Thiyl radicals in organic synthesis

Leveraging of Sulfur Anions in Photoinduced Molecular Transformations

This perspective describes recent advances in the use of sulfur anions to promote molecular transformations under irradiation with visible light. The topics are classified by the following reaction modes performed by the key sulfur anions: (1) C-S coupling via electron donor-acceptor (EDA) interactions, (2) photoinduced molecular transformation via sulfur anion EDA catalysis, (3) sulfur anions as photoredox and hydrogen atom transfer (HAT) catalysts, and 4) dithiocarbamate and xanthate as nucleophilic catalysts for photoinduced radical cascade reactions.

Production of Alkyl Aryl Sulfides from Aromatic Disulfides and Alkyl Carboxylates via a Disilathiane-Disulfide Interchange Reaction

The results of this study show that disilathiane is an effective mediator in the synthesis of alkyl aryl sulfides with disulfides and alkyl carboxylates. Mechanistic studies suggest that disilathiane promotes cleavage of the sulfur-sulfur bond of disulfides to generate thiosilane as a key intermediate. Diselenides were also applicable to this transformation to produce the corresponding selenides.

Dibenzocycloheptanones construction through a removable P-centered radical: synthesis of allocolchicine analogues

Dibenzocycloheptanones containing a tricyclic 6-7-6-system are present in numerous biologically active natural molecules. However, the simple and efficient preparation of derivatives containing a dibenzocycloheptanone scaffold remains difficult to date. Herein, we report a versatile strategy for the construction of these challenging seven-membered rings using a 7-endo-trig cyclization which is initiated by a phosphorus-centered radical. This approach provides a step-economical regime for the facile assembly of a wide range of phosphorylated dibenzocycloheptanones. Remarkably, we also have devised a traceless addition/exchange strategy for the preparation of dephosphorylated products at room temperature with excellent yields. Therefore, this protocol allows for the concise synthesis of biorelevant allocochicine derivatives.

Metal Sulfide Photocatalysts for Lignocellulose Valorization

Transition metal sulfides are an extraordinarily vital class of semiconductors with a wide range of applications in the photocatalytic field. A great number of recent advances in photocatalytic transformations of lignocellulosic biomass, the largest renewable carbon resource, into high-quality fuels and value-added chemicals has been achieved over metal sulfide semiconductors. Herein, the progress and breakthroughs in metal-sulfide-based photocatalytic systems for lignocellulose valorization with an emphasis on selective depolymerization of lignin and oxidative coupling of some important bioplatforms are highligted. The key issues that control reaction pathways and mechanisms are carefully examined. The functions of metal sulfides in the elementary reactions, including CO-bond cleavage, selective oxidations, CC coupling, and CH activation, are discussed to offer insights to guide the rational design of active and selective photocatalysts for sustainable chemistry. The prospects of sulfide photocatalysts in biomass valorization are also analyzed and briefly discussed.

A visible-light-induced thiol addition/aerobic oxidation cascade reaction of epoxides and thiols for the synthesis of β-hydroxylsulfoxides

A photochemical thiol addition/aerobic oxidation cascade reaction has been developed. This protocol enables efficient oxidative coupling of epoxides and thiols to access structurally valuable β-hydroxylsulfoxides. A broad range of functional groups are compatible to obtain moderate to good yields of the target products. Mechanistic studies revealed a sequential reaction pathway involving base-promoted thiol addition of thiols to epoxides and visible-light-induced aerobic oxygenation of thioethers.

Thiol-Mediated α-Amino Radical Formation via Visible-Light-Activated Ion-Pair Charge-Transfer Complexes

Visible-light-activated electron donor-acceptor complexes offer distinct reaction pathways for the synthesis of complex molecules under mild conditions. Herein, we report a method for the reductive generation of α-amino radicals via the reaction of a visible-light-activated ion-pair charge-transfer complex formed between an in situ-generated alkyl-iminium ion and a thiophenolate. This distinct activation mode is demonstrated through the development of a multicomponent coupling reaction to form substituted aminomethyl-cyclopentanes from secondary amines, cyclopropyl aldehydes, and alkenes. The operationally straightforward transformation displays broad scope and provides a means to generate cyclic amine-containing scaffolds from readily available feedstocks.

Recent Advances in C-F Bond Cleavage Enabled by Visible Light Photoredox Catalysis

The creation of new bonds via C-F bond cleavage of readily available per- or oligofluorinated compounds has received growing interest. Using such a strategy, a myriad of valuable partially fluorinated products can be prepared, which otherwise are difficult to make by the conventional C-F bond formation methods. Visible light photoredox catalysis has been proven as an important and powerful tool for defluorinative reactions due to its mild, easy to handle, and environmentally benign characteristics. Compared to the classical C-F activation that proceeds via two-electron processes, radicals are the key intermediates using visible light photoredox catalysis, providing new modes for the cleavage of C-F bonds. In this review, a summary of the visible light-promoted C-F bond cleavage since 2018 was presented. The contents were classified by the fluorosubstrates, including polyfluorinated arenes, gem-difluoroalkenes, trifluoromethyl arenes, and trifluoromethyl alkenes. An emphasis is placed on the discussion of the mechanisms and limitations of these reactions. Finally, my personal perspective on the future development of this rapidly emerging field was provided.

Site-Selective α-C-H Functionalization of Trialkylamines via Reversible Hydrogen Atom Transfer Catalysis

Trialkylamines are widely found in naturally occurring alkaloids, synthetic agrochemicals, biological probes, and especially pharmaceuticals agents and preclinical candidates. Despite the recent breakthrough of catalytic alkylation of dialkylamines, the selective α-C(sp³)-H bond functionalization of widely available trialkylamine scaffolds holds promise to streamline complex trialkylamine synthesis, accelerate drug discovery, and execute late-stage pharmaceutical modification with complementary reactivity. However, the canonical methods always result in functionalization at the less-crowded site. Herein, we describe a solution to switch the reaction site through fundamentally overcoming the steric control that dominates such processes. By rapidly establishing an equilibrium between α-amino C(sp³)-H bonds and a highly electrophilic thiol radical via reversible hydrogen atom transfer, we leverage a slower radical-trapping step with electron-deficient olefins to selectively forge a C(sp³)-C(sp³) bond with the more-crowded α-amino radical, with the overall selectivity guided by the Curtin-Hammett principle. This subtle reaction profile has unlocked a new strategic concept in direct C-H functionalization arena for forging C-C bonds from a diverse set of trialkylamines with high levels of site selectivity and preparative utility. Simple correlation of site selectivity and ¹³C NMR shift serves as a qualitative predictive guide. The broad consequences of this dynamic system, together with the ability to forge N-substituted quaternary carbon centers and implement late-stage functionalization techniques, hold potential to streamline complex trialkylamine synthesis and accelerate small-molecule drug discovery.

Matrix-isolated trifluoromethylthiyl radical: sulfur atom transfer, isomerization and oxidation reactions

By high-vacuum flash pyrolysis of bis(trifluoromethyl)disulfane oxide (CF₃S(O)SCF₃) at 400 °C, the elusive trifluoromethylthiyl radical (CF₃S˙) has been efficiently generated in the gas phase. Subsequent isolation of CF₃S˙ in cryogenic matrixes (Ne, Ar, and N₂) allows a first time characterization with IR and UV-vis spectroscopy by combining with computations at the CCSD(T)/aug-cc-pV(T + d)Z level. In addition to the photo-induced sulfur atom transfer (SAT) from CF₃S˙ to N₂ and CO and the isomerization to ˙CF₂SF, the O₂-oxidation via the intermediacy of the novel thiylperoxy radical CF₃SOO˙ has been observed.

The organocatalytic synthesis of perfluorophenylsulfides via the thiolation of trimethyl(perfluorophenyl)silanes and thiosulfonates

The organic superbase t-Bu-P₄-catalyzed direct thiolation of trimethyl(perfluorophenyl)silanes and thiosulfonates was developed. Yields of perfluorophenylsulfides of up to 97% under catalysis of 5 mol% t-Bu-P₄ were achieved. This method was shown to provide an efficient way to construct the perfluorophenyl-sulfur bond under mild metal-free reaction conditions.

Visible-Light-Promoted Oxidative Annulation of Naphthols and Alkynes: Synthesis of Functionalized Naphthofurans

A visible-light-mediated site-selective oxidative annulation of naphthols with alkynes for the synthesis of functionalized naphthofurans has been developed. The reaction relies on the in situ formation of an electron donor acceptor pair between phenylacetylene and thiophenol as the light-absorbing system to obviate the requirement of an added photocatalyst. The protocol facilitates the transformation of 1-naphthol and 2-naphthol as well as 1,4-naphthoquinone into a wide variety of highly functionalized naphthofurans.

Additives for Ambient 3D Printing with Visible Light

With 3D printing, the desire is to be "limited only by imagination," and although remarkable advancements have been made in recent years, the scope of printable materials remains narrow compared to other forms of manufacturing. Light-driven polymerization methods for 3D printing are particularly attractive due to unparalleled speed and resolution, yet the reliance on high-energy UV/violet light in contemporary processes limits the number of compatible materials due to pervasive absorption, scattering, and degradation at these short wavelengths. Such issues can be addressed with visible-light photopolymerizations. However, these lower-energy methods often suffer from slow reaction times and sensitivity to oxygen, precluding their utility in 3D printing processes that require rapid hardening (curing) to maximize build speed and resolution. Herein, multifunctional thiols are identified as simple additives to enable rapid high-resolution visible-light 3D printing under ambient (atmospheric O₂ ) conditions that rival modern UV/violet-based technology. The present process is universal, providing access to commercially relevant acrylic resins with a range of disparate mechanical responses from strong and stiff to soft and extensible. Pushing forward, the insight presented within this study will inform the development of next-generation 3D-printing materials, such as multicomponent hydrogels and composites.

Fluorine-Retentive Strategies for the Functionalization of gem-Difluoroalkenes

gem-Difluoroalkenes are readily available fluorinated building blocks, and the fluorine-induced electronic perturbations of the alkenes enables a wide array of selective functionalization reactions. However, many reactions of gem-difluoroalkenes result in a net C─F functionalization to generate monofluorovinyl products or addition of F to generate trifluoromethyl-containing products. In contrast, fluorine-retentive strategies for the functionalization of gem-difluoroalkenes remain less generally developed, and is now becoming a rapidly developing area. This review will present the development of fluorine-retentive strategies including electrophilic, nucleophilic, radical, and transition metal catalytic strategies with an emphasis on key physical organic and mechanistic aspects that enable reactivities.

Kinetic Analysis of a Cysteine-Derived Thiyl-Catalyzed Asymmetric Vinylcyclopropane Cycloaddition Reflects Numerous Attractive Noncovalent Interactions

Kinetic studies of a vinylcyclopropane (VCP) cycloaddition, catalyzed by peptide-based thiyl radicals, are described. Reactions were analyzed by using reaction progress kinetic analysis, revealing that ring-opening of the VCP is both rate- and enantio-determining. These conclusions are further corroborated by studies involving racemic and enantiopure VCP starting material. Noncovalent interactions play key roles throughout: both the peptide catalyst and VCP exhibit unproductive self-aggregation, which appears to be disrupted by binding between the catalyst and VCP. This in turn explains the requirement for the key catalyst feature, a substituent at the 4-position of the proline residue, which is required for both turnover/rate and selectivity.

Cysteinyl radicals in chemical synthesis and in nature

Nature harnesses the unique properties of cysteinyl radical intermediates for a diverse range of essential biological transformations including DNA biosynthesis and repair, metabolism, and biological photochemistry. In parallel, the synthetic accessibility and redox chemistry of cysteinyl radicals renders them versatile reactive intermediates for use in a vast array of synthetic applications such as lipidation, glycosylation and fluorescent labelling of proteins, peptide macrocyclization and stapling, desulfurisation of peptides and proteins, and development of novel therapeutics. This review provides the reader with an overview of the role of cysteinyl radical intermediates in both chemical synthesis and biological systems, with a critical focus on mechanistic details. Direct insights from biological systems, where applied to chemical synthesis, are highlighted and potential avenues from nature which are yet to be explored synthetically are presented.

Light-Induced Single-Electron Transfer Processes involving Sulfur Anions as Catalysts

Photoredox catalysis has evolved as an attractive approach to enable a wide variety of chemical reactions with high selectivity under mild conditions. The development of novel photocatalytic systems is key to obtaining new reactivity and improving their catalytic performances. In this context, cost-effective organic anion-based photocatalysts have recently attracted increasing interest. In particular, sulfur-based anionic catalysts are of interest due to their unique redox properties. This Perspective highlights and discusses recent advances in light-induced single-electron-transfer processes directly involving sulfur anions as catalysts. The content of this Perspective is organized along the different photoinduced electron-transfer pathways between catalysts and substrates.

Photo-crosslinking polymers by dynamic covalent disulfide bonds

A simple and general strategy to construct photo-crosslinkable polymers by introducing sidechain 1,2-dithiolanes based on natural thioctic acid is presented. The disulfide five-membered rings act both as light-absorbing and dynamic covalent crosslinking units, enabling efficient photo-crosslinking and reversible chemical decrosslinking of polydimethylsiloxane polymers.

Selective deoxygenative alkylation of alcohols via photocatalytic domino radical fragmentations

The delivery of alkyl radicals through photocatalytic deoxygenation of primary alcohols under mild conditions is a so far unmet challenge. In this report, we present a one-pot strategy for deoxygenative Giese reaction of alcohols with electron-deficient alkenes, by using xanthate salts as alcohol-activating groups for radical generation under visible-light photoredox conditions in the presence of triphenylphosphine. The convenient generation of xanthate salts and high reactivity of sequential C–S/C–O bond homolytic cleavage enable efficient deoxygenation of primary, secondary and tertiary alcohols with diverse functionality and structure to generate the corresponding alkyl radicals, including methyl radical. Moreover, chemoselective radical monodeoxygenation of diols is achieved via selective formation of xanthate salts.

The generation of alkyl radicals through deoxygenation of abundant alcohols via photoredox catalysis is of interest. In this study, the authors report a one-pot strategy for visible-light-promoted photoredox coupling of alcohols with electron-deficient alkenes, assisted by carbon disulfide and triphenylphosphine.

Synthetic exploration of sulfinyl radicals using sulfinyl sulfones

Sulfinyl radicals – one of the fundamental classes of S-centered radicals – have eluded synthetic application in organic chemistry for over 60 years, despite their potential to assemble valuable sulfoxide compounds. Here we report the successful generation and use of sulfinyl radicals in a dual radical addition/radical coupling with unsaturated hydrocarbons, where readily-accessed sulfinyl sulfones serve as the sulfinyl radical precursor. The strategy provides an entry to a variety of previously inaccessible linear and cyclic disulfurized adducts in a single step, and demonstrates tolerance to an extensive range of hydrocarbons and functional groups. Experimental and theoretical mechanistic investigations suggest that these reactions proceed through sequential sulfonyl and sulfinyl radical addition.

Sulfinyl radicals are an underexplored synthon in organic chemistry due to the fact that they reversibly add to pi systems and undergo homodimerization. Here the authors synthesize sulfonyl sulfones, previously thought to be unstable, and demonstrate their broad use as sulfinyl radical precursors in disulfurizations of alkenes and alkynes.

o-Semiquinone radical anion isolated as an amorphous porous solid

The use of metal cations is a commonly applied strategy to create S > 1/2 stable molecular systems containing semiquinone radicals. Persistent mono-semiquinonato complexes of diamagnetic metal ions (S = 1/2) have been hitherto less common and mostly limited to the complexes of heavy metal ions. In this work, a mono-semiquinonato complex of aluminum, derived from 1,2-dihydroxybenzene, is obtained using a surprisingly short and uncomplicated procedure. The isolated product is an amorphous and porous solid that exhibits very good stability under ambient conditions. To characterise its molecular and electronic structure, 9.7, 34 and 406 GHz EPR spectroscopy was used in concert with computational techniques (DFT and DLPNO-CCSD). It was revealed that the radical complex is composed of two chemically equivalent aluminum cations and two catechol-like ligands with the unpaired electron uniformly distributed between the two organic molecules. The good stability and porous structure make this complex applicable in heterogeneous aerobic reactions.

Photoinduced Hydrocarboxylation via Thiol-Catalyzed Delivery of Formate Across Activated Alkenes

Herein we disclose a new photochemical process to prepare carboxylic acids from formate salts and alkenes. This redox-neutral hydrocarboxylation proceeds in high yields across diverse functionalized alkene substrates with excellent regioselectivity. This operationally simple procedure can be readily scaled in batch at low photocatalyst loading (0.01% photocatalyst). Furthermore, this new reaction can leverage commercially available formate carbon isotologues to enable the direct synthesis of isotopically labeled carboxylic acids. Mechanistic studies support the working model involving a thiol-catalyzed radical chain process wherein the atoms from formate are delivered across the alkene substrate via CO₂^•- as a key reactive intermediate.

Remote Site-Selective Radical C(sp3 )-H Monodeuteration of Amides using D2 O

Site-selective incorporation of deuterium into biologically active compounds is of high interest in pharmaceutical industry. We present a mild and environmentally benign metal-free method for the remote selective radical C-H monodeuteration of aliphatic C-H bonds in various amides with inexpensive heavy water (D2 O) as the deuterium source. The method uses the easily installed N-allylsulfonyl moiety as an N-radical precursor that generates the remote C-radical via site-selective 1,5- or 1,6-hydrogen atom transfer (HAT). Methyl thioglycolate, that readily exchanges its proton with D2 O, serves as the radical deuteration reagent and as a chain-carrier. The highly site-selective monodeuteration has been applied to different types of unactivated sp3 -C-H bonds and also to the deuteration of C-H bonds next to heteroatoms. The potential utility of this method is further demonstrated by the site-selective incorporation of deuterium into natural product derivatives and drugs.

Photochemical Hydrothiolation of Amorphadiene and Formal Synthesis of Artemisinin via a Pummerer Rearrangement

A new access to artemisinin is reported based on a selective photochemical hydrothiolation of amorphadiene, a waste product of the industrial semisynthetic route. This study highlights the discovery of two distinctive activation pathways under solvent-free conditions or using a photocatalyst promoting H-abstraction. Subsequently, a chemoselective oxidation of the resulting photochemically generated thioether, followed by a Pummerer rearrangement, affords dihydroartemisinic aldehyde, a key intermediate in the synthesis of artemisinin.

Non-innocent Radical Ion Intermediates in Photoredox Catalysis: Parallel Reduction Modes Enable Coupling of Diverse Aryl Chlorides

We describe a photocatalytic system that elicits potent photoreductant activity from conventional photocatalysts by leveraging radical anion intermediates generated in situ. The combination of an isophthalonitrile photocatalyst and sodium formate promotes diverse aryl radical coupling reactions from abundant but difficult to reduce aryl chloride substrates. Mechanistic studies reveal two parallel pathways for substrate reduction both enabled by a key terminal reductant byproduct, carbon dioxide radical anion.

Hydroalkylation of Unactivated Olefins via Visible-Light-Driven Dual Hydrogen Atom Transfer Catalysis

Radical hydroalkylation of olefins enabled by hydrogen atom transfer (HAT) catalysis represents a straightforward means to access C(sp³)-rich molecules from abundant feedstock chemicals without the need for prefunctionalization. While Giese-type hydroalkylation of activated olefins initiated by HAT of hydridic carbon-hydrogen bonds is well-precedented, hydroalkylation of unactivated olefins in a similar fashion remains elusive, primarily owing to a lack of general methods to overcome the inherent polarity-mismatch in this scenario. Here, we report the use of visible-light-driven dual HAT catalysis to achieve this goal, where catalytic amounts of an amine-borane and an in situ generated thiol were utilized as the hydrogen atom abstractor and donor, respectively. The reaction is completely atom-economical and exhibits a broad scope. Experimental and computational studies support the proposed mechanism and suggest that hydrogen-bonding between the amine-borane and substrates is beneficial to improving the reaction efficiency.

Photochemical radical cyclization reactions with imines, hydrazones, oximes and related compounds

Photochemical reactions are a key method to generate radical intermediates. Often under these conditions no toxic reagents are necessary. During recent years, photo-redox catalytic reactions considerably push this research domain. These reaction conditions are particularly mild and safe which enables the transformation of poly-functional substrates into complex products. The synthesis of heterocyclic compounds is particularly important since they play an important role in the research of biologically active products. In this review, photochemical radical cyclization reactions of imines and related compounds such as oximes, hydrazones and chloroimines are presented. Reaction mechanisms are discussed and the structural diversity and complexity of the products are presented. Radical intermediates are mainly generated in two ways: (1) electronic excitation is achieved by light absorption of the substrates. (2) The application of photoredox catalysis is now systematically studied for these reactions. Recently, also excitation of charge transfer complexes has been studied in this context from many perspectives.

Unexpected Substituent Effects in Spiro-Compound Formation: Steering N-Aryl Propynamides and DMSO toward Site-Specific Sulfination in Quinolin-2-ones or Spiro[4,5]trienones

A highly substituent-dependent rearrangement allows for the novel and SOCl₂-induced divergent synthesis of 3-methylthioquinolin-2-ones and 3-methylthiospiro[4.5]trienones through intramolecular electrophilic cyclization of N-aryl propyamides. DMSO acts as both solvent and sulfur source, and use of DMSO-h₆/d₆ enables the incorporation of SCH₃ or SCD₃ moieties to the 3-position of the heterocyclic framework. Different para-substituents trigger divergent reaction pathways leading to the formation of quinolin-2-ones for mild substituents and spiro[4,5]trienones for both electron-withdrawing and -donating substituents, respectively. On the basis of both computational and experimental results, a new mechanism has been put forward that accounts for the exclusive spirolization/defluorination process and the surprising substituent effects.

Visible-Light-Driven C-S Bond Formation Based on Electron Donor-Acceptor Excitation and Hydrogen Atom Transfer Combined System

Developed herein is a visible-light-driven synthesis of sulfides by an electron donor-acceptor/single electron transfer and hydrogen atom transfer combined system without transition metals and strong oxidants. This reaction proceeds through the excitation of an electron donor-acceptor complex between a thiolate and an aryl halide, followed by the hydrogen atom transfer from an alkane to the generated aryl radical.

Photoactive Metal-Organic Frameworks for the Selective Synthesis of Thioethers: Coupled with Phosphine to Modulate Thiyl Radical Generation

Metal-organic framework (MOF) materials are intriguing photocatalysts to trigger radical-mediated chemical transformations. We report herein the synthesis and characterization of a series of isomorphic MOFs which show a novel structure, wide visible-light absorption, high chemical stability, and specific redox potential. The prepared MOFs were explored for the photoinduced single-electron oxidation of thiol compounds, generating reactive thiyl radicals to afford thioethers via a convenient thiol-olefin reaction. Importantly, we provide a widely applicable strategy by combing a photoactive MOF with phosphine to modulate the generation of thiyl radical in the reaction, thereby producing a single product of the thioether without the formation of a disulfide byproduct due to the dimerization of thiyl radicals. The photocatalytic reaction takes advantage of this strategy, showing great generality where tens of thiols and olefins have been examined as coupling partners. In addition, the strategy has also been demonstrated to be effective for the reactions catalyzed by other MOFs. Mechanism studies reveal that the selective synthesis of C-S products relies on a synergy between the photoinduced generation of a thiyl radical over the MOF and the in situ cleavage of S-S bond into a S-H bond by phosphine. It is notable that the synthesized MOFs show advanced performance in comparison with classical MOFs. The work not only provides a series of novel MOF photocatalysts that are capable of photoinduced thiol-olefin coupling but also indicates the great potential of MOFs for photochemical transformations mediated by reactive radicals.

Nickel-Catalyzed Reductive Thiolation of Unactivated Alkyl Bromides and Arenesulfonyl Cyanides

The cross-electrophile coupling between unactivated alkyl bromides with arenesulfonyl cyanides catalyzed by Ni(acac)₂ under reductive conditions to form unsymmetrical sulfides is developed. This approach for sulfide synthesis is practical, relies on available, unfunctionalized materials such as alkyl (pseudo)halides, and is scalable. This catalytic strategy provides a complementary method for the preparation of unsymmetrical alkyl-aryl sulfides under mild conditions with good functional group tolerance.

An aryl thiol-vinyl azide coupling reaction and a thiol-vinyl azide coupling/cyclization cascade: efficient synthesis of β-ketosulfides and arene-fused 5-methylene-2-pyrrolidinone derivatives

The addition reaction of thiol to vinyl azide has been extensively studied. Variously substituted aryl thiols are all viable for this coupling process. The scope of the other partner is successfully expanded from α-aryl vinyl azide to α-alkyl vinyl azide. A thiol-vinyl azide coupling/cyclization cascade is realized with substituted aryl vinyl azides carrying a 2-methoxycarbonyl group. The value of β-ketosulfide products was demonstrated by its application in S-heterocycle synthesis.

Nutrition and sulfur

Sulfur is unusual in that it is a mineral that may be taken into the body in both inorganic and organic combinations. It has been available within the environment throughout the development of lifeforms and as such has become integrated into virtually every aspect of biochemical function. It is essential for the nature and maintenance of structure, assists in communication within the organism, is vital as a catalytic assistant in intermediary metabolism and the mechanism of energy flow as well as being involved in internal defense against potentially damaging reactive species and invading foreign chemicals. Recent studies have suggested extended roles for sulfur-containing molecules within living systems. As such, questions have been raised as to whether or not humans are receiving sufficient sulfur within their diet. Sulfur appears to have been the "poor relation" with regards to mineral nutrition. This may be because of difficulties encountered over its multifarious functions, the many chemical guises in which it may be ingested and its complex biochemical interconversions once taken into the body. No established daily requirements have been determined, unlike many minerals, although suggestions have been proposed. Owing to its widespread distribution within dietary components its intake has almost been taken for granted. In the majority of individuals partaking of a balanced diet the supply is deemed adequate, but those opting for specialized or restrictive diets may experience occasional and low-level shortages. In these instances, the careful use of sulfur supplements may be of benefit.

Thiol- and Disulfide-Based Stimulus-Responsive Soft Materials and Self-Assembling Systems

Properties and applications of synthetic thiol- and disulfide-based materials, principally polymers, are reviewed. Emphasis is placed on soft and self-assembling materials in which interconversion of the thiol and disulfide groups initiates stimulus-responses and/or self-healing for biomedical and non-biomedical applications.

Direct Allylic C(sp3 )-H and Vinylic C(sp2 )-H Thiolation with Hydrogen Evolution by Quantum Dots and Visible Light

Direct allylic C-H thiolation is straightforward for allylic C(sp³ )-S bond formation. However, strong interactions between thiol and transition metal catalysts lead to deactivation of the catalytic cycle or oxidation of sulfur atom under oxidative condition. Thus, direct allylic C(sp³ )-H thiolation has proved difficult. Represented herein is an exceptional for direct, efficient, atom- and step-economic thiolation of allylic C(sp³ )-H and thiol S-H under visible light irradiation. Radical trapping experiments and electron paramagnetic resonance (EPR) spectroscopy identified the allylic radical and thiyl radical generated on the surface of photocatalyst quantum dots (QDs). The C-S bond formation does not require external oxidants and radical initiators, and hydrogen (H₂ ) is produced as byproduct. When vinylic C(sp² )-H was used instead of allylic C(sp³ )-H bond, the radical-radical cross-coupling of C(sp² )-H and S-H was achieved with liberation of H₂ . Such a unique transformation opens up a door toward direct C-H and S-H coupling for valuable organosulfur chemistry.

Solvent-Switched Oxidation Selectivities with O2 : Controlled Synthesis of α-Difluoro(thio)methylated Alcohols and Ketones

The solvent-switched hydroxylation and oxygenation of α-difluoro(thio)methylated carbanions with molecular oxygen under mild conditions are reported. This strategy tames the redox reactions of the in situ generated hydroperoxy difluoromethylsulfides, in which solvent-bonding can alter their reactivity and switch the oxidation selectivities. These controllable three-component reactions of gem-difluoroalkenes, thiols and molecular oxygen afford various useful α-difluoro(thio)methylated alcohols and ketones in high yields. Significantly, this protocol has been applied in the synthesis different bioactive molecules. Mechanism studies enable the detection of the hydroperoxy difluoromethylsulfide intermediates and exclude the thiol-based radical pathway.

Transition metal-free formal hydro/deuteromethylthiolation of unactivated alkenes

Methylthioether is involved in the methylthiotransfer process in organisms, and therefore its functionality is of paramount importance to living organisms. Several methods for the installation of the methylthio group in small molecules have been reported previously; however, procedures starting from unactivated alkenes are rare. Herein, we report a formal hydro/deuteromethylthiolation of alkenes by using dimethyl(methylthio)sulfonium trifluoromethanesulfonate as the stimulator and sodium borohydride/deuteride as the hydrogen/deuterium source. The process represents a mild, transition metal-free and methanethiol-free route towards the synthesis of methylthioethers from unactivated alkenes.

Insight into the Thiol-yne Kinetics via a Computational Approach

Thiol-yne reactions have drawn attention because of the click nature as well as the regular step-growth network nature of their products, despite the radical-mediated reactant. However, the factors governing the reaction pathways have not been examined using quantum chemical tools in a comprehensive manner. Thereupon, we have systematically investigated the mechanism of thiol-yne reactions, focusing on the structural influences of thiol and alkyne functionalities. The reaction kinetics, structure-reactivity relations, and E/Z diastereoselectivity of the products have been enlightened for the first cycle of the thiol-yne polymerization reaction. For this reason, a diverse set of 11 thiol-yne reactions with four thiols and eight alkynes was modeled by means of density functional theory. We performed a benchmark study and determined the M06-2X/6-31+G(d,p) level of theory as the best cost-effective methodology to model such reactions. Results reveal that spin density, the stabilities of sulfur radicals for propagation, and the stability of alkenyl intermediate radicals for the chain transfer are the determining factors of each reaction rate. Intramolecular π-π stacking interactions at transition-state structures are found to be responsible for Z diastereoselectivity.