2,2 -Bithiophene halogenation

Halogen-substituted thiophenium ions have low stability and can be observed by NMR only at temperatures below -30 °C (86MRC699). Disproportionation of the 2,5-dibromo-2H-thiophenium ion with formation of 5-bromo- and 3,5-dibromo-2H-thiophenium ions was observed on increasing the temperature from —50 to —10 °C (86MRC699). For chlorothiophenium ions, the disproportionation could not be observed so distinctly. Note that 2,4-dichloro-2H-thiophenium ion (5) is stable at room temperature and is the main transformation product of the less stable 2,5-dichloro-2H-thiophenium ion (2j). The latter fact was used for the preparation of difficultly available 2,4-dichlorothiophene (6) from the 2,5-isomer 3,5,4 -trichloro-2,2 -bithiophene (7) was isolated as a byproduct resulted, probably, from the reaction of ion 5 with dichloride 6 (Scheme 7) (90G365). 

An application of the developed methodology to simple substrates for metal halogen exchange reactions but also to metallation procedures afforded reliable results. Thus, the technique was utilized for a detailed kinetic investigation of a double-sided Halogen Dance (Fig. 8, A) reaction towards 4,4 -dibromo-2,2 -bithiophene (Bobrovsky et al., 2008). 

Bakeev, K. A. (Ed.). (2005). Process Analytical Technology - Spectroscopic tools and implementation strategies for the Chemical and Pharmaceutical Industries, Blackwell Publishing Ltd., ISBN 9781405121033, Oxford/UK Bobrovsky, R. Hametner, C. Kalt, W. Froehlich J. (2008). Selective Halogen Dance Reactions at 5,5 -Dibromo-2,2 -bithiophene. Heterocycles Vol.76, No.2, pp. 1249-1259, ISSN 0385-5414... 

In an effort to overcome steric interactions due to -substituents in cyclotetra(2,3-thienylene)s 4.6 (see above), Marsella et al. synthesized expanded thiophene-fused didehydro [12]annulene 4.32, which incorporated two additional ethynylene groups between the -positions of the cycUc stmcture (Scheme 1.44) [408]. Regiospeciflc halogenation of a 2,2 -bithiophene and Sonogashira-type cross-coupling with the ethynylated counter part were key elements of the synthetic strategy, which was also used to prepare a further extended macrocycle 4.33 (Chart 1.54), profiting from quadrupolar interactions of the central phenylene units [409]. 

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