Electrochemical oxidation of 1-chloro(bromo)-2,2,6,6-tetramethylpiperidines

Electrochemical oxidation of 1-haloamines of the 2,2,6,6-tetramethylpiperidine series results in the formation of relatively stable radical-cations, detected by the methods of cyclic voltammetry and ESR spectroscopy. The final products of electrochemical oxidation of these haloamines are stable nitroxyl radicals.     

Separation of the mineral oil aromatic hydrocarbons of three and more aromatic rings from those of one or two aromatic rings

An analytical method was developed for the quantitation of the mineral oil aromatic hydrocarbons in cosmetic raw materials separating those of one or two aromatic rings from those of three and more aromatic rings. Normal phase high performance liquid chromatography was used with donor‐acceptor complex chromatography. The composition of both fractions and the quantities of respective compounds were determined by comprehensive two dimensional gas chromatography with time of flight mass spectrometry and by liquid chromatography coupled to gas chromatography with flame ionization detection.     

Molecular geometry and optical activity of N-nitroso-2,2,6,6-tetramethylpiperidines generated by spontaneous crystallization and inclusion complexation with optically active diols

Three sterically strained N-nitrosamines and their inclusion complexes with optically active diols (TADDOLs) were obtained and their solid state crystal structures are described. Owing to the formation of N-nitroso-4-hydroxy-2,2,6,6-tetramethylpiperidine 2 as spontaneously resolvable conglomerate crystals (space group P3₂) its solid state CD was measured. The crystal structures of the inclusion complexes revealed that in all cases the guest nitrosamines assume chiral conformations as seen by their chiroptical spectra. The optically active nitrosamines are configurationally labile and rapidly racemize in solution. The solid state structures revealed that in order to avoid an allylic 1,3-strain [A^(1,3)], caused by an interaction of the nitrosamino group with the methyl substituents, the piperidine ring in 1 and 2 assumes a chair conformation significantly flattened at the amino nitrogen whereas in the 4-oxo derivative 3 the piperidine ring assumes a twist-boat conformation.     

Diels-Alder reaction between indoles and cyclohexadienes photocatalyzed by pi,pi aromatic ketones

[structure: see text] Diels-Alder reactions between indoles and cyclohexadienes can be photocatalyzed by benzoylthiophenes. Both experimental and theoretical studies show that the reactions occur through ternary excited triplet state complexes (triplet triplexes).     

Reactivity of N-alkanoyloxy-2,2,6,6-tetramethylpiperidines (O-acylTEMPOs) towards hydride-transferring or metallic alkylating reagents; unprecedented stability and application to chemoselective transformations

Owing to the unprecedented stability of O-acylTEMPOs towards hydride-transferring and metallic alkylating reagents such as LiAlH4 and RMgX, chemoselective transformation of diacid mixed alkyl/TEMP-1-yl esters, where O-acylTEMPOs remained intact, is achieved with these reagents, giving the corresponding carbinols, respectively.     

Substituent effects on non-covalent interactions with aromatic rings: insights from computational chemistry

Non-covalent interactions with aromatic rings pervade modern chemical research. The strength and orientation of these interactions can be tuned and controlled through substituent effects. Computational studies of model complexes have provided a detailed understanding of the origin and nature of these substituent effects, and pinpointed flaws in entrenched models of these interactions in the literature. Here, we provide a brief review of efforts over the last decade to unravel the origin of substituent effects in π-stacking, XH/π, and ion/π interactions through detailed computational studies. We highlight recent progress that has been made, while also uncovering areas where future studies are warranted.     

Carbolithiation of gem-aryl disubstituted methylenecyclopropanes

A novel carbolithiation of gem-aryl disubstituted methylenecyclopropanes was disclosed by the treatment with BuLi in THF at -78 degree C. The corresponding addition products can be obtained in good to high yields by quenching with various electrophiles.     

Formation of aromatic rings through enamine annulation

Condensation of pyrrolidine enamine of ketones with 1, 4-diacetoxy-2-butanone provides a new, concise synthetic route to substituted benzenes, dihydroindenes, tetrahydronaphthalenes, and di- and octahydrophenanthrenes. The reaction produced modest yields with regiocontrol of the secondary amine substituent.     

A study of aromatic three membered rings

The resonance energies (REs) of neutral three membered ring analogs of the cyclopropenyl cation, computed using block localized wave function (BLW) methods, reveal considerable variations. The RE's of cyclopropenes substituted with exocyclic double bonded groups C?X, (X = O, NH, CH₂, S, PH, SiH₂) increase with the electronegativity of X in the same row (SiH₂ < PH < S and CH₂ < NH < O). The extra cyclic resonance energies (ECREs) (an energetic measure of aromaticity based on comparisons with the RE's of acyclic models) of these derivatives range from +10.5 kcal/mol for cyclopropenone (X = O) (somewhat aromatic; the benzene ECRE is 29.3 kcal/mol) to ?2.4 kcal/mol (slightly antiaromatic) for X = SiH₂. Additional disubstitution of the C?C double bond by X′ groups (X′ = CH₃, NH₂, OH, SiH₃, PH₂, SH) increases the REs considerably, but has only small effects on the ECREs. Even the ECRE of deltic acid (X = O, X′ = OH) is only increased to +13.3 kcal/mol. The conclusion based on ECRE's, that all 12 of the three membered rings are only marginally aromatic/antiaromatic, is supported by the satisfactorily plot (R² = 0.92) of ECRE against values of NICS(0)_πzz (a superior nucleus chemical independent shift magnetic index of aromaticity), which range only from ?6.1 ppm (diatropic) for deltic acid (cf., ?35.5 ppm for benzene and ?14.2 ppm for the parent cyclopropenium ion) to +8.9 ppm (paratropic) for the silicon derivative, X = SiH₂, X′ = SiH₃. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Polyaromatic ribbons from oligo-alkynes via selective radical cascade: stitching aromatic rings with polyacetylene bridges

Selective radical generation in conjugated oligomeric o-aryleneethynylenes initiates an intramolecular cascade which involves five fast radical cyclizations followed by aromatization via a 1,5-H shift with a >93% yield per step. This radical cascade transformation opens a new avenue for the systematic and controlled preparation of functionalized graphene nanoribbons where, potentially, each of the peripheral aromatic rings can be different.     

Intramolecular reactions of 2-indolylacyl radicals: cyclisation upon aromatic rings

The generation of 2-indolylacyl radicals from the corresponding selenoesters under reductive (tributyltin hydride-AIBN) and nonreductive (hexabutylditin, 300 W) conditions and their behaviour in cyclisation reactions upon benzene rings attached either to the indole nitrogen or the C-3 ring position have been studied.     

Crosslinkable aromatic polyimides containing 2,3-dicyanopyrazine rings

New crosslinkable aromatic polyimides were synthesized from 2,3-dicyano-5,6-di(aminophenyl)pyrazine ( 1b ) and 2,3-dicyanopyrazino [5,6–9,10] diaminophenanthrene ( 2b ). They were characterized by viscosimetry, IR, NMR, x-ray, DTA, TMA, TGA, and isothermal gravimetric analysis. The polyimides derived from 1b were amorphous whereas those prepared from 2b showed microcrystalline or crystalline character. The polymers were soluble in polar aprotic solvents (DMF, NMP, DMSO) at ambient temperature or upon heating. They dissolved also completely or partially in certain hot inorganic and organic acids (H₂SO₄, CCl₃COOH). Their glass transition temperatures were in the range of 257–370°C. The crosslinked polymers obtained after an appropriate thermal treatment, were stable up to 397–426°C in N₂ or air and afforded anaerobic char yields of 62–75% at 800°C. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1303–1311, 1997     

1,2,3-Oxadiazole rings in the aromatic compounds: A quantum-chemical investigation

Quantum chemical method PBE0/cc-pVTZ in conjunction with the polarizable continuum model (PCM) was applied to the calculation of the equilibrium structural parameters of the benzene and naphthalene 1,2,3-oxadiazole derivatives, as well as relevant diazocarbonyl compounds both in the free state and dissolved in dichloromethane. The calculated ionization potentials of benzo-1,2,3-oxadiazole (9.49 eV), 6-diazocyclohexa-2,4-dien-1-one (8.16 eV) and (6-oxocyclohexa-2,4-dien-1-ylidene)carbonyl (8.34 eV) differ from the experimental values no more than by 0.09 eV. The calculations indicate the possible existence of naphtho [2,3-d:6,7-d′]bis(1,2,3-oxadiazole) in the gas phase and in aprotic solvents.     

Insertion of arynes into carbon-halogen sigma-bonds: regioselective acylation of aromatic rings

Arynes were found to insert into carbon-halogen sigma-bonds of various acid halides, enabling acyl and halogen moieties to be introduced simultaneously into adjacent positions of aromatic rings.     

Synthesis of N-acyl- and N-aroyl-2-oxazolidones

The action on -chloroethyl N-alkylcarbamates of salts of xanthic and dithiocarbamic acids or caustic soda has given good yields of the otherwise difficult to obtain N-acyl-2-oxazolidones.     

Properties of excited states of aromatic rings containing nitrogen

Quantum chemical calculations with the semiempirical molecular orbital (MO) method SINDO1 were performed on excited states of the following five- and six-membered heterocycles containing nitrogen: imidazol, pyrazol, pyridine, pyridazine, pyrimidine, pyrazine, and s-triazine. The geometries and adiabatic excitation energies of T₁, S₁, S₂ were calculated. We also present charges, bond orders, and dipole moments. Consequences of these results for UV spectroscopic data are discussed.