Homolytic S?S Bond Dissociation of 11 Bis(thiocarbonyl)disulfides R‐C(S)?S?S?C(S)R and Prediction of A Novel Rubber Vulcanization Accelerator

The structures and energetics of eight substituted bis(thiocarbonyl)disulfides (RCS₂)₂, their associated radicals RCS₂^., and their coordination compounds with a lithium cation have been studied at the G3X(MP2) level of theory for R=H, Me, F, Cl, OMe, SMe, NMe₂, and PMe₂. The effects of substituents on the dissociation of (RCS₂)₂ to RCS₂^. were analyzed using isodesmic stabilization reactions. Electron‐donating groups with an unshared pair of electrons have a pronounced stabilization effect on both (RCS₂)₂ and RCS₂^.. The S? S bond dissociation enthalpy of tetramethylthiuram disulfide (TMTD, R=NMe₂) is the lowest in the above series (155 kJ mol^?1), attributed to the particular stability of the formed Me₂NCS₂^. radical. Both (RCS₂)₂ and the fragmented radicals RCS₂^. form stable chelate complexes with a Li⁺ cation. The S? S homolytic bond cleavage in (RCS₂)₂ is facilitated by the reaction [Li(RCS₂)₂]⁺+Li⁺→2 [Li(RCS₂)]^.+. Three other substituted bis(thiocarbonyl) disulfides with the unconventional substituents R=OSF₅, Gu¹, and Gu² have been explored to find suitable alternative rubber vulcanization accelerators. Bis(thiocarbonyl)disulfide with a guanidine‐type substituent, (Gu¹CS₂)₂, is predicted to be an effective accelerator in sulfur vulcanization of rubber. Compared to TMTD, (Gu¹CS₂)₂ is calculated to have a lower bond dissociation enthalpy and smaller associated barrier for the S? S homolysis.     

Revealing Unexpected Mechanisms for Nucleophilic Attack on S?S and Se?Se Bridges

The reactivity of disulfide and diselenide derivatives towards F^? and CN^? nucleophiles has been investigated by means of B3PW91/6‐311+G(2df,p) calculations. This theoretical survey shows that these processes, in contrast with the generally accepted view of disulfide and diselenide linkages, do not always lead to S? S or Se? Se bond cleavage. In fact, S? S or Se? Se bond fission is the most favorable process only when the substituents attached to the S or the Se atoms are not very electronegative. Highly electronegative substituents (X) strongly favor S? X bond fission. This significant difference in the observed reactivity patterns is directly related to the change in the nature of the LUMO orbital of the disulfide or diselenide derivative as the electronegativity of the substituents increases. For weakly electronegative substituents, the LUMO is a σ‐type S? S (or Se? Se) antibonding orbital, but as the electronegativity of the substituents increases the π‐type S? X antibonding orbital stabilizes and becomes the LUMO. The observed reactivity also changes with the nature of the nucleophile and with the S or Se atom that undergoes the nucleophilic attack in asymmetric disulfides and diselenides. The activation strain model provides interesting insights into these processes. There are significant similarities between the reactivity of disulfides and diselenides, although some dissimilarities are also observed, usually related to the different interaction energies between the fragments produced in the fragmentation process.     

Magnetic Anisotropy of the C?C Single Bond

Anisotropic effects are broadly used in NMR spectroscopy for structure elucidation. With the development of computational methods it has become possible to quantify the effects and obtain further insight into their origin. Some classical interpretations have been questioned. Herein, we show that the classical “anisotropy cone” representing the anisotropic effect of the C? C single bond should be revised: deshielding at its side and shielding along its end are observed. Consequently, methyl, methylene, and methyne hydrogen atoms are not deshielded by C? C bonds as is conventionally explained in NMR spectroscopy textbooks. They are just less shielded than by the C? H bonds attached at the same carbon. In addition, this anisotropic effect is dependent on the environment and care should be taken when drawing conclusions based on it. For example, it differs for the staggered and eclipsed conformations of ethane in HCCH planes, as well as for cyclohexane. In fact, it is not the anisotropy of the C2? C3/C5? C6 bonds that determines the chemical shift difference of axial and equatorial protons of a rigid cyclohexane ring, but magnetic contributions from all bonds.     

Interplay of Magnetic Exchange Interactions and Ni?S?Ni Bond Angles in Polynuclear Nickel(II) Complexes

The ability of bridging thiophenolate groups (RS^?) to transmit magnetic exchange interactions between paramagnetic Ni^II ions is examined. Specific attention is paid to complexes with large Ni? SR? Ni angles. For this purpose, dinuclear [Ni₂L¹(μ‐OAc)?I₂][I₅] ( 2 ) and trinuclear [Ni₃L²(OAc)₂][BPh₄]₂ ( 3 ), where H₂L¹ and H₂L² represent 24‐membered macrocyclic amino‐thiophenol ligands, are prepared and fully characterized by IR‐ and UV/Vis spectroscopy, X‐ray crystallography, static magnetization M measurements and high‐field electron spin resonance (HF‐ESR). The dinuclear complex 2 has a central N₃Ni₂(μ‐S)₂(μ‐OAc)Ni₂N₃ core with a mean Ni? S? Ni angle of 92°. The macrocycle L² supports a trinuclear complex 3 , with distorted octahedral N₂O₂S₂ and N₂O₃S coordination environments for one central and two terminal Ni^II ions, respectively. The Ni? S? Ni angles are at 132.8° and 133.5°. We find that the variation of the bond angles has a very strong impact on the magnetic properties of the Ni complexes. In the case of the Ni₂‐complex, temperature T and magnetic field B dependencies of M reveal a ferromagnetic coupling J=?29 cm^?1 between two Ni^II ions (H=JS₁S₂). HF‐ESR measurements yield a negative axial magnetic anisotropy (D<0) which implies a bistable (easy axis) magnetic ground state. In contrast, for the Ni₃‐complex we find an appreciable antiferromagnetic coupling J′=97 cm^?1 between the Ni^II ions and a positive axial magnetic anisotropy (D>0) which implies an easy plane situation.     

Mechanistic Studies of C?C Bond Cleavage of Nitriles by Dinuclear Metal Cryptates

We previously reported that dinuclear copper(II) cryptate [Cu₂L]⁴⁺ cleaves the C? C bond of acetonitrile at room temperature to produce a cyano‐bridged dinuclear cryptate and methanol, whereby the reaction mechanism has not yet become clear. We have now systemically investigated this reaction, and four cryptates, [Cu₂L](ClO₄)₄ ( 1 ), [Zn₂L](ClO₄)₄ ( 2 ), [Cu₂L(H₂O)₂](CF₃SO₃)₄ ( 5 ), and [Cu₂L(OH)(OH₂)](ClO₄)₃ ( 6 ) are reported here. Cryptates 1 and 2 can cleave the C? C bonds of acetonitrile, propionitrile, and benzonitrile at room temperature under open atmospheric conditions to give cyano‐bridged cryptates [Cu₂L(CN)](ClO₄)₃ ( 3 ) and [Zn₂L(CN)](ClO₄)₃ ( 4 ), respectively, and the corresponding alcohol. In contrast, 5 and 6 do not show any C? C bond activation of nitriles, as the interior axial positions of Cu^II in 5 and 6 are occupied by water/OH^?. The C? C bond cleavage of (S)‐(+)‐2‐methylbutyronitrile by 2 produced (R)‐(?)‐2‐butanol only; that is, the cleavage reaction proceeds through an S_N2 pathway (Walden inversion).     

Cyclic S?N compounds and phosphorus reagents: Part X. Synthesis and characterization of phosphinimino-substituted S?N heterocycles

Room temperature reactions of S₄N₄ with (amino) diphenylphosphines, (R)Ph₂P, have basically yielded two different types of S N heterocycles under two different stoichiometric conditions. Phosphiniminocyclotrithiatriazenes, (R)Ph₂PN S₃N₃ (R = C₄H₈N , C₅H₁₀N , C₆H₁₂N , CH₃NC₄H₈N , (C₆H₁₁)₂ N , and (C₆H₅CH₂)₂N ) have been obtained (yield 45–76%) from a 1:2 mole ratio (S₄N₄:(R)Ph₂P) reaction, while the disubstituted S₄N₄ derivatives, 1,5-[Ph₂(R)PN]₂S₄N₄ (R = C₄N₈N , C₅H₁₀N , and C₆H₁₂N ) have been obtained (yield 30–45%) only from a 1:3.5–4 mole ratio reaction. All the 1,5-[Ph₂(R)PN]₂S₄N₄ derivatives prepared in this study undergo a room temperature solution phase transformation to the corresponding (R)Ph₂PN S₃N₃ heterocycles.     

Studies on the Synthesis and Characterization of Pd?TiO2?SiO2 Nanocomposite for Electroanalytical Applications

A nanocomposite of Pd? TiO₂? SiO₂ is developed through a sol‐gel process from the reaction products of titanium isopropoxide followed by mixing the same with palladium linked 3‐glycidoxypropyltrimethoxysilane. The reaction product is sonicated and calcinated to obtain the nanocomposite of Pd? TiO₂? SiO₂. The calcination at 600 °C yielded an amorphous structure whereas at 900 °C it resulted into a nanocrystalline structure. The nanocomposite of palladium was further characterized by TEM, XRD, IR and EDS. The material acts as an efficient electrocatalyst. Electrocatalysis of ascorbic acid is observed at 0.1 V vs. Ag/AgCl, shows linearity between 1 µM and 1 mM in 0.1 M phosphate buffer (pH 7.0).     

Density Functional Studies of the C?F Bond Activation of CF3 Radical by Bare Co+

The C? F bond activation mechanism of CF₃ radical by bare Co⁺ has been studied by density functional theory. Three local minima and two first‐order saddle points were located for the potential energy surface (PES) of [Co, C, F₃]⁺. The activation barrier involving C? F bond activation was calculated to be only 14.73 kJ/mol, while the largest barrier of 149.29 kJ/mol on the FES involves Co? C bond rupture. The bonding mechanism between Co⁺, C and F atoms were discussed based on Mulliken population. The relevant bond dissociation energy and thermochemistry data were calculated with the limited experimental values, and the results are in good agreement with the experimental findings.     

Iridium Catalysis for C?H Bond Arylation of Heteroarenes with Iodoarenes

Efficient couplings using equimolar quantities of each coupling partner and multiple C? H bond arylation reactions are achieved with an Ir‐based catalytic system for the C? H bond arylation of electron‐rich heteroarenes with iodoarenes to construct extended π‐systems. The dramatic ligand effect on reaction efficiency leads to the discovery that Crabtree's catalyst (see scheme) is the optimal catalyst precursor.

     

Cuprous Oxide Catalyzed Oxidative C?C Bond Cleavage for C?N Bond Formation: Synthesis of Cyclic Imides from Ketones and Amines

Selective oxidative cleavage of a C C bond offers a straightforward method to functionalize organic skeletons. Reported herein is the oxidative C C bond cleavage of ketone for C N bond formation over a cuprous oxide catalyst with molecular oxygen as the oxidant. A wide range of ketones and amines are converted into cyclic imides with moderate to excellent yields. In‐depth studies show that both α‐C H and β‐C H bonds adjacent to the carbonyl groups are indispensable for the C C bond cleavage. DFT calculations indicate the reaction is initiated with the oxidation of the α‐C H bond. Amines lower the activation energy of the C C bond cleavage, and thus promote the reaction. New insight into the C C bond cleavage mechanism is presented.     

Intramolecular Aminocyanation of Alkenes by N?CN Bond Cleavage

A metal‐free, Lewis acid promoted intramolecular aminocyanation of alkenes was developed. B(C₆F₅)₃ activates N‐sulfonyl cyanamides, thus leading to a formal cleavage of the N CN bonds in conjunction with vicinal addition of sulfonamide and nitrile groups across an alkene. This method enables atom‐economical access to indolines and tetrahydroquinolines in excellent yields, and provides a complementary strategy for regioselective alkene difunctionalizations with sulfonamide and nitrile groups. Labeling experiments with ¹³C suggest a fully intramolecular cyclization pattern due to the lack of label scrambling in double crossover experiments. Catalysis with Lewis acid is realized and the reaction can be conducted under air.     

Copper‐Catalyzed Aerobic Oxidative C?C Bond Cleavage for C?N Bond Formation: From Ketones to Amides

A novel copper‐catalyzed aerobic oxidative C(CO) C(alkyl) bond cleavage reaction of aryl alkyl ketones for C N bond formation is described. A series of acetophenone derivatives as well as more challenging aryl ketones with long‐chain alkyl substituents could be selectively cleaved and converted into the corresponding amides, which are frequently found in biologically active compounds and pharmaceuticals.     

B?H,C?H,and B?C Bond Activation: The Role of Two Adjacent Agostic Interactions

Tuning the nature of the linker in a L∼BHR phosphinoborane compound led to the isolation of a ruthenium complex stabilized by two adjacent, δ‐C H and ε‐B_sp2 H, agostic interactions. Such a unique coordination mode stabilizes a 14‐electron “RuH₂P₂” fragment through connected σ‐bonds of different polarity, and affords selective B H, C H, and B C bond activation as illustrated by reactivity studies with H₂ and boranes.     

Amide‐Directed Tandem C?C/C?N Bond Formation through C?H Activation

The transformation of C? H bonds into other chemical bonds is of great significance in synthetic chemistry. C? H bond‐activation processes provide a straightforward and atom‐economic strategy for the construction of complex structures; as such, they have attracted widespread interest over the past decade. As a prevalent directing group in the field of C? H activation, the amide group not only offers excellent regiodirecting ability, but is also a potential C? N bond precursor. As a consequence, a variety of nitrogen‐containing heterocycles have been obtained by using these reactions. This Focus Review addresses the recent research into the amide‐directed tandem C? C/C? N bond‐formation process through C? H activation. The large body of research in this field over the past three years has established it as one of the most‐important topics in organic chemistry.     

Asymmetric Intramolecular Carbocyanation of Alkenes by C?C Bond Activation

Versatility : Ni⁰ and Pd⁰ complexes act as catalysts in the intramolecular aryl‐ and acylcyanation reactions, respectively, of alkenes (see scheme). These reactions not only proceed with high yield and selectivity, they also tolerate a wide range of functional groups and can furnish valuable heterocycles such as oxindoles, which are precursors for a myriad of natural and/or biologically active products.

     

Demonstration of the Heterolytic O?O Bond Cleavage of Putative Nonheme Iron(II)?OOH(R) Complexes for Fenton and Enzymatic Reactions

One‐electron reduction of mononuclear nonheme iron(III) hydroperoxo (Fe^III OOH) and iron(III) alkylperoxo (Fe^III OOR) complexes by ferrocene (Fc) derivatives resulted in the formation of the corresponding iron(IV) oxo complexes. The conversion rates were dependent on the concentration and oxidation potentials of the electron donors, thus indicating that the reduction of the iron(III) (hydro/alkyl)peroxo complexes to their one‐electron reduced iron(II) (hydro/alkyl)peroxo species is the rate‐determining step, followed by the heterolytic O O bond cleavage of the putative iron(II) (hydro/alkyl)peroxo species to give the iron(IV) oxo complexes. Product analysis supported the heterolytic O O bond‐cleavage mechanism. The present results provide the first example showing the one‐electron reduction of iron(III) (hydro/alkyl)peroxo complexes and the heterolytic O O bond cleavage of iron(II) (hydro/alkyl)peroxo species to form iron(IV) oxo intermediates which occur in nonheme iron enzymatic and Fenton reactions.