Polarity Transduction Enables the Formal Electronically Mismatched Radical Addition to Alkenes

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.