Asymmetric catalytic variants of sunlight-driven photochemical processes hold extraordinary potential for the sustainable preparation of chiral molecules. However, the involvement of short-lived electronically excited states inherent to any photochemical reaction makes it challenging for a chiral catalyst to dictate the stereochemistry of the products. Here, we report that readily available chiral organic catalysts, with well-known utility in thermal asymmetric processes, can also confer a high level of stereocontrol in synthetically relevant intermolecular carbon-carbon bond-forming reactions driven by visible light. A unique mechanism of catalysis is proposed, wherein the catalyst is involved actively in both the photochemical activation of the substrates (by inducing the transient formation of chiral electron donor-acceptor complexes) and the stereoselectivity-defining event. We use this approach to enable transformations that are extremely difficult under thermal conditions, such as the asymmetric α-alkylation of aldehydes with alkyl halides, the formation of all-carbon quaternary stereocentres and the control of remote stereochemistry.

Photochemical activity of a key donor–acceptor complex can drive stereoselective catalytic α-alkylation of aldehydes

Photoredox catalysis, especially in combination with transition metal catalysis, can produce redox states of transition metal catalysts to facilitate challenging bond formations that are not readily accessible in conventional redox catalysis. For arene functionalization, metallophotoredox catalysis has successfully made use of the same leaving groups as those valuable in conventional cross-coupling catalysis, such as bromide. Yet the redox potentials of common photoredox catalysts are not sufficient to reduce most aryl bromides, so synthetically useful aryl radicals are often not directly available. Therefore, the development of a distinct leaving group more appropriately matched in redox potential could enable new reactivity manifolds for metallophotoredox catalysis, especially if arylcopper(iii) complexes are accessible, from which the most challenging bond-forming reactions can occur. Here we show the conceptual advantages of aryl thianthrenium salts for metallophotoredox catalysis, and their utility in site-selective late-stage aromatic fluorination. Despite the potential of fluorinated compounds in pharmaceuticals and agrochemicals, the formation of C–F bonds remains challenging. It has now been shown that aryl sulfonium salts, which can be made by site-selective C–H functionalization, have advantageous photoredox reactivity compared to conventional (pseudo)halides and can be used for late-stage C–H fluorination.

Photoredox catalysis with aryl sulfonium salts enables site-selective late-stage fluorination

Single-electron transfer is often accompanied by bond cleavage or bond formation reactions converting initially formed ion radicals into radicals. Electron transfer and bond breaking may involve a transient ion radical intermediate or occur in a concerted manner, as sketched in Figure 1.1 The question of the distinction between these two mechanisms and of the nature of the molecular factors that favor one mechanism over the other arises for thermal (electrochemical, homogeneous)2 as well as for photoinduced3,4 reactions.5 With aliphatic molecules, injection of an electron leads to a purely dissociative state, and accordingly, reductive cleavage follows a concerted mechanism. With molecules containing low lying orbitals, such as π* orbitals, able to host transitorily the incoming electron, reductive cleavage may follow one or the other of the two mechanisms. When the cleavage of the anion radical is fast, the rate-determining step of the stepwise pathway is the initial electron transfer. Under these conditions, the thermodynamic factor governing the competition between the two mechanisms is the standard free energy of anion radical cleavage, ∆GC 0 (eq 1)

Can Single-Electron Transfer Break an Aromatic Carbon−Heteroatom Bond in One Step? A Novel Example of Transition between Stepwise and Concerted Mechanisms in the Reduction of Aromatic Iodides

Standard reduction potentials, SRPs, of the halogen atoms have been calculated in water on the basis of an appropriate thermochemical cycle. Using the best up-to-date thermodynamic data available in the literature, we have calculated EX•/X−o values of 3.66, 2.59, 2.04, and 1.37 V vs SHE for F•, Cl•, Br•, and I•, respectively. Additionally, we have computed the SRPs of Cl•, Br•, and I• in acetonitrile (CH3CN) and dimethylformamide (DMF) by correcting the values obtained in water for the free energies of transfer of X• and X− from water to the nonaqueous solvent S and the intersolvent potential between water and S. From the values of EX•/X−o in CH3CN and DMF, the SRPs of a series of alkyl halides of relevance to atom transfer radical polymerization and other important processes such as pollution abatement have been calculated in these two solvents. This has been done with the aid of a thermochemical cycle involving the gas-phase homolytic dissociation of the C−X bond, solvation of RX, R•, and X•, and redu...

/pdf/estimation-of-standard-reduction-potentials-of-halogen-atoms-56fj2qaskp.pdf

Estimation of standard reduction potentials of halogen atoms and alkyl halides.

Recent rate data for very fast cleaving of aryl chloride and bromide anion radicals may be accommodated satisfactorily within rate constant versus ArX/ArX•- standard potential existing correlations provided the standard potential is determined experimentally Cyclic voltammetry is used for this purpose, taking careful account of the electron transfer/fragmentation reaction mixed character of the kinetics The ensuing activation/driving force relationships allow the determination of the intrinsic barriers, the magnitude of which are discussed in the framework of a new Morse curve model that includes and emphasizes the role of bond bending

Fragmentation of Aryl Halide π Anion Radicals. Bending of the Cleaving Bond and Activation vs Driving Force Relationships

The cyclic voltammetric investigation of a series of α-substituted acetophenones allowed the identification of the concerted and stepwise character of the dissociative electron transfer reaction, and, in the stepwise cases, the determination of the cleavage rate constants and the standard potentials for the formation of the anion radical. Analysis of the data, using thermodynamical parameters derived from experiment and from literature points to three mechanism governing factors, the oxidability of the leaving group, the bond dissociation energy of the bond being broken, and the LUMO energy. The first of these factors appears to be largely predominant in many cases in the control of the concerted vs stepwise dichotomy. The fluoro substituent provides a reverse example where the bond strength overcomes the unfavorable effect of the leaving group oxidability. It is also an exception, in terms of anion radical cleavage reactivity, where the strength of the C−F bond significantly contributes to slow down the ...

Concerted and Stepwise Dissociative Electron Transfers. Oxidability of the Leaving Group and Strength of the Breaking Bond as Mechanism and Reactivity Governing Factors Illustrated by the Electrochemical Reduction of α-Substituted Acetophenones

Solvent reorganization as a governing factor in the kinetics of intramolecular dissociative electron transfers. cleavage of anion radicals of alpha -s ubstituted acetophenones

Evidence for Orbital Symmetry Restrictions in Intramolecular Dissociative Electron Transfer

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Concerted and Stepwise Dissociative Electron Transfers. Oxidability of the Leaving Group and Strength of the Breaking Bond as Mechanism and Reactivity Governing Factors Illustrated by the Electrochemical Reduction of α-Substituted Acetophenones

Solvent reorganization as a governing factor in the kinetics of intramolecular dissociative electron transfers. cleavage of anion radicals of alpha -s ubstituted acetophenones

Evidence for Orbital Symmetry Restrictions in Intramolecular Dissociative Electron Transfer