Sterically Demanding Planar Chiral P,N Ligands by Diastereoselective Ortho Lithiation of Pentaphenylferrocenyloxazolines and Their Application to Palladium-Catalyzed Substitutions with Cyclic Allylic Acetates

Abstract

Functionalized 1′,2′,3′,4′,5′-pentaphenylferrocenes are known as valuable ligands and catalysts. Planar chiral derivatives—e.g. Fu’s pentaphenylferrocene fused pyridines or palladacycles developed in our group—have previously been described to be very efficient asymmetric catalysts for a number of applications. Nevertheless, protocols for diastereoselective ortho lithiations of 1′,2′,3′,4′,5′-pentaphenylferrocenes are still unknown. Such protocols could significantly increase the arsenal of accessible planar chiral pentaphenylferrocenes. In this full paper, we describe such diastereoselective ortho lithiations of pentaphenylferrocenyloxazolines. Both possible diastereomers can be produced in almost diastereomerically pure form depending on the choice of a Lewis base additive, which is required in the lithiation step for a sufficient reactivity. This development was utilized to prepare a set of planar chiral pentaphenylferrocene-based phosphino-oxazoline ligands. The latter were investigated in Pd-catalyzed allylic substitution reactions with cyclic allylic acetates to allow for a first comparison with related planar chiral ligands and provided good enantioselectivity with this challenging substrate class.