]] for the Fries rearrangement has not been determined. Evidence for inter- and [[intramolecular]] mechanisms have been obtained by [[Crossover experiment (chemistry)|crossover experiments]] with mixed reactants.
The Reaction progress is not dependent on [[solvent]] or [[Substrate (chemistry)|substrate]]. A widely accepted mechanism involves a [[carbocation]] intermediate.
In the first reaction step a [[Lewis acid]] for instance [[aluminium chloride]] {{chem|AlCl|3}} co-ordinates to the [[carbonyl]] oxygen atom of the [[acyl]] group. This oxygen atom is more [[electron]] rich than the [[phenol]]ic oxygen atom and is the preferred [[Lewis base]]. This interaction [[Chemical polarity|polarizes]] the [[covalent bond|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 aromatic ring. 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.
[[image:Fries-Umlagerung 2a.svg|600px|center|Riarrangiamento di Fries parte prima]]
A low reaction temperature favors [[Arene substitution patterns|para substitution]] and with high temperatures the [[Arene substitution patterns|ortho]] product prevails, this can be rationalised as exhibiting classic [[Thermodynamic versus kinetic reaction control]] as the ortho product can form a more stable bidentate complex with the Aluminium.<ref>{{cite book |title= Aromatic Chemistry (Oxford Chemistry Primers)|last= Sainsbury|first= Malcolm|year= 1992|publisher= Oxford University Press|isbn=0198556748 |page= 65}}</ref> Formation of the ortho product is also favoured in non-polar solvents; as the solvent polarity increases, the ratio of the para product also increases.<ref>{{cite book |title= Strategic Applications of Named Reactions in Organic Synthesis: Background and Detailed Mechanisms|last= Kürti|first= László|last2= Czakó|first2= Barbara|year= 2005|publisher= Elsevier Academic Press|isbn= 0123694833|page= 181}}</ref>
Sostituzione ''para'':
[[image:Fries-Umlagerung 3.svg|600px|center|Riarrangiamento di Fries parte prima]]
Sostituzione ''orto'':
[[image:Fries-Umlagerung 4.svg|400px|center|Riarrangiamento di Fries parte prima]]
==Applicazioni==
[[Phenol]]s react to form [[ester]]s instead of hydroxyarylketones when reacted with [[acyl halide]]s under [[Friedel-Crafts acylation]] conditions. Therefore, this reaction is of industrial importance for the synthesis of hydroxyarylketones, which are important intermediates for several pharmaceuticals. As an alternative to [[aluminium chloride]], other [[Lewis acid]]s 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 [[catalyst]]s altogether research into alternative
[[heterogeneous catalysis|heterogeneous]] catalysts is actively pursued.
== Limiti==
In all instances only [[ester]]s can be used with stable acyl components that can withstand the harsh conditions of the Fries rearrangement. If the aromatic or the acyl component is heavily substituted then the [[chemical yield]] will drop due to [[steric hindrance|steric]] constraints. Deactivating meta-directing groups on the benzene group will also have an adverse effect as can be expected for a [[Friedel-Crafts reaction|Friedel–Crafts acylation]].
== Riarrangiamento foto-Fries ==
In addition to the ordinary thermal phenyl ester reaction a so-called [[photochemistry|photochemical]] '''photo-Fries rearrangement''' exists<ref>Bellus, D. ''Advances in Photochemistry''; John Wiley & Sons: Chichester, 1971; Vol. 8, 109–159.</ref> that involves a [[radical (chemistry)|radical]] [[reaction mechanism]]. This reaction is also possible with deactivating [[substituent]]s on the aromatic group. Because the yields are low this procedure is not used in commercial production. However, photo-Fries rearrangement may occur naturally, for example when a plastic bottle made of polycarbonate (PC) is exposed to the sun, particularly to UV light at a wavelength of about 310 nm, if the plastic has been heated to 40° Celsius or above (as might occur in a car with windows closed on a hot summer day). In this case, photolysis of the ester groups would lead to leaching of phthalate from the plastic.<ref>Norma Searle, "Environmental effects on polymeric materials," pp. 313–358, in ''Plastics and the Environment'', edited by Anthony Andrade, Wiley, 2003.</ref>
[[Image:Fries rearrangement (photo).svg|center|600px|Photo Fries rearrangement]]
== Riarrangiamento di Fries anionico ==
In addition to Lewis acid and photo-catalysed Fries rearrangements, there also exists an anionic Fries rearrangement. In this reaction, the aryl ester undergoes ortho-metalation with a strong base, which then rearranges in a nucleophilic attack mechanism.
== Voci correlate ==
*Reazione del tipo [[Reazione di Friedel-Crafts|alchilazione di Friedel–Crafts]]:
**[[Riarrangiamento di Hofmann-Martius]]
** Riarrangiamento di Fries
**[[Riarrangiamento di Fischer-Hepp]]
* [[Reazione di Duff]]
== Note ==
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