An excited-state molecule offers a completely different reactivity than in its ground state. It can be both a better oxidant and a better reductant, thus enabling novel transformations due to the ability to generate radical intermediates under mild conditions. In modern photochemistry, and in particular in photoredox catalysis, such ability has been widely exploited. For example, a photocatalyst, upon light excitation, can trigger single-electron transfer (SET) events with bench-stable substrates leading to the formation of radicals. This strategy has been successfully merged with asymmetric organocatalysis and metal catalysis, providing new opportunities for making chiral molecules. Recently, we showcased a complementary approach based on the direct excitation of certain chiral organocatalytic intermediates and substrates. Upon light-excitation, these intermediates reveal novel reactivity manifolds that trigger the formation of radical intermediates enabling transformations unavailable under thermal conditions.
Following this line of research, we exploited the direct excitation of substrates and organometallic intermediates to unlock novel reactivity, enabling asymmetric metal-catalyzed transformations unfeasible under the thermal domain. Specifically, we utilized the chemistry of 4-alkyl-1,4-dihydropyridines (4-alkyl-DHPs) in either excited-state and ground-state as source of electrons and radicals precursor.
Initially we focused on the exploitation of the excited-state properties of 4-alkyl-DHPs, serving as strong photoreductants and radical source, to enable an asymmetric nickel-catalyzed acyl cross-coupling. Readily available symmetrical anhydrides served as acyl precursors to access highly enantioenriched chiral α,α-disubstituted ketones. Despite the many advantages offered, the use of asymmetric nickel catalysis within the photochemical framework has been rarely employed. Therefore, this work represents one of the few examples in this area and the first report that employs the direct excitation of substrates.
Later we studied how, by means of light-excitation, it is possible to divert the well-established reactivity of an organoiridium chiral complex, enabling mechanistically original radical processes unattainable in the thermal domain. In particular, we probed a particular chiral η3-allyliridium(III) complex because of its marked electrophilic character. This complex was characterized by spectroscopical and electrochemical analysis estimating an excited-state oxidation potential of ̴ +1.24 V. Therefore, on light excitation, this Ir(III) complex turns its ground-state electrophilic features revealing photooxidants properties. This novel catalytic function was exploited to develop a light-driven enantioselective alkyl-alkyl cross-coupling between benzyl allylic alcohols and α-amino radicals, derived from 4-alkyl-DHPs, which served as ground-state radical sources. Overall, the reaction proceeded under blue light irradiation at room temperature affording the desired product in high yields and enantiomeric excess.