Photochemical transformations rely on the ability of organic molecules or catalysts to absorb light and reach electronically excited states. Since the chemical and physical properties of excited-state molecules significantly differ from the ground state, light-mediated chemistry can offer interesting new reactivity patterns that are unavailable under thermal activation.
The main objective of this doctoral studies was to implement new organocatalytic methods for the construction of carbon-carbon bonds proceeding via radical pathways. In particular, I investigated different photochemical strategies available to organocatalysts and organic intermediates for generating radicals and so enabling unconventional reactivity not feasible under thermal conditions.
The first project (Chapter II) details how the tools of amine catalysis and photoredox catalysis could be combined to develop a general system for stereoselective radical conjugate addition to ground-state chiral iminium ions.
In the second part of the doctoral studies (discussed in Chapter III), a C-H allylic benzylation reaction proceeding via cross-coupling between benzyl and allyl radicals was implemented. These two intermediates were generated by a single dithiophosphoric acid catalyst which used a sequential electron donor-acceptor complex and hydrogen atom transfer activation mechanism.
In the final project (Chapter IV), a C-H allylation of pyridines was developed. A dithiophosphoric acid played three catalytic roles, sequentially acting as (i) Brønsted acid, (ii) excited-state reductant, and (iii) hydrogen atom abstractor to activate allylic C-H bonds. Pyridinyl radicals emerging from protonation and subsequent reduction of pyridine were coupled with allylic radicals to afford the products after rearomatization.
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