Switchable Decarboxylative Heck-Type Reaction and Oxo-alkylation of Styrenes with N-Hydroxyphthalimide Esters under Photocatalysis

Cu(II) Promoted C(sp3 )-H Activation in Unactivated Cycloalkanes: Oxo-Alkylation of Styrenes to Synthesize β-Disubstituted Ketones

We report the Cu(II) catalyzed synthesis of β-disubstituted ketones from styrene via oxo-alkylation with unactivated cycloalkanes as the alkylating agent in presence of tert-butylhydroperoxide (TBHP) and 1-methylimidazole as oxidant and base respectively. β-disubstituted ketones are known to be synthesized by using either expensive Ru/Ir complexes, or low-cost metal complexes (e. g., Fe, Mn) with activated species like aldehyde, acid, alcohol, or phthalimide derivatives as the alkylating agent, however, use of unactivated cycloalkanes directly as the alkylating agent remains challenging. A wide range of aliphatic C-H substrates as well as various olefinic arenes and heteroarene (35 substrates including 14 new substrates) are well-tolerated in this method. Hammett analysis shed more light on the substitution effect in the olefinic part on the overall mechanism. Furthermore, the controlled experiments, kinetic isotope effect study, and theoretical calculations (DFT) enable us to gain deeper insight of mechanistic intricacies of this new simple and atom-economic methodology.

A Chemoselective Polarity-Mismatched Photocatalytic C(sp3 )-C(sp2 ) Cross-Coupling Enabled by Synergistic Boron Activation

We report the development of a C(sp3 )-C(sp2 ) coupling reaction using styrene boronic acids and redox-active esters under photoredox catalysis. The reaction proceeds through an unusual polarity-mismatched radical addition mechanism that is orthogonal to established processes. Synergistic activation of the radical precursor and organoboron are critical mechanistic events. Activation of an N-hydroxyphthalimide (NHPI) ester by coordination to boron enables electron transfer, with decomposition leading to a nucleofuge rebound, activating the organoboron to radical addition. The unique mechanism enables chemoselective coupling of styrene boronic acids in the presence of other alkene radical acceptors. The scope and limitations of the reaction, and a detailed mechanistic investigation are presented.

Iminoacylation of Alkenes via Photoredox N-Heterocyclic Carbene Catalysis

The iminoacylation of alkenes via photoredox N-heterocyclic carbene catalysis is developed with the employment of alkene-tethered α-imino-oxy acids and acyl imidazoles. The corresponding substituted 3,4-dihydro-2H-pyrroles were afforded in moderate to good yields with good to high diastereoselectivities in most cases. The reaction involves the 5-exo-trig radical cyclization of an alkene-tethered iminyl radical and the following coupling with a ketyl radical from acyl imidazole under NHC catalysis.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Visible-light-mediated decarboxylative (E)-alkenylation of aliphatic carboxylic acids with aryl styryl sulfones under metal-free conditions

The decarboxylative alkenylation of aliphatic carboxylic acids with aryl styryl sulfones is efficiently catalyzed by riboflavin tetraacetate under visible light irradiation at room temperature. This metal-free protocol is cost-efficient, environmentally friendly and provides the corresponding olefins with excellent (E)-diastereocontrol. The methodology can also be used to prepare internal alkynes regioselectively by using alkynyl sulfones as radical acceptors. The suitability as building blocks of the olefins obtained was demonstrated by performing an (E)- to (Z) photoisomerization, an iron-catalyzed allylic substitution of the phenoxy group derived from the 2-phenoxycarboxylic acid substrates, as well as syn-epoxidations, and diastereoselective intramolecular iodoarylations. Based on control experiments and DFT calculations, we proposed a reaction mechanism that accounts for the regio- and diastereo-selectivity observed.

Photocatalytic 1,2-oxo-alkylation reaction of styrenes with diazoacetates

We report on the photocatalytic 1,2-difunctionalization reaction of styrenes with acceptor-only diazoalkanes. In the presence of DABCO and tBuOOH, the carbene reactivity of diazoalkanes can be suppressed and a 1,2 oxo-alkylation reaction can be achieved (32 examples, up to 94% yield) without the formation of cyclopropane by-products via the formation of radical intermediates from ethyl diazoacetate.

Multicomponent reactions and photo/electrochemistry join forces: atom economy meets energy efficiency

Visible-light photoredox catalysis has been regarded as an extremely powerful tool in organic chemistry, bringing the spotlight back to radical processes. The versatility of photocatalyzed reactions has already been demonstrated to be effective in providing alternative routes for cross-coupling as well as multicomponent reactions. The photocatalyst allows the generation of high-energy intermediates through light irradiation rather than using highly reactive reagents or harsh reaction conditions. In a similar vein, organic electrochemistry has experienced a fruitful renaissance as a tool for generating reactive intermediates without the need for any catalyst. Such milder approaches pose the basis toward higher selectivity and broader applicability. In photocatalyzed and electrochemical multicomponent reactions, the generation of the radical species acts as a starter of the cascade of events. This allows for diverse reactivity and the use of reagents is usually not covered by classical methods. Owing to the availability of cheaper and more standardized photo- and electrochemical reactors, as well as easily scalable flow-setups, it is not surprising that these two fields have become areas of increased research interest. Keeping these in view, this review is aimed at providing an overview of the synthetic approaches in the design of MCRs involving photoredox catalysis and/or electrochemical activation as a crucial step with particular focus on the choice of the difunctionalized reagent.

An organophotoredox-catalyzed redox-neutral cascade involving N-(acyloxy)phthalimides and allenamides: synthesis of indoles

An organophotoredox-catalyzed radical cascade of allenamides and alkyl N-(acyloxy)phthalimides for the synthesis of indoles is documented. The method features mild and robust reaction conditions, and exhibits broad scope. The tandem process enriches the limited repertoire of alkyl NHPI ester addition on electron-rich π-bonds as well as radical chemistry involving allenamides.

Photocatalytic cross-couplings via the cleavage of N-O bonds

N-(Acyloxy)phthalimide and oxime derivatives containing N-O bonds are important chemicals and synthetic intermediates, and visible light photoredox reductions of the N-O bonds provide carbon- or nitrogen-centered radicals for N-(acyloxy)phthalimide derivatives and iminyl radicals for oxime derivatives. This feature article summarises the recent progress in the visible light photoredox organic reactions, including decarboxylative addition reactions, alkylation, allylation, alkenylation, alkynylation, arylation, heteroarylation and cascade annulation of N-(acyloxy)phthalimide derivatives through the formation of carbon-carbon bonds, decarboxylative borylation, amination, oxygenation, sulfuration, selenylation, fluorination and iodination of N-(acyloxy)phthalimide derivatives through the formation of carbon-heteroatom bonds, and additions to arenes and alkenes, hydrogen atom transfer and the cleavage of α-carbon-carbon bonds via the iminyl radical intermediates for oxime derivatives.

Visible light-induced alkylpyridylation of styrenes via a reductive radical three-component coupling

A visible light-induced and metal-free strategy for the intermolecular three-compoment alkylpyridylation of styrenes is reported. Hantzsch ester was found to be key to initiate the overall reductive radical coupling reaction. This radical process realized difunctionalization of styrenes, selectively yielding alkylated pyridines in good to excellent yields with a wide tolerance of functional groups, mild reaction conditions and simple operation. This new reaction complements existing visible light-induced variants of styrenes with NHP esters and expands the capabilities of radical-based cross-coupling reactions of pyridines.

Organophotoredox-Catalyzed Three-Component Coupling of Heteroatom Nucleophiles, Alkenes, and Aliphatic Redox Active Esters

This manuscript describes a visible-light-mediated organophotoredox catalytic process for vicinal difunctionalization of alkenes using heteroatom nucleophiles and aliphatic redox active esters. A wide range of heteroatom nucleophiles including alcohols, water, carboxylic acids, amides, and halogens can be used for this reaction. This radical relay type reaction allows forging of C(sp³)-C(sp³) with a carbon-centered radical and C(sp³)-heteroatom bonds with a benzyl cation on the vinylarenes with complete regioselectivity in a single step.

Organo-photoredox-Catalyzed Atom-Transfer Radical Substitution of Alkenes with α-Carbonyl Alkyl Halides

A light-driven atom-transfer radical substitution (ATRS) and carboesterification reaction of alkenes with alkyl halides has been developed using PTH as the organo-photoredox catalyst. Two types of products were obtained, depending on the additive and solvent used during the reaction. Primary, secondary, and tertiary alkyl halides reacted to give the ATRS products. This protocol has several advantages: it requires mild reaction conditions and a low catalyst loading and exhibits a broad substrate scope and good functional group tolerance. Mechanistic studies indicate that alkyl radicals might be generated as the key intermediates via photocatalysis, providing a new direction for ATRS reactions.

Generation of Alkyl Radicals: From the Tyranny of Tin to the Photon Democracy

Alkyl radicals are key intermediates in organic synthesis. Their classic generation from alkyl halides has a severe drawback due to the employment of toxic tin hydrides to the point that "flight from the tyranny of tin" in radical processes was considered for a long time an unavoidable issue. This review summarizes the main alternative approaches for the generation of unstabilized alkyl radicals, using photons as traceless promoters. The recent development in photochemical and photocatalyzed processes enabled the discovery of a plethora of new alkyl radical precursors, opening the world of radical chemistry to a broader community, thus allowing a new era of photon democracy.

Total Synthesis of (+)-Polyoxamic Acid via Visible-Light-Mediated Photocatalytic β-Scission and 1,5-Hydrogen Atom Transfer of Glucose Derivative

A total synthesis of polyoxamic acid has been achieved. The key feature of the synthetic route is a visible-light-mediated β-scission and carbon-to-carbon 1,5-hydrogen atom transfer (1,5-HAT) to provide the functionalized alditol under mild conditions. This type of carbon-to-carbon 1,5-HAT initiated by C(sp³)-centered radicals has been scarcely reported. Furthermore, the reaction was adapted for flow chemistry, facilitating the total synthesis of polyoxamic acid.

Aminoalkyl radicals as halogen-atom transfer agents for activation of alkyl and aryl halides

Organic halides are important building blocks in synthesis, but their use in (photo)redox chemistry is limited by their low reduction potentials. Halogen-atom transfer remains the most reliable approach to exploit these substrates in radical processes despite its requirement for hazardous reagents and initiators such as tributyltin hydride. In this study, we demonstrate that α-aminoalkyl radicals, easily accessible from simple amines, promote the homolytic activation of carbon-halogen bonds with a reactivity profile mirroring that of classical tin radicals. This strategy conveniently engages alkyl and aryl halides in a wide range of redox transformations to construct sp3-sp3, sp3-sp2, and sp2-sp2 carbon-carbon bonds under mild conditions with high chemoselectivity.

The photoredox alkylarylation of styrenes with alkyl N-hydroxyphthalimide esters and arenes involving C-H functionalization

The In(OTf)₃-promoted three-component photoredox alkylarylation of styrenes with alkyl NHP esters and arenes to access alkylated arene derivatives through C-C bond cleavage and C-H functionalization is reported. By utilizing visible-light photoredox catalysis, alkyl N-hydroxyphthalimide esters serving as alkyl carbon-centered radicals and a wide range of arenes (e.g., indoles, pyrrole, and electron-rich arenes) as nucleophiles were used to enable the introduction of various alkyl groups and aryl groups across the C[double bond, length as m-dash]C bonds with excellent selectivity and functional group tolerance.

Photoredox-catalyzed stereoselective alkylation of enamides with N-hydroxyphthalimide esters via decarboxylative cross-coupling reactions

Stereoselective β-C(sp2)-H alkylation of enamides with redox-active N-hydroxyphthalimide esters via a photoredox-catalyzed decarboxylative cross-coupling reaction is demonstrated. This methodology features operational simplicity, broad substrate scopes, and excellent stereoselectivities and functional group tolerance, affording a diverse array of geometrically defined and synthetically valuable enamides bearing primary, secondary or tertiary alkyl groups in satisfactory yields.

Construction of C(sp2 )-C(sp3 ) Bond between Quinoxalin-2(1H)-ones and N-Hydroxyphthalimide Esters via Photocatalytic Decarboxylative Coupling

A novel visible-light-driven decarboxylative coupling of alkyl N-hydroxyphthalimide esters (NHP esters) with quinoxalin-2(1H)-ones has been developed. This C(sp² )-C(sp³ ) bond-forming transformation exhibits excellent substrate generality with respect to both the coupling partners. Of note, a series of 3-primary alkyl-substituted quinoxalin-2(1H)-ones that were difficult to synthesize by previous methods could be obtained in moderate to excellent yields. Additionally, the mild conditions, easy availability of substrates, wide functional group tolerance and operational simplicity make this protocol practical in the synthesis of 3-alkylated quinoxalin-2(1H)-ones.

Bond-Forming and -Breaking Reactions at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate Salts

Organosulfur compounds have long played a vital role in organic chemistry and in the development of novel chemical structures and architectures. Prominent among these organosulfur compounds are those involving a sulfur(IV) center, which have been the subject of countless investigations over more than a hundred years. In addition to a long list of textbook sulfur-based reactions, there has been a sustained interest in the chemistry of organosulfur(IV) compounds in recent years. Of particular interest within organosulfur chemistry is the ease with which the synthetic chemist can effect a wide range of transformations through either bond formation or bond cleavage at sulfur. This review aims to cover the developments of the past decade in the chemistry of organic sulfur(IV) molecules and provide insight into both the wide range of reactions which critically rely on this versatile element and the diverse scaffolds that can thereby be synthesized.

Visible-Light-Promoted Oxo-difluoroalkylation of Alkenes with DMSO as the Oxidant

Visible-light-promoted oxo-difluoroalkylation (acetylation and acetamidation) of alkenes with dimethyl sulfoxide as both the solvent and the oxidant was developed, affording the corresponding α,α-difluoro-γ-ketoacetates and acetamides in modest yields. Both terminal and internal alkenes worked well for the reaction. This reaction features simple starting materials, a green oxidant, mild reaction conditions, and highly functional products.

Photoinduced C(sp3 )-N Bond Cleavage Leading to the Stereoselective Syntheses of Alkenes

Herein we report a versatile Mizoroki-Heck-type photoinduced C(sp³ )-N bond cleavage reaction. Under visible-light irradiation (455 nm, blue LEDs) at room temperature, alkyl Katritzky salts react smoothly with alkenes in a 1:1 molar ratio in the presence of 1.0 mol % of commercially available photoredox catalyst without the need for any base, affording the corresponding alkyl-substituted alkenes in good yields with broad functional-group compatibility. Notably, the E/Z-selectivity of the alkene products can be controlled by an appropriate choice of photoredox catalyst.

One-Pot Electrochemical Nickel-Catalyzed Decarboxylative Sp2-Sp3 Cross-Coupling

A one-pot electrochemical nickel-catalyzed decarboxylative sp²-sp³ cross-coupling reaction has been developed using redox-active esters prepared in situ from alkyl carboxylates and  N-hydroxyphthalimide tetramethyluronium hexafluorophosphate (PITU). This undivided cell one-pot method enables C-C bond formation using inexpensive, benchtop-stable reagents with isolated yields up to 95% with good functional group tolerance, which includes nitrile, ketone, ester, alkene and selectivity over other aromatic halogens.

Deaminative (Carbonylative) Alkyl-Heck-type Reactions Enabled by Photocatalytic C-N Bond Activation

The palladium-catalyzed Heck reaction is a well-known, Nobel Prize winning transformation for producing alkenes. Unlike the alkenyl and aryl variants of the Heck reaction, the alkyl-Heck reaction is still underdeveloped owing to the competitive side reactions of alkyl-palladium species. Herein, we describe the development of a deaminative alkyl-Heck-type reaction that proceeds through C-N bond activation by visible-light photoredox catalysis. A variety of aliphatic primary amines were found to be efficient starting materials for this new process, affording the corresponding alkene products in good yields under mild reaction conditions. Moreover, this strategy was successfully applied to deaminative carbonylative alkyl-Heck-type reactions.

Photocatalytic decarboxylative [2 + 2 + 1] annulation of 1,6-enynes with N-hydroxyphthalimide esters for the synthesis of indene-containing polycyclic compounds

An efficient photoredox-mediated [2 + 2 + 1] annulation of 1,6-enynes with N-hydroxyphthalimide esters was reported for the synthesis of spiro and non-spiro indene-containing polycyclic frameworks.

An efficient approach to access 1,1,2-triarylethanes enabled by the organo-photoredox-catalyzed decarboxylative addition reaction

An efficient approach to access 1,1,2-triarylethanes enabled by organo-photoredox-catalyzed decarboxylative 1,6-conjugate addition of carboxylic acids to p-QMs.

Visible-light-triggered direct keto-difluoroacetylation of styrenes with (fluorosulfonyl)difluoroacetate and dimethyl sulfoxide leads to α-difluoroacetylated ketones

Photoredox-catalyzed direct keto-difluoroacetylation of styrenes with methyl (fluorosulfonyl)difluoroacetate and dimethyl sulfoxide to deliver various α-difluoroacetylated ketones are disclosed.

A photoredox catalyzed iminyl radical-triggered C–C bond cleavage/addition/Kornblum oxidation cascade of oxime esters and styrenes: synthesis of ketonitriles

A photoredox-catalyzed iminyl radical-triggered C–C bond cleavage/addition/Kornblum oxidation cascade of cycloketone oxime esters and styrenes in DMSO is described for the first time.