Isomerisation reactions mechanism

This stereospecific oxidation does not occur for all dioximes, probably due to isomerisation of the dioxime during the reaction or to different reaction mechanisms involved in the use of different oxidants. When the lipophilic-hydrophilic balance of the two furoxan isomers is appropriate, they are easily separated by chromatography or fractional crystallisation. For example, the synthesis of 4-hydroxymethyl-3-furoxancarboxamide (CAS 1609), one of the most promising furoxancarboxamide vasodilators (see later), passes through the intermediate formation of a mixture of the two isomeric methyl hydroxymethylfuroxancarboxylic esters, which can easily be separated by recrystallisation from isopropyl acetate [18]. 

The hydrogenation steps can be preceded or succeeded by isomerization steps, which typically require the presence of hydrogen in the liquid phase, because hydrogen plays a role for the reaction mechanism, even though it is not consumed in the isomerisation itself. Lack of hydrogen on the catalyst surface typically leads to severe deactivation. 

Network structure and reaction mechanisms in high pressure vulcanisation (HPV) and peroxide vulcanisation of BR was studied by 13C solid-state NMR [43]. Different samples of polybutadiene (51% trans, 38% cis, and 11 % vinyl) were peroxide cured with dicumyl peroxide on a silica carrier and by the HPV conditions of 250 °C and 293 MPa. The 13C NMR spectra from peroxide and HPV cures were compared to a control samples heated to 250 °C for 6 minutes under atmospheric pressure. Although no new isolated strong peaks were detected in either the peroxide or HPV vulcanisations, small increases in both spectra were observed at 29.5, 36.0, 46.5, and 48.0 ppm. These peaks compare favourably with calculated shifts from structures that arise from main chain radical addition to the pendent vinyl groups. These assignments are further reinforced by the observation that the vinyl carbon concentration is substantially reduced during vulcanisation in both peroxide and HPV curing. Two peaks at 39.5 and 42.5 ppm appear only in the peroxide spectrum. Cis-trans isomerisation was absent in both cures. 

Plant cells show an extensive repertoire of chemical reaction mechanisms epoxi-dation, reduction, oxidation, hydroxylation, isomerisation. It is self-evident that plant cell cultures synthesize as enantioselectively as their mother organisms. Besides the well-known flavour extracts and single substances, also presently unknown naturally flavour chemicals and mixtures of these are in principle obtainable. Therefore the rapid progress in investigating this area is not surprising [26],... 

In an equally unambiguous paper. Grant and Lambert showed that it was atomically held oxygen which was responsible for the epoxidation of ethylene and that the molecularly held species, though present, was merely a spectator [24]. The intermediate responsible for isomerisation was, therefore, CH2-CH2-O-Ag, and Grant and Lambert stated as much in their paper [24]. They produced the reaction mechanism in their paper, in which they proposed that selective oxidation results from an electrophilic attack by an O(a) on the olefinic n-bond. ... 

The 1,3 "dipolar" redox isomerisation of C-captor-substituted amide chlorides is general, first order and independent of solvent polarity it is the fastest with the strongest captor groups (ref. 10). Since ionisation induced by counterion complexation of these amide chlorides blocks the isomerisation, either homolysis of CCl-bonds in opposition or a concerned elimination to the interceptable dipole and readdition of HC1 could explain the reaction mechanism. But what is the driving force of the rearrangement Again the Electronegativity Effect (ref. 2) is certainly... 

It is well established that unsatnrated fatty acids undergo oxidation, via a radical reaction mechanism. Carotenoids undergo similar reactions and indeed do this so readily they can act as antioxidants in food materials. This antioxidant ability of carotenoids derives from their ability to form a resonance stabilised free radical. In certain controlled conditions chemical oxidation of carotenoids can give rise to epoxide formation and isomerisation of this to a furanoxide (Wong,... 

First, in the experimental conditions we have used, the hydrogenation of both B2N and B3N is 100 percent to the but5T onitrile (BN). Secondly, the initial results collected clearly show that the apparent reaction orders, for both studied substrates, are different from zero and hence demonstrate that the product competes with the reactant for the adsorption of the catal)dic site. Thirdly, in the hydrogen pressure range explored, the isomerization of B3N is never observed on any catalyst (Pd-1, Pd-3) or on the support free of metal. This is an evidence that no side isomerisation reaction occurs neither on the metal nor on the support. Finally, we may also notice the reactivity order of the substrates as previously observed on Raney nickel (14-15) B3N is more rapidly reduced than B2N. This fact precludes a hydrogenation mechanism of B3N through isomerisation to B2N. 

In order to learn more about the reaction mechanism and the scope of the aniline rearrangement, additional substrates were tested. Experiments with toluidines (methylpyridines) showed a substantial isomerisation and disproportionation of the substrate (Table 4) due to the methyl-shift reaction on the highly acidic catalyst. Selectivities are low due to disproportionation reaction of the reactants and a large amount of di- and trimethylated reactants and several addition products were formed (which explains the missing 60% in the reaction of o-toluidine). 

It may be assumed that dialdehyde formation can be achieved because the isomerisation of the intermediate nonconjugated unsaturated aldehyde to the conjugated aldehyde (which would only give monoaldehyde by hydrogenation) is not catalyzed by the modified rhodium catalyst (see also chapter on reaction mechanism of the hydroformylation reaction, p. 4), whereas HCo(CO)4 catalyzes the isomerisation. 

It is commonly accepted that reactions of CH with unsaturated hydrocarbons proceed without any entrance barrier via the formation of an initial intermediate which decomposes via hydrogen elimination. These reactions are exothermic and show several exit pathways due to the existence of isomers of final products. However there is debate on the exact nature of the reaction mechanism. As for the reactions of CH with acetylene and ethylene, the most likely mechanism seems to be the addition of the CH radical on the carbon multiple bonds to form a 3-carbon-atom cycle. It is believed that this cyclic intermediate isomerises to give linear intermediates which decompose to give the final products. The insertion of the CH radical in the C-H bond of the molecule is not thought to be a favourable entrance chaimel. For the reactions of CH with aromatic compounds a similar mechanism is expected. With respect to anthracene the observed behaviour of the rate coefficient is usually indicative of a reaction which... 

This view on the reaction mechanism was strongly supported by Collyer and Blow [56] who compared their findings with the enzyme from Arthrobacter B3278 with CarrelTs results [52]. In their view, the mechanism-based inactivator , which is supposed to become armed upon isomerisation of the aldose to the a,)3-unsaturated ketose, then alkylate His-53 (which corresponds to His-54 of the Streptomyces ruhiginosus enzyme), does not bind in a productive conformation which would involve interaction of 0-3 with one of the cations in the active site. This is not possible if this compoimd does not bear a hydroxy fimc-tion in this position. They reasoned that the alkylation step could occur independently of the isomerisation as, in their opinion, His-53 is not involved in the latter. Furthermore, the authors disagreed on the orientation of the substrate d-... 

The reaction of 2-bromo-5-nitrothiazole with weakly basic secondary aliphatic amines gave the expected 2-amino products. The isomeric 5-bromo-2-nitrothiazole with such amines gave mixtures of the expected 5-amino products along with 2-aminated 5-nitrothiazole rearrangement products. A mechanism was proposed which involves the slow thermal isomerisation of the 5-bromo-2-nitrothiazole to the much more reactive 2-bromo isomer which competes, in the case of relatively weak amine nucleophiles, with direct but slow displacement of the 5-bromo group to form the normal displacement product <96JHC1191>. 

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