Sulfur-containing analogues of vitamin E (thiachromanols), either linked or not to a catechol moiety, were synthesized and their hydrogen-atom donating ability evaluated. The determination of the O--H bond dissociation enthalpy (BDE) of the alpha-tocopherol analogue 4 by the electron paramagnetic resonance (EPR) equilibration technique provided a value of 78.9 kcal mol(-1), that is, approximately 1.8 kcal mol(-1) higher than that of alpha-tocopherol. The kinetic rate constants for the reaction with peroxyl radicals (kinh), measured by inhibited autoxidation studies, showed that thiachromanols react 2.5 times slower than the corresponding tocopherols, in agreement with the higher BDE value. This behavior was explained, on the basis of crystallographic analyses and DFT calculations, in terms of a change in the molecular geometry caused by insertion of a sulfur atom into the framework of vitamin E. This behavior implies a greater deviation of the condensed ring from coplanarity with the aromatic ring, thus giving rise to a decrease in the conjugative stabilization of the phenoxyl radical and consequently to an increase in the O--H bond strength. Although less reactive than tocopherols, thiachromanols may, however, act as bimodal antioxidants as a result of the hydroperoxide decomposing ability of the sulfur atom. 相似文献
ONOO. is an important intermediate in the autoxidation of nitrogen monoxide by dioxygen. A formerly unknown red isomer of N2O4, ONOONO (see figure), formed in 2‐methylbutane at 113 K from nitrogen monoxide and dioxygen, is converted to O2NNO2 upon warming.
Novel 2,2‐bis‐C‐functionalized chain glucosid‐3‐uloses have been synthesized in the form of ketal. The synthesis is rather convenient through one pot multistep reactions and no expensive reagents are involved. The products are formed via autoxidation‐Michael addition mechanism and their structures were characterized by X‐ray crystallographic analysis. Their crystal stuctures are stablilized by intermolecular hydrogen bonds and double‐strand supramolecular stacking is observed. 相似文献
Summary: The preparation of polystyrene block methyl methacrylate copolymers (PS-b-PMMA) is described. The polystyrene segment was prepared by anionic polymerization and the methylmethacrylate segment was prepared via free radical autoxidation of a borane agent attached to the styrene chain. 1 The chemistry involves a transformation of the anionic polymerization process to borane chemistry by firstly producing polystyrene with chain end unsaturated alkyl functional groups prepared using a n-butyllithium initiator and termination with allylchlorodimethylsilane. Secondly, the unsaturated macroinitiator end was hydroborated by 9-borabicyclo[3.3.1]nonane (9-BBN) to produce a borane terminated PS. Thirdly, the borane group at the chain end was selectively oxidized and converted to polymeric radicals in the presence of methyl methacrylate which then initiated radical polymerization to produce block copolymers. The polymer obtained was characterized using several chromatographic techniques including LC-CC (liquid chromatography under critical conditions) for the polystyrene segments and two-dimensional chromatography with LC-CC in the first dimension and SEC in the second. The results show that block formation was successful although significant homopolymerization of methyl methacrylate is also obtained. 相似文献
Racemic (2R,4R)-, (2S,4S)-3-benzoyl-1,2,4,5-tetraphenylpentane-1,5-dione results from the alkali-induced condensation of acetophenone with desoxybenzoin in the presence of air. The structure which resembles that of an irregular five-bladed paddlewheel is confirmed by X-ray crystallography. 相似文献
Aerobic oxidation of toluene (PhCH3) is investigated by complementary experimental and theoretical methodologies. Whereas the reaction of the chain‐carrying benzylperoxyl radicals with the substrate produces predominantly benzyl hydroperoxide, benzyl alcohol and benzaldehyde originate mainly from subsequent propagation of the hydroperoxide product. Nevertheless, a significant fraction of benzaldehyde is also produced in primary PhCH3 propagation, presumably via proton rather than hydrogen transfer. An equimolar amount of benzyl alcohol, together with benzoic acid, is additionally produced in the tertiary propagation of PhCHO with benzylperoxyl radicals. The “hot” oxy radicals generated in this step can also abstract aromatic hydrogen atoms from PhCH3, and this results in production of cresols, known inhibitors of radical‐chain reactions. The very fast benzyl peroxyl‐initiated co‐oxidation of benzyl alcohol generates HO2. radicals, along with benzaldehyde. This reaction also causes a decrease in the overall oxidation rate, due to the fast chain‐terminating reaction of HO2. with the benzylperoxyl radicals, which causes a loss of chain carriers. Moreover, due to the fast equilibrium PhCH2OOH+HO2.?PhCH2OO.+H2O2, and the much lower reactivity of H2O2 compared to PhCH2OOH, the fast co‐oxidation of the alcohol means that HO2. gradually takes over the role of benzylperoxyl as principal chain carrier. This drastically changes the autoxidation mechanism and, among other things, causes a sharp decrease in the hydroperoxide yield. 相似文献
The antioxidation activity of vitamin E, vitamin C and lipophilic derivatives of vitaminC, i.e. ascorbyl--6-caprylate, 6-laurate and 6-palmitate, on the autoxidation of linoleic acidwere studied in SDS, Triton X-100 and CTAB micelles. The effect of the lipophilicity ofthe antioxidants and micellar effects on the antioxidation activity are discussed. 相似文献