Brønsted acid-mediated radical processes in organic synthesis

This highlight focuses on recent applications of Brønsted acid-mediated radical processes across a range of reaction manifolds, including transition metal-based catalysis, photochemistry, and polymer science. A brief overview of significant research developments in the area and a discussion of the key mechanistic features of notable transformations are provided.

Rhodium(II) azavinyl carbenes and their recent application to organic synthesis

This highlight solely focusses on the synthetic applications of azavinyl rhodium(II) carbenes derived from N-sulfonyl triazoles. An overview of their use in intermolecular reactions to form a variety of heterocycles is examined, in addition to intramolecular annulations and rearrangements.

RGO/ZnO nanocomposite: an efficient, sustainable, heterogeneous, amphiphilic catalyst for synthesis of 3-substituted indoles in water

A nanocomposite consisting of reduced graphene oxide and zinc oxide nanoparticles (RGO/ZnO) with unique structural features was developed as an efficient, sustainable, amphiphilic, heterogeneous catalyst for the synthesis of various 3-substituted indoles in water. The catalyst was recycled six times without significant loss in catalytic activity. The higher environmental compatibility and sustainability factors such as smaller E-factor and higher atom economy make the present methodology a true green and sustainable process for the synthesis of various biologically important 3-substituted indoles.

Control of excitation and quenching in multi-colour electrogenerated chemiluminescence systems through choice of co-reactant

We demonstrate a new approach to manipulate the selective emission in mixed electrogenerated chemiluminescence (ECL) systems, where subtle changes in co-reactant properties are exploited to control the relative electron-transfer processes of excitation and quenching. Two closely related tertiary-amine co-reactants, tri-n-propylamine and N,N-diisopropylethylamine, generate remarkably different emission profiles: one provides distinct green and red ECL from [Ir(ppy)3] (ppy=2-phenylpyridinato-C2,N) and a [Ru(bpy)3](2+) (bpy=2,2'-bipyridine) derivative at different applied potentials, whereas the other generates both emissions simultaneously across a wide potential range. These phenomena can be rationalized through the relative exergonicities of electron-transfer quenching of the excited states, in conjunction with the change in concentration of the quenchers over the applied potential range.

Hydrotelluration of acetylenic esters: structural characterization of stereoisomers of methyl/ethyl beta-(aryltelluro)acrylates

Synthesis and complete characterization of some ester functionalized vinylic tellurides bearing an aryl ligand with varying steric and electronic effects bound to tellurium is described. Hydrotelluration of methyl propiolate using Ar2Te2/NaBH4 in methanol results in a mixture of stereoisomers of methyl β-(aryltelluro)acrylates, ArTeCH[double bond, length as m-dash]CHCOOMe (Ar = 4-MeOC6H4, 1A; 1-C10H7, 2A; 2,4,6-Me3C6H2, 3A; C5H5FeC5H4, 4A; 4-Me2NC6H4, 5A; and 2-C4H3S, 6A). The same reaction in ethanol provides isomeric mixtures of the ethyl esters ArTeCH[double bond, length as m-dash]CHCOOEt (1B–6B). However, in the reactions between methyl propiolate and Ar2Te2 (Ar = 2,4,6-Me3C6H2, 4-Me2NC6H4) in isopropanol or t-butanol, no exchange of alkyl groups between the parent ester and the solvent is observed, instead detelluration of the Ar2Te2 to Ar2Te is a competing reaction along with almost exclusive formation of the (Z)-isomers (3Aa, 5Aa). The geometry of the separated stereoisomers is established in solution, with the help of 1H, 13C and 125Te NMR spectrometry. Of particular interest is the observation that 125Te chemical shifts {deshielded in (Z) compared to (E); Δδ = 106–136 ppm} and the geminal heteronuclear coupling constants {2J(1H–125Te) values for (E) are more than seven times that of the corresponding (Z) isomer} can be used to distinguish between liquid isomers. Structural characterization in the solid state by single-crystal X-ray diffraction for the 2Ba, 3Aa, 3Ba, 5Aa, 8 (Z)-isomers as well as for both stereoisomers of 4-Me2NC6H4TeCH[double bond, length as m-dash]CHCOOEt (5Ba and 5Bb) is also presented. The carbonyl O atom of the ester group is invariably involved, at least in the solid state, in a secondary bonding interaction with the Te(II) atom. While an intermolecular Te⋯O interaction gives rise to one-dimensional supramolecular arrays in the crystal lattice of 5Bb with (E) configuration, it is realized intramolecularly in the case of the (Z)-isomers due to the cis position of the chalcogen atoms.

Enhancing thermal stability and mechanical properties of lyotropic liquid crystals through incorporation of a polymerizable surfactant.

We present a facile method to prepare thermally stable and mechanically robust crosslinked lyotropic liquid crystals (LLCs) through incorporation of a polymerizable amphiphile into a binary LLC system comprising commercially available surfactant Brij 97 and water. Thermal stability and mechanical properties of the polymerized LLCs were significantly enhanced after polymerization of the incorporated polymerizable surfactant. The effect of incorporating a polymerizable amphiphile on the phase behavior of the LLC system was studied in detail. In situ photo-rheology was used to monitor the change in the mechanical properties of the LLCs, namely the storage modulus, loss modulus, and viscosity, upon polymerization. The retention of the LLC nanostructures was evaluated by small angle X-ray scattering (SAXS). The ability to control the thermal stability and mechanical strength of LLCs simply by adding a polymerizable amphiphile, without tedious organic synthesis or harsh polymerization conditions, could prove highly advantageous in the preparation of robust nanomaterials with well-defined periodic structures.

Synthesis and fractionated crystallization of organic-inorganic hybrid composite materials from an amphiphilic polyethylene-block-poly (ethylene oxide) diblock copolymer

Mesostructurally ordered inorganic–organic hybrid composite materials were successfully synthesized by utilizing a low-molecular-weight amphiphilic polyethylene-block-poly(ethylene oxide) (PE–PEO) diblock copolymer as the directing agent. The hybrid composites were formed via the sol–gel reaction of inorganic precursor tetraethoxysilane (TEOS) in an acidic ethanol/water solution with various amounts of PE–PEO. In these composite materials, the hydrophobic PE block of the PE–PEO copolymer forms separate microphase on the nanoscales within the rigid matrix of silica network. The crystallization of the PE block is strictly restricted within the microphase by the rigid silica matrix and takes place through homogeneous nucleation under the nanoscale confinement environment.

Inorganic-organic hybrids of the p,p'-diphenylmethylenediphosphinate, pcp2-. synthesis, characterization and XRPD structures of [Sn(pcp)] and [Cu(pcp)]

Two new inorganic-organic polymeric hybrids [Sn(pcp)] and [Cu(pcp)], pcp = CH₂(PhPO₂)₂^2-, have been synthesized and structurally chracterized. The tin derivative has been obtained by reaction of the p,p'-diphenylmethylenediphosphinic acid (H₂pcp) in water with SnCl₂·2H₂O, while the copper derivative has been synthesized through a hydrothermal reaction from the same H₂pcp acid and Cu(O₂CMe)₂·H₂O. The structures of these compounds have been solved "ab initio" by X-ray powder diffraction (XRPD) data. [Sn(pcp)] has a ladder-like polymeric structure, with tin(II) centers bridged by diphenylmethylenediphosphinate ligands, and alternating six- and eight-membered rings. The hemilectic coordination around the metal shows the tin(II) lone pair to be operative, resulting in significant interaction mainly with a C-C bond of one phenyl ring. The [Cu(pcp)] complex displays a polymeric columnar structure formed by two intersecting sinusoidal ribbons of copper(II) ions bridged by the bifunctional phosphinate ligands. The intersections of the ribbons are made of dimeric units of pentacoordinated copper ions. Crystal data for [Sn(pcp)]: monoclinic, space group P2₁Ic, a = 11.2851(1), b = 15.4495(6), c = 8.6830(1) Å, ^β= 107.546(1)°, V = 1443.44(9) Å, Z = 4. Crystal data for [Cu(pcp)]: triclinic, space group P, a = 10.7126(4), b = 13.0719(4), c = 4.9272(3) Å, α= 92.067(5), β= 95.902(7), γ= 87.847(4)°, V = 685.47(7), Z = 2. The tin compound has been characterized by ¹¹⁹Sn MAS NMR (magic-angle spinning NMR), revealing asymmetry in the valence electron cloud about tin. Low-temperature magnetic measurements of the copper compound have indicated the presence of weak antiferromagnetic interactions below 50 K.

Enzymatic synthesis of isopropyl acetate catalysed by immobilized bacillus cereus lipase in organic medium

Selective production of fragrance fatty acid ester from isopropanol and acetic acid has been achieved using silica-immobilizedlipase of Bacillus cereus MTCC 8372. A purified thermoalkalophilic extracellular lipase was immobilized by adsorption onto the silica. The effects of various parameters like molar ratio of substrates (isopropanol and acetic acid; 25 to 100 mM), concentration ofbiocatalyst (25–125 mg/mL), reaction time, reaction temperature, organic solvents,molecular sieves, and initial water activity werestudied for optimal ester synthesis. Under optimized conditions, 66.0mM of isopropyl acetate was produced when isopropanol and acetic acid were used at 100mM: 75mM in 9h at 55◦C in n-heptane under continuous shaking (160 rpm) using bound lipase(25mg). Addition of molecular sieves (3 °A ×1.5mm) resulted in a marked increase in ester synthesis (73.0mM). Ester synthesiswas enhanced by water activity associated with pre-equilibrated saturated salt solution of LiCl. The immobilized lipase retained more than 50% of its activity after the 6th cycle of reuse.