Alkoxide and thiolate group transfer reactions of a (PPP)Ni scaffold via metal ligand cooperation

Abstract

Carbon monoxide is an important C1 source employed in various industrial processes to produce useful organic compounds. Monsanto, Cativa, Fischer-Tropsh and hydroformylation are well known chemical processes utilizing CO. For the last several decades, there has been significant efforts to develop efficient catalytic systems for such processes. However, many of the industrial catalysts utilize expensive nobel metals, such as Ru, Rh, Ir and Pt, thus leading to high costs for catalysts. In addition, due to the fundamental stable bond strength between said metals and a substrate, the desired chemical transformations typically operate at high temperatures and high pressures. In nature, similar reactions occur under relatively milder conditions, such as room temperature and 1 atm. These reactions are catalyzed by biological catalysts, enzymes utilizing the earth abundant $1^{st}$ row transition metals. Acetyl-CoA synthase (ACS) catalyzes the biosynthesis of acetyl-CoA from the sequential coupling reactions of a methyl group and CO with the coenzyme A. The major chemical transformations including C-C bond formation take place at a low-valent nickel center in the active site of ACS. Details of the mechanism, the order of substrate binding and the oxidation state of nickel are still unclear. For better understanding of the chemistry involved in ACS and ultimately developing synthetic catalysts using organonickel complexes, the C-C bond formation chemistry and the thiolate group transfer at a low-valent nickel carbonyl species were studied by various groups for several decades. Recent advances in understanding the reaction mechanism proposed for ACS catalysis and studying the relevant model systems are reviewed in Chapter 1. Also, the initial study of the work, reaction of CO with (PNP)NiCO species will be explained. In chapter 2, a general background of metal-ligand cooperation studied by several groups will be described. Then, the alkoxide and phenolate group transfer from a nickel(II) center to a central phosphine moiety of the ligand, resulting a P-O bond formation at the ligand with two electron reduction at a nickel center will be introduced. This unique metal-ligand cooperative behavior was also achieved with phenylthiolate and resulted in P-S bond formation and cleavage. Therefore, we attempted to store the thiolate moiety in the ligand and explore the C-S bond formation of acetyl and thiolate which is similar to the final step that occurs in ACS. In Chapter 3, P-S bond formation/cleavage mediated by a nickel ion supported by a PPP ligand (PPP = $P[2-P^iPr_2-C_6H_4]_2^-$) will be explained. A mononuclear thiolato nickel complex, (PPP)Ni(SAr) was prepared by treating the chloride starting material with NaSPh. Upon carbonylation, this complex produces a nickel(0) monocarbonyl species, $(PP^{SAr}P)Ni(CO)$ in which the thiolate migrates onto the central P of the ligand to give a P-S bond and 2-electron reduction of a nickel(II) center. The reaction undergoes via a pseudo first-order decay with respect to consumption of a nickel(II) thiolato species suggesting an intramolecular reaction under the excess CO(g) conditions. The reverse reaction involving P-S bond cleavage with concomitant decarbonylation occurs to regenerate (PPP)Ni(SAr) in benzene. Reaction of $(PP^{SPh}P)NiCO$ with trityl chloride results in Ph3CSPh formation, whereas the reaction with MeI gives methylation at a phosphide moiety or a thiolate group. In chapter 4, two pathways of alkoxide migration occurring at a nickel(II) center supported by a PPP ligand are presented. In the first route, the addition of a π-acidic ligand to a (PPP)Ni alkoxide species reveals the formation of a P-O bond. To demonstrate a P-O bond formation, a nickel(II) isopropoxide species (PPP)Ni($O^iPr$) was prepared. Upon addition of a π-acidic isocyanide ligand $CN^tBu$, a nickel(0) isocyanide species ($PP^{OiPr}P$)Ni($CN^tBu$) was generated

P-O bond formation occurred via reductive elimination (RE). When CO is present, migratory insertion (MI) occurs instead. The reaction of (PPP)Ni($O^iPr$) with CO(g) results in the formation of (PPP)Ni($COO^iPr$), representing an alternative pathway. The corresponding RE product ($PP^{OiPr}P$)Ni(CO) can be independently produced from the substitution reaction of {($PP^{OiPr}P)Ni}_2(μ-N_2$) with CO(g). While two different carbonylation pathways in (PPP)Ni($O^iPr$) seem feasible, C-O bond forming migratory insertion singly occurs.In the last chapter, interconversion between Ni(0) and Ni(II) carbonyl species supported by a PPP ligand is demonstrated. Without any change in the first coordination sphere, a nickel’s redox change occurs by 2-electron donation to induce the P-O bond cleavage. The phosphide and phosphinite ligands undergo significant geometrical change with the P-O bond formation and cleavage induced by the Lewis acid, B($C_6F_5$)3.