New Synthetic Routes toward Enantiopure Nitrogen Donor Ligands
journal contributionposted on 08.12.2006, 00:00 by Xavier Sala, Anna M. Rodríguez, Montserrat Rodríguez, Isabel Romero, Teodor Parella, Alexander von Zelewsky, Antoni Llobet, Jordi Benet-Buchholz
New polypyridylic chiral ligands, having either C3 or lower symmetry, have been prepared via a de novo construction of the pyridine nucleus by means of Kröhnke methodology in the key step. The chiral moieties of these ligands originate from the monoterpen chiral pool, namely (−)-α-pinene ((−)-14, (−)-15) and (−)-myrtenal ((−)-9, (−)-10). Extension of the above-mentioned asymmetric synthesis procedure to the preparation of enantiopure derivatives of some commonly used polypyridylic ligands has been achieved through a new aldehyde building block ((−)-16). As an example, the synthesis of a chiral derivative of N,N-bis(2-pyridylmethyl)ethylamine (bpea) ligand, (−)-19, has been performed to illustrate the viability of the method. The coordinative ability of the ligands has been tested through the synthesis and characterization of complexes [Mn((−)-19)Br2], (−)-20, and [RuCl((−)-10)(bpy)](BF4), (−)-21. Some preliminary results related to the enantioselective catalytic epoxidation of styrene with the ruthenium complex are also presented.