Acridine/Lewis Acid Complexes as Powerful Photocatalysts: A Combined Experimental and Mechanistic Study.

A class of generated Lewis acid (LA) activated acridine complexes is reported, which act as potent photochemical catalysts for the oxidation of a variety of protected secondary amines. Acridine/LA complexes exhibit tunable excited state reduction potentials ranging from +2.07 to 2.38 V vs. SCE. The ytterbium triflate complex of 3,6-di--butyl-9-mesitylacridine catalyzes a photochemical Giese-type reaction of Boc-protected secondary amines with challenging conjugate acceptors such as acrylates, that are inaccessible to the analogous acridinium (-Bu-Mes-Acr) catalyzed reaction. The mechanism of this reaction was investigated using a suite of physical organic probes including intramolecular C kinetic isotope effects (KIEs), variable time normalization analysis (VTNA) kinetics, determination of redox potentials, and computational studies. In the reaction catalyzed by -Bu-Mes-Acr, mechanistic studies are consistent with single-electron transfer (SET) from the ground-state reduced -Bu-Mes-Acr to the α-keto radical intermediate as the first irreversible step in the catalytic cycle. Intriguingly, we find that the reduced acridine/LA complexes are better ground state reductants (-0.72 to -0.74 V vs SCE) relative to -Bu-Mes-Acr (-0.59 V vs SCE) and predict that the increased substrate reactivity stems from a lower energy barrier for this key SET event.