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Mechanism of the Stereoselective α‑Alkylation of Aldehydes Driven by the Photochemical Activity of Enamines
Herein
we describe our efforts to elucidate the key mechanistic
aspects of the previously reported enantioselective photochemical
α-alkylation of aldehydes with electron-poor organic halides.
The chemistry exploits the potential of chiral enamines, key organocatalytic
intermediates in thermal asymmetric processes, to directly participate
in the photoexcitation of substrates either by forming a photoactive
electron donor–acceptor complex or by directly reaching an
electronically excited state upon light absorption. These photochemical
mechanisms generate radicals from closed-shell precursors under mild
conditions. At the same time, the ground-state chiral enamines provide
effective stereochemical control over the enantioselective radical-trapping
process. We use 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. Measurements of the quantum yield
reveal that a radical chain mechanism is operative, while reaction-profile
analysis and rate-order assessment indicate the trapping of the carbon-centered
radical by the enamine, to form the carbon–carbon bond, as
rate-determining. Our kinetic studies unveil the existence of a delicate
interplay between the light-triggered initiation step and the radical
chain propagation manifold, both mediated by the chiral enamines