Scholl reaction

The Scholl reaction is a coupling reaction between two arene compounds with the aid of a Lewis acid and a protic acid. [cite journal
author = Roland Scholl, Christian Seer
title = Abspaltung aromatisch gebundenen Wasserstoffs und Verknüpfung aromatischer Kerne durch Aluminiumchlorid
journal = Justus Liebig's Annalen der Chemie
volume = 394
pages = 111–177
year = 1912
issue = 2
doi = 10.1002/jlac.19123940202] [M B Smith, J March. "March's Advanced Organic Chemistry" (Wiley, 2001) (ISBN 0-471-58589-0)]

The protic acid is often an impurity in the Lewis Acid and also formed in the course of a Scholl reaction. Reagents are iron(III) chloride in dichloromethane, copper(II) chloride, PIFA and boron trifluoride etherate in dichloromethane, Molybdenum(V) chloride and lead tetraacetate with BF₃ in acetonitrile "Controlling the Scholl Reaction" Benjamin T. King, Jií Kroulík, Charles R. Robertson, Pawel Rempala, Cameron L. Hilton, Justin D. Korinek, and Lisa M. Gortari J. Org. Chem.; 2007; 72(7) pp 2279 - 2288; (Article) DOI|10.1021/jo061515x] .

Given the high reaction temperature and the requirement for strongly acidic catalysts the chemical yield often are low and the method is not a popular one. Intramolecular reactions fair better than the intermolecular ones, for instance in the organic synthesis of 9-phenylfluorene:

Or the formation of the pyrene "dibenzo-(a.1)-pyrene" from the anthracene] "1-phenylbenz(a)anthracene" (66% yield). [Vingiello, F. A.; Yanez, J.; Campbell, J. A. "J. Org. Chem." 1971, "36", 2053-2056. (DOI|10.1021/jo00814a005)]

One study showed that the reaction lends itself to cascade reactions to form more complex polycyclic aromatic hydrocarbons ["Nanosized Molecular Propellers by Cyclodehydrogenation of Polyphenylene Dendrimers" Christopher D. Simpson, Gunter Mattersteig, Kai Martin, Lileta Gherghel, Roland E. Bauer, Hans Joachim Räder, and Klaus Müllen J. Am. Chem. Soc.; 2004; 126(10) pp 3139 - 3147; (Article) DOI|10.1021/ja036732j]

In certain applications such as triphenylene synthesis this reaction is advocated as an alternative for the Suzuki reaction. A recurring problem is oligomerization of the product which can be prevented by blocking tert-butyl substituents :

The exact reaction mechanism is not known but could very well proceed through an arenium ion. Just as in electrophilic aromatic substitution, Activating groups such as methoxy improve yield and selectivity :

References

ee also

* Friedel-Crafts alkylation