The Mechanism of Visible-Light Photochemical Rearrangements of Conjugated Amide Enolates

Deprotonation of conjugated amides forms colored solutions of lithium enolates (trienolates) that absorb light in the visible range of the electromagnetic spectrum. Irradiation of these extended enolates at a suitable wavelength induces a family of skeletal rearrangements, including bond migrations and ring expansions, that proceed through a biradical intermediate. Combined experimental and computational studies reveal that excitation of the extended enolate chromophore promotes homolysis of the C─N bond of the amide via an intersection seam between the S₁ and S₀ potential energy surfaces. The resulting singlet biradical undergoes rapid recombination to generate rearranged amide products, with regioselectivity governed by the molecular framework. Acyclic trienolates undergo competing [1,3]-, [1,5]-, and [1,7]-migrations, but incorporation of a ring constrains radical recombination, resulting in regioselective re-cyclizations. Lack of stereospecificity during the rearrangements and cyclopropylbenzyl ring openings, coupled with a lack of cross-reactivity in TEMPO trapping and a crossover experiment, indicates the formation of short-lived, solvent-caged radical pairs rather than freely diffusing radicals. These findings establish a general mechanistic rationalization of visible-light-driven rearrangements of amide enolates, and suggest that ring topology can be used to control the outcome of the biradical recombination.