Shining Light on Copper: Unique Opportunities for Visible-Light-Catalyzed Atom Transfer Radical Addition Reactions and Related Processes

Abstract

Visible-light photoredox catalysis offers exciting opportunities to achieve challenging carbon-carbon bond formations under mild and ecologically benign conditions. Desired features of photoredox catalysts are photostability, long excited-state lifetimes, strong absorption in the visible region, and high reduction or oxidation potentials to achieve electron transfer to substrates, thus generating radicals that can undergo synthetic organic transformations. These requirements are met in a convincing way by Ru(II)(phenanthroline)3- and Ir(III)(phenylpyridine)3-type complexes and, as a low-cost alternative, by organic dyes that offer a metal-free catalyst but suffer in general from lower photostability. Cu(I)(phenanthroline)2 complexes have been recognized for more than 30 years as photoresponsive compounds with highly negative Cu(I)* → Cu(II) oxidation potentials, but nevertheless, they have not been widely considered as suitable photoredox catalysts, mainly because their excited lifetimes are shorter by a factor of 5 to 10 compared with Ru(II) and Ir(III) complexes, their absorption in the visible region is weak, and their low Cu(II) → Cu(I) reduction potentials might impede the closure of a catalytic cycle for a given process. Contrasting again with Ru(II)L3 and Ir(III)L3 complexes, Cu(I)L2 assemblies undergo more rapid ligand exchange in solution, thus potentially reducing the concentration of the photoactive species. Focusing on atom transfer radical addition (ATRA) reactions and related processes, we highlight recent developments that show the utility of Cu(I)(phenanthroline)2 complexes as photoredox catalysts, demonstrating that despite their short excited-state lifetimes and weak absorption such complexes are efficient at low catalyst loadings. Moreover, some of the inherent disadvantages stated above can even be turned to advantages: (1) the low Cu(II) → Cu(I) reduction potential might efficiently promote reactions via a radical chain pathway, and (2) the tendency for ligand exchange in Cu(I)L2 assemblies allows the efficient synthesis of heteroleptic Cu(I)LL' complexes to tune the steric and electronic properties and also might coordinate and thus activate substrates in the course of a reaction in addition to electron transfer. Moreover, new photoredox cycles have also been discovered beyond the visible-light-induced Cu(I)* → Cu(II) electron transfer that is arguably best known: examples of the Cu(II)* → Cu(I) and Cu(I)* → Cu(0) transitions have been realized, greatly broadening the potential for copper-based photoredox-catalyzed transformations. Finally, a number of organic transformations that are unique to Cu(I) photoredox catalysts have been discovered.

Blue‐Emissive Cobalt(III) Complexes and Their Use in the Photocatalytic Trifluoromethylation of Polycyclic Aromatic Hydrocarbons

“…[3] Many strategies have been used to improve the photophysical properties of these complexes with the most popular focusing on the manipulation of the energies of non-emissive 3 MC states relative to emissive triplet metal-to-ligand charge transfer ( 3 MLCT) states. Following these approaches,Ru II , [5,6] Cr III , [7] and Fe((III), low spin) [8] complexes with strongly s-donating tridentate ligands have been shown to have dramatically longer emission lifetimes.A nother popular strategy to induce emission in 1 st row transition metals is to use d [10] metal-ions to avoid nonemissive d-d transition, for example,Cu I , [9] Ni 0 , [10] and Zn II ; [11] however, their MLCT excited states often undergo strong geometrical distortion [12] and non-radiative relaxation to the ground state can be rapid. Following these approaches,Ru II , [5,6] Cr III , [7] and Fe((III), low spin) [8] complexes with strongly s-donating tridentate ligands have been shown to have dramatically longer emission lifetimes.A nother popular strategy to induce emission in 1 st row transition metals is to use d [10] metal-ions to avoid nonemissive d-d transition, for example,Cu I , [9] Ni 0 , [10] and Zn II ; [11] however, their MLCT excited states often undergo strong geometrical distortion [12] and non-radiative relaxation to the ground state can be rapid.…”

“…Following these approaches,Ru II , [5,6] Cr III , [7] and Fe((III), low spin) [8] complexes with strongly s-donating tridentate ligands have been shown to have dramatically longer emission lifetimes.A nother popular strategy to induce emission in 1 st row transition metals is to use d [10] metal-ions to avoid nonemissive d-d transition, for example,Cu I , [9] Ni 0 , [10] and Zn II ; [11] however, their MLCT excited states often undergo strong geometrical distortion [12] and non-radiative relaxation to the ground state can be rapid. [16] Some of these complexes have been explored as photoredox catalysts in Diels-Alder reactions, [7] alkylations, and oxidative cyclizations, [17] trifluoromethylation of alkenes, [9] and polymerization. [16] Some of these complexes have been explored as photoredox catalysts in Diels-Alder reactions, [7] alkylations, and oxidative cyclizations, [17] trifluoromethylation of alkenes, [9] and polymerization.…”

Photokatalyse mit sichtbarem Licht: Welche Bedeutung hat sie für die organische Synthese?

“…[241] Alternativ kçnnen [2+ +2]-Cycloadditionen durch einen Energietransfer (Sensibilisierung;S chema 1d)e rreicht werden, [242] wie es fürP rozesse unter UV-Bestrahlung mit Sensibilisatoren wie Benzophenonen seit langem bekannt ist. [248,249] Im klassischen Prozess wird ein Radikal R 1 C aus R 1 X 251 durch Abstraktion von XC über einen Radikalstarter wie AIBN oder Peroxid erzeugt, das mit dem Alken 252 einen radikalischen Kettenmechanismus auslçst und das ATRA-Produkt 253 hervorbringt (Schema 69). kçnnen ebenfalls effizient [2+ +2]-Cycloadditionen durch Energietransfer fçrdern, [192,244] was auch [2+ +2]-Prozesse mit Styrolen 249 besonders effektiv macht (Schema 68).…”

“…[248,249] 7.1.1. In Anbetracht der Konzentrationen der einzelnen Komponenten (typischerweise Katalysator/Substrat = 1:100) scheint dieser Wega us statistischer Sicht am wahrscheinlichsten zu sein.…”

Visible‐Light‐Driven, Copper‐Catalyzed Decarboxylative C(sp3)−H Alkylation of Glycine and Peptides

“…[6] Thed irect alkylation of C(sp 3 )ÀHp rovides ac onvenient alternative for C(sp 3 )ÀC(sp 3 )c ross-coupling.V ery recently, the photoinduced alkylation of C(sp 3 )ÀHw as developed by the groups of MacMillan [7] and Yu, [8] using either alkyl bromides or benzyl chlorides as electrophiles.MacMillan and co-workers also reported its applications in alkylated modification of peptides and pharmaceutical compounds. [9] In 2017, Fu and coworkers reported the first visible-light-induced Cu-catalyzed intramolecular C(sp 3 ) À Nc oupling by the decarboxylation of an alkyl NHP ester (Figure 1b). Compared with Ir and Ru, the use of Cu as ap hotoredox catalyst is uncommon.…”