Visible-Light-Triggered Directly Reductive Arylation of Carbonyl/Iminyl Derivatives through Photocatalytic PCET

Abstract

The first visible-light-mediated radical-radical cross-coupling strategy that enables the direct arylation of carbonyl/iminyl derivatives in the presence of EtN has been realized. Such an atom-economical protocol furnishes a broad scope of arylation products such as secondary/tertiary alcohols and amines via a PCET process that facilitates the challenging reduction of C═X (X = O, N). Mechanistic investigation indicates two photocatalytic redox cycles were involved in the process, and EtN was proved to serve as a dual reductant and proton donor. Moreover, the isolated byproducts and controlled experiments could be considered as powerful supporting evidence for our hypothesis.

Photocatalytic Barbier reaction – visible-light induced allylation and benzylation of aldehydes and ketones

“…Another strategy to enable photocatalytic two-electron processes would be a reductive radical–radical cross coupling where one electron is transferred to each starting material, generating two radical intermediates that can recombine to give the desired product ( Scheme 1b ). 12 Photocatalytic reductions of aromatic aldehydes to the corresponding alcohols have been known since a report by Pac et al in 1983 ( ref. 13 ) and in 1990 the formation of diols as homocoupling products of ketyl radical anions has been observed.…”

“…After having been used only rarely in photoredox catalysis for many years, there has been an increasing number of reports about photocatalytic reductions of aldehydes and ketones recently. Ketyl radicals have often been used for radical–radical coupling reactions, 12 , 15 e.g. in the work of Rueping and co-workers about a photoredox-catalyzed reductive dimerization of aldehydes and ketones ( Scheme 1c ) 15 b or in the reductive arylation of carbonyl derivatives by Xia et al in 2017.…”

Visible‐Light Photocatalysis of the Ketyl Radical Coupling Reaction

“…Creatively,i n2 017 the Xia group achieved the first photoredox-induced ketyl radical( or a-aminor adical) and aryl radicali on coupling reaction to realize the arylation of carbonyl/iminyl derivatives (Scheme16). [39] Ab road range of aromatic aldehydes, ketones,and imines can be transformed into secondary/tertiary alcohols and amines under the mild reactionc onditions. The Ir III photocatalyst is initially excited to *Ir III by absorption of visible light, and it then undergoes single-electron reduction by Et 3 Nt op roduce the Ir II species with the generation of radical cation 117.R eduction of the C= Xb ond is enabled by interaction with a-ammonium radical 118 (or Lewis acidic cation radical 117)t hrough hydrogenbond bindingb yaPCET process, and this generatest he ketyl radicalo ra-amino radical 120.M oreover,asecond redox cycle starts with the oxidation of another Et 3 Nm olecule by excited *Ir III to generate radicalc ation 117 and the Ir II species.…”

Recent developments in transition-metal photoredox-catalysed reactions of carbonyl derivatives

“…*Ru II (bpy) 3 2+ ( E 1/2 * II/I = +0.77 V vs SCE) 66 is sufficiently oxidizing to accept an electron from α -silylamine 32.1 ( E 1/2 red = +0.41 V vs SCE for N -phenyl- N -((trimethylsilyl)methyl)aniline), 98 generating Ru I (bpy) 3 + species and a cation radical 32.2 , which undergoes rapid silyl transfer with Rh-bounded ketone 32.5 to afford an α -aminomethyl radical 32.3 and an electron deficient silylated intermediate 32.6 . A SET from Ru I (bpy) 3 + ( E 1/2 II/I = −1.33 V vs SCE) 66 to 32.6 forms a rhodium coordinated, silylated ketyl radical 32.7 , which subsequently undergoes a radical-radical coupling reaction with the α -aminomethyl radical 32.3 to afford Rh-bound coupled product 32.8 .…”