Thiol-mediated radical cyclizations

Redox-active molecules as organocatalysts for selective oxidative transformations - an unperceived organocatalysis field

Organocatalysis is widely recognized as a key synthetic methodology in organic chemistry. It allows chemists to avoid the use of precious and (or) toxic metals by taking advantage of the catalytic activity of small and synthetically available molecules. Today, the term organocatalysis is mainly associated with redox-neutral asymmetric catalysis of C-C bond-forming processes, such as aldol reactions, Michael reactions, cycloaddition reactions, etc. Organophotoredox catalysis has emerged recently as another important catalysis type which has gained much attention and has been quite well-reviewed. At the same time, there are a significant number of other processes, especially oxidative, catalyzed by redox-active organic molecules in the ground state (without light excitation). Unfortunately, many of such processes are not associated in the literature with the organocatalysis field and thus many achievements are not fully consolidated and systematized. The present article is aimed at overviewing the current state-of-art and perspectives of oxidative organocatalysis by redox-active molecules with the emphasis on challenging chemo-, regio- and stereoselective CH-functionalization processes. The catalytic systems based on N-oxyl radicals, amines, thiols, oxaziridines, ketone/peroxide, quinones, and iodine(I/III) compounds are the most developed catalyst types which are covered here.

Solvent-mediated switching between oxidative addition and addition–oxidation: access to β-hydroxysulfides and β-arylsulfones by the addition of thiols to olefins in the presence of Oxone

Solvent-switching allows formation of either β-hydroxy-2-arylethyl aryl sulfides or 2-arylethyl aryl sulfones exclusively in thiol–ene ‘click’ reactions conducted with Oxone.

Metalated isocyanides: formation, structure, and reactivity

Metalated isocyanides are highly versatile organometallics. Central to the reactivity of metalated isocyanides is the presence of two orthogonally reactive carbons, a highly nucleophilic "carbanion" inductively stabilized by a carbene-like isocyanide carbon. The two reactivities are harnessed in the attack of metalated isocyanides on π-electrophiles where an initial nucleophilic attack leads to an electron pair that cyclizes onto the terminal isocyanide carbon in a rapid route to diverse, nitrogenous heterocycles. Harnessing the potent nucleophilicity of metalated isocyanides while preventing electrophilic attack on the terminal isocyanide carbon has largely been driven by empirical heuristics. This review provides a foundational understanding by surveying the formation, structure, and properties of metalated isocyanides. The focus on the interplay between the structure and reactivity of metalated isocyanides is anticipated to facilitate the development and deployment of these exceptional nucleophiles in complex bond constructions.

Investigations on Gold-Catalyzed Thioalkyne Activation Toward Facile Synthesis of Ketene Dithioacetals

Nucleophilic addition to thioalkynes was investigated under various catalytic conditions with gold(I) complexes being identified as the optimal catalysts. Structural evaluation of the product revealed an unexpected cis-addition, arising from a gold-associated thioketene intermediate. Based on this interesting mechanistic insight, a gold(I)-catalyzed thioether addition to thioalkynes was developed as a novel approach to prepare ketene dithioacetals with good yields and high efficiency.

Thiyl radical-mediated cyclization of ω-alkynyl O-tert-butyldiphenylsilyloximes

Treatment of ω-alkynyl O-tert-butyldiphenylsilyloximes with odorless thiol and AIBN gave cyclic O-silylhydroxylamines via a radical process in good yields.

Cascade Photoredox/Iodide Catalysis: Access to Difluoro-γ-lactams via Aminodifluoroalkylation of Alkenes

The novel cascade photoredox/iodide catalytic system enables the alkene to serve as a radical acceptor capable of achieving aminodifluoroalkylation of alkenes. Cheap iodide salts play a vital role in this reaction, which could tune carbocation reactivity through reversible C-I bond formation for controlling reaction selectivity, and a series of competitive reactions are completely eliminated in the presence of multiple reactivity pathways. The present dual catalytic protocol affords a very convenient method for direct synthesis of various difluoro-γ-lactams from simple and readily available starting materials under mild reaction conditions.

Copper-catalyzed radical coupling of 1,3-dicarbonyl compounds with terminal alkenes for the synthesis of tetracarbonyl compounds

A novel and efficient copper-catalyzed radical cross-coupling of 1,3-dicarbonyl compounds with terminal alkenes for the synthesis of tetracarbonyl compounds with a quaternary carbon atom has been developed. Mechanistically, this transformation involves the construction of two C-C bonds and two C[double bond, length as m-dash]O bonds in a one-pot process. The reaction tolerates a wide range of functional groups and proceeds under mild conditions.

Highly selective synthesis of 6-substituted benzothiophenes by Sc(OTf)3-catalyzed intermolecular cyclization and sulfur migration

A series of 6-substituted benzo[b]thiophenes was synthesized using a Sc(OTf)₃-catalyzed intermolecular cyclization between para-substituted N-(arylthio)succinimides and alkynes taking advantage of a unique and selective 1,2-sulfur migration occuring via an electrophilic ipso cyclization to a key spirocyclic thiete intermediate.

Autoinductive thiolation/oxygenation of alkenes at room temperature

Spontaneous O₂ fixation by just mixing activated alkenes and thiols at room temperature is rationally demonstrated.

Aerobic oxidation in nanomicelles of aryl alkynes, in water at room temperature

On the basis of the far higher solubility of oxygen gas inside the hydrocarbon core of nanomicelles, metal and peroxide free aerobic oxidation of aryl alkynes to β-ketosulfones has been achieved in water at room temperature. Many examples are offered that illustrate broad functional group tolerance. The overall process is environmentally friendly, documented by the associated low E Factors.

Tuneable radical cyclisations: a tin-free approach towards tricyclic and spirocyclic heterocycles via a common precursor

A novel common precursor approach towards both tricyclic and spirocyclic heterocycles is described. Cyclisations are based on thiyl radical/isocyanide methodology and avoid the use of tin.

Hydrothiolation of benzyl mercaptan to arylacetylene: application to the synthesis of (E) and (Z)-isomers of ON 01910·Na (Rigosertib®), a phase III clinical stage anti-cancer agent

A stereoselective and efficient method for free radical addition of benzyl thiol to aryl acetylene in the presence of Et3B-hexane has been developed for the synthesis of (Z) and (E)-styryl benzyl sulfides where base catalyzed hydrothiolations have failed. The scope of this reaction was successfully extended for the synthesis of (E)-ON 01910·Na, a phase III clinical stage anti-cancer agent and its inactive geometrical isomer (Z)-ON 01910·Na. It is interesting to note that all the E-isomers synthesized have shown better cytotoxicity profile on cancer cells compared to the Z-isomers.

Thiol-yne coupling: revisiting old concepts as a breakthrough for up-to-date applications

Radical thiol-yne coupling (TYC) has emerged as one of the most appealing click chemistry procedures, appearing as a sound candidate for replacing/complementing other popular click reactions such as the thiol-ene coupling (TEC) and the Cu-catalysed azide-alkyne cycloaddition (CuAAC). Radical TYC is indeed a metal-free reaction suitable for biomedical applications, and its mechanistic features often make it more efficient than its TEC sister reaction and more suitable for multifaceted derivatisations in the materials chemistry and bioconjugation realms. This article reviews the fascinating results obtained in those fields in very recent years.

Aminomethylation of Michael acceptors: complementary radical and polar approaches mediated by dialkylzincs

Phtalimidomethyl iodide and substituted maleimidomethyl iodide were used as radical precursors in dialkylzinc-mediated radical addition to diethyl fumarate. The reactions led stereoselectively to functionalized pyrrolizidines. The radical mechanism was supported by spin-trapping experiments and rationalized by theoretical calculations. Radical additions, on the one hand, and carbozincation followed by transmetalation with copper(I), on the other, were shown to be complementary methods to achieve the formal aminomethylation of activated unsaturated compounds.

Enantiospecific synthesis of [2.2]paracyclophane-4-thiol and derivatives

This paper describes a simple route to enantiomerically enriched [2.2]paracyclophane-4-thiol via the stereospecific introduction of a chiral sulfoxide to the [2.2]paracyclophane skeleton. The first synthesis of an enantiomerically enriched planar chiral benzothiazole is also reported.