Development of Electron-Deficient Olefin Ligands for Nickel-Catalyzed Aziridine Cross-Coupling Reactions

Abstract

Ligands play a vital role in transition metal catalysis: they modulate the steric and electronic properties of the metal catalysts, thus enabling the desired reactivity and selectivity. The dominant ligand classes include phosphines, amines, and NHCs, which render the metal centers more electron-rich through σ-donation. On the contrary, although the π-accepting, electron-deficient olefins (EDOs) are known to promote bond-forming reductive elimination, they have not been widely utilized as ligands for catalysis. Development of these EDOs into a modular class of ligands would allow for novel reactivity that cannot be achieved with existing ligand classes.

In an effort to develop cross couplings with non-traditional electrophiles, we discovered that an electron-deficient olefin, dimethylfumarate, is the optimal ligand for nickel-catalyzed Negishi alkylation of styrenyl aziridines. Mechanistic studies revealed a stereoablative mechanism of this reaction and that the sulfonamide group is involved in directing the C‒C formation. Furthermore, the critical role of dimethylfumarate is most likely be promoting the reductive elimination.

To expand the substrate scope to the less activated alkyl aziridines, we designed an N-protecting group, cinsyl (Cn), which contains an electron-deficient olefin as the directing group. Effective arylations and alkylations of Cn-aziridines can be achieved utilizing the nickel catalyst and organozinc reagents. The stereoablative mechanism going through radical intermediates was again observed.

The modular framework of dimethylfumarate allows us to modify the ligand structure and achieve more challenging transformation. We found that an indenylsultam-derived ligand, Fro-DO, enables cross coupling with 1,1-disubstituted styrenyl aziridines to generate all-carbon quaternary centers. Solid-state analysis revealed a unique U-shape structure of this ligand, which may be responsible for the improved reactivity and selectivity. Additionally, utilization of a chiral camphorsultam-derived EDO ligand provided modest but promising enantioselectivity of this reaction.

In this thesis work, we have demonstrated that EDOs can be developed into a novel ligand class for transition metal catalysis. The structural platform allows for rapid ligand modification and reaction evaluation. We expect that future exploration of these EDO ligands will unlock new reactivity and selectivity that has not been possible with current technology.