For more than a century, the Ullman coupling has allowed for the generation of biaryl products from aryl halides. Although exceptionally valuable, this reaction has been used primarily for accessing symmetrical biaryls, while unsymmetrical biaryls have been synthesized through traditional cross coupling methods (Suzuki, Stille, Negishi, and Kumada couplings), reacting an aryl organometallic reagent with an aryl halide. A general cross Ullman reaction, which couples two different aryl electrophiles in good yields, without the use of an excess of one of the starting materials, has proven elusive. Herein, a key advance in cross electrophile coupling is presented, incorporating the use of two homogeneous metal catalysts to achieve the desired unsymmetrical biaryl products.  This reaction tolerates the use of electron rich and electron poor aryl substrates, in addition to pyridyl halides, and its selectivity relies on the use of both catalysts. When a single metal catalyst is employed under the reaction conditions, dimer is predominately generated. While revealing another rare example of multimetallic catalysis, this method serves as a new potential selectivity model for cross electrophile coupling.

Quest for selectivity: Multimetallic catalysis for the cross coupling of aryl electrophiles

248th National Meeting (2014)

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