Synthesis of Phosphino(Silyl) Ligated Nickel and Manganese Complexes for the Catalytic Hydrofunctionalization of Alkenes

Abstract

Transition metal catalysts play a key role in the synthesis of value-added products from abundant raw materials. While homogeneous catalysts that feature scarce metals such as Pd, Pt, Rh, and Ru have proven effective, recent focus on sustainability has led to interest in utilizing Earth-abundant 3d-metals such as Mn, Fe, Co, and Ni. Multidentate phosphino(silyl) ligands under investigation in the Turculet group have proven useful in 3d-metal mediated catalysis. This document details the development of new tridentate PSiN and bidentate PSi supported Ni and Mn complexes for application in hydrofunctionalization catalysis.

Nickel complexes supported by a new PSiN ligand that features a quinolyl donor, as well as complexes supported by the bidentate CyPSi (CyPSi = κ2-(2-Cy2PC6H4)SiiPr2) ligand were shown to be effective pre-catalysts for alkene tandem isomerization-hydroboration. Deuterium labeling experiments support a Ni-mediated alkene chain-walking mechanism involving reversible alkene insertion/β-hydride elimination. Borylation occurs exclusively at a terminal position, affording high selectivity. Nickel complexes supported by a new PSiInd ligand featuring an indolyl backbone were also pursued, and these complexes along with (CyPSi)Ni species were screened in alkene hydrogenation catalysis. A variety of sterically hindered, unfunctionalized alkenes were readily hydrogenated under mild conditions. Deuteration experiments highlight the occurrence of background chain-walking, similar to that observed in the previous hydroboration studies.

The synthesis of chiral phosphino(silyl) Ni complexes for application in asymmetric catalysis was also targeted. In this regard, a new (BIPHEN-SilaPhos)Ni(η3-C8H13) complex is described. This complex and the previously synthesized ((S,S)-TADDOL-SilaPhos)Ni(η3-C8H13) were applied in the asymmetric hydrogenation of (Z)-2-acetamido-3-arylacrylates to access chiral α-amino acid esters. SilaPhos ligation represents a new approach to chiral ligands featuring chirality at a Si donor. The (S,S)-TADDOL-SilaPhos ligated Ni complex afforded the desired products in near quantitative yields with excellent enantioselectivity (up to 98:2 er). Both direct and transfer hydrogenation with iPrOH as the hydrogen source are shown to be viable pathways for this reactivity.

Progress towards the synthesis of Mn complexes supported by multidentate phosphino(silyl) ligation is also described. Mn(I) tricarbonyl complexes supported by CyPSiP (CyPSiP = κ3-(2-Cy2PC6H4)2SiMe) and PSiN ligation were synthesized and structurally characterized. The utility of Mn pre-catalysts in alkyne semi-hydrogenation and alkene hydrogenation was investigated. In situ generated Mn(II) dialkyl complexes featuring CyPSiP and PSiN ligation are shown to be active in the catalytic hydrogenation of a range of terminal alkenes.