Biaryl compounds, transition-metal-catalyzed cross-coupling

Transition metal-catalyzed cross-coupling is now recognized to be one of the most powerful carbon-carbon bond-formation reactions [1], The palladium-catalyzed coupling of aryl halides or their synthetic equivalents, for example aryl triflates, with arylmetals is very often employed in the synthesis of biaryl molecules, whose skeletons are found in a wide range of important compounds including natural products and organic functional materials [1-3]. 

Over the past century, several methods have been developed for the synthesis of biaryl compounds [7]. Among these are the Ullmann-type coupling [8,9], the Scholl reaction [10], the Gomberg-Bachmann reaction [11], and recently transition-metal-catalyzed cross-coupling reactions [12], In particular, palladium-catalyzed crosscoupling reactions have been successfully applied to the synthesis of biaryls due to their generally high yields and excellent selectivities. 

Axially chiral biaryls are found widely in the key structure of useful chiral ligands and catalysts [1] and biologically active compounds [2], The conventional method for their catalytic asymmetric synthesis is based on an asymmetric cross-coupling approach [3], As a conceptually new approach to axially chiral biaryls, asymmetric aromatic ring construction via transition-metal-catalyzed [2 - - 2 + 2] cycloaddition has recently appeared [4], In 1999, Sato et al. reported two types of nickel-catalyzed [2-1-2-1-2] cycloaddition for the synthesis of functionalized biaryls [5], One was the cycloaddition of aryl-substituted monoyne 1 with acetylene, leading to biaryl 2 (Scheme 9.1), and the other was the cycloaddition of aryl-substituted 1,6-diyne 3 with acetylene, leading to biaryl 4 (Scheme 9.2) [5],... 

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