Synthesis of Co and Ni Complexes for Stereoselective Hydrogenation Catalysis

Abstract

The chemistry of PSiP pincer complexes of second- and third-row transition metals has been well established within the context of bond activation chemistry and catalysis. Recent developments have begun to establish the chemistry of the first-row transition metals in conjunction with PSiP ligation, with examples of Fe, Co, and Ni demonstrating capable catalytic performance in a variety of hydrofunctionalization reactions. This document details the development of PSiP and bidentate PSi supported Co and Ni complexes for highly active catalytic hydrogenation. Additionally, an example of mixed phosphino-phosphonite ligation is shown to support Co catalyzed asymmetric hydrogenation.

In terms of (PSiP) Co chemistry, work in this document has improved on previously reported alkene hydrogenation catalysis through the synthesis of (PSiP)Co complexes supported by alkyne ligation. These complexes are shown to be active catalysts for a broad scope of functionalized alkenes under mild conditions (1 atm H2, room temperature). A computational study into the mechanism of alkene hydrogenation by these complexes suggests that the silyl donor can act in a non-innocent manner to improve catalyst turnover.

A (PSiP)Ni hydride complex has been shown to act as a catalyst for the (E)-selective semihydrogenation of alkynes. This catalyst operates under exceptionally mild conditions (1 atm H2, 1 mol % Ni, room temperature) and demonstrates excellent selectivity for (E)-selective semihydrogenation of alkynes across a broad substrate scope. A brief mechanistic investigation reveals facile insertion of alkynes into the Ni-H bond, and that electron withdrawing substituents on the substrate hinder the reaction.

A mixed phosphine-phosphonite ligand was coordinated to a series of Co complexes that could support the asymmetric hydrogenation of α-dehydroamino acid derivatives. A cationic CoI complex was synthesized and applied towards this transformation where it was found to provide high enantioselectivity across a broad, functionalized, substrate scope. Deuterium labeling experiments provide evidence for a dihydride mechanism resulting from oxidative addition of dihydrogen.

Lastly, a new class of chiral ligand featuring chirality at Si has been developed and applied towards the Ni catalyzed asymmetric hydrogenation of functionalized enamides. Chiral phosphino(silyl) (PSi) ligation is shown to induce enantioselectivity and high catalytic turnover for the asymmetric hydrogenation of a dehydroamino acid methyl ester and a tri-substituted carbocyclic alkene. An in situ generated species is shown to provide the highest activity and enantioselectivity.