Some new C2-symmetric bicyclo[2.2.1]heptadiene ligands: synthesis and catalytic activity in rhodium(I)-catalyzed asymmetric 1,4- and 1,2-additions

C2-Symmetric bicyclo[2.2.1]hepta-2,5-dienes with various substituents (R=Bn, i-Bu, c-Hex, allyl) are prepared starting from the corresponding enantiomerically pure bis-triflate (R=OTf). These chiral ligands are tested and compared in rhodium(I)-catalyzed 1,4- and 1,2-addition of phenylboronic acid to cyclic enones and aryl aldehydes, respectively. Some interesting reactivity and selectivity effects due to the introduction of sterically demanding or olefinic substituents are reported. Moreover, remarkably high catalytic activity is observed for the rhodium(I)-catalyzed 1,2-addition.