Mechanistic Studies of Two Ni-Catalyzed Reactions: Reductive Amidation of Esters with Nitroarenes and Hydrosilylation of Alkenes

Mechanistic investigations into homogeneously catalyzed reactions are essential for the fur-ther development of synthetic methodologies. Insights in the overall reaction pathway and vital steps such as the rate-determining and selectivity-determining steps allow for rational design of improved catalysts. Moreover, mechanistic investigations may reveal the existence of unexpected intermediates in the catalytic cycle. The discovery of such intermediates may inspire new methodologies that would not have been considered if their formation was un-known. Because of economic advantages over heavier congeners, there has been a recent surge in the use of nickel -based homogeneous catalysts. Although much progress has been made into methodology development, the mechanistic understanding of the new reactivities is relatively primitive. In-depth mechanistic investigations into nickel catalyzed reactions are therefore of considerable interest. Chapters 2-4 deal with the mechanism of reductive coupling of nitroarenes and esters to form amides. In chapter 2, the initial reduction pathway that occurs before the reaction with ester is investigated, revealing the crucial intermediate to be azobenzene and the source of the proton in the final amide product. In chapter 3, kinetics, Hammett plots, competition experi-ments and stoichiometric reactions with a potential intermediate are described. These studies reveal azobenzene is reductively cleaved by low-valent nickel before the ester gets involved in the reaction. Moreover, ZnCl2, a byproduct of the reduction of the nickel precursor by zinc, is revealed to play an important role in several steps of the reaction. In chapter 4, the computa-tional investigation based on the experimental data is discussed. By combining the computa-tional exploration of the potential energy surface with qualitative kinetic analysis, a catalytic cycle for the reaction between azobenzene and ester could be proposed. The finding that the reaction between azobenzene and ester goes through nickel imide inter-mediates is intriguing as nickel imides are known to react with a variety of substrates and azo-benzene would be a relatively benign precursor to them. Chapter 5 reports a preliminary explo-ration of several types of reactivities can be expected to work on the basis that the reactivity with imides is already known. Several types of these reactivities seem to be inaccessible as they are incompatible with the highly reductive nature of the nickel catalyst that is required for the azobenzene activation. However, some other reactivities show promise. Chapter 6 reports an investigation in the alkene hydrosilylation catalyzed by a nickel pincer complex developed in our group. Previous investigations had indicated a nickel alkyl complex was the resting state. However, a combination of reaction progress analysis, transmission elec-tron microscopy and competition reactions reveal that this is merely a precatalyst. Under cata-lytic conditions, it slightly decomposes under to form the true catalyst, which is tentatively identified as nickel nanoparticles.