Synthesis and Investigation of Light Responsive Molecules Containing Cyclopropenium Ions

The present thesis describes recent advances in the pursuit of novel light-responsive molecules containing cyclopropenium ions. In an effort to understand the underlying factors regarding the photophysical properties of cyclopropenium ions, emphasis was placed on the previously reported “Janus sponge”, where systematic structural modifications to four individual components of the molecule led to measurable and predictable changes in molar extinction coefficients, quantum yields, and Stokes shifts. Using time-dependent density functional theory calculations, the origin of these trends were traced to internal charge transfer. Additionally, modulating hydrogen bonding between intermolecular, bifurcated, and intramolecular interactions by choice of counterion was used to alter the quantum yield of cyclopropenium ion-containing fluorophores. The basis of this switchability was examined using X-ray diffraction analysis, 1H NMR spectroscopy, density functional theory calculations, and fluorescence spectroscopy. Notably, this work led to the development of the first cyclopropenium ion containing “true” proton sponge. As an extension, light responsive molecules are not isolated to fluorescence. This thesis also outlines the development of the first cyclopropenium ion containing an azo group. Key findings include the fact that cyclopropenium ion containing azo compounds are stable and cyclopropenium ions red-shift the absorbance wavelength in comparison to azobenzene by 75 nm. The synthetic, structural, electronic, and photophysical properties of these compounds are discussed

Development and Investigation of the Fluorescence of Cyclopropenium Ions

The work presented herein employs cyclopropenium ions as a central design element towards the goal of developing fluorescent, superbasic and boronium-substituted compounds. A novel guanidine-cyclopropenimine proton sponge with exceptional basicity is reported that was further utilized to develop a stable tetracoordinate boronium-substituted proton sponge. A large focus of this thesis was also placed on the development of the recently discovered fluorescence of cyclopropenium ions leading to a new class of small molecule organic fluorophores. Among this new platform of fluorescent compounds, a specific fluorophore featured an impressive photophysical profile that bodes well for future applications in fluorescent imaging techniques. Insight into the structure, electronics, bonding and photophysical properties of these derivatives is offered

Synthesis of Cyclopropenium-Appended Organocatalysts and Applications

Enclosed within this dissertation is the development and application of multiple cyclopropenium-containing compounds formally belonging to two closely-related classes of organocatalysts, namely thioureas and squaramides. The former catalyst, coined as a thiourea-cyclopropenium, is deployed in pyranylation reactions of alcohols and phenols, as well as Friedel–Crafts alkylation, while the latter—a squaramide-cyclopropenium catalyst—targets oxime ether bond formation. Accompanying these innovative synthetic methodologies are comprehensive experimental and computational mechanistic studies that work in synergy to delineate numerous key features, all of which provide valuable information with respect to understanding the multifaceted nature of catalysis. Experimental and spectral data are provided for all new compounds

Development of a Class of Cyclopropenimine Based Compounds for Application in Catalysis

The present thesis outlines our latest findings in the pursuit of novel bis(diisopropylamino)cyclopropenimine (DAC) compounds. Particular focus was placed on the synthesis and investigation of DAC-substituted proton sponges, as well as their application in organo-catalysis. Herein, we report the synthesis of a non-symmetric DAC-functionalized proton sponge coined “Janus” sponge. Theoretical and experimental investigation of this sponge provided a monoprotonated salt, without a N-HN intramolecular hydrogen bond and a relatively low freebase strain. Instead, DFT calculations and X-ray crystallography revealed the presence of a hydrogen bond to the Cl- counter ion, leading to the unprecedented ¬in-out geometry of the molecule. Furthermore, the salt of the Janus sponge was found to be highly fluorescent both in the solid state and solution. Its experimentally measured pKa of 23.8 was found to be in good agreement with the calculated value of 23.9. The use of Janus, as well as the previously synthesized DACN (a naphthalene DAC derivative) sponge in phase transfer catalysis was also explored. The DACN proton sponge was found to be a highly efficient bifunctional phase-transfer catalyst, facilitating the movement of charged intermediates from the interface to the organic layer via favourable partitioning of hydrophilic/hydrophobic surface areas

Higher-Order Cyclopropenimine Superbases: Direct Neutral Brønsted Base Catalyzed Michael Reactions with α‑Aryl Esters

The synthesis and characterization of six new classes of higher-order superbases, including five that incorporate cyclopropenimine functionality, has been achieved. We propose a nomenclature that designates these as the CG₂, GC₂, PC₃, PC₁, C₃, and GP₂ classes of superbases. The p<i>K</i>_BH+ values were measured to be between 29.0 and 35.6 in acetonitrile. Linear correlations of ten superbase basicities vs that of their substituents demonstrated the insulating effect of the cyclopropenimine core. The molecular structures of several of these materials were obtained by single-crystal X-ray analysis, revealing interesting aspects of conformational bias and noncovalent organization. The types of superbasic cores and substituents were each reliably shown to affect selectivity for deprotonation over alkylation. Higher-order cyclopropenimine and guanidine superbase stability to hydrolysis was found to correlate to basicity. Finally, a GC₂ base was found to catalyze conjugate additions of α-aryl ester pronucleophiles, representing the first report of a neutral Brønsted base to catalyze such reactions

Higher-Order Cyclopropenimine Superbases: Direct Neutral Brønsted Base Catalyzed Michael Reactions with α‑Aryl Esters

The synthesis and characterization of six new classes of higher-order superbases, including five that incorporate cyclopropenimine functionality, has been achieved. We propose a nomenclature that designates these as the CG₂, GC₂, PC₃, PC₁, C₃, and GP₂ classes of superbases. The p<i>K</i>_BH+ values were measured to be between 29.0 and 35.6 in acetonitrile. Linear correlations of ten superbase basicities vs that of their substituents demonstrated the insulating effect of the cyclopropenimine core. The molecular structures of several of these materials were obtained by single-crystal X-ray analysis, revealing interesting aspects of conformational bias and noncovalent organization. The types of superbasic cores and substituents were each reliably shown to affect selectivity for deprotonation over alkylation. Higher-order cyclopropenimine and guanidine superbase stability to hydrolysis was found to correlate to basicity. Finally, a GC₂ base was found to catalyze conjugate additions of α-aryl ester pronucleophiles, representing the first report of a neutral Brønsted base to catalyze such reactions