Ultrasound effects on the photopinacolization of benzophenone

Ultrasonic irradiation is able to modify the course of several photochemical reactions, especially bimolecular, proceeding via triplet states. These effects were illustrated in the study of benzophenone photopinacolization in ethanol. The rates and yields increase when sonication is applied simultaneously to UV irradiation. An explanation is based on a 2-fold effect: (i) light-absorbing transient species undergo sonolytic decomposition, making the photoconversion more efficient, and (ii) sonication induces the triplet state quenching, as shown by Stern-Volmer plots from experiments run in the presence of naphthalene, probably due to the easier collisional deactivation processes favored by the homogeneous distribution of the activated species.     

Repairing the Thiol‐Ene Coupling Reaction

Thiol‐ene coupling (TEC) reactions emerged as one of the most useful processes for coupling different molecular units under reaction mild conditions. However, TEC reactions involving weak C? H bonds (allylic and benzylic fragments) are difficult to run and often low yielding. Mechanistic studies demonstrate that hydrogen‐atom transfer processes at allylic and benzylic positions are responsible for the lack of efficiency of the radical‐chain process. These competing reactions cannot be prevented, but reported herein is a method to repair the chain process by running the reaction in the presence of triethylborane and catechol. Under these reaction conditions, a unique repair mechanism leads to an efficient chain reaction, which is demonstrated with a broad range of anomeric O‐allyl sugar derivatives including mono‐, di‐, and tetrasaccharides bearing various functionalities and protecting groups.     

Iridium-catalyzed silylation of unactivated C–H bonds

The functionalization of primary C–H bonds has been a longstanding challenge in catalysis. Our group has developed a series of silylations of primary C–H bonds that occur with site selectivity and diastereoselectivity resulting from an approach to run the reactions as intramolecular processes. These reactions have become practical by using an alcohol or amine as a docking site for a hydrosilyl group, thereby leading to intramolecular silylations of C–H bonds at positions dictated by the presence common functional groups in the reactants. Oxidation of the C–Si bond leads to the introduction of alcohol functionality at the position of the primary C–H bond of the reactant. The development, scope, and applications of these functionalization reactions is described in this minireview.     

Radiolyis of polytetrafluoroethylene and polystyrene catalytic supports in presence of tritiated water

The behavior of hydrophobic catalyst supports in the isotopic exchange reactions H/D/T was analyzed by the determination of radiolysis processes in the presence of HTO. In this study, the behavior of polytetrafluoroethylene and styrenedivynilbenzene copolymer catalyst supports in the presence of HTO was analyzed. The primary and secondary radiolytic effects were evaluated using original computational methods. The radiolytical processes were analyzed by exposure of the catalytic supports immersed in water to gamma radiation, and also by analysis of chemical modification by FTIR spectroscopy, fluoride formation and sol/gel fraction determination.     

搅拌对Belousov-Zhabotinskii振荡反应的影响及反应机理研究

研究了非流动条件下搅拌对B-Z振荡反应的影响。实验发现即使在氮气保护下, 搅拌仍对振荡行为(振幅和周期等)有明显影响。为解释搅拌的影响作用, 对B-Z反应的Oregonator模型作了修正和补充并依据非平衡态热力学的一般原理将搅拌影响唯象地归结为反应速率常数的相对变化。修正后的模型不仅较好地克服了原模型存在的一些问题, 而且在引入搅拌作用以后得到了与实验现象定性一致的数值模拟结果。     

Simulation of organic reactions: from the degradation of chemicals to combinatorial synthesis

Organic reactions can be run under a variety of conditions, from laboratory experiments, through technical processes, to combinatorial chemistry. The scope is further extended when the metabolism of compounds and the reactions in the mass spectrometer are included. We present here several concepts: reactors, phases, and modes, which, together with a kinetic modeling, allow the treatment of such a broad scope of organic reactions. These concepts have been implemented in a knowledge-based system, EROS. Several applications of this system to the wide world of organic reactions are given.     

Theoretical verification of nonthermal microwave effects on intramolecular reactions

There have been a growing number of articles that report dramatic improvements in the experimental performance of chemical reactions by microwave irradiation compared to that under conventional heating conditions. We theoretically examined whether nonthermal microwave effects on intramolecular reactions exist or not, in particular, on Newman-Kwart rearrangements and intramolecular Diels-Alder reactions. The reaction rates of the former calculated by the transition state theory, which consider only the thermal effects of microwaves, agree quantitatively with experimental data, and thus, the increases in reaction rates can be ascribed to dielectric heating of the solvent by microwaves. In contrast, for the latter, the temperature dependence of reaction rates can be explained qualitatively by thermal effects but the possibility of nonthermal effects still remains regardless of whether competitive processes are present or not. The effective intramolecular potential energy surface in the presence of a microwave field suggests that nonthermal effects arising from potential distortion are vanishingly small in intramolecular reactions. It is useful in the elucidation of the reaction mechanisms of microwave synthesis to apply the present theoretical approach with reference to the experiments where thermal and nonthermal effects are separated by screening microwave fields.     

Pd(0)-Catalyzed PMHS reductions of aromatic acid chlorides to aldehydes

[reaction: see text] Contrary to previous reports, polymethylhydrosiloxane (PMHS) under Pd(0) catalysis can efficiently reduce aryl acid chlorides to their corresponding aldehydes without requiring an additional reductant, provided the reactions are run in the presence of fluoride.     

Searching for New Reactivity (Nobel Lecture)

The processes for the selective oxidation of olefins have long been among the most useful tools for day‐to‐day organic synthesis. Herein, the focus is on the asymmetric‐epoxidation (AE) and asymmetric‐dihydroxylation (AD) reactions developed by Sharpless and co‐workers. The reactions have a wide scope, are simple to run, and involve readily available starting materials. Ligand‐accelerated catalysis is crucial to these reactions and might be the agent for uncovering more catalytic processes. In addition to the selectivity benefits of catalysis, the phenomenon of turnover (amplification) raises its potential impact. The author and his co‐workers developed small, highly enantioselective catalysts that were unfettered by the “lock‐and‐key” selectivity of Natures enzymes, and tolerant of substrates throughout the entire range of olefin substitution patterns.     

Chemical polarisation of 15N and 13C nuclei in diazo coupling reactions

Chemical polarisation of nitrogen and carbon nuclei in the decomposition and diazo coupling reactions of benzenediazonium fluoborate has been investigated. If homo- and heterolytic processes run in parallel in a multistage reaction sequence, then the nuclear polarisation created in a free-radical reaction can be nearly quantitatively carried over to products of rapid ionic reactions such as azo dyes.     

Nickel-on-charcoal-catalyzed aromatic aminations and Kumada couplings: mechanistic and synthetic aspects

Protocols for aromatic aminations and Kumada couplings catalyzed by 'heterogeneous' nickel-on-charcoal (Ni/C) have been revised, making them simpler and more time efficient. For both types of reactions, reduction of the catalyst precursor Ni(II)/C using n-BuLi prior to addition of a substrate can be avoided. Instead, in amination reactions, the amine in combination with lithium tert-butoxide was found to convert Ni(II)/C to active Ni(0). For Kumada couplings, direct reduction of Ni(II)/C by the Grignard reagent is easily achieved. Reactions run in the presence of triarylphosphine ligands of varying substitution patterns and with differing electronic properties provided insight into the mechanism of these nickel-catalyzed transformations. Ligandless Kumada couplings were facile with aryl Grignards, which may be a consequence of pi-complexation of nickel by the aryl group in the reagent. Larger scale reactions of both types of couplings have been successfully performed, suggesting that Ni/C-based processes can be scaled-up as needed.     

Regioselectivity in arene-catalyzed reductive lithiation of acetals of chlorobenzaldehydes

The regioselectivity of arene-catalyzed reductive lithiation of acetals of chlorobenzaldehydes strongly depends on the form of lithium metal employed as a reducing agent. According to previous findings, naphthalene catalyzed reductions run in the presence of lithium powder (high Na content) led to competitive metalations of both aromatic carbon-chlorine and benzylic carbon-oxygen bonds. At variance with these results, naphthalene catalyzed reductions run in the presence of lithium wire (either high or low Na content) led to highly regioselective metalation of aromatic carbon-chlorine bonds. These results disclose new possibilities of selective applications of arene-catalyzed reductive lithiation reactions.     

Pro- and antioxidant characteristics of natural thiols

In aqueous solutions at physiological temperature, the mechanism of antioxidative action of natural thiols (glutathione, cysteine, and homocysteine) mainly involves reactions with reactive oxygen species (ROS), peroxyl radicals and hydrogen peroxide. Reduction of hydrogen peroxide by thiols is accompanied by radical generation. The kinetic characteristics of these processes, including those for the reactions of hydrogen peroxide and glutathione immobilized on solid supports such as sodium montmorillonite (clay) and cellulose were determined. Prooxidative effects of thiols are related with the reactions of thiyl radicals formed in the exchange reactions of thiols with other radicals and in the reactions between thiols and hydroperoxides. Thiyl radicals are known to react easily with double bonds. Resveratrol and caffeic acid, phenolic antioxidants containing double bond in their molecules, were shown to be consumed when reacted with glutathione and the process accelerated in the presence of hydrogen peroxide.     

Iridium‐Catalyzed Allylic Substitutions with Cyclometalated Phosphoramidite Complexes Bearing a Dibenzocyclooctatetraene Ligand: Preparation of (π‐Allyl)Ir Complexes and Computational and NMR Spectroscopic Studies

(π‐Allyl)Ir complexes derived from dibenzocyclooctatetraene and phosphoramidites by cyclometalation are effective catalysts for allylic substitution reactions of linear monosubstituted allylic carbonates. These catalysts provide exceptionally high degrees of regioselectivity and allow the reactions to be run under aerobic conditions. A series of (π‐allyl)Ir complexes were prepared and characterized by X‐ray crystal structure analyses. An allylic amination with aniline displayed different resting states depending on the presence of a strong base. DFT calculations were carried out on the mechanistic aspects of these reactions. The results suggest that for the (π‐allyl)Ir complexes, the formation and reactions with nucleophiles proceed with comparable rates.     

KINETICS OF CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER REACTIONS IN PHOspHOLIPID BILAYER VESICLES

Abstract The effects of electrostatic surface charge and valinomycin addition in the presence of K* on the kinetics and the inside-outside asymmetry properties of light-induced electron transfer reactions between chlorophyll triplet state and benzoquinone, ferricyanide and methyl viologen in large unilamellar vesicles have been investigated using laser flash photolysis. Modifying the surface charge of the bilayers by incorporating charged surfactants or decreasing the ionic strength of the suspending medium caused large changes in the dynamics of the electron transfer reactions, which could be interpreted in terms of electrostatic interactions between reactants, products and membrane components, and the existence of a spontaneous transmembrane electrical potential corresponding to an excess of negative charge at the outer surface of the vesicle bilayer. The presence of valinomycin had more specific effects on these reactions, which were consistent with an electrostatic influence of the presence of the positively-charged K⁺-valinomycin complex within the bilayer on the dynamics of only those triplet quenching and radical formation and decay processes which occur in this region of the vesicle structure.     

On the origin of frequency dispersion at the interface between mercury electrode and aqueous solutions of alkali halides

The origin of frequency dispersion of electrochemical impedance is investigated at the interface of mercury and aqueous solutions of single alkali halides. It is found that in the presence of each one of KI, CsI, CsF and CsBr salts, the interface presents certain potential regions where frequency dispersion effects are detected and others where the ideal capacitor behavior is closely approximated. Frequency dispersion effects are contributed by interfacial processes such as anion and cation adsorption, mercury halide film formation and dissolution and charge transfer reactions. The discrimination between frequency dispersion due to charge transfer processes occurring at the Hg/solution interface and that due to reactant adsorption itself is generally difficult and depends on the reaction mechanism, provided that a discrete adsorption step is anticipated.     

Acceleration of the diels-alder reaction by clays suspended in organic solvents

Rapid and stereoselective Diels-Alder reactions can be run in ethanol or methylene chloride in the presence of Fe^III- doped K10 montmorillonite.     

Improved synthesis of functional CTVs and cryptophanes using Sc(OTf)3 as catalyst

Functional cyclotriveratrylene (CTV) and cryptophane derivatives are synthesized in the presence of scandium triflate [Sc(OTf)3]. This route allows the preparation of new derivatives that could not be prepared or easily obtained by using the previously reported experimental procedures. With a catalytic amount of scandium triflate (1% mol), CTVs were obtained with yields similar to or higher than those reported previously in reactions run under strong acidic conditions. Cryptophanes were also synthesized in fairly good yields by performing the ring-closure step in the presence of a stoichiometric amount of Sc(OTf)3. Interestingly, this novel approach strongly reduces the formation of side products and gives rise to novel functionalized molecules for the construction of supramolecular host-guest systems.     

Aspects of mechanism and structure in the formation of muonated radicals

Radiolysis effects associated with the implantation of positive muons in liquid samples are reviewed. The major processes to be considered resemble the normal reactions induced by ionising radiation; these are electron ejection and the subsequent chemical reaction of electrons and holes. The thermal or near-thermal reactions are described in terms of a general scheme which includes the various possible routes to the formation of muonium and of muonated organic radicals. The scheme emphasizes the consequences of the extreme dilution of the muon probe, and its outstanding reactivity towards solvents or solutes of even weak basicity, it its role as lightweight proton. The mechanism of radical formation in materials with widely different properties may be predicted from the general scheme. For the few compounds in which the mechanism has been elucidated experimentally this is compared with chemical expectations. Characteristics of the compounds and of their cosolvents which favour or inhibit the different possibilities are discussed in terms of the interplay between reaction mechanism and molecular structure. The balance between competitive processes may be altered by the presence of inert solvent. In protic solvents the balance may also be altered by proton exchange, and indirectly by a modification of reactivities due to hydrogen bonding. Mechanisms involving intermediate muonic cation or radical anion states are established in particular cases. These ionic states may also be the origin of the diamagnetic fraction in certain circumstances.     

The effect of transition state bond order on sigmatropic shifts and cycloaddition reactions

A recently developed classical model for the calculation of activation free energies of chemical reactions is applied to the study of sigmatropic shifts and cycloaddition reactions. The model does not require the consideratlon of parallel or perpendicular effects along the reaction coordinate to interpret substituent effects. These reactions can be treated as any other ordinary chemical reation, and the effect of substituents is explained in terms of an increase in the bond order of the transition state, n³, due to an increase in the electronic charge produced by the substituents on the carbon skeleton. Secondary isotope effects are also explained in terms of n³. The model estimates absolute ΔG³ for the main reactions in good agreement with experiment, under the assumption that the processes are concerted and synchronous and consequently n³ = O.5. The nonlinear character of the free energy relationships for these reactions is attributed to the variation of n³ upon substitution. Some of the apparent violations of the Woodward-Hoffmann rules on the conservation of orbital symmetry in concerted reactions can be treated in a similar way. The synchronism of these reactions is also discussed.