Chiral Boronic Esters Coupled

ACS MEETING NEWS A powerful way to form C–C bonds is by way of transition-metal-catalyzed cross-coupling reactions employing air- and moisture-tolerant organoboranes. But examples of successful coupling reactions with secondary organoboranes are rare, and chiral versions are rarer still. Cathleen M. Crudden of Queen's University, in Kingston, Ontario, has now devised the first example of the cross-coupling of chiral secondary boronic esters that proceeds with retention of stereochemistry (J. Am. Chem. Soc., DOI: 10.1021/ja8094075). Crudden and coworkers first used a rhodium catalyst to react vinyl arenes with pinacolborane to give chiral benzylic boronic esters. They then coupled these intermediates to aryl iodides with a palladium catalyst in the presence of a triphenylphosphine ligand and silver oxide base. The resulting chiral 1,1-diarylethanes are difficult to prepare by other methods, Crudden said. The reaction tolerates a range of functional groups on either the boronic ester or the aryl iodide. The team was surprised to find that the reaction is quite specific for secondary organoboronic esters—the corresponding primary boronic esters aren't coupled under the same conditions.