Copper‐Mediated Aryl–Aryl Bond Formation Leading to Biaryls: A Century after the Ullmann Breakthrough

This chapter outlines developments in the copper-mediated aryl—aryl coupling methods. These can be classified into several major categories depending on the types of reactions and precursors involved: (i) coupling of aryl halides and diazonium salts; (ii) coupling of aryltin, arylboron, or arylsilicon reagents; (iii) coupling via arylation involving C—H or C—C bond fission; and (iv) oxidative coupling of 2-naphthols. The third category can be further divided into direct C—H arylations using prefunctionalized or nonfunctionalized arylating agents and decarboxylative coupling of benzoic acid derivatives. In addition to cross-coupling reactions, each category includes homocoupling reactions, useful tools for the construction of symmetrical biaryl frameworks from a single precursor. The chapter is devoted to a brief survey of Ullmann coupling and related aryl—aryl bond formation reactions. The later sections survey the modern copper mediated coupling reactions beyond the classical Ullmann biaryl synthesis.