C−N,C−S and S−S Bond Cleavage by Rhodium PCcarbeneP Pincer Complexes

Rhodium PC_carbeneP complexes 1‐L {L=PPh₃, PPh₂(C₆F₅)} react with isothiocyanate, carbodiimide and disulphide to enable C?S, C?N and S?S bond cleavage. The cleaved molecules are sequestered by the metal center and the pincer alkylidene linkage, forming η²‐coordinated sulfide or imide centered pincer complexes. When a C?S or S?S bond is cleaved, the resulting complexes can bridge two rhodium centers through sulphur forming dimeric complexes and eliminating a monodentate phosphine ligand.     

Electro‐Oxidative S−H/S−H Cross‐Coupling with Hydrogen Evolution: Facile Access to Unsymmetrical Disulfides

Sulfur is an essential element because it exists widely in proteins. The disulfide bond is an important moiety in many different types of significant organic molecules. A new approach for oxidant‐ and catalyst‐free S?H/S?H cross‐coupling, with hydrogen evolution, to construct unsymmetrical disulfides was developed. Under the conditions of an undivided cell at room temperature, a series of unsymmetrical disulfides were prepared with up to 87 % yield from the direct coupling of an aryl mercaptan and alkyl mercaptan. Gram‐scale synthesis also highlights the synthetic utility of this electrochemical strategy.     

Electrocatalytic Oxidant‐Free Dehydrogenative C−H/S−H Cross‐Coupling

An environmentally friendly electrocatalytic protocol has been developed for dehydrogenative C−H/S−H cross‐coupling. This method enabled C−S bond formation under catalyst‐ and oxidant‐free conditions. Under undivided electrolysis conditions, various aryl/heteroaryl thiols and electron‐rich arenes afforded the C−S bond‐formation products in 24–99 % yield. A preliminary mechanistic study indicated that the generation of aryl radical cation intermediates is key to the success of this transformation.     

Site‐Selective C−S Bond Formation at C−Br over C−OTf and C−Cl Enabled by an Air‐Stable,Easily Recoverable,and Recyclable Palladium(I) Catalyst

This report widens the repertoire of emerging Pd^I catalysis to carbon–heteroatom, that is, C?S bond formation. While Pd⁰‐catalyzed protocols may suffer from the formation of poisonous sulfide‐bound off‐cycle intermediates and lack of selectivity, the mechanistically diverse Pd^I catalysis concept circumvents these challenges and allows for C?S bond formation (S–aryl and S–alkyl) of a wide range of aryl halides. Site‐selective thiolations of C?Br sites in the presence of C?Cl and C?OTf were achieved in a general and a priori predictable fashion. Computational, spectroscopic, X‐ray, and reactivity data support dinuclear Pd^I catalysis to be operative. Contrary to air‐sensitive Pd⁰, the active Pd^I species was easily recovered in the open atmosphere and subjected to multiple rounds of recycling.     

S—H⋯S hydrogen‐bond chain in thio­salicylic acid

In crystalline thio­salicylic acid (2‐mercapto­benzoic acid), C₇H₆O₂S, the carboxyl­ic acid groups form hydrogen‐bonded dimers, whereas the S—H groups form an infinite S—H?S—H?S—H hydrogen‐bond chain, with an S?S distance of 3.986 (3) Å.     

Transition‐Metal‐Free Activation of Amides by N−C Bond Cleavage

The amide bond N?C activation represents a powerful strategy in organic synthesis to functionalize the historically inert amide linkage. This personal account highlights recent remarkable advances in transition‐metal‐free activation of amides by N?C bond cleavage, focusing on both (1) mechanistic aspects of ground‐state‐destabilization of the amide bond enabling formation of tetrahedral intermediates directly from amides with unprecedented selectivity, and (2) synthetic utility of the developed transformations. Direct nucleophilic addition to amides enables a myriad of powerful methods for the formation of C?C, C?N, C?O and C?S bonds, providing a straightforward and more synthetically useful alternative to acyl‐metals.     

RASS‐Enabled S/P−C and S−N Bond Formation for DEL Synthesis

DNA encoded libraries (DEL) have shown promise as a valuable technology for democratizing the hit discovery process. Although DEL provides relatively inexpensive access to libraries of unprecedented size, their production has been hampered by the idiosyncratic needs of the encoding DNA tag relegating DEL compatible chemistry to dilute aqueous environments. Recently reversible adsorption to solid support (RASS) has been demonstrated as a promising method to expand DEL reactivity using standard organic synthesis protocols. Here we demonstrate a suite of on‐DNA chemistries to incorporate medicinally relevant and C?S, C?P and N?S linkages into DELs, which are underrepresented in the canonical methods.     

Heteroacene Synthesis through C−S Cross‐Coupling/5‐endo‐dig Cyclization

Highly efficient, one‐step synthesis of sulfur‐containing heteroacenes was achieved through palladium‐catalyzed C?S cross‐coupling of bis‐alkynes with thioacetate as hydrogen sulfide surrogate. Heteroacenes consisting of three, five, and seven fused aromatic rings were obtained in a single catalytic step by four‐, six‐, and eightfold C?S bond formation.     

Copper‐Catalyzed Oxidative Reaction of β‐Keto Sulfones with Alcohols via C−S Bond Cleavage: Reaction Development and Mechanism Study

A Cu‐catalyzed cascade oxidative radical process of β‐keto sulfones with alcohols has been achieved by using oxygen as an oxidant. In this reaction, β‐keto sulfones were converted into sulfinate esters under the oxidative conditions via cleavage of C?S bond. Experimental and computational studies demonstrate that a new pathway is involved in this reaction, which proceeds through the formation of the key four‐coordinated Cu^II intermediate, O?O bond homolysis induced C?S bond cleavage and Cu‐catalyzed esterification to form the final products. This reaction provides a new strategy to sulfonate esters and enriches the research content of C?S bond cleavage and transformations.     

S‐Alkylation of Soft Scorpionates

The alkylation reactions of soft scorpionates are reported. The hydrotris(S‐alkyl‐methimazolyl)borate dications (alkyl=methyl, allyl, benzyl), which were prepared by the reaction of Tm^Me anion and primary alkyl halides, have been isolated and structurally characterised. The reaction is, however, not universally successful. DFT analysis of these alkylation reactions (C?S versus B? H alkylation) indicates that the observed outcome is driven by kinetic factors. Extending the study to incorporate alternative imine thiones (mercaptobenzothiazole, bz; thiazoline, tz) led to the structural characterisation of di[aquo‐μ‐aquohydrotris(mercaptobenzothiazolyl)boratosodium], which contains sodium atoms in the κ³‐S,S,S coordination mode. Alkylation of Na[Tbz] and Na[tzTtz] leads to decomposition resulting in the formation of the simple S‐alkylated heterocycles. The analysis of the species involved in these reactions shows an inherent weakness in the B? N bond in soft scorpionates, which has implications for their use in more advanced chemistry.     

Continuous‐Flow Electrosynthesis of Benzofused S‐Heterocycles by Dehydrogenative C−S Cross‐Coupling

Reported herein is the synthesis of benzofused six‐membered S‐heterocycles by intramolecular dehydrogenative C?S coupling using a modular flow electrolysis cell. The continuous‐flow electrosynthesis not only ensures efficient product formation, but also obviates the need for transition‐metal catalysts, oxidizing reagents, and supporting electrolytes. Reaction scale‐up is conveniently achieved through extended electrolysis without changing the reaction conditions and equipment.     

Visible‐Light Photoredox Catalysis: Direct Synthesis of Sulfonated Oxindoles from N‐Arylacrylamides and Arylsulfinic Acids by Means of a Cascade C−S/C−C Formation Process

A novel photocatalytic synthesis of sulfonated oxindoles from N‐arylacrylamides and arylsulfinic acids was developed by means of a cascade C?S/C?C bond‐formation process. This method provides mild, efficient, and atom‐economical access to various sulfonated oxindoles in water.     

C−C Bond‐Forming Strategy by Manganese‐Catalyzed Oxidative Ring‐Opening Cyanation and Ethynylation of Cyclobutanol Derivatives

A novel C?C bond‐forming strategy employing manganese‐catalyzed ring‐opening of cyclobutanol substrates, followed by cyanation or ethynylation, is described. A cyano C1 unit and ethynyl C2 unit are regiospecifically introduced to the γ‐position of ketones at room temperature, providing a mild yet powerful method for production of elusive aliphatic nitriles and alkynes. All transformations described are based on a common sequence: 1) oxidative ring‐opening of cyclobutanol substrates by C?C bond cleavage; 2) radical addition to triple bonds bearing an arylsulfonyl group; and 3) radical‐mediated C?S bond cleavage.     

Copper‐Catalyzed Reaction Cascade of Thiophenol Hydroxylation and S‐Arylation through Disulfide‐Directed C−H Activation

Copper‐catalyzed thiophenol C?H activation is described. Through an initial attempt to conduct C‐arylation with arylboronic acid, a rather surprising sequential C?H activation and S‐arylation was discovered. Mechanistic investigation revealed the disulfide intermediate as the key component in directing C?H oxidation. The overall reaction proceeded under mild conditions with molecular oxygen as the oxidant. Discovery of disulfide as the directing group provides a potential new direction for catalytic C?H functionalization under mild conditions.     

Copper‐Catalyzed Oxidative C−H Bond Functionalization of N‐Allylbenzamide for Regioselective C−N and C−O Bond Formation

Copper‐catalyzed oxidative couplings of N‐allylbenzamides for C?N and C?O bond formations have been developed through C?H bond functionalization. To demonstrate the utility of this approach, it was applied to the synthesis of β‐aminoimides and imides. To the best of our knowledge, these are the first examples in which different classes of N‐containing compounds have been directly prepared from the readily available N‐allylbenzamides using an inexpensive catalyst/oxidant/base (CuSO₄/TBHP/Cs₂CO₃) system.     

Natural bond orbital analysis of some S‐nitrosothiols biological molecules

Theoretical study of several S‐nitrosothiols biological molecules has been performed using quantum computational ab initio RHF and density functional B3LYP and B3PW91 methods with 6‐31G(d,p) basis set. Geometries obtained from DFT calculations were used to perform natural bond orbital (NBO) analysis. It is noted that the weakness in the S? N sigma bond is due to n_O1→σ delocalization and is responsible for the longer S? N bond length in S‐nitrosothiols. It is also noted that decreased occupancy of the localized σ_SN orbital in the idealized Lewis structure or increased occupancy of σ of the non‐Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are related with the resulting p character of the corresponding sulfur natural hybrid orbital of σ_SN bond orbital. In addition, the charge transfer energy decreases with the increasing of the Hammett constants of substituent groups, and the partial charge distribution on the skeletal atoms shows that the electrostatic repulsion or attraction between atoms can give a significant contribution to the intramolecular and intermolecular interaction. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

RhI‐Catalyzed Intramolecular Carbonylative C−H/C−I Coupling of 2‐Iodobiphenyls Using Furfural as a Carbonyl Source

Synthesis of fluoren‐9‐ones by a Rh‐catalyzed intramolecular C?H/C?I carbonylative coupling of 2‐iodobiphenyls using furfural as a carbonyl source is presented. The findings indicate that the rate‐determining step is not a C?H bond cleavage but, rather, the oxidative addition of the C?I bond to a Rh^I center.     

A Combined Experimental and Theoretical Electron Density Study of Intra‐ and Intermolecular Interactions in Thiourea S,S‐Dioxide

The thiourea S,S‐dioxide molecule is recognized as a zwitterion with a high dipole moment and an unusually long C? S bond. The molecule has a most interesting set of intermolecular interactions in the crystalline state—a relatively strong O???H? N hydrogen bond and very weak intermolecular C???S and N???O interactions. The molecule has C_s symmetry, and each oxygen atom is hydrogen‐bonded to two hydrogen atoms with O???H? N distances of 2.837 and 2.826 Å and angles of 176.61 and 158.38°. The electron density distribution is obtained both from Xray diffraction data at 110 K and from a periodic density functional theory (DFT) calculation. Bond characterization is made in terms of the analysis of topological properties. The covalent characters of the C? N, N? H, C? S, and S? O bonds are apparent, and the agreement on the topological properties between experiment and theory is adequate. The features of the Laplacian distributions, bond paths, and atomic domains are comparable. In a systematic approach, DFT calculations are performed based on a monomer, a dimer, a heptamer, and a crystal to see the effect on the electron density distribution due to the intermolecular interactions. The dipole moment of the molecule is enhanced in the solid state. The typical values of ρ_b and H_b of the hydrogen bonds and weak intermolecular C???S and N???O interactions are given. All the interactions are verified by the location of the bond critical point and its associated topological properties. The isovalue surface of Laplacian charge density and the detailed atomic graph around each atomic site reveal the shape of the valence‐shell charge concentration and provide a reasonable interpretation of the bonding of each atom.     

Enantioselective Cross‐Exchange between C−I and C−C σ Bonds

A palladium‐catalyzed enantioselective intramolecular σ‐bond cross‐exchange between C?I and C?C bonds is realized, providing chiral indanones bearing an alkyl iodide group and an all‐carbon quaternary stereocenter. Pd/TADDOL‐derived phosphoramidite is found to be an efficient catalytic system for both C?C bond cleavage and alkyl iodide reductive elimination. In addition to aryl iodides, aryl bromides can also be used for this transformation in the presence of KI. Density‐functional theory (DFT) calculation studies support the ring‐opening of cyclobutanones occuring through an oxidative addition/reductive elimination process involving Pd^IV species.     

Hydrogenation and Transfer Hydrogenation Promoted by Tethered Ru−S Complexes: From Cooperative Dihydrogen Activation to Hydride Abstraction/Proton Release from Dihydrogen Surrogates

Hydrogenation and transfer hydrogenation of imines with cyclohexa‐1,4‐dienes, as well as with a representative Hantzsch ester dihydrogen surrogate, are reported. Both processes are catalyzed by tethered Ru?S complexes but differ in the activation mode of the dihydrogen source: cooperative activation of the H?H bond at the Ru?S bond leads to the corresponding Ru?H complex and protonation of the sulfur atom, whereas the same cationic Ru?S catalyst abstracts a hydride from a donor‐substituted cyclohexa‐1,4‐diene to form the neutral Ru?H complex and a low‐energy Wheland intermediate. A sequence of proton and hydride transfers on the imine substrate then yields an amine. The reaction pathways are analyzed computationally, and the established mechanistic pictures are in agreement with the experimental observations.