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Mechanism of the Stereoselective α‑Alkylation of Aldehydes Driven by the Photochemical Activity of Enamines
journal contribution
posted on 2016-06-06, 00:00 authored by Ana Bahamonde, Paolo MelchiorreHerein
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.