Development and Mechanistic Understanding of Photochemical Reactions

Resumen

My doctorate studies aimed at using the photochemical activity of transient intermediates or molecular aggregations, such as electron donor-acceptor (EDA) complexes, to unable unconventional relativities. An EDA complex is a ground-state association characterized by the appearance of a new absorption band, the charge transfer band, which generally falls in the visible region. The irradiation of the charge transfer band of an EDA complex is related with a single-electron transfer from the donor to the acceptor. As a result, the EDA photoexcitation with visible light triggers the formation of radical species. I focused on the synthetic potential of this photochemical strategy, and on the mechanistic understanding of the developed photochemical reactions. Chapter III of this dissertation describes my first project where we focused on expanding the photochemical strategy for the enantioselective α-alkylation of aldehydes, previously describe by the group, to include ketones as substrates. This transformation relies on the generation of EDA complexes, formed between enamines, generated upon condensation of a primary amine and a ketone, and electron-poor alkyl halides. The resulting methodology provided a rare example of direct enantioselective α-alkylation of unmodified ketones. Subsequently, Chapter IV describes a further expansion of the EDA-based alkylation methodology. Specifically, we used the ability of indoles to act as donor counterpart for the formation of EDA complexes with electron-poor alkyl bromides. Conceptually, this study demonstrated that other molecules than enamines could serve as suitable donors in EDA complex formation. Synthetically, the photochemical activity of the indole-based EDA complexes promoted the formation of indole alkylation products. Mechanistically, we could isolate and fully characterize, by X-ray crystallographic analysis, the EDA complex that was promoting this photochemical transformation. The second part of my PhD work was characterized by a progressive transition from reaction development towards a mechanistically-oriented approach. In particular, I focused on detailed mechanistic studies of some photochemical enantioselective transformations previously developed in the group. Initially, I focused on studying the mechanism of the photochemical α-alkylation of aldehydes. As discussed in Chapter V, I used a combination of conventional photophysical investigations, nuclear magnetic resonance spectroscopy, and kinetic studies to gain a better understanding of the factors governing these enantioselective photochemical catalytic processes. We found that these α-alkylation reactions rely on the generation of open-shell species via two different enamine-based photochemical mechanisms: the direct photo-excitation of the enamine or the excitation of a photoactive EDA complex, generated between enamines and electron-poor alkyl halides. Quantum yield measurements revealed that a radical chain mechanism was operative. In addition, kinetic studies established the trapping of the carbon-centered radical by the enamine, to form the carbon-carbon bond, was the turnover-limiting step. Our group recently developed an iminium ion-mediated radical conjugate addition to β,β-disubstituted cyclic enones. The last chapter of this thesis will detail how a combination of spectroscopic and kinetic studies have been used to elucidate the key mechanistic aspects of this transformation. The chemistry exploits the ability of the chiral primary amine catalyst, purposely adorned with a redox-active carbazole moiety, to facilitate the stereoselective interception of photochemically generated carbon-centered radicals by means of an electron-relay mechanism. Our studies uncovered an unanticipated turnover-limiting step.