In order to clarify the rate-determining step, we conducted deuterium labeling studies (Figure ). When a 1:1 mixture of 2b and 2b-D was employed, the negative secondary kinetic isotope effect was observed (kH/kD = 0.85). Thus, the radical addition to imine, in which the carbon center of the C-H or C-D bond is converted from C(sp 2 ) into C(sp 3 ), is suggested to be the rate-determining step. According to these results, we propose a reaction mechanism (Figure ). First, protonated imines are excited by visible light and the resultant excited state of the chiral phosphoric acid-imine complex, which has high reduction potential, oxidizes benzothiazoline by single electron transfer (SET) to generate the amino radical and the radical cation of benzothiazoline. Both of these effectively interact with chiral phosphate anion, and the radical transfer proceeds enantioselectively to afford chiral amine and benzothiazole. Under these reaction conditions, the racemization of benzothiazoline occurs and the matched enantiomer selectively participates in the radical addition.