Polarity Transduction Enables the Formal Electronically Mismatched Radical Addition to Alkenes

Metal-Free Electrochemistry-Driven Decarboxylative Primary Alkyl-Alkoxylation of Olefins

Here, a primary alkylative difunctionalization of olefins based on the decarboxylation of carboxylate ions to obtain alkyl radicals by electrochemical anodic oxidation is reported. The reaction employs quaternary ammonium carboxylates as the source of alkyl radicals and does not require additional oxidizing agents or electrolytes. The reaction exhibits a broad substrate range and functional group compatibility. It gently converts mono- or disubstituted styrene substrates and alkyl carboxylate anions of various carbon chain lengths and substituents to products under the reaction conditions. Furthermore, it is important to note that not only alcohols but carboxylic acids and water can also serve as nucleophilic reagents to participate in the reaction and yield the corresponding products. Preliminary mechanistic studies have demonstrated that the reaction is enabled by the lower oxidation potential of the carboxylate anion compared to that of the olefin. The anodic oxidation of the carboxylate anion occurs prior to the oxidation of the olefin, followed by decarboxylation to obtain alkyl radicals.

Visible-Light-Mediated Decarboxylative (Amino)Alkylation of Azomethine Imines

Herein, we report an efficient, organophotocatalyzed decarboxylative (amino)alkylation of azomethine imines using readily available carboxylic acids as alkylating agents. This transformation exhibits wide scope, and a variety of carboxylic acids, including glycine derivatives, were employed as radical precursors. The use of 4CzIPN as the photocatalyst allowed the application of nonbenzylic secondary and tertiary carboxylic acids also, overcoming previous limitations. The wide scope, applicability of nonprefunctionalized precursors, and mild conditions are the highlights of this method. The intermediacy of key radical intermediates was established by radical trapping experiments.

Microfluidic Photocatalytic Ring Expansion of Sulfonium Salts for the Synthesis of Cyclic Sulfides

Cyclic sulfides are relevant building blocks in medicinal and synthetic chemistry, with applications ranging from drug discovery to materials science. However, the synthesis of medium-sized cyclic sulfides (6-8-membered rings) remains largely underdeveloped. Herein, we report a photocatalytic ring-expansion strategy for sulfonium salts, granting access to six-, seven-, and eight-membered cyclic sulfides with very high regio- and diastereocontrol. The implementation of the method under continuous flow was key to increasing the efficiency and minimizing product decomposition. Mechanistic investigations revealed the formation of benzylic radicals and carbocation intermediates that control the high regio- and diastereoselectivity observed. Finally, the synthetic utility of this approach was demonstrated in the synthesis of cyclic sulfoxides and sulfones, which can be easily obtained from the corresponding sulfide products.

Alkyl Azetidines Via Batch and Flow Photochemistry

Alkyl azetidines have been prepared by photochemical modifications of azetidine-2-carboxylic acids in batch and in flow. The reaction has been realized in milligram, gram, and even multigram quantities. The obtained azetidines are valuable building blocks for drug discovery.

Enhancing Electron Donor-Acceptor Complex Photoactivation with a Stable Perylene Diimide Metal-Organic Framework

Electron donor-acceptor complexes are commonly employed to facilitate photoinduced radical-mediated organic reactions. However, achieving these photochemical processes with catalytic amounts of donors or acceptors can be challenging, especially when aiming to reduce catalyst loadings. Herein, we have unveiled a framework-based heterogenization approach that significantly enhances the photoredox activity of perylene diimide species in radical addition reactions with alkyl silicates by promoting faster and more efficient electron donor-acceptor complex formation. Besides offering broad substrate scope in alkene hydroalkylation, the newly developed heterogeneous photocatalysis substantially improves the catalyst turnover numbers in comparison to previous homogeneous photocatalytic systems and demonstrates outstanding catalyst recyclability. These research findings pave the way for the advancement of various efficient and practical organic transformations using framework-supported organocatalysts.

Metal-Catalyzed Hydrogen Atom Transfer (MHAT) Hydroalkylation with Electron-Deficient Alkynes

We present a novel strategy for olefin construction via the reductive coupling of electron-neutral alkenes with electron-deficient alkynes under metal-catalyzed hydrogen atom transfer conditions. This methodology provides selective access to both trans and the more challenging-to-synthesize cis isomers and permits the olefin to be installed next to sterically hindered centers, key factors in the synthesis of biologically active compounds. The reaction exhibits broad functional group tolerance and proceeds under mild, nontoxic conditions with high atom efficiency.

Pnictogen and Chalcogen Salts as Alkylating Agents

Alkylation reactions and their products are considered crucial in various contexts. Synthetically, the alkylation of a nucleophile is usually promoted using hazardous alkyl halides. Here, we aim to highlight the potential of pnictogen (ammonium or phosphonium) and chalcogen salts (sulfonium, selenonium, and telluronium) to function as alkylating agents. These compounds can be considered as non-volatile electrophilic alkyl reservoirs. We will center our discussion on the strategies developed in recent years to expand the synthetic utility of these salts in terms of transferable alkyl groups, substrate scope, and product selectivity.

Arylthianthrenium Salts for Triplet Energy Transfer Catalysis

Sigma bond cleavage through electronically excited states allows synthetically useful transformations with two radical species. Direct excitation of simple aryl halides to form both aryl and halogen radicals necessitates UV-C light, so undesired side reactions are often observed and specific equipment is required. Moreover, only aryl halides with extended π systems and comparatively low triplet energy are applicable to synthetically useful energy transfer catalysis. Here we show the conceptual advantages of arylthianthrenium salts (ArTTs) for energy transfer catalysis with high energy efficiency compared to conventional aryl (pseudo)halides and their utility in arylation reactions of ethylene. The fundamental advance is enabled by the low triplet energy of ArTTs that may originate in large part from the electronic interplay between the distinct sulfur atoms in the tricyclic thianthrene scaffold, which is not accessible in either simple (pseudo)halides or other conventional sulfonium salts.

Carbosulfonylation of Alkynes: A Direct Conversion of sp-C to sp3-C through Visible Light-Mediated 3-Component Reaction

A 3-component metal-free carbosulfonylation of alkynes is reported using readily available alkyl carboxylic acids and arylsulfinates under visible light irradiation. This photochemical approach gives direct conversion of sp-C to sp3-C yielding highly functionalized alkyl sulfones. It employs feedstock chemicals as starting materials and shows a broad substrate scope and moderate diastereoselectivity. The method's utility is highlighted in the synthesis of sedum alkaloids. A single photocatalyst is proposed to be active in two distinct photocatalytic cycles operating in tandem.

Base-Mediated Divergent Synthesis of Spiro-heterocycles Using Pronucleophiles and Ethylene via Thianthrenation

Spirocyclic compounds are abundant in biologically active products. However, the divergent synthesis of spirocyclic compounds using low-cost and abundant available starting materials remains a challenge. Herein, we report an effective method for producing spirocyclic motifs using a cyclic β-carbonyl ester or amide and ethylene via thianthrenation. This strategy highlights the exciting possibility of utilizing abundant ethylene as a C-2 synthon and allows regulating the core structure of the spirocyclic compound by simply altering the base type.

Easy Access to Functionalized Indolines and Tetrahydroquinolines via a Photochemical Cascade Cyclization Reaction

Herein, the development of a light-mediated synthesis of functionalized indolines and tetrahydroquinolines is reported. These structural motifs are considered as highly valuable targets, attributed to their widespread occurrence in pharmaceuticals and natural products. The gold-mediated approach offers a direct route to functionalized indolines in yields of up to 81% under mild photochemical conditions. Thereby, easily accessible Boc-protected N-aryl-allylamine and homoallylamine derivatives were reacted with sp3-hybridized haloalkanes in an intermolecular cascade cyclization reaction. A broad scope of substrates, including a variety of different substituents on the aromatic backbone as well as various haloalkanes, could be utilized. Indoline derivatives, which are functionalized in position 2, are also accessible by applying ortho-allylic anilines. Moreover, the synthetic appeal was demonstrated for a total synthesis of the anti-inflammatory agent AN669 in three reaction steps in an overall yield of 64%.

Shedding light on thermally-activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) is a hot research topic in view of its impressive applications in a wide variety of fields from organic LEDs to photodynamic therapy and metal-free photocatalysis. TADF is a rare and fragile phenomenon that requires a delicate equilibrium between tiny singlet-triplet gaps, sizable spin-orbit couplings, conformational flexibility and a balanced contribution of charge transfer and local excited states. To make the picture more complex, this precarious equilibrium is non-trivially affected by the interaction of the TADF dye with its local environment. The concurrent optimization of the dye and of the embedding medium is therefore of paramount importance to boost practical applications of TADF. Towards this aim, refined theoretical and computational approaches must be cleverly exploited, paying attention to the reliability of adopted approximations. In this perspective, we will address some of the most important issues in the field. Specifically, we will critically review theoretical and computational approaches to TADF rates, highlighting the limits of widespread approaches. Environmental effects on the TADF photophysics are discussed in detail, focusing on the major role played by dielectric and conformational disorder in liquid solutions and amorphous matrices.

Alkene Thianthrenation Unlocks Diverse Cation Synthons: Recent Progress and New Opportunities

Oxidative alkene functionalization reactions are a fundamental class of complexity-building organic transformations. However, the majority of established approaches rely on electrophilic reagents that limit the diversity of groups that can be installed. Recent advances have established a new approach that instead relies on the transformation of alkenes into thianthrene-derived cationic electrophiles. These linchpin intermediates can be generated selectively and undergo a diverse array of mechanistically distinct reactions with abundant nucleophiles. Taken together, this unlocks a suite of net oxidative alkene transformations that have been elusive using conventional strategies. This Minireview describes these advances and is organized around the three distinct synthons formally accessible from alkenes via thianthrenation: 1) alkenyl cations; 2) vicinal dications; 3) allyl cations. Throughout the Minireview, we illustrate how thianthrenium salts address key limitations endemic to classic alkene-derived electrophiles and highlight the mechanistic origins of these distinctions wherever possible.

Triply Selective & Sequential Diversification at Csp 3: Expansion of Alkyl Germane Reactivity for C-C & C-Heteroatom Bond Formation

We report the triply selective and sequential diversification of a single Csp 3 carbon carrying Cl, Bpin and GeEt3 for the modular and programmable construction of sp3-rich molecules. Various functionalizations of Csp 3-Cl and Csp 3-BPin (e.g. alkylation, arylation, homologation, amination, hydroxylation) were tolerated by the Csp 3-GeEt3 group. Moreover, the methodological repertoire of alkyl germane functionalization was significantly expanded beyond the hitherto known Giese addition and arylation to alkynylation, alkenylation, cyanation, halogenation, azidation, C-S bond formation as well as the first demonstration of stereo-selective functionalization of a Csp 3-[Ge] bond.

Nucleophilic functionalization of thianthrenium salts under basic conditions

In recent years, S-(alkyl)thianthrenium salts have become an important means of functionalizing alcohol compounds. However, additional transition metal catalysts and/or visible light are required. Herein, a direct thioetherification/amination reaction of thianthrenium salts is realized under metal-free conditions. This strategy exhibits good functional-group tolerance, operational simplicity, and an extensive range of compatible substrates.

Iron-Catalyzed Enantioselective Multicomponent Cross-Couplings of α-Boryl Radicals

Despite recent interest in the development of iron-catalyzed transformations, methods that use iron-based catalysts capable of controlling the enantioselectivity in carbon-carbon cross-couplings are underdeveloped. Herein, we report a practical and simple protocol that uses commercially available and expensive iron salts in combination with chiral bisphosphine ligands to enable the regio- and enantioselective (up to 91:9) multicomponent cross-coupling of vinyl boronates, (fluoro)alkyl halides, and Grignard reagents. Preliminary mechanistic studies are consistent with rapid formation of an α-boryl radical followed by reversible radical addition to monoaryl bisphosphine-Fe(II) and subsequent enantioselective inner-sphere reductive elimination. From a broader perspective, this work provides a blueprint to develop asymmetric Fe-catalyzed multicomponent cross-couplings via the use of alkenes as linchpins to translocate alkyl radicals, modify their steric and electronic properties, and induce stereocontrol.

Red-light-mediated Barton decarboxylation reaction and one-pot wavelength-selective transformations

In organic chemistry, selecting mild conditions for transformations and saving energy are increasingly important for achieving sustainable development goals. Herein, we describe a red-light-mediated Barton decarboxylation using readily available red-light-emitting diodes as the energy source and zinc tetraphenylporphyrin as the catalyst, avoiding explosive or hazardous reagents or external heating. Mechanistic studies suggest that the reaction probably proceeds via Dexter energy transfer between the activated catalyst and the Barton ester. Furthermore, a one-pot wavelength-selective reaction within the visible light range is developed in combination with a blue-light-mediated photoredox reaction, demonstrating the compatibility of two photochemical transformations based on mechanistic differences. This one-pot process expands the limits of the decarboxylative Giese reaction beyond polarity matching.

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.

Oxetane Synthesis via Alcohol C-H Functionalization

Oxetanes are strained heterocycles with unique properties that have triggered significant advances in medicinal chemistry. However, their synthesis still presents significant challenges that limit the use of this class of compounds in practical applications. In this Letter, we present a methodology that introduces a new synthetic disconnection to access oxetanes from native alcohol substrates. The generality of the approach is demonstrated by the application in late-stage functionalization chemistry, which is further exploited to develop a single-step synthesis of a known bioactive synthetic steroid derivative that previously required at least four synthetic steps from available precursors.

Fe-Catalyzed Hydroallylation of Unactivated Alkenes with Vinyl Cyclopropanes

Catalytic, reductive C-C bond formation between alkenes and vinyl cyclopropane (VCP) through hydrogen atom transfer (MHAT) is developed. Despite VCP's use as probes in radical-clock experiments, translation of this manifold into synthetic methods for accessing elusive C-C bonds remains largely unexplored. This work represents the first foray into this front where the high chemoselectivity of MHAT for alkene over VCP was pivotal for realizing the strategy. This method exhibits a broad scope, high functional group tolerance, and useful applications.

Radical Hydrocarboxylation of Unactivated Alkenes via Photocatalytic Formate Activation

Herein we disclose a strategy to promote the hydrocarboxylation of unactivated alkenes using photochemical activation of formate salts. We illustrate that an alternative initiation mechanism circumvents the limitations of prior approaches and enables hydrocarboxylation of this challenging substrate class. Specifically, we found that accessing the requisite thiyl radical initiator without an exogenous chromophore eliminates major byproducts that have plagued attempts to exploit similar reactivity for unactivated alkene substrates. This redox-neutral method is technically simple to execute and effective across a broad range of alkene substrates. Feedstock alkenes, such as ethylene, are hydrocarboxylated at ambient temperature and pressure. A series of radical cyclization experiments indicate how the reactivity described in this report can be diverted by more complex radical processes.