Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C–H Bonds Elaboration

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.

Catalyst-free [2 + 2] photocycloadditions between benzils and olefins under visible light

“…At any rate, in this case the reaction showed lower diastereoselectivity [22]. As far as compound S3 is concerned, it is formed only in traces (never exceeding 10% yield) and its formation can be explained by taking into consideration a hydrogen atom transfer (HAT) reactivity [30,31]. Indeed, the excited state of benzil has been reported to activate hydrocarbons by homolytic cleavage of a C(sp 3 )−H bond [25].…”

Thermal Functionalization of Alkanes with Carbon Electrophiles

“…2). A myriad of functional groups, such as CF3 (5), amide (7), ether (8), fluorine and chlorine (11)(12)(13), acetal (14), Bpin (18) and phenol (19) 20 , 57 for (±)-cuspareine and 58 for an estrogen receptor 21 . The method was also effective for late-stage functionalization of drug compounds by introduction of 2-pyrrolidine to indomethacine (59) and a butyl amide to clofibrate (60).…”

“…1b). [7][8][9][10] To our knowledge, a HAT-provoked transition-metal-catalyzed alkane/carbon-electrophile coupling process remains unexplored under thermal conditions 11 , likely due to the lack of a ground-state oxidant that is compatible with catalytic reductive coupling process. Herein, we disclose that use of di-tBu-peroxide (DTBP) as the HAT oxidant and Zn as the reductant enables Ni-catalyzed thermal functionalization of a broad range of alkane C(sp 3 )H bonds with organohalides to forge C(sp 3 )-C bonds.…”

Accelerated and Scalable C(sp3)–H Amination via Decatungstate Photocatalysis Using a Flow Photoreactor Equipped with High-Intensity LEDs

“… 10 While thermocatalytic approaches have proved to be the method of choice for the functionalization of C(sp 2 )–H bonds, photocatalysis has rapidly emerged as a powerful strategy for the selective activation of C(sp 3 )–H bonds. 11 Two seminal approaches to establish these coveted C(sp 3 )–N bonds were independently reported by Zuo and co-workers 12 − 14 and Kokotos et al 15 The methods exploited, respectively, a cerium salt 16 and phenylglyoxylic acid 17 to generate H atom abstractors. These species are subsequently capable of homolytically cleaving C(sp 3 )–H bonds and afford a carbon-centered radical, which is finally trapped by an azodicarboxylate to afford the targeted C(sp 3 )–N bond.…”