Jernej Iskra

Oxidative Halogenation with "Green" Oxidants: Oxygen and Hydrogen Peroxide

It is difficult to imagine organic chemistry without organo-halogen compounds and the molecular halogens needed for their preparation. The halogens have very different reactivity, with iodine usually requiring some form of activation, while others are reactive and hazardous chemicals. To avoid their use, various modified reagents have been discovered (N-bromo- and N-chlorosuccinimide, Selectfluor..), but halogens are used to prepare these reagents and when they are used the atom economy is poor. A better approach, which is based on biomimetric research on oxidative halogenation in nature, consists of generating the halogenating reagent in situ under acidic conditions from a halide salt. The result of such a reaction has been halogenation with 100 % halogen atom economy. Suitable oxidants for the oxidation of halides are hydrogen peroxide and oxygen.

Bromination of ketones with H2O2–HBr “on water”

Various 1,3-diketones, β-ketoesters, cyclic ketones, aryl alkyl and dialkyl ketones were effectively brominated with an aqueous H2O2–HBr system “on water” at room temperature without the need for a catalyst or organic solvent. The resultant brominated ketones were isolated in yields of 69–97% with high selectivity for monobromination vs. dibromination. Reactivity was manipulated by using more diluted aqueous solutions of H2O2 and HBr and the use of an excess of HBr where necessary. Dilution also increases selectivity of ring bromination vs. α-bromination of aryl ketones with an activated phenyl ring. Finally, an aqueous H2O2–HBr system was used for a tandem oxidation–bromination process and alcohols were transformed into α-bromoketones. This simple but effective “on water” bromination of ketones with an aqueous H2O2–HBr system is characterised by the use of inexpensive reagents, a lower impact on the environment and the absence of organic waste that make it a good alternative to existing bromination methods.

One-pot synthesis of non-symmetric tetraoxanes with the H2O2/MTO/fluorous alcohol system☆

Abstract 1,2,4,5-Tetraoxanes, potent antimalarial drugs, were selectively synthesized in fluorous alcohols (2,2,2-trifluoroethanol–TFE and 1,1,1,3,3,3-hexafluoro-2-propanol–HFIP). A use of these solvents enabled for the first time a one-pot synthesis of non-symmetric tetraoxanes in good yields from simple ketones and aldehydes with 2 equiv. of 30% hydrogen peroxide and 0.1 mol% of methyltrioxorhenium (MTO).

Selective aerobic oxidative dibromination of alkenes with aqueous HBr and sodium nitrite as a catalyst

Various alkenes (internal, terminal, aryl and alkyl substituted) and 1,2-diphenylethyne were efficiently and selectively dibrominated using 2 equivalents of 48% aqueous hydrobromic acid, with air as an oxidant and sodium nitrite as a catalyst. Despite the presence of water, only trans dibromination occurred producing anti-1,2-dibromoalkanes and (E)-1,2-dibromo-1,2-diphenylethene. A comparison of resource demand, waste production and environmental, health and safety issues of the NaNO2 catalyzed aerobic bromination with molecular bromine and other oxidative bromination methods revealed that the proposed method is not only selective and effective but has a better environmental profile.

α-Substituted organic peroxides: synthetic strategies for a biologically important class of gem-dihydroperoxide and perketal derivatives

In this paper we review the recent developments in the synthesis of alpha-substituted hydroperoxides. Alpha-substituted hydroperoxides are interesting compounds due to their chemistry and bioactivity and as intermediates for the synthesis of other peroxides, of which cyclic peroxides are of major importance. Although the emphasis of this report will be on the derivatives of gem-dihydroperoxides, perketals, as well as the less studied nitrogen and sulfur derivatives, will also be covered.

Methyltrioxorhenium-catalysed epoxidation of alkenes: enhancement of reactivity in hexafluoro-2-propanol

Abstract Methyltrioxorhenium-catalysed epoxidation of alkenes with hydrogen peroxide can be improved by using hexafluoro-2-propanol as a solvent. Quantitative conversions of cyclic and terminal olefins can be obtained with only 30% H 2 O 2 and 0.1 mol% of catalyst.

Room Temperature Regioselective Iodination of Aromatic Ethers Mediated by SelectfluorTM Reagent F-TEDA-BF4

Abstract Monosubstituted phenyl ethers were regioselectively iodinated with a mixture of iodine and F-TEDA in acetonitrile at room temperature at the para position, while 1-methoxy-4-substituted benzene derivatives were converted to 2-iodo products in high yield.

Aerobic oxidative iodination of ketones catalysed by sodium nitrite “on water” or in a micelle-based aqueous system

Selective and efficient aerobic oxidative iodination of ketones in aqueous media was achieved by using molecular iodine as the source of iodine atoms, air as the terminal oxidant, sodium nitrite (NaNO2) as the catalyst and H2SO4 as the activator of the overall catalytic process. The efficiency of the reaction, resulting in α-iodo ketones, was significantly improved in an aqueous solution of the anionic amphiphile sodium dodecyl sulfate (SDS), capable of self-assembly into micelle-based aggregates, thus forming a reactive micellar system. The regioselectivity of iodofunctionalization of aryl methyl ketones was regulated by the reaction medium used: in an aqueous micelle-based system the methyl group was iodinated, while in anhydrous MeCN aryl iodides were formed with high selectvity.

Solvent effects in the fluorination of aromatic molecules with ‘F-TEDA-BF4’

The type of functionalization of an aromatic molecule achieved with 1-chloromethyl-4-fluoro-1,4-diazobicyclo-(2,2,2)octane bis-tetrafluoroborate, ‘F-TEDA-BF4’, in trifluoroacetic acid depends on its structure: naphthalene and phenanthrene gave fluorinated products, anthracene gave the trifluoroacetate, while an addition process occurred with 9-methoxy-phenanthrene in methanol, and an addition-elimination process in trifluoroacetic acid.

A hybrid organic–inorganic sol–gel coating for protecting aluminium alloy 7075-T6 against corrosion in Harrison’s solution

A two-step procedure for synthesizing a new type of hybrid coating is presented. Coatings were synthesized from tetraethyl orthosilicate, 3-(trimethoxysilyl)propyl methacrylate and zirconium(IV) propoxide (ZTP). Zirconium(IV) propoxide was chelated with methacrylic acid. The synthesis, which proceeded at room temperature, was optimized using in situ Fourier transform infrared spectroscopy (FTIR). The effects of ZTP content, ageing and Zr/Si ratio were investigated. The viscosity of the sols increased with the ratio of Zr to Si. The terminal stability of the hybrid coatings was determined by thermogravimetric analysis and their composition and morphology analyzed by FTIR and scanning electron microscopy combined with energy dispersive X-ray spectroscopy. Sols were deposited on the aluminium AA7075-T6 substrate by spin-coating. The corrosion properties in dilute Harrison’s solution of samples coated by hybrid coatings were determined by electrochemical polarization methods. The coatings substantially improved the corrosion resistance of the substrate, as shown by a broad range of passivity extending up to high electrode potentials. The degree of protection was dependent on the content of zirconium and the time of ageing. The strong corrosion protection was ascribed to the bonding between silicon, oxygen and zirconium (Si–O–Si, Si–O–Zr), as proved by the FTIR analysis.