University of British Columbia. Chemistry

Kinetic studies were conducted on three unrelated reaction types using traditional and modified reaction monitoring tools. The Aza-Piancatelli rearrangement was studied through ReactIR and HPLC-MS to obtain a better understanding of why the substrate scope was limited. It was found that the Lewis acid catalyzed reaction is often zero-order, dependent on the lanthanide metal used. Off-cycle binding of the nucleophile to the Lewis acid was proposed to help explain the zero-order profile. Differences between Lewis and Brønsted acid catalysts were found through subsequent experiments assessing catalyst deactivation and the chemoselectivity of the products in the Aza-Piancatelli rearrangement. An automated sampling system was created for hands-free reaction monitoring and offline analysis by HPLC-MS to provide detailed information about more complicated reactions. The automated sampling system was modified for the study of microwave assisted reactions. This application allowed for more information to be derived from the field of poorly-understood microwave chemistry than allowed by previous technology. Comparisons were made between microwave-assisted and conventionally heated reactions, using a Claisen rearrangement as a model reaction. As expected, it was found that the Claisen rearrangement of allylphenyl ethers displayed similar kinetics between the two heating modes. The technology was also used briefly to search for the existence of non-thermal effects. It was shown that the sampling apparatus could be useful for collecting data observed from microwave-specific effects. Mechanistic studies were also conducted on the Kinugasa reaction to obtain a better understanding of why the reaction generally behaves poorly in regards to the formation of β-lactam product. To study the reaction, samples for HPLC-MS analysis were taken manually, then by a liquid handler, and then through direct-injection to the HPLC. It was found that its side-product formation was directly coupled to the desired product formation, suggesting that both the product and imine side-product stem from a common intermediate. Another little-known side-product was isolated, suggesting the common intermediate could be intercepted by select nucleophiles to form an amide. This finding will direct future attempts to find conditions to favor either β-lactam or amide formation.

This thesis describes a novel method to generate trifluoromethanesulfenyl arene or heteroarene products via Ni-catalyzed ortho-selective C-X (X = Cl or Br) activation. Successful C-X activation requires directing groups, but it is highly selective and allows aryl chlorides to be used and shows an appreciable substrate scope. The protocol tolerates various nitrogen-containing directing groups including imines, pyridines, pyrimidines, amides and oxazolines. The method is also compatible with aryl halides bearing substituents with a wide rage of electronic properties, including electron-donating or withdrawing abilities, as well as potentially sensitive functional groups. It also produces trifluoromethylthiolated arenes in good-to-excellent yields at ambient temperatures. [formula omitted]

Titanium dioxide nanoparticles (NPs) and iridium(III) complexes have been prepared and studied as photocatalysts towards enhanced efficiency and mechanistic understanding of photocatalysis. Core-shell palladium-titanium dioxide NPs, Pd@TiO₂, were prepared using monodisperse Pd@SiO₂ core-shell NPs as a template. The Pd cores and porous, high surface area TiO₂ shells are expected to prevent Pd loss and increase surface-substrate interactions, respectively, thereby improving photocatalytic efficiency. Carbon dioxide was photocatalytically reduced in water by the Pd@TiO₂ NPs with methane being the major product. Iridium(III) complexes were tailored to increase the excited state lifetime through minor ligand modification. [Ir(ppy)₂phen]PF₆, [Ir(ppy)₂dtbbpy]PF₆, [Ir(ppy)₂dmbpy]PF₆, and [Ir(ppy)₂bpy]PF₆ were prepared where ppy = 2-phenylpyridine, bpy = 2,2ʹ-bipyridine, dmbpy = 4,4ʹ-dimethyl-2,2ʹ-bipyridine, dtbbpy = 4,4’-di-tert-butyl-2,2ʹ-bipyridine and phen = 1,10-phenanthroline. Their excited state lifetimes range from 0.3 µs to 0.7 µs and correlate to the rate of single-electron transfer (SET) from excited state to substrate, CF₃SO₂Cl (1.9 × 10⁸ M‾¹ s‾¹ to 9.3 × 10⁸ M‾¹ s‾¹), but the rate of final product formation is unchanged. The photocatalyzed trifluoromethylation of quinoline was used as a prototypical reaction. The unchanged rate of product formation indicates that SET involving the excited state is not rate-limiting in this system. Further increase in the SET rate was attempted by attaching pyrene onto a ligand in the complex for Reversible Electron Energy Transfer (REET) to increase the photocatalyst excited state lifetime more significantly. [Ir(npy)₂bpyethylpyr]PF₆, [Ir(npy)₂bpypyr]PF₆, and [Ir(npy)₂dmbpy]PF₆ were prepared where npy = 2-(naphthalen-1-yl)pyridine, dmbpy = 4,4ʹ-dimethyl-2,2ʹ-bipyridine, bpyethylpyr = 4-methyl-4ʹ-[2-(pyren-1-yl)ethyl]-2,2ʹ-bipyridine and bpypyr = 4-(1ʹʹ-pyrenyl)-2,2ʹ-bipyridine. The excited state lifetimes are 13.8 µs, 4.8 µs and 3.2 µs, respectively. Excited state lifetime and transient absorption studies indicate that only the complex with pyrene separated from bpy by an alkyl bridge displays REET. The rates of SET and product formation in the trifluoromethylation of quinoline are slower upon incorporation of pyrene. This is attributed to new, less reactive excited states and increased photocatalyst size, which slows down diffusion. The findings presented herein reveal the importance of photophysical and structural properties, such as photocatalyst size and excited state lifetime, in SET and photocatalytic efficiency, thereby contributing to guided optimization of photocatalytic systems.