Synthesis of Tetra-ortho-Substituted Biaryls Using Aryltriolborates

Abstract

Tetra-ortho-substituted biaryls were synthesized by cross-coupling between 2,6-disubstituted bromoarenes and aryltriolborates possessing substituents at ortho-carbon. The use of a copper(I) halide such as CuCl (20 mol%) with a palladium catalyst was found to be highly effective to give such sterically hindered biaryls in good yields. Transition-metal-catalyzed cross-coupling reactions are effective synthetic methods for the formation of C- C bonds. 1 Cross-coupling reaction between arylmetal compounds and aryl electrophiles is a recent variant of traditional Ullman coupling for the synthesis of biaryls. Although this protocol has been extensively studied using a variety of organometallic reagents and electrophiles, 1 interest has recently been shown in the use of nonmetallic boron compounds because of their high stability in air and water and compatibility with a broad range of functional groups. Tetra-ortho- substituted biaryls are important fragments of organic functional materials 2 and many biologically active compounds such as michellamine and steganone. 3 A recent advance is the use of electron-rich and sterically demanding ligands, such as tri-tert- butylphosphine, 4 dialkylarylphosphines, 5-9 N- heterocyclic carbenes, 10-12 and other ligands, 13-14 for synthesis of sterically hindered biaryl compounds. However, the use of large amounts of a base, especially a strong base, may be a major limitation for these applications. The development of an efficient, mild and operationally simple catalyst system avoiding the use of large amounts of a base remains a challenge and has becomes an urgent issue.