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

A class of <i>in-situ</i> 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-<i>t</i>-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 (<i>t</i>-Bu-Mes-Acr^⊕) catalyzed reaction. The mechanism of this reaction was investigated using a suite of physical organic probes including intramolecular ¹³C kinetic isotope effects (KIEs), variable time normalization analysis (VTNA) kinetics, determination of redox potentials, and computational studies. In the reaction catalyzed by <i>t</i>-Bu-Mes-Acr^⊕, mechanistic studies are consistent with single-electron transfer (SET) from the ground-state reduced <i>t</i>-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 <i>t</i>-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.