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

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.