Quantum Chain Reactions and δ-Hydrogen Abstraction of Aromatic Ketones: Insights into Solid to Solid Transformations and Efficiency in Crystals
ABSTRACT OF THE DISSERTATION
Quantum Chain Reactions and Delta-Hydrogen Abstraction of Aromatic Ketones:
Insights into Solid to Solid Transformations and Efficiency in Crystals
Amy Esther Nielsen
Doctor of Philosophy in Chemistry
University of California, Los Angeles 2014
Professor Miguel A. Garcia-Garibay, Chair
Solid state photoreactions of ketones have long been of equal fascination and frustration for researchers. In the 19th century, Trommsdorff reported on his observations of the yellowing and cracking of crystals of α-santonin when they were exposed to ambient sunlight. While this reaction took place with great visual effect, it was not until many years later that the cause of the bursting of the crystals was more fully elucidated. Solid-state photochemistry has generally been plagued with issues stemming from an inability to rationally design photoreactions as, until recently, few analytical methods were available for analysis of reactions in the solid state that are analogous to those commonly used to analyze solution phase experiments. As a result, solid state photochemistry has historically been heavily reliant on product analysis for investigations into mechanisms. This usually involves dissolving the crystal in solution. While this is adequate for solution phase experiments, reaction outcomes in crystals are impacted by the crystalline environment in which the reaction occurs, and dissolving the crystal effectively erases any de novo crystallographic information inherent to the reaction. Research in the Garcia-Garibay group has found a way to circumvent many of these issues via the utilization of nanocrystalline suspensions for solid state photoreaction studies, an advance which has already expanded our insight into the mechanisms of organic reactions in crystals. Analysis of solid state photochemistry via nanocrystalline suspensions offers the opportunity to gain more detailed knowledge into reaction mechanisms by providing a method to conduct spectroscopic and actinometric analysis into solid state reactions without losing any of the structural information contained in the crystal lattice.
Chapter 2 of this thesis will discuss a solid state photochemical study originally reported by Wagner and co-workers in 1989. In their original study, the Norrish-Yang-like photochemical cyclization of α-o-tolyl and α-mesityl acetophenones to the corresponding 2-indanols was explored in both solution phase and the bulk solid, though a lack of methodology for spectroscopic and kinetic analysis at the time made it difficult to gain understanding of the mechanisms at work in the solid state reaction. Utilizing our methodology of photolysis of nanocrystalline suspensions, we were able to analyze the efficiency of the cyclization reaction in the solid state, and discovered a unique trend that correlated with the steric bulk of α-o-tolyl acetophenones, but showed the inverse trend in α-mesityl acetophenones under identical conditions.
Chapters 3 and 4 will discuss a quantum chain reaction known to take place in the conversion of diarylcyclopropenones to diarylacetylenes, stemming from work that was previously published in our group. In Chapter 3, in work done in collaboration with Dr. Gregory Kuzmanich, the photochemical decarbonylation of alkyl-tethered diphenylcyclopropenone dimers to form tethered diphenylacetylenes will be discussed. Both solution and solid state photolysis are explored, and evidence is shown for a Dexter mediated energy transfer mechanism. In Chapter 4, as an extension to this preliminary study, the reactions of aryl-tethered diphenylcyclcopropenones have also been examined, with respect to applications in materials chemistry; in this system we demonstrate evidence of a through bond energy transfer mechanism.
In Chapter 5, in work done in collaboration with Dr. Antoine Stopin, the reactions of biarylcyclopropenones with substituents of varying steric bulk are described, in an effort to better understand and test the limitations of the topochemical postulate in solid state photochemistry, which indicates that reaction in crystals may only occur with a minimum amount of molecular movement without rupturing the crystal lattice. In two cases shown here, evidence is shown for the occurrence of solid-to-solid reconstructive phase transformations taking place, despite steric bulk. This indicates that, even though there are structural limits to the strength of the crystal lattice, reactions can take place in a solid-to-solid manner in some substrates that can be activated to have a high potential energy.