Abstract
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 (PCHAT) 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 PCsHAT 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.
Highlights
The selective manipulation of C−H bonds (especially C(sp3)− H bonds) represents a remarkable challenge in synthetic campaigns because organic molecules contain many of these bonds of different nature
Even though the use of temporary directing groups, that is functions that are reversibly bound to the substrate to drive selectivity, has been proposed,[6] the direct aliphatic C−H bond elaboration in organic molecules still remains the unfound Holy Grail in chemistry.[3,7−12] Notably, this is an intense area of research, because it is a godsend for late-stage functionalization[13−17] and in function-oriented synthesis[18] thanks to the innate atom-economy related to the direct elaboration of C−H bonds
Irradiation of 1,3-dioxolane 91.3 in ethyl acetate solution in the presence of AQ-2-COOH (10 mol %) followed by quenching with sodium thiosulfate allowed synthesizing the corresponding ester 91.4 in good yield (69%). It is apparent from the examples described in this review that the photocatalyzed hydrogen atom transfer approach has had and will have in future years a primary role in synthetic planning, in both academic and industrial settings
Summary
The selective manipulation of C−H bonds (especially C(sp3)− H bonds) represents a remarkable challenge in synthetic campaigns because organic molecules contain many of these bonds of different nature. A typical reactivity mode that can be exploited to forge a C(sp3)−C(sp3) bond is the radical addition of nucleophilic radicals onto Michael acceptors In this scenario, the Ccentered radical generated via photocatalyzed HAT is trapped by an electrophilic olefin and the resulting radical adduct is quenched via back hydrogen atom transfer (or sequential electron/proton transfer) from the reduced form of the photocatalyst (PC−H; see Scheme 5), closing the photocatalytic cycle. The carbazole moiety is oxidized by an intramolecular electron transfer with the unstable radical cation formed by radical addition onto the chiral iminium ion intermediate, functioning as an electron donor Another class of widely used oxygen-based hydrogen donors is that of cyclic ethers, wherein the HAT step occurs at the labile α-to-O C−H bonds. Both ClAQ241 and EY221 were likewise effective PCsHAT to trigger the C−H to C−C bond conversion in tetrahydrothiophene
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