Nickel catalysts Ullmann coupling

The classical methodology of the Ullmann biaryl synthesis was significantly improved by Semmelhack et al. using zero-valent nickel complexes [28]. Aryl iodides were coupled in high yield using bis(1,5-cyclooctadiene)nickel(0) as catalyst. Yields were further improved by the application of tetrakis(triphenylphosph-ine)nickel(0) [29] (Scheme 40). 

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