Enantioselective conjugate additions of α-amino radicals via cooperative photoredox and Lewis acid catalysis

Enantioselective Photocatalytic [3 + 2] Cycloadditions of Aryl Cyclopropyl Ketones

Control of stereochemistry in photocycloaddition reactions remains a substantial challenge; almost all successful catalytic examples to date have involved [2 + 2] photocycloadditions of enones. We report a method for the asymmetric [3 + 2] photocycloaddition of aryl cyclopropyl ketones that enables the enantiocontrolled construction of densely substituted cyclopentane structures not synthetically accessible using other catalytic methods. These results show that the dual-catalyst strategy developed in our laboratory broadens synthetic chemists' access to classes of photochemical cycloadditions that have not previously been feasible in enantioselective form.

Enantioselective Visible-Light-Induced Radical-Addition Reactions to 3-Alkylidene Indolin-2-ones

The title compounds underwent a facile and high-yielding addition reaction (19 examples, 66-99% yield) with various N-(trimethylsilyl)methyl-substituted amines upon irradiation with visible light and catalysis by a metal complex. If the alkylidene substituent is non-symmetric and if the reaction is performed in the presence of a chiral hydrogen-bonding template, products are obtained with significant enantioselectivity (58-72% ee) as a mixture of diastereoisomers. Mechanistic studies suggest a closed catalytic cycle for the photoactive metal complex. However, the silyl transfer from the amine occurs not only to the product, but also to the substrate, and interferes with the desired chirality transfer.

Organophotocatalysis: Insights into the Mechanistic Aspects of Thiourea-Mediated Intermolecular [2+2] Photocycloadditions

Mechanistic investigations of the intermolecular [2+2] photocycloaddition of coumarin with tetramethylethylene mediated by thiourea catalysts reveal that the reaction is enabled by a combination of minimized aggregation, enhanced intersystem crossing, and altered excited-state lifetime(s). These results clarify how the excited-state reactivity can be manipulated through catalyst-substrate interactions and reveal a third mechanistic pathway for thiourea-mediated organo-photocatalysis.

Brønsted acid-catalysed conjugate addition of photochemically generated α-amino radicals to alkenylpyridines

The conjugate addition of α-amino radicals to alkenylpyridines has been accomplished by the synergistic merger of Brønsted acid and visible light photoredox catalysis. Key to reaction development was the protonation of the alkenylpyridines that transiently generated a highly reactive, electrophilic pseudo-iminium ion intermediate. Initial investigations using chiral phosphoric acids provide clues on the feasibility of an enantioselective catalytic variant.

Thiaporphyrin-mediated photocatalysis using red light

Thiophene-containing porphyrin compounds are capable of catalytic, photo-reductive dehalogenation on an array of α-halo ketone model substrates with low catalyst loadings (0.1 mol%), in the presence of low energy, red light (>645 nm).

Enantioselective rhodium/ruthenium photoredox catalysis en route to chiral 1,2-aminoalcohols

A visible-light-induced enantioselective redox coupling of α-silylalkylamines with 2-acyl imidazoles is catalyzed by a combination of a chiral Rh Lewis acid and a Ru photoredox sensitizer.

Visible light-promoted C–H functionalization of ethers and electron-deficient arenes

Photocatalyst-free visible-light-mediated C–H arylation of ethers via C_(Sp2)–C_(Sp3) C–H functionalization involving hydrogen atom transfer (HAT) pathway.

Visible light-induced aerobic C–N bond activation: a photocatalytic strategy for the preparation of 2-arylpyridines and 2-arylquinolines

An efficient method for accessing arylpyridines and arylquinolines via visible light-induced aerobic C–N bond activation is described.

α-Arylation/Heteroarylation of Chiral α-Aminomethyltrifluoroborates by Synergistic Iridium Photoredox/Nickel Cross-Coupling Catalysis

Direct access to complex, enantiopure benzylamine architectures using a synergistic iridium photoredox/nickel cross-coupling dual catalysis strategy has been developed. New C(sp(3))-C(sp(2)) bonds are forged starting from abundant and inexpensive natural amino acids.

Metal-Catalyzed β-Functionalization of Michael Acceptors through Reductive Radical Addition Reactions

Transition-metal-catalyzed radical reactions are becoming increasingly important in modern organic chemistry. They offer fascinating and unconventional ways for connecting molecular fragments that are often complementary to traditional methods. In particular, reductive radical additions to α,β-unsaturated compounds have recently gained substantial attention as a result of their broad applicability in organic synthesis. This Minireview critically discusses the recent landmark achievements in this field in context with earlier reports that laid the foundation for today's developments.

Boron chemistry in a new light

Photocatalysis has recently opened up new avenues for the generation of radical species under visible light irradiation conditions. A particularly fascinating class of photocatalyzed transformations relies on the activation of stable boron species with visible-light since it allows the creation of boryl and/or carbon radicals through single electron transfer or energy transfer without the need for specific and costly equipment. This new paradigm has found numerous applications in synthetic organic chemistry, catalysis, and macromolecular chemistry. In this minireview, the concepts underlying photoactivation of boron-species as well as applications to the creation of C-H, C-C, C-O, B-C and B-S bond are discussed.

Characterizing Chain Processes in Visible Light Photoredox Catalysis

The recognition that Ru(bpy)32+ andsimilar visible light absorbing transition metal complexes can be photocatalysts for a variety of synthetically useful organic reactions has resulted in a recent resurgence of interest in photoredox catalysis. However, many of the critical mechanistic aspects of this class of reactions remain poorly understood. In particular, the degree to which visible light photoredox reactions involve radical chain processes has been a point of some disagreement that has not been subjected to systematic analysis. We have now performed quantum yield measurements to demonstrate that threerepresentative, mechanistically distinct photoredox processes involve product-forming chain reactions. Moreover, we show that the combination of quantum yield and luminescence quenching experiments provides a rapid method to estimate the length of these chains. Together, these measurements constitute a robust, operationally facile strategy for characterizing chain processes in a wide range of visible light photoredox reactions.

Recent synthetic additions to the visible light photoredox catalysis toolbox

The boom in visible light photoredox catalysis (VLPC) research has demonstrated that this novel synthetic approach is here to stay. VLPC enables reactive radical intermediates to be catalytically generated at ambient temperature, a feat not generally allowed through traditional pyrolysis- or radical initiator-based methodologies. VLPC has vastly extended the range of substrates and reaction schemes that have been traditionally the domain of radical reactions. In this review the photophysics background of VLPC will be briefly discussed, followed by a report on recent inroads of VLPC into decarboxylative couplings and radical C-H functionalization of aromatic compounds. The bulk of the review will be dedicated to advances in synergistic catalysis involving VLPC, namely the combination of photoredox catalysis with organocatalysis, including β-functionalization of carbonyl groups, functionalization of weak aliphatic C-H bonds, and anti-Markovnikov hydrofunctionalization of alkenes; dual catalysis with gold or with nickel, photoredox catalysis as an oxidation promoter in transition metal catalysis, and acid-catalyzed enantioselective radical addition to π systems.