Chiral N-Heterocyclic Carbene Ligands Derived from 2,2‘-Bipiperidine and Partially Reduced Biisoquinoline:  Rhodium and Iridium Complexes in Asymmetric Catalysis

New and reliable procedures for the preparation of both enantiomers of 2,2‘-bipiperidine have been developed. Chiral imidazolinium salts derived from 2,2‘-bipiperidine and partially reduced biisoquinoline were prepared in high yields. Rhodium and iridium complexes of new chiral N-heterocyclic carbenes were obtained by transmetalation from corresponding silver(I) complexes. The structures of these complexes were verified by X-ray diffraction. The novel carbenes seem to range between the very electron-rich bis(amido)carbenes and the imidazole-derived carbenes with regard to the electronic ligand properties (IR evidence). Rhodium and iridium complexes were applied to a variety of catalytic asymmetric reactions. Modest enantioselectivities of up to 28% ee were observed.