Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C-H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C-H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C-C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

The adoption of hydrogen atom transfer (HAT) in a photocatalytic approach, in which an excited catalyst is responsible for substrate activation, offers unique opportunities in organic synthesis, enabling the straightforward activation of R–H (R = C, Si, S) bonds in desired reagents. Either a direct strategy, based on the intrinsic reactivity of a limited number of photocatalysts in the excited state, or an indirect one, in which a photocatalytic cycle is used for the generation of a thermal hydrogen abstractor, can be exploited. This microreview summarizes the most recent advances (mainly from the last two years) in this rapidly developing area of research, collecting the selected examples according to the nature of the species promoting the HAT process. From the synthetic point of view, this area has led to the development of a plethora of strategies for C–C, C–Si, C–N, C–S, and C–halogen (particularly, fluorine) bond formation, as well as for oxidation reactions.

This review explores the benefits of flow chemistry and dispels the notion that it is a mysterious “black box”, demonstrating how it can push the boundaries of organic synthesis through the understanding of its governing principles.

Applications of photocatalytic Hydrogen Atom Transfer (HAT) methodologies for the Late-Stage Functionalization (LSF) of complex molecules have been discussed in this perspective.

Uranyl nitrate hexahydrate performs as an efficient photocatalyst in the direct C–H to C–C bond conversion under blue light irradiation via Hydrogen Atom Transfer (HAT). This uranyl salt enables the remarkable smooth functionalization of unactivated (cyclo)alkanes, ethers, acetals and amides via radical addition onto electrophilic olefins. Dedicated electrochemical measurements on compounds and intermediates involved in the process were carried out to support the mechanistic proposal.