Unlike nitrogen atoms, the trivalent phosphorus atom in phosphine (PR3) can act as a chiral center due to its high inversion energy. In addition, phosphines have a lone pair and vacant d orbitals; therefore, phosphines show good coordination ability toward transition metals. Hence, phosphines are widely used as ligands for transition metals. From this viewpoint, various P-stereogenic phosphines have been synthesized and are very useful as chiral ligands for transition metal-catalyzed asymmetric reactions. Recently, we have focused on P-stereogenic phosphines and synthesized optically active polymers , oligomers , and cyclic compounds  containing these units in the main skeleton. In particular, a practical synthetic route to P-stereogenic diphosphacrowns has been developed .
Since the first report on the synthesis of dibenzo-18-crown-6 by Pedersen, numerous other crown ether derivatives have been reported. However, to our knowledge, there have been no examples of the synthesis of P-stereogenic phosphacrowns possessing chiral phosphorus atoms in the ring skeletons. The prepared P-stereogenic diphosphacrowns have a chiral ring structure as well as chiral heteroatoms (phosphorus atoms) that interact directly with guest ions and molecules, leading to the new type of optically active molecule .
- For example, see: Imoto, H.; Morisaki, Y.; Chujo, Y. Chem. Commun. 2010, 46, 7542-7544.
- For example, see: Morisaki, Y.; Ouchi, Y.; Naka, K.; Chujo, Y. Chem. Asian J. 2007, 2, 1166-1173.
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