The rearrangement mechanism

The rearrangement mechanisms of 5-perfluoroalkyl-l,2,4-oxadiazoles such as the five-to-six membered ring-rearrangements by hydrazinolysis and the photoinduced competitive rearrangements have been investigated <06JOC8106 06JOC2740>. 

A carbon labelling study has elucidated the rearrangement mechanism for formation of chalcone (97) which accompanies formation of (91) by the expected vicinyl elimination of trimethylsilyl and benzotriazolyl groups from 2-benzotriazolyl-2-aryl-3-ketopropylsilanes, on reaction with fluoride ion in DMF. ° Thus, it has been possible to distinguish between the two alternative mechanisms depicted in Scheme 11 (via intermediates (93) or (95), respectively, by determining the fate of the labelled quaternary carbon of substrate (89). The results are consistent with the formation of a cyclopropane intermediate (95) which subsequently ring opens, with relief of strain, to form delocalized carbanion (96), from which the chalcone (97) is obtained (labelled... 

Exactly the same result was obtained when the homopolymers were oxidized at — 25°C with a N,N,N, N -tetraethylethylenediamine-cuprous chloride catalyst, conditions which have been reported to cause coupling of DMP homopolymers solely by rearrangement (14). The NMR spectrum of this polymer is shown in Figure 3, together with the spectra of a mixture of homopolymers and of a random copolymer formed by simultaneous oxidation of the monomers. Apparently, dissociation and redistribution occur often enough to determine the structure of the product in this system, even under conditions that favor coupling of polymer molecules by the rearrangement mechanism. 

The rearrangements mechanism and the sequence of cage atoms migrations were proved unequivocally by carrying out the reactions with selectively deuterated 7,8-dicarba-nido-undecaborates(l-), the use of nB and H NMR spectroscopy and X-ray structures, as well as the counter synthesis of a rearrangement product. 

The rearrangement mechanism of a cobalt(III) nitrosarco-phaginate in the basic conditions to yield a contracted Ne-cage, as well as the intermediate reaction products are described in Ref. 106 (see Chapter 4). 

Variable-temperature ID and 2D CP/MAS NMR spectra of Gp2Ti(771-G5H5)2 have been reported. The compound is fluxional. The rearrangement mechanism of the 7/-Cp rings in this complex proceeds via a single sigmatropic process.1879... 

We were particularly interested in finding the mechanisms of the structural transitions between the different nanocrystals. Based on the small values of the activation energies, Hudgins et al. suggested that the rearrangement mechanisms might involve a sequence of surface diffusion... 

Molecular orbital studies on the orbital interactions between two 3 radicals have been carried out (86CC140) in order to rationalize the rearrangement mechanism (Section XXI.A. 1) by which 3 is converted to 2 in solution. These studies show that the SOMO, which is of ir character, has the largest coefficients on the sulfur atoms, and thus association can be considered to arise through S. . . S bonding interactions. Coupled... 

Subsequently Winstein directly observed 1-methyl-444 and 5-methyl-substituted 445 norbomadienyl ions, to elucidate the rearrangement mechanism of ion 439 into ion 440. At —105 °C cation 446 is rearranged into a mixture (55 45) of 1-methyl-444 and 5-methyl-cation 445. At —75 °C ion 444 is rearranged irreversibly into ion 445 which at —45 °C is decomposed but not converted into ion 440. Ion 445 was obtained from another precursor — quadricyclanoi 447 with intermediate rearrangement of the 7-methyl-7-quadricyclyl ion. TTie structure of ions 444 and 445 is established by PMR analysis and by conversion of ion 445 into syn-methyl ether 448 which is different from the anti-isomer 441 formed from ion 440. 

Oxazoles with an acyl substituent at C4 can undergo a thermal rearrangement essentially exchanging the C4 and C5 substituents. The facility of this rearrangement is improved when the C5 substituent is a heteroatom (-OR, -SR, -Cl). Dewar and Turchi observed deuterium scrambling of a labelled oxazole ester under thermal conditions. They propose a nitrile ylide intermediate in the rearrangement mechanism. This... 

Propose the rearrangement mechanism from isoxazole to oxazole. ... 

No changes in fabric dimensions and, therefore, in fabric count were observed. Accordingly, the rearrangement occurred only at the level of the warp yarn the overall fabric construction was not affected by the rearrangement mechanism. Since weft yarn did not undergo changes, either in crimping or in yarn cross-section, the fabric thickness measured was no different after... 

What followed were decades of investigation to determine the mechanism through which such a rearrangement would occur. Notably, the work of Meerwein and van Emster demonstrated the presence of a cationic intermediate that preceded the 1,2-alkyl migration. Through his studies of equilibrium isomerism of bomyl chloride, camphene hydrochloride (3), and isobomyl chloride (6), Meerwein established that the rearrangement mechanism relied on ionization, and exhibited a heavily solvent-dependant kinetic profile. These studies, which laid the foundation for modem... 

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