We have demonstrated the potential of organocatalysis to make chiral molecules with high stereoselectivity using the reactivity of photochemically generated radicals. These open-shell intermediates were generated by two different approaches: i) upon direct excitation of organocatalytic intermediates while exploiting the ensuing single-electron-transfer mechanisms, or ii) by using an external photoredox catalyst.
In a first example, we developed an asymmetric photochemical radical cascade process. Our system harnesses the oxidative potential of visible-light excited chiral iminium ions to activate alkenes, which are generally recalcitrant to thermal activation strategies, and form reactive radicals that trigger a stereocontrolled cascade. To further explore the synthetic potential of asymmetric radical cascade processes, we applied this photochemical organocatalytic activation mode to activate allenes by SET oxidation. This report offers the first example of the use of allene radical cations in asymmetric transformations.
Finally, we established a general procedure for the enantioselective radical conjugate addition to simple α,β-unsaturated aldehydes. We envisioned that employing an external photocatalyst as an oxidant to generate a radical, we could intercept the radical with an electrophilic ground-state chiral iminium ion.
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