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Thesis advisor: Shih-Yuan Liu / Despite extensive research in B–N-containing aromatic systems (most notably 1,2-azaborines) for their potential use in biomedicine and materials science, development of their oxygen counterparts, 1,2-oxaborines, remains underdeveloped. Presented herein is a straightforward route to access 1,2-oxaborines via a ring-closing metathesis strategy. Attempts to utilize the 1,2-oxaborine as a 1,3-diene in the Diels–Alder cycloaddition for potential application as a 4C + 1O synthon are also presented. Lastly, investigations regarding the aromaticity of the B–O heterocycles is probed using computations and isothermal reaction calorimetry. / Thesis (MS) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

The Diels-Alder dimerization of trans-1-substituted butadienes is expected to yield a cyclohexene adduct with the substituents in the 3 and 4 positions cis to one another. This prediction is based on past observations of other Diels-Alder additions. The cis-isomer is the only one consistent with a two-stage mechanism proposed by Woodward and Katz. In the case of trans-phenylbutadiene, the expected adduct is cis-3-phenyl 1-4-(trans-styry1) cyclohexene. Alder, Haydn and Vogt, however, reported that the corresponding trans-isomer is the dimerization product. There is reason to believe that the observation of the trans-isomer may have been the result of product isomerization during purification. In view of the unexpected nature of the product and its implications with respect to the Diels-Alder mechanism, the dimerization reaction was reinvestigated. trans-Phenylbutadiene was heated at 1300 for six hours. Analysis by thin layer chromatography on silica gel impregnated with silver nitrate revealed two major components as well as several minor ones. The two major components were isolated by column chromatography on silica gel impregnated with silver nitrate. They were present in roughly equal proportions. One of them was purified and found to be identical with cis-3-pheny l~4-(trans-styry l) cyclohexene prepared by an independent method. The second major component, not yet isolated in pure form, is probably the corresponding trans-isomer on the basis of IR and NMR spectra as well as the fact that the trans-isomer was found by Alder et al. An evaluation of the significance of this reaction with respect to the Diels-Alder mechanism will have to await positive identification of the second component as well as a more careful determination of its relative proportion to the cis-isomer.

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Light as a “green” source of energy has become increasingly attractive throughout the past century and has shown versatility for the application of activating chemical reactions. Compared with traditional energy sources, it provides a more direct, selective and controllable method. My PhD study was focused on the study of photochemistry of organic materials in two different systems. The first system is regarding reversible photoacids which generate protons on irradiation. With the aim of systematically studying these novel types of long lived photoacids, a series of photoacids was designed, synthesized and whose chemical mechanism was thoroughly investigated. This type of photoacid changes from a weak acid to a strong acid with a pH change of several units, which achieves nearly complete proton dissociation upon visible light irradiation. The whole process is reversible and the half-life of the proton-dissociation state is long enough to be used in many applications. Besides fundamental studies, different applications based on this type of photoacids were also completed. An esterification reaction was catalyzed and the volume of a pH-sensitive polymer was altered due to the large amount of photo generated protons from this photoacid. A reversible electrical conductivity change of polyaniline (PANI) was also achieved by doping with this reversible photoacid. In order to induce a large conductivity increase, an irreversible photoacid generator (PAG) was embedded in a novel PANI/PAG/PVA novel composition. In this system, Poly (vinyl alcohol) (PVA) forms a hydrogen-bonding network to facilitate proton transfer between the PAG and PANI. A final electrical conductivity of 10-1 S cm-1 was successfully achieved after irradiation. The second system in which I explored photochemistry of organic molecules concerns Photoretro-Diels-Alder (PrDA) reactions and a variety of Diels-Alder (DA) adducts were designed for these studies. UV light was used to trigger the retro-Diels-Alder reactions. Quantum yield of iv each DA adducts was investigated. This revealed that the photo-reactivity of this process depends on the electron-donating ability of the diene and the electron-withdrawing ability of the dienophile component. Mechanistic studies of this PrDA reaction reveal that a charge-separated intermediate is generated from a singlet excited state. This was applied to an unsaturated cyclic α-diketones (DKs), which underwent PrDA reactions and generated anthracene derivatives and carbon monoxide (CO), which itself plays profound and important roles in biological systems. These unsaturated cyclic α-diketones (DKs) encapsulated in micelles are effective CO-releasing molecules (CORMs) and are capable of carrying and releasing CO in cellular systems. This novel type of organic CORMs has potentially low toxicity and generates fluorescence, which provides a useful tool for the study of the biological functions of CO.

The goal of this thesis was to design thermally responsive polyol resins that would be compatible with isocyanates. Two approaches were made to reach this goal, the first involved functionalizing soybean oil and the second involved post-polymerization modification of a methacrylate based resin. A soybean based coating with thermally responsive Diels-Alder linkages has been prepared following an automotive two-component formulation. The resulting coatings displayed the capability to be healed following physical deformation by a thermal stimulus, and such a material has significant potential for end users. Various curing agents were employed, and resulted in variation of scratch resistance and re-healablity. Different thermally responsive soybean resins were synthesized to have varying amounts of reversible and nonreversible linkages when incorporated into the coating. Additionally, different isocyanates were added at differing ratios of NCO:OH in search of the optimum coating. It was found through the analysis of re-healabilty, hardness, gloss, and adhesion that the optimal combination was an acetylated resin (no irreversible crosslinks) with 54% reversible Diels Alder linkages at an NCO:OH ratio of 5:1 using isophorone diiscocyanate. Materials were evaluated via differential scanning calorimetry (DSC), scratch resistance, Koenig hardness, gloss measurements, and topographical analysis. In the second project, copolymerization of methyl methacrylate and 2-isocyanatoethyl methacrylate via free radical polymerization was done to synthesize a polymer with pendant isocyanates. The isocyanate was used as a chemical handle to incorporate Diels-Alder linkages into the PMMA resin. The PMMA resin with Diels-Alder linkages was successfully synthesized and incorporated into a polyurethane gel as proven via 1H NMR and IR. The gel showed thermal reversibility at 120°C due to retro-DA reaction via DSC as well as thermally reversible bulk properties.

The corrosion of infrastructure imposes a significant monetary cost, and at times human cost, upon society. Methods to improve corrosion resistance of materials are described herein which utilize the reversibility of the Diels-Alder reaction to impart thermal responsiveness upon materials. Such stimuli responsiveness can potentially play a role in self healing properties which lead to reduced cracking and thus increased corrosion protection. Reversible Diels-Alder chemistry was utilized to manipulate the surface energy of glass substrates. Hydrophobic dieneophiles were prepared and attached to glass slides and capillaries to yield a nonwetting surface. Thermal treatment of the surfaces cleaved the Diels-Alder linkage, and resulted in the fabrication of a hydrophilic surface. Preliminary analysis utilized contact angle (CA) measurements to monitor the change in surface energy, and observed a hydrophilic state (CA - 70±3°) before attachment of the dieneophile to a hydrophobic state (CA - 101±9°) followed by regeneration of the hydrophilic state (CA - 70±6°) upon cleavage of the Diels-Alder linkage. The treatments were then applied to glass capillaries, with effective treatment confirmed by fluid column measurements. Patterned treatments were also demonstrated to provide effective fluid flow gating. Reversible Diels-Alder linkages were incorporated into polymer thermoset binding resins in order to provide a means by which a crosslinked thermoset could undergo stimuli responsive reversible crosslinking. The binder systems which were utilized included two types of amine curing agents, polydimethylsiloxane (PDMS) and Jeffamine® polyetheramines (PEA), and two types of epoxy resins, EPON resin based on diglycidyl ether of Bisphenol-A and epoxidized soybean oil. Various dienes and dienophiles were employed to functionalize the selected binder systems and were met with various degrees of success. The synthetic technique which proved to be the most promising was the Diels-Alder modification of the epoxidized soybean oil.

Thèse numérisée par la Direction des bibliothèques de l'Université de Montréal.

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