Expanding the C2-symmetric bicyclo[2.2.1]hepta-2,5-diene ligand family:: Concise synthesis and catalytic activity in rhodium-catalyzed asymmetric addition

New C-2-symmetric bicyclo[2.2.1]hepta-2,5-dienes bearing methyl and phenyl substituents at the 2 and 5 positions were prepared enantiomerically pure through a two-step sequence starting from the readily available bicyclo[2.2.1] hepta-2,5dione. Due to the instability or volatility of these dienes, their isolation was achieved through the formation of the corresponding stable [RhCl(diene)](2) complexes. These chiral rhodium complexes displayed high activity and enantioselectivity (up to 99% ee) in the rhodium-catalyzed 1,4-addition and 1,2-addition of phenylboronic acid to cyclic enones and N-sulfonylimines, respectively.