Peroxycarbenium-mediated C-C bond formation: applications to the synthesis of hydroperoxides and peroxides

The Lewis acid-mediated reaction of alkene nucleophiles with peroxyacetals provides an effective route for the synthesis of homologated peroxides and hydroperoxides. In the presence of Lewis acids such as TiCl(4), SnCl(4), and trimethylsilyl triflate, peroxyacetals and peroxyketals undergo reaction with allyltrimethylsilane, silyl enol ethers, and silyl ketene acetals to afford homoallyl peroxides, 3-peroxyketones, and 3-peroxyalkanoates, respectively. Reactions of peroxyacetals are Lewis acid dependent; TiCl(4) promotes formation of ethers while SnCl(4) and trimethylsilyl triflate promote formation of peroxides. Lewis acid-promoted reactions of silylated hydroperoxyacetals furnish silylated hydroperoxides, which can be deprotected to homologated hydroperoxides. Hydroperoxyketals undergo Lewis acid-mediated allylation to furnish 1,2-dioxolanes via attack of hydroperoxide on the intermediate carbocation. Lewis acid-mediated cyclization of unsaturated peroxyacetals furnishes 1,2-dioxanes, 1,2-dioxepanes, and 1,2-dioxacanes through 6-endo/exo, 7-endo/endo, and 8-endo/endo pathways. The corresponding reactions involving 6-endo/endo and 5-endo/exo pathways were unsuccessful.     

Digitising chemical synthesis in automated and robotic flow

Continuous flow chemical synthesis is already known to have many attributes that give it superiority over batch processes in several respects. To expand these advantages with those from automation will only drive such enabling technologies further into the faster producing, more efficient 21^st century chemical world. In this report we present several examples of algorithmic chemical search, along with flow platforms that link hardware and digital chemical operations on software. This enables organic syntheses to be automatically carried out and optimised with as little human intervention as possible. By applying such enabling technologies to the production of small organic molecules and pharmaceutical compounds in end-to-end multistep processes, a range of reaction types can be accessed and, thus, the flexibility of these single, compact flow designs may be revealed. Automated systems can allow several reactions to take place on the same setup, enabling direct comparison of reactions under different conditions. Moreover, the production of new and known target compounds can be made faster and more efficient, the recipes of which can then be stored as digital files. Some of the automating software has employed machine-powered learning to assist the chemist in developing intelligent algorithms and artificial intelligence (AI) driven synthetic route planning. This ultimately produces a continuous flow platform that can design its own viable pathway to a particular molecule and then carry it out on its own, allowing the chemists, at the same time, to apply their expertise to other pressing challenges in their fields.

Automated flow chemistry: humans and machines working together to create faster, safer, more efficient laboratories of the future where computers help to design and control experiments, allowing researchers to focus on other scientific pursuits.     

Reactions in the rechargeable lithium-O2 battery with alkyl carbonate electrolytes

The nonaqueous rechargeable lithium-O(2) battery containing an alkyl carbonate electrolyte discharges by formation of C(3)H(6)(OCO(2)Li)(2), Li(2)CO(3), HCO(2)Li, CH(3)CO(2)Li, CO(2), and H(2)O at the cathode, due to electrolyte decomposition. Charging involves oxidation of C(3)H(6)(OCO(2)Li)(2), Li(2)CO(3), HCO(2)Li, CH(3)CO(2)Li accompanied by CO(2) and H(2)O evolution. Mechanisms are proposed for the reactions on discharge and charge. The different pathways for discharge and charge are consistent with the widely observed voltage gap in Li-O(2) cells. Oxidation of C(3)H(6)(OCO(2)Li)(2) involves terminal carbonate groups leaving behind the OC(3)H(6)O moiety that reacts to form a thick gel on the Li anode. Li(2)CO(3), HCO(2)Li, CH(3)CO(2)Li, and C(3)H(6)(OCO(2)Li)(2) accumulate in the cathode on cycling correlating with capacity fading and cell failure. The latter is compounded by continuous consumption of the electrolyte on each discharge.     

Fluorescence from the 2B1 state of NO2 excited at 4545 Å

The 4545 Å line of the Ar⁺ laser has been used to excite a ^pR₁ line of a vibrational band of the ²B₁-²A₁ system of nitrogen dioxide. Fluorescence from the upper level to the ground electronic state forms a long progression in the bending vibration. This progression can be followed from v″₂ = 0 up to v″₂ = 12, and the fluorescence intensity passes through several local minima as a function of v″₂. The main features of the intensity distribution can be reproduced using a model Hamiltonian which separates the bending and rotation from the stretching vibrations. This Hamiltonian can also be used to reproduce the fluorescence frequencies by adjusting the potential function of the lower state.     

Ion Exchange in Hydrometallurgical Processing: An Overview and Selected Applications

Ion exchange has traditionally been employed for the purification of water and the removal of metal contaminants from dilute waste streams. More recently, its use in removing trace metallic impurities from hydrometallurgical process streams (with typical background metal concentrations of 50–100 g/L) has increased substantially. It is also used as a primary recovery and concentration unit operation for certain commodities, where both technical and cost advantages become apparent for complex flow sheets. This overview discusses selected modern applications of ion exchange in hydrometallurgical processes for uranium, precious metals, copper, cobalt, nickel and zinc, and identifies some opportunities for the future.     

The Addition of Nitriles to a Molecular Digermene: Reversible Addition and Comparison to Surface Reactivity

The addition of acetonitrile, propionitrile, and acrylonitrile to tetramesityldigermene was investigated and compared to the addition of acetonitrile and acrylonitrile to germanium dimers on the Ge(100)‐2×1 surface. In each case, a 1,2,3‐azadigermetine was formed as the major product. As on the surface, the addition of nitriles to digermenes was found to be reversible, providing the first example of a reversible cycloaddition of a ditetrelene. No evidence for a six‐membered cyclic ketenimine was observed as noted in the surface chemistry, suggesting that the surface ketenimine might only form between two adjacent dimers rather than on a single dimer. The comparative chemistry provides important insights that are not possible by the independent study of each system.     

Microwave heating of ceramic composites

The microwave heating of a ceramic composite is modelled andanalysed. The composite consists of many small ceramic particlesembedded in a ceramic cement. The composite is assumed to bewell insulated, and each particle is assumed to be in imperfectthermal contact with the surrounding cement. Based on thesetwo assumptions an asymptotic theory exploiting the small Biotnumber and small non-dimensional contact conductance is developed.Our asymptotic theory yields a set of nonlinear partial differentialequations which govern the temperature in the composite. Theseare reduced to a set of coupled nonlinear ordinary differentialequations in which the surface area of each particle entersas a parameter. Recent experiments with such composites haveshown that the steady-state temperature of the composite isstrongly dependent upon the radii of the embedded particles.Our model captures this effect. In fact, our analysis showsthat the assumption of imperfect thermal contact between theparticles and the ceramic cement is essential for this trendto be established.     

Neural correlates of semantic and syntactic processes in the comprehension of case marked pronouns: Evidence from German and Dutch

Background  

It is well known that both semantic and syntactic information play a role in pronoun resolution in sentences. However, it is unclear what the relative contribution of these sources of information is for the establishment of a coreferential relationship between the pronoun and the antecedent in combination with a local structural case constraint on the pronoun (i.e. case assignment of a pronoun under preposition governing). In a prepositional phrase in German and Dutch, it is the preposition that assigns case to the pronoun. Furthermore, in these languages different overtly case-marked pronouns are used to refer to male and female persons. Thus, one can manipulate biological/syntactic gender features separately from case marking features.