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theses

This dissertation details our efforts in deploying both experimental (mass spectrometry) and computational (Gaussian) methods to study the kinetic and thermodynamic properties of organic species in the gas phase. Most organic reactions can be broadly considered as a combination of an electrophile with a nucleophile. Therefore, the quantification of the nucleophilicity and electrophilicity of organic substances is of ongoing interest to organic chemists. The nucleophilicity-electrophilicity scale in solution has been well defined. However, gas-phase nucleophilicity-electrophilicity studies are scarce. One of our main focuses is to build a nucleophilicity-electrophilicity database with intrinsic reactivity parameters. More specifically, we have measured the rate constants (k) of the association reactions between benzylhydrylium electrophiles and amine nucleophiles in the gas phase. Kinetics isotope effect (KIE) studies have been carried out to establish the nature of the product. Potential pitfalls of using the association reactions to quantify gas-phase reactivities are discussed, and an improved reaction model has been proposed and studied. These results are discussed in Chapter 2. In recent years, triazolylidene carbenes have been widely used in organocatalysis. Although the triazolylidenes have been studied in a wide range of catalytic transformations, the fundamental properties of these species remain largely unknown. In order to probe their intrinsic properties, we calculated and measured the gas phase acidities of a series triazolium precatalysts (the conjugate acids of triazolylidene carbenes). The relationship between the thermodynamic properties and the catalytic reactivities has also been investigated. We find that the gas phase acidities of the triazolium precatalysts are influenced by the subtle electronic properties of their substituents. Moreover, there are correlations between the gas phase acidities and the selectivities of two triazolylidene carbene-catalyzed Umpolung reactions. These correlations are the first of their kind and can be used to guide future catalyst design. These results are discussed in Chapter 3. In Chapter 4, we explore the possibility of using a charge-tagged N-heterocyclic carbene (NHC) to catalyze Umpolung reactions, such as the benzoin condensation and Stetter reaction, in the gas phase. We designed and synthesized thiazolylidene catalysts with charge tags, which allowed us to track NHC-catalyzed reactions in vacuo by mass spectrometry. Last, in Chapter 5, a comprehensive fundamental study of two charge-tagged triazolylidene catalyst is described. These charge-tagged species are novel triazolylidene derivatives with a carboxylate tail. The relative stabilities of various isomers are probed by calculations in both gas-phase and condensed-phase environments; comparisons are made to known condensed phase structural data. Measurement of the proton affinities of the carboxylate-tagged carbenes is used, in combination with calculations, to establish the gas-phase structure of these species.

ETD_8528

Ph.D.

Includes bibliographical references

by Yijie Niu

doi:10.7282/T3XG9VBJ

ETD doctoral

The author owns the copyright to this work.