Racemic but Tropos (Chirally Flexible) BIPHEP Ligands for Rh(I)-Complexes: Highly Enantioselective Ene-Type Cyclization of 1,6-Enynes
journal contributionposted on 14.10.2004, 00:00 by Koichi Mikami, Shohei Kataoka, Yukinori Yusa, Kohsuke Aikawa
The tropos (chirally flexible) or atropos (chirally rigid) nature of BIPHEP−Rh complexes at room temperature critically depends on the amines complexed. The aliphatic DPEN complex is atropos, whereas the aromatic DABN complex is tropos. BIPHEP−Rh chirality can thus be controlled by DABN at room temperature. The amine-free BIPHEP−Rh complex is tropos. At 5 °C, even amine-free BIPHEP−Rh complexes are atropos and hence can be used as enantiopure catalysts to give high enantioselectivity in ene-type cyclization of 1,6-enynes.