Halogenation of Five-Membered Heterocycles

Finally, M. R. Grimmett from the University of Otago in New Zealand deals comprehensively with the halogenation of five-membered heterocycles in the first part of a survey that will ultimately cover the halogenation of all heterocycles. 

Similar elimination reactions are employed in the synthesis of the benzologs of five-membered heterocyclic compounds. For example, /3-phenylethyl alcohols having hydroxyl, sulfhydryl, and amino groups in the ortbo position ate cyclized to the dihydro derivatives of benzofuran, benzothiophene, and benzopyrrole, respectively. Likewise, dehydro-halogenation of /3-(o-hydroxyphenyl)-ethyl bromide gives dihydtobenzo-furan in 72% yield. ... 

Lithiobenzothiophenes can be generated, and reacted with electrophiles, if the temperature is kept low. Direct deprotonation of benzothiophenes follows the usual pattern for five-membered heterocycles and takes place adjacent to the hetero atom, and in concord with this pattern, metal-halogen exchange processes favour a 2- over a 3-halogen. 2-Trialkylstannylbenzofurans are useful for palladium-catalysed coupling with aromatic halides. ... 

Wang et al. reported that FeCls- and FeBrs-promoted cyclization/halogenation of alkynyl diethyl acetals formed ( )-2-(l-halobenzylidene or alkylidene)-substi-tuted five-membered carbo- and heterocycles. It was found that the 1 1 molar ratio... 

This chapter discusses the transition metal catalyzed functionalization of such systems that fall outside the topic of Chapters 6 and 7, as well as certain other compound classes (e.g. purines, pyrones). In contrast to the abundant literature of the chemistry of five and six membered systems, the transition metal catalyzed transformations of other heterocycles have not been studied so far in the same depth, probably due to the limited availability of their halogen derivatives compared to haloazines and haloazoles. Purine compounds and their structural analogues constitute an exception, since their biological importance proved to be a strong drive for synthetic chemist worldwide.1... 

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