According to the proposed mechanism (Fig. ), the reaction commences with excitation of the photocatalyst by the visible light irradiation, followed by reductive quenching of the excited-state photocatalyst by i Pr2NEt. This mechanistic step was confirmed by the steady-state fluorescence quenching studies (Fig. ), which identified i Pr2NEt as the only reaction component that effectively quenches the excited photocatalyst. Accordingly, the Stern-Volmer quenching constant for i Pr2NEt (KSV = 8129 M -1 ) was found to be almost three orders of magnitude higher than for carbohydrate substrate 2a and the methyl acrylate somophile (KSV = 9 M -1 and 15 M -1 , respectively). Upon quenching through a singleelectron transfer, i Pr2NEt is oxidized to an N-centered cation-radical species, which undergoes facile deprotonation to furnish α-amino C-centered radical as an active XAT agent. The latter abstracts an iodine atom from the carbohydrate substrate 2 to furnish an α-silyl C-radical (2-CH2-rad). This radical intermediate then engages in the regiodetermining 1,6-HAT reaction to produce the C4-centered αhydroxy C-radical (2-C4-rad). Finally, the C4-radical adds to the somophile during the stereodetermining C-C bond-forming step of the reaction, followed by one-electron reduction of the formed α-carbonyl Cradical (3-C-rad) by the reduced photocatalyst and subsequent protonation. These steps furnish the desired C4-functionalized product 3 and complete the photocatalytic cycle. In order to rule out the contribution of a competitive intermolecular HAT pathway towards 2-C4-rad, a mixture of activated (2e) and deactivated (4a) carbohydrate substrates was subjected to the standard reaction conditions, producing the expected C4-functionalized product 3e only from the activated substrate, along with dehalogenated side-product 4e (Fig. ). The deactivated substrate did not engage in the reaction and no traces of the otherwise expected product 3a were observed, confirming the intramolecular nature of the regiodetermining HAT step.