Photoredox Promoted Barbier-Type Reaction of Alkyl Iodides with N-Alkyl and N-Aryl Imines

Unlocking the reactivity of the C-In bond: alkyl indium reagents as a source of radicals under photocatalytic conditions

Generation of organic radicals from organometallic compounds is a key step in metallaphotoredox cross-coupling reactions. The ability of organoindium compounds to serve as sources of alkyl radicals under light promoted oxidative conditions is described. Organoindium reagents were used in dual photocatalytic/nickel cross-coupling with aryl bromides. These reagents can be conveniently obtained from primary, secondary and tertiary alkyl bromides and chlorides using a novel indium(i) bromide/lithium bromide system. Both steps, the formation of organoindiums and their cross-coupling are insensitive towards air and moisture and tolerate a wide variety of functional groups.

One-Pot Domino Catalysis to Construct Alkyl/Aryl Pyrroles Initiated by Pd-TMM Annulation of Unactivated Imines

Herein, a one-pot domino catalyzed three-component process is described, which is initiated by a palladium/zinc cooperatively catalyzed cycloaddition between trimethylenemethane (TMM) and unactivated alkyl/aryl imines, followed by one-pot isomerization and Zn(OTf)2-catalyzed DDQ oxidation, furnishing valuable substituted pyrroles. We disclose that the palladium/zinc cooperative catalysis affords a dual-Zn(OTf)2-stabilized azapalladacycle, wherein the Pd-N bond is polarized by Zn(OTf)2, facilitating a unique outer-sphere allylic amination. Moreover, subsequent DDQ dehydrogenation can be feasibly promoted by zinc catalysis.

Light-Mediated Radical Addition to Azomethine Compounds: Novel Reactivity and Activation Modes

Azomethines is a class of compounds, which have traditionally served as electrophilic substrates, but their reactions with radicals have long been limited. Photocatalysis provided ample opportunities for promoting these reactions, with wide variety of reagents serving as precursors of radicals. Besides regular addition mode at the azomethine fragment, the oxidative pathway, in which the C=N bond remains in the product, has become possible by proper selection of redox catalyst. This review summarizes new developments in this rapidly developing field over the past five years. New concepts on activation of the C=N bond towards radical attack are discussed.

Photocatalytic Decarboxylative Alkylation of Cyclic Imine-BF3 Complexes: A Modular Route to Functionalized Azacycles

Alkyl radicals generated via an acridine photocatalyzed decarboxylation reaction of feedstock carboxylic acids engage with a range of cyclic imine-BF3 complexes to provide α-functionalized azacycles in an operationally simple process. A three-component variant of this transformation incorporating [1.1.1]propellane as an additional reaction partner enables the synthesis of valuable bicyclopentane (BCP)-containing azacycles. Reactions exhibit good functional group compatibility, enabling late-stage modification of complex bioactive molecules.

Photoredox Activation of Fluorinated Organozinc Reagents: Hydrofluoroalkylation of Unactivated and Electron Deficient Alkenes

Hydrofluoroalkylation of alkenes with organozinc reagents under photocatalytic conditions is described. The fluorinated alkyl radicals were generated from organozincs by the single electron oxidation of the carbon-zinc bond. The radical addition step is followed either by hydrogen atom transfer for unactivated olefins or by a reduction/protonation sequence for strongly accepting arylidenemalononitriles.

Multicomponent Synthesis of α-Branched Amines via a Zinc-Mediated Carbonyl Alkylative Amination Reaction

Methods for the synthesis of α-branched alkylamines are important due to their ubiquity in biologically active molecules. Despite the development of many methods for amine preparation, C(sp3)-rich nitrogen-containing compounds continue to pose challenges for synthesis. While carbonyl reductive amination (CRA) between ketones and alkylamines is the cornerstone method for α-branched alkylamine synthesis, it is sometimes limited by the sterically demanding condensation step between dialkyl ketones and amines and the more restricted availability of ketones compared to aldehydes. We recently reported a "higher-order" variant of this transformation, carbonyl alkylative amination (CAA), which utilized a halogen atom transfer (XAT)-mediated radical mechanism, enabling the streamlined synthesis of complex α-branched alkylamines. Despite the efficacy of this visible-light-driven approach, it displayed scalability issues, and competitive reductive amination was a problem for certain substrate classes, limiting applicability. Here, we report a change in the reaction regime that expands the CAA platform through the realization of an extremely broad zinc-mediated CAA reaction. This new strategy enabled elimination of competitive CRA, simplified purification, and improved reaction scope. Furthermore, this new reaction harnessed carboxylic acid derivatives as alkyl donors and facilitated the synthesis of α-trialkyl tertiary amines, which cannot be accessed via CRA. This Zn-mediated CAA reaction can be carried out at a variety of scales, from a 10 μmol setup in microtiter plates enabling high-throughput experimentation, to the gram-scale synthesis of medicinally-relevant compounds. We believe that this transformation enables robust, efficient, and economical access to α-branched alkylamines and provides a viable alternative to the current benchmark methods.

One-Pot Transformation of Aldehydes to Ketones via Minisci-Type Reaction of Imines

A method for the conversion of aldehydes to ketones via the preliminary formation of aldiminines is described. The imines are involved in acid promoted Minisci-type reaction with alkyl radicals generated from esters of N-hydroxylphthalimide under photoredox conditions. Aminyl radical cations formed after the addition of the iminium ions are believed to be key intermediates, which determine the reaction outcome.