Asymmetric intramolecular reductive coupling of bisimines templated by chiral diborons†

An asymmetric intramolecular reductive coupling of bisimines has been accomplished for the first time under mild conditions with bis((+)-pinanediolato)diboron as the template, providing the unprecedented chiral dihydrophenanthrene-9,10-cis-diamines in high yields and excellent enantioselectivities. The chiral exocyclic cis-diamine products have served as effective chiral ligands for asymmetric catalysis. A DFT study highlights the crucial roles of the uncommon twisted-boat pathway (instead of the common chair type) and the steric effect in exclusively forming the cis-diamines and achieving high enantioselectivity. This reductive coupling protocol represents a significant expansion of the diboron-promoted [3,3]-sigmatropic rearrangement.