Pentamethylated and pentaphenylated azaferro- and azaruthenocenes: simple and general methodology for the preparation of enantiopure derivatives

A methodology for the enantioselective synthesis of planar chiral 2-substituted 1',2',3',4',5'-pentamethylazaferro- and azaruthenocenes is reported. The key step is the introduction of an enantiopure chiral sulfoxide auxiliary via a selective ortho-lithiation with subsequent chromatographic separation of the resulting diastereomers. Cleaving off the sulfoxide auxiliary by treatment with t-BuLi generates an optically pure anion which may be quenched with a variety of electrophiles to give the optically pure azametallocene derivatives. In addition, it was attempted to extend the methodology to encompass 1',2',3',4',5'-pentaphenyl derivatives. It was, however, not possible to attach the chiral sulfoxide auxiliary onto the pentaphenylated azaferrocene, and for the azaruthenocene case, only one diastereomer could be isolated. Most importantly, the sulfoxide group could be cleaved off and the resulting chiral azaruthenocenyl anion was quenched with paraformaldehyde and iodine, resulting in products with ee values of 85% and 99%, respectively.