Single-Step Construction of Planar and Central Chirality in Metallocenes by Rhodium-Catalyzed Desymmetric Cyclization

One-step asymmetric synthesis of metallocenes with planar
and central
chirality was developed by rhodium-catalyzed desymmetric cyclization.
The reactions of prochiral 1,2-dialkenyl metallocenes with organoboronic
acids were carried out in dioxane at 25 °C in the presence of
an Rh/(S,S)-Fc-tfb complex. Metallocenes,
having one chiral plane and three chiral centers, were obtained in
high yields with excellent diastereo- and enantioselectivity (>50:1
dr, up to >99.5% ee). Mechanistic studies have shown the reactions
proceed through a desymmetric conjugate addition–Michael cyclization
sequence. Both planar chirality and the first central chirality are
constructed simultaneously in a single desymmetric addition step.
The synthetic utility is demonstrated by the asymmetric synthesis
of a novel chiral chain diene ligand, self-replication of which was
successfully achieved.