4b was obtained quantitatively with 85% ee using 1.3 equiv of racemic 3a (Figure ). If the reaction proceeds by one transition state, the racemization of 3a is necessary. In order to confirm the racemization, we performed the deuterium scrambling experiment using 3a. Upon mixing 3a-D2 (deuterium content: 93%-D) with a stoichiometric amount of chiral phosphoric acid 1c in mesitylene at rt for 24 h, the deuterium content at the benzylic position of 3a decreased to 27% (Figure ). This result clearly shows that the chiral center of benzothiazoline was racemized under the reaction conditions by way of enamine form (Scheme S1). The UV-vis spectra of the mixture of 1a and 2b showed new absorbance in the visible region (Figure ). Therefore, the complex of 1a and 2b formed by hydrogen bonding (2b@1a) absorbed visible light. In contrast, little difference was observed by the addition of 3a. These results suggest that the reaction proceeded not by the formation of an electron donor-acceptor complex between 2b and 3a but by the direct excitation of 2b@1a. In order to verify this assumption, we conducted a fluorescence quenching experiment of 2b@1a. The Stern-Volmer plot revealed that the fluorescence of 2b@1a was quenched by 3a (Figure ). In addition, we estimated the oxidation potential of the excited state of 2b@1a. The cyclic voltammogram of 2b@1a exhibited an irreversible reduction wave at Ep/2 = -1.04 V (vs SCE, Figure ). Furthermore, the fluorescence spectrum of 2b@1a excited at 450 nm intersected with the absorbance spectrum at 472 nm, which indicates that the lowest excited state energy E00* is 2.62 eV (Figure ). The reduction potential of 2b@1a at the lowest excited state was estimated to be +1.58 V vs SCE. Thus, 3a (Eox = 0.70 V vs SCE 13a ) could be oxidized by the excited state of 2b@1a. There are the two possible pathways for the radical addition: (1) to radical anion of 2@1a (A) or (2) to neutral 2@1a (Figure , see Supporting Information). In order to determine which pathway is plausible, a highly reductive but low oxidative photocatalyst was added to the reaction. In the presence of 5 mol% of tris(2-phenylpyridyl)iridium (Ir(ppy)3) as the photocatalyst, almost the same enantioselectivity was observed (Figure ). Using Ir(ppy)3, protonated imines were firstly reduced by the excited state of Ir(ppy)3 (E*ox = -1.73 V, E*red = 0.31 V vs SCE), and 3a (Eox = 0.70 V vs SCE) could not be oxidized. Thus, the radical-radical coupling between A and the alkyl radical from the radical cation of 3a is the plausible pathway for the reaction.