Bergman’s cycloaromatization

By carefully adapting the unique chemistry of Bergman s cycloaromatization, it seemed possible to synthesize the rigid-rod polymer as a thin film of high quality and high purity. 

There are a few reports of poly(naphthalene) thin films. Yoshino and co-workers. used electrochemical polymerization to obtain poly(2,6-naphthalene) film from a solution of naphthalene and nitrobenzene with a composite electrolyte of copper(II) chloride and lithium hexafluoroarsenate. Zotti and co-workers prepared poly( 1,4-naphthalene) film by anionic coupling of naphthalene on. platinum or glassy carbon electrodes with tetrabutylammonium tetrafluoroborate as an electrolyte in anhydrous acetonitrile and 1,2-dichloroethane. Recently, Hara and Toshima prepared a purple-colored poly( 1,4-naphthalene) film by electrochemical polymerization of naphthalene using a mixed electrolyte of aluminum chloride and cuprous chloride. Although the film was contaminated with the electrolyte, the polymer had very high thermal stability (decomposition temperature of 546°C). The only catalyst-free poly(naphthalene) which utilized a unique chemistry, Bergman s cycloaromatization, was obtained by Tour and co-workers recently (vide infra). 

Finally, just a few words dedicated to the synthesis of polyphenylenes, extremely important polymers, and in particular substituted polyphenylenes such as PPV, which exhibit superb thermal and chemical resihence, semiconduchng properties upon doping and applicahons such as OLEDs. Contrary to their linear acenes counterparts, long polyphenylenes can be obtained e.g., by Bergman s method consisting in the thermal cycloaromatization of enediynes (Lockhart et al, 1981). 

Klein and Kdnig," for example, have reported the synthesis of a sulfonamide substituted 1,2-aryldiyne 111, where the sulfonamide group, being a cr-acceptor, should facilitate the Bergman cycloaromatization. However, as reported by Zaleski for a series of l,S-bis substituted oct-4-ene-2,6-diyne derivatives 113, steric hindrance and hydrogen bond formation between the terminal substituents can influence the temperature of the cyclization, and therefore the reactivity of the sulfonamide compound 111 is comparable with the one of the unsubstituted analog 112, since the electron withdrawing effect of the substituent is compensated by its steric hindrance (Scheme 19.30). 

Nicolaou, K.C. Liu, A. Zeng, Z. McComb, S. Redox-controlled Bergman cycloaromatizations. Designed enediynes with DNA-cleaving properties and antitumor activity. J. Am. Chem. Soc. 1992, 114, 9T19-91 1. 

Zeidan, T.A., Kovalenko, S.V., Manoharan, M. and Alabugin, I.V. (2006) Ortho effect in the Bergman cyclization comparison of experimental approaches and dissection of cycloaromatization kinetics. Journal of Organic Chemistry, 71(3), 962-975. 

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