Isomerization isotopic labelling

Parallel and reversible reactions. The isomerization of allyl phenyl sulfide is a degenerate rearrangement made detectable by isotopic labeling of one end of the allyl group, permitting kinetic monitoring by NMR techniques.12... 

In addition to this skeletal isomerization reaction, Anderson and Avery (24) showed that in a suitable isotopically labeled hydrocarbon, a reaction leading to positional isomerization occurred. Thus, with n-butane-l-13C as the reactant, the isomerization products were 2-(methyl-13C) propane, and 7i-butane-2-13C ... 

Proportions of Isotopically Labeled Products from Isomerization of Hexanes over 0.2% Platinum /Alumina Catalyst at 273°C ... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

Wentrup and co-workers have carried out systematic flash vacuum thermolysis studies with a series of fused tetrazoles. Investigations of the isomeric tetrazolo[l,5- ]pyrazine 17 and tetrazolo[l,5-f]pyrimidine 20 showed that, in both cases, ring contraction takes place to afford imidazoles in high yields, but isotope labeling experiments revealed that the mechanisms of the openings of the two ring systems are different <2002JOC8538>. 

Characterization of ion structures by bimolecular reactions, in which an ion is allowed to react with a neutral gas of known structure and the ionic products are analysed by mass spectrometry, depends on isomeric species having distinctive reactivities which reflect the functional group(s) that are present. This method is conceptually analogous to the use of structure-specific test reagents in classical solution chemistry. Sometimes a group may be transferred to a particular ion from the gas (methylene transfer is commonly encountered) on other occasions, hydrogen transfer is monitored. The latter is conveniently combined with isotopic labelling. 

A Global scheme for solvolysis 2 Clocks for reactions of ion pairs 3 Addition of solvent to carbocation-anion pairs i Protonation of a carbocation-anion pair 11 Isomerization of ion pair reaction intermediates Reorganization of ion pairs in water 13 Internal return of isotopically labeled ion pairs Racemization of ion pairs 22 Concluding remarks 24 Acknowledgements 24 References 24... 

The observation of hidden reactions during solvolysis, through the use of chiral or isotopically labeled substrates has created considerable excitement in communities interested in the mechanisms of nonenzymatic and enzyme catalyzed reactions. These hidden reactions reveal something interesting about reaction mechanisms. However, chemists and biochemists are still working on the problem of extracting simple and definitive conclusions from analysis of data for these isomerization reactions. 

Compounds labeled with multiple isotopes of the same atom provide the opportunity to observe virtual isomerization reactions that go undetected in conventional studies.There have been relatively few modern studies of these virtual isomerization reactions," " because the reactions themselves are not common and because of difficulties in drawing general conclusions from the observation of the scrambling of isotopic label during solvolysis (Scheme 8). 

The reaction of alkenes with ozone constitutes an important method of cleaving carbon-carbon double bonds.138 Application of low-temperature spectroscopic techniques has provided information about the rather unstable species that are intermediates in the ozonolysis process. These studies, along with isotope labeling results, have provided an understanding of the reaction mechanism.139 The two key intermediates in ozonolysis are the 1,2,3-trioxolane, or initial ozonide, and the 1,2,4-trioxolane, or ozonide. The first step of the reaction is a cycloaddition to give the 1,2,3-trioxolane. This is followed by a fragmentation and recombination to give the isomeric 1,2,4-trioxolane. The first step is a... 

Recent data on other alloys confirm the overall classification presented above, but at the same time lead to some refinement of the picture. For example, the most diluted Pt-Au alloys revealed isomerization, identified as running via 3C intermediates. This evidence was obtained (248) by establishing the fact that pentane isomerizes on most diluted Pt-Au alloys with 100% selectivity, whereas this molecule can only isomerize via the 3C complexes. This conclusion has been confirmed by the isotopic labeling method (269). It is therefore reasonable to assume that this isomerization can also proceed on isolated Pt sites, as can a part of dehydrocyclization and the dehydrogenation. We must conclude on the basis of this information that on metals like Pt, the fast multisite and the slow one-site mechanisms of hydrocarbon reactions may operate in parallel with each other. 

Dianions of the above types may not fall into the category of homoenolate in a strictly formal sense. Nevertheless the amide dianion does show a behavior typical of the homoenolate. Thus, the reaction of the isotopically labeled stannylpropionate results in scrambling of the label probably via a cyclopropane intermediate Eq. (47) [44]. As the result of such an equilibration, isomerization of a-methyl and a-phenyl substituted propionate homoenolates may occur to give the thermodynamically more favorable isomers, respectively. 

From the observation that the generally exothermic addition reactions do not occur readily we may infer that they require appreciable activation energies. These latter range from 17 to 23 kcal. for the additions of HI to olefins53 to 30 to 40 kcal. for the additions of HBr or HC1 to olefins, f It has been demonstrated by Okabe and McNesby24 that if sufficient energy is available in the molecule (e.g., by vacuum UV photolysis) then direct elimination of CH4 or H2 is possible from alkanes. In these cases, isotopic labeling experiments have shown that the two H atoms may come from adjacent C atoms to form the olefin, or even from the same C atom to form the isomeric carbene ... 

According to an early report, sulfated zirconia promoted with 1.5% Fe and 0.5% Mn increased the rate of isomerization of n-butane to isobutane by several orders of magnitude at modest temperature (28°C).299 This reactivity is surprising, since the isomerization of n-butane in strong liquid acids takes place at a rate much lower than that of higher alkanes, which is due to the involvement of the primary carbocationic intermediate. In addition, other solid acids, such as zeolites, did not show activity under such mild conditions. Evidence by isotope labeling studies with double-labeled n-butane unequivocally shows, however, that the isomerization of... 

Studies with unpromoted and Pt-promoted solid acids, specifically, H morde-nite,308 Pt Beta-zeolite,313 and acidic polyoxometalate cesium salts314,315 applied in the presence of hydrogen, showed that the isomerization of n-butane follows a monomolecular pathway. This consists of dehydrogenation on the metal site and isomerization on acidic sites. A new study using isotope labeling provided direct... 

Allylic amide isomerization, 117 Allylic amine isomerization ab initio calculations, 110 catalytic cycle, 104 cobalt-catalyzed, 98 double-bond migration, 104 isotope-labeling experiments, 103 kinetics, 103 mechanism, 103 model system, 110 NMR study, 104 rhodium-catalyzed, 9, 98 Allylnickel halides, 170 Allylpalladium intermediates, 193 Allylsilane protodesilylation, 305 Aluminum, chiral catalysts, 216, 234, 310 Amide dimers, NMR spectra, 282, 284 Amines ... 

Isotope-labeling experiments, amine isomerization, 103 Itatonic acid, 28... 

Lesage, D., Virelizier, H., Jankowski, C.K., Tabet, J.C. 1998. Identification of isomeric tributylphosphate dimers formed by radiolysis using tandem mass spectrometry and stable isotopic labelling. Eur. Mass Spectrom. 4 47-54. 

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