Isotopic labelling studies rearrangement

Van den Berg et al. suggested an intramolecular Stevens rearrangement of the oxo-nium ylide to ethyl methyl ether to interpret carbon-carbon bond formation463 [Eq. (3.55)]. Olah and coworkers, however, provided evidence (based on isotopic labeling studies) that the oxonium ylide undergoes intermolecular methylation to ethyl dimethyloxonium ion [Eq. (3.56)] instead of Stevens rearrangement 447... 

It is curious that there have been more fundamental developments in 1,3,5-triazine synthesis by this method than in any of the other sections, yet it is the oldest of all the synthetic routes. Cyanuric acid was originally prepared by the pyrolysis of uric acid (7) (see Section 2.20.1.2). The mechanism was proposed by Brandenberger and coworkers on the basis of isotopic labelling studies, and was reviewed by Modest (B-61MI22000, p. 706). The rearrangement of pyrimidines to 1,3,5-triazines is the most important advance in this area. 

The reaction of SO2 with rf dioxygen complexes gives a chelated sulphate complex. Isotopic labelling studies with 62 show that one of the terminal oxygen atoms of the sulphate originates from the dioxygen complex and the other from the SO2. This has been interpreted in terms of the rearrangement of a five membered cyclic intermediate . ... 

The same products are formed by pyrolysis of the o-, m-, or p-methylbenzaldehyde p-toluenesulfonylhydrazones. Suggest mechanisms that could account for these skeletal rearrangements. Are any of your mechanisms consistent with the following isotopic labeling study ... 

Fundamental studies of the mechanism of propane ammoxidation over Mo-V-Nb-0 based catalysts show that the mechanism is the same as earlier studies foimd for other propane ammoxidation catalysts, notably those based on V-Sb-O (see above). Isotope labeling studies using C-labeled propane show that the reaction proceeds through propylene as the only intermediate (156). There is no dimerization or skeletal rearrangement of C3 moities rather intermediate propylene is converted directly to acrylonitrile. Also pulse reaction studies show that the reaction occurs by a redox process wherein lattice oxygens are used in the... 

In related chemistry, Hughes o i observed the novel rearrangement shown in Scheme 5. Isotopic labeling studies have shown that this rearrangement involves an interchange of substituents between the... 

Despite both intramolecular and intermolecular mechanisms for the Chapman rearrangement having been postulated [28], it is the intramolecular one that has been confirmed by crossover experiments and isotopic labeling studies [29]. It has also been observed that the rearrangement follows first-order kinetics involving a nucleophilic aromatic substitution step [30]. 

The first mass spectrometric investigation of the thiazole ring was done by Clarke et al. (271). Shortly after, Cooks et al., in a study devoted to bicydic aromatic systems, demonstrated the influence of the benzo ring in benzothiazole (272). Since this time, many studies have been devoted to the influence of various types of substitution upon fragmentation schemes and rearrangements, in the case of alkylthiazoles by Buttery (273) arylthiazoles by Aune et al. (276), Rix et al. (277), Khnulnitskii et al. (278) functional derivatives by Salmona el al. (279) and Entenmann (280) and thiazoles isotopically labeled with deuterium and C by Bojesen et al. (113). More recently, Witzhum et al. have detected the presence of simple derivatives of thiazole in food aromas by mass spectrometry (281). 

The occurrence and extent of rearrangement of the 2-butyl cation have also been investigated by solvolysis studies using isotopic labeling. When 2-butyl tosylate is solvolyzed in acetic acid, C-2/C-3 rearrangement occurs only to the extent of 9% in the 2-butyl acetate which is isolated.Thus, under these conditions, most of the reaction proceeds by direct participation of the solvent. 

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. 

There exist early examples of this transformation [507, 508], but due to the symmetric structure of the alkene part, only isotope labeling, etc., allowed the exclusion of a prototropic rearrangement. Furthermore, due to the high reaction temperatures of 340 °C and above, several different products are formed. A low-temperature version (77 K) of this reaction via the radical cation has been reported [509]. The chirality transfer has been studied and a detailed mechanistic investigation has been conducted [510] typical experiments in that context were the reactions of substrates such as 155 and 157 (Scheme 1.70). 

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