Enantioselective Synthesis of Tunable Chiral Clickphine P,N-Ligands and Their Application in Ir-Catalyzed Asymmetric Hydrogenation

A small library of highly tunable
chiral Clickphine P,N-ligands
has been prepared in an enantioselective fashion by CuI-catalyzed asymmetric propargylic amination using a single chiral
complex and a subsequent in situ cycloaddition click reaction. The
scope of the propargylic amination to yield optically active triazolyl
amines is described. The amines are transformed in a one-pot procedure
to the corresponding Ir–Clickphine complexes, which serve as
catalysts for the asymmetric hydrogenation of di-, tri-, and tetrasubstituted
unfunctionalized alkenes. Enantioselectivities of up to 90% ee were
obtained in these hydrogenations, which are among the best reported
in the case of the tetrasubstituted substrate 2-(4′-methoxyphenyl)-3-methylbut-2-ene
(9) (87% ee). This is a demonstration of the effective
use of the chiral pool, as from one chiral catalyst a library of chiral
Ir complexes has been synthesized that can hydrogenate various alkenes
with high selectivity.