Mass spectrometric and computational studies of N-heterocycles
Creators: name: Wang, Ning
Publisher: name: No Publisher Supplied
Year: 2019

description: This dissertation focuses on the experimental and theoretical studies of thermodynamic properties and reactivity of two types of heterocyclic compounds: N-heterocyclic carbenes (NHCs) and imidazoles.In recent years, triazolylidene carbenes have drawn broad attention in stereoselective organo-catalysis. These carbenes have dominated over thiazolylidene and imidazolylidene carbenes with drastic improvement in the achievable stereoselectivity in a variety of NHC-catalyzed reactions. Although intensively applied and studied in a great variety of catalytic transformations, the intrinsic properties of these carbenes remain mostly unknown. Our group therefore measured and calculated the gas phase acidities of a series triazolium pre-catalysts (the conjugate acids of triazolylidene carbenes), both achiral and chiral, with mass spectrometric experiments and quantum mechanical methods to probe their fundamental properties. It was found that the gas phase acidities of the triazolium pre-catalysts can be tuned by the subtle electronic properties of their substituents on the triazolium ring. The correlation between the thermodynamic properties and the catalytic reactivities has also been discovered and further investigated. We have found the correlations between the gas phase acidities of these triazolium pre-catalysts and the stereoselectivity of two triazolylidene carbene-catalyzed Umpolung reactions. These correlations, the first of their kind, can be potentially used to provide guidance for future stereoselective catalyst design. These results are discussed in Chapter 2.Carbon dioxide capture and sequestration (CCS) has become a research “hotspot” due to the increasing concern regarding their roles in the global greenhouse effect and subsequent climate change and environment destruction. We investigated the role that imidazolate anion plays in the absorption of carbon dioxide. A series of imidazoles with structural variations were characterized with both mass spectrometric experiments and computer modeling, in the gas phase and in an ionic liquid medium. By establishing correlations between the imidazolate properties and CO2 absorption capacity, we hope to provide guidance for a future generation of greenhouse gas capture systems with improved efficiency and robustness. During this process, we have also analyzed the substitution effects on the imidazolate properties and CO2 capture, and confirmed that mass spectrometry-based experiments can be designed to investigate tautomerism of imidazoles. These results are discussed in Chapter 3.The difference in reactivity of organic species in solution phase compared to gas phase has interested many organic chemists. Gas phase studies using mass spectrometry allow us to penetrate the intrinsic property of various organic species and elucidate reaction mechanisms without solvent effects. Only charged species could be detected by mass spectroscopy, and organocatalysts with charged handles are therefore important for such purposes. In Chapter 4, we discussed how we could utilize the charge-handled NHCs in Umpolung reactions such as Stetter reaction and benzoin condensation in the gas phase. Selected imidazolylidene, thiazolylidene and triazolylidene carbenes, either synthesized ab initio or commercially available, were examined both experimentally and theoretically.Finally, in Chapter 5, we reported the preliminary data collected that explore the relationship between the gas-phase properties of ionic liquids (cations/anions) with proton conductivity/open-circuit potential (OCP) of ionic liquid-based electrolytes used in anhydrous fuel cells.