Syntheses of ferrocene-motif N-P/S mixed-donor ligands and their applications in construction of chiral carbon-carbon and carbon-heteroatom bonding via asymmetric allylic substitutions

Abstract

P/S chelating mixed-donor ligands have attracted recent attention in synthetic applications and showed excellent results in several asymmetric catalyses, such as transition-metal-catalyzed asymmetric allylic substitutions (AAS), asymmetric hydrogenation and aza-Diels-Alder reaction etc. In this thesis, we have designed and synthesized a family of newly developed fine-tunable phosphinamidite-thioether ligands with a ferrocene motif, which are easily accessible from the readily available Ugi amine as a chiral building block. The class of ferrocenyl phosphinamidite-thioether ligands is named as (S,pR)-FerroNPS which reflects its characteristic scaffold, chelating moiety, chirality on central and planar asymmetry. The transition-metal-catalyzed asymmetric allylic substitutions (AAS) have become one of the most powerful tools for the enantioselective formation of carbon-carbon and carbon-heteroatom bonds due to its versatile variation of nucleophiles. The family of (S,pR)-FerroNPS ligands have been successfully applied in the palladium-catalyzed asymmetric allylic alkylation (AAA) as the prototypical reaction to evaluate their usefulness leading to excellent isolated yield and enantiomeric excess in up to 99%. In a similar vein, the use of a mono-fluorinated nucleophile, a potential biologically active functionality, gave the corresponding optically active product in up to 93% ee. In addition to carbon-carbon bond formation, various amines were examined to afford the desired products containing a carbon-nitrogen bond in excellent yields and enantioselectivities (up to 96%). There are very limited cases in which the enantioselective version of palladium-catalyzed C-O bond formation has been accomplished by an allyic substitution protocol. It has been demonstrated that our Pd-(S,pR)-FerroNPS catalysts are generally effective and versatile to achieve C-O bond forming reaction with up to 96% ee. Significantly, hitherto unreported electronic effect of non-conjugated substituents on benzylic alcohols on enantioselectivity was observed.