Synthesis and Application of Chiral Palladium-Phosphane Precatalyst in Enantioselective C-N Cross-Coupling

Abstract

The various strategies and reaction conditions towards the synthesis of benzene-, naphthalene-, and phenanthrene-based dicyclohexylbiaryl phosphanes are presented. A chiral third generation Buchwald-type precatalyst has been synthesized, employing the non-commercially available ligand, (R)-dicyclohexyl(2'-methoxy-[1,1'-binaphth-2-yl])phosphane, (R)-Cy2MOP, and a dimeric methanesulfonate-bridged palladacycle based on a 2-aminobiphenyl scaffold. Application of the palladacyclic precatalyst in an enantioselective variant of the BuchwaldHartwig reaction is demonstrated in the desymmetrization of prochiral α-(2-bromobenzyl)malonamides via intramolecular N-arylation. The scope of the catalysis in the presence of the precatalyst has been investigated with efforts towards optimizing yields, catalyst loading, and enantioselectivities. Attempts towards the isolation of a potential reaction intermediate in the form of a stable amido-bound palladium complex are discussed. Finally, strategies towards gaining mechanistic insight on the origin of the enantiomeric excess are discussed.