Acridine/Lewis\nAcid Complexes as Powerful Photocatalysts:\nA Combined Experimental and Mechanistic Study

A class of in situ generated Lewis acid (LA) activated\nacridine\ncomplexes is reported, which act as potent photochemical catalysts\nfor the oxidation of a variety of protected secondary amines. Acridine/LA\ncomplexes exhibit tunable excited state reduction potentials ranging\nfrom +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\nreaction of Boc-protected secondary amines with challenging conjugate\nacceptors such as acrylates, that are inaccessible to the analogous\nacridinium (<i>t</i>-Bu-Mes-Acr<sup>⊕</sup>) catalyzed\nreaction. The mechanism of this reaction was investigated using a\nsuite of physical organic probes including intramolecular <sup>13</sup>C kinetic isotope effects (KIEs), variable time normalization analysis\n(VTNA) kinetics, determination of redox potentials, and computational\nstudies. In the reaction catalyzed by <i>t</i>-Bu-Mes-Acr<sup>⊕</sup>, mechanistic studies are consistent with single-electron\ntransfer (SET) from the ground-state reduced <i>t</i>-Bu-Mes-Acr<sup>•</sup> to the α-keto radical intermediate as the first\nirreversible step in the catalytic cycle. Intriguingly, we find that\nthe reduced acridine/LA complexes are better ground state reductants\n(−0.72 to −0.74 V vs SCE) relative to <i>t</i>-Bu-Mes-Acr<sup>•</sup> (−0.59 V vs SCE) and predict\nthat the increased substrate reactivity stems from a lower energy\nbarrier for this key SET event.