Mechanisms for radical reactions initiating from N-hydroxyphthalimide esters

Red-shifted photoredox generation and trapping of alkyl radicals towards bioorthogonality

The photocatalytic generation and trapping of alkyl radicals is a powerful synthetic tool in organic chemistry, but it remains underexplored in biological settings. Here, we present two photoredox systems that leverage green- or red-light irradiation for the activation and subsequent Giese coupling of redox-active alkyl phthalimide esters. Besides utilizing mild low-energy light sources, these reactions operate with biocompatible BnNAH or NADH as electron donor. Notably, they display compatibility with air, water and biologically relevant conditions, including cell-culture media or even cell lysates. This work marks a significant step towards integrating synthetic alkyl-radical chemistry into biological settings.

Enantioselective Desymmetrization of Biaryls via Cooperative Photoredox/Brønsted Acid Catalysis and Its Application to the Total Synthesis of Ancistrobrevolines

Photoredox catalysis has emerged as a powerful tool for forming and breaking chemical bonds, further taking hold with its integration with asymmetric catalysis. While the dual-catalytic approach has led to successful examples of the control of stereogenic centers, the control of stereogenic axes has remained underexplored. In this study, an acylimine intermediate was generated through photoredox catalysis, and a symmetric substrate, 2-arylresorcinol, was desymmetrized with the aid of chiral phosphoric acid catalysis. Using this approach, a stereogenic center and stereogenic axis were successfully controlled to provide a natural-product-driven compound. The origins of enantioselectivity and diastereoselectivity were investigated through a density functional theory study of four possible enantiodetermining transition states. Consequently, the first total syntheses of the ring-contracted naphthylisoquinoline alkaloid ancistrobrevolines A and B were accomplished concisely. This approach provides not only a novel methodology and strategy to synthesize naphthylisoquinoline alkaloids but also a direction to advance catalytic research and total synthesis studies.

Straightforward Superbase-Mediated Reductive O-Phosphorylation of Aromatic and Heteroaromatic Ketones with Red Phosphorus in the Superbase Suspension KOH/DMSO(H2O)

It was shown for the first time that diaryl(hetaryl)ketones are capable of directly phosphorylating with red phosphorus in the superbase suspension KOH/DMSO(H2O) at 85 °C for 1.5 h to afford potassium bis(diaryl(hetaryl)methyl)phosphates that were earlier inaccessible in a yield of up to 45%. The ESR data demonstrate that unlike previously published phosphorylation with elemental phosphorus, this new phosphorylation reaction proceeds via a single electron transfer from polyphospide anions to diaryl(hetaryl)ketones. This is the first example of the C-O-P bond generation during the phosphorylation with elemental phosphorus in strongly basic media, which usually provides C-P bond formation.

Photocatalysis Enables Chemodivergent Radical Polar Crossover: Ritter-Type Amidation vs Heck-Type Olefin Carbofunctionalizations

Three-component alkene difunctionalization reactions constitute an ideal platform to rapidly build molecular complexity, enabling the simultaneous introduction of two distinct, orthogonal functional groups into the C═C bond in a single step. Herein, a photoredox catalyzed Ritter-type carboamidation of electronically diverse styrenes harnessing non-stabilized, nucleophilic primary radicals generated from readily-accessible carboxylic acid-derived redox active esters is reported. Furthermore, it is found that Heck-type products are chemoselectively obtained by simply switching aryl olefin acceptors with 1,1-diarylolefins. In the context of photocatalytic chemodivergent radical polar crossover, the synthesis of various trisubstituted alkenes was achieved, simultaneously revealing a divergence in the activation of redox-active esters toward reduction. In-depth mechanistic studies demonstrated both transformation pathways, while DFT calculations indicated the origin of product switchability. Both Ritter-type and Heck-type olefin carbofunctionalizations are scalable up to 4 mmol scale in batch and continuous flow, proving the synthetic utility of the methodology.

Photochemical Alkylamination of Olefins through Reactivity-Based Sorting of Alkyl Radicals

Alkyl radicals represent some of the most intriguing prospects in organic synthesis, showing diverse patterns of reactivity for versatile transformations. In light of this, the methyl radical, in addition to being a methylating agent, is also a good proposition for hydrogen atom transfer (HAT). Similarly, acetonitrile also has dual facets to its reactivity, acting as an amination reagent in the Ritter reaction while also being the progenitor to cyanomethyl radicals through HAT. We hereby take advantage of the merging of the dual reactivities of these radicals, allowing facile access to amines of various types from olefins when conjugated with a photoredox Ritter amination.

Redox-Selective Macromolecular Electrolysis for Sequential Functionalization and Deconstruction

This study demonstrates that selective macromolecular electrolysis can be achieved on copolymers containing redox-orthogonal targets by controlling the externally applied voltage. We designed macromolecules containing phthalimide (E1/2 = -1.8 V vs Ag/AgNO3) and tetrachlorophthalimide (E1/2 = -1.3 V vs Ag/AgNO3) (meth)acrylates that have significantly different reduction potentials such that they are separately redox-addressable. The polymer-centered radicals generated by decarboxylation can either undergo (1) hydrogen atom transfer to form olefinic repeat units or (2) β-scission to deconstruct the polymer backbone. Our results reveal selective electrochemical control over postpolymerization modifications, which enables sequential transformations that tune the glass transition temperature of electrochemically generated copolymers over a range of -54 to 125 °C. This method was also shown to maintain its selectivity in a polymer blend and provided access to copolymers (poly(styrene-co-propylene-co-ethylene)) that would be challenging to prepare in other ways. These results demonstrate the potential of macromolecular electrolysis for selective material functionalization and degradation. This approach expands the toolbox for postpolymerization modification and targeted polymer degradation with applications in macromolecular information processing, spatiotemporal patterning, and producing materials with complex architectures that are driven by external stimuli.

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.