A practical transition metal-free aryl-aryl coupling method: arynes as key intermediates

Upon treatment of various aryllithium intermediates with 1,2-dibromobenzene or 1-bromo2-iodobenzene, dissymmetrical ortho, ortho'-di-, tri- and even tetrasubstituted bromo- or iodobiaryls become readily available. The crucial steps in all these reactions were the nucleophilic addition of the organolithium precursor to a transient aryne species released from it by beta-elimination of a lithium halide and, stabilization of the resulting 2-biaryllithium intermediate by in situ transfer of bromine or iodine from the starting material. This straightforward transition metal-free access to biaryls allows the preparation of highly valuable halobiaryls on a gram scale in excellent yields. These precursors can be subsequently functionalized by highly regioselective halogen/metal permutations into a vast variety of target molecules. This was demonstrated in the synthesis of several mono- and diphosphine ligands.