Other Rearrangements

The net structural change is the same for both mechanisms. The energy requirements of the cyclopropanone and semibenzilic mechanisms may be fairly closely balanced, as instances of the operation of the semibenzilic mechanism have been reported even for compounds with hydrogen available for enolization. Included in the evidence that the cyclopropanone mechanisms usually operates, in preference to the semibenzilic mechanism, is the demonstration, in several instances, that a 

Numbers refer to percentage of label at each carbon. 

Charge-site-displacement reactions take place in EE ions (Equation 8.105), 

Even hidden displacements can occur. In the loss of water from CI-protonated oximes, HjO is displaced by the migration of the phenyl group to the protonated hydroxyl (Equation 8.113 Maquestiau et aL 1980). Only syn-isomers were found to undergo this type of reaction. 

Unknown 8.13. Formulate a displacement rearrangement which will rationalize an abundant ion in the mass spectrum of (i-ionone, Unknown 4.13. (For the answer see Equation 9.29.) 

EE ions can also undergo such group migrations with elimination, as shown in Equations 8.117-8.120 (8.118 Wood et al. 1992). 

A prime driving force for such eliminations is the stability of the product ion. The most common examples involve formation of a new bond between large groups, particularly aromatic rings, to increase the resonance stabilization of the ionic product. Beynon et al. (1959) first pointed out the common loss of CO from aromatic compounds such as quinones. For anthraquinone (Equation 8.121) at least the second CO loss must involve a new bond between the rings the TE of the neutral product is 7.6 eV. 

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This is clearly demonstrated in the pinacolinic deamination (cf. p. 114) of an optically active form of the amino-alcohol (50). Such reactions proceed from a conformation (antiperiplanar 50a or 50b) in which the migrating (Ph) and leaving (NH2 as N2 cf. p. 114) groups are TRANS to each other. Rearrangement via a bridged carbocation would necessarily lead to 100% inversion at the migration terminus in the product ketone (5lab), whichever initial conformation, (50a) or (50b), was involved ... 

In other cases, however, the chemistry of the cycloalkanylidenes can be governed by the geometric constraints inherent in the system. Strain can raise the activation barrier to the 1,2-H shift, permitting the intervention of other rearrangements. The reaction of cyclobutylidene (100) is a case in point Eq. 37.16116... 

It should be noted that the silylhydrides 106 109 were the only products of the reaction shown in Eq. (30). The isomeric agostic species, related to the niobium species 64 and 65 were not identified, and all attempts to convert 106-109 into these or other rearranged species at elevated temperatures resulted in decomposition. 

The catalysts Co-CMS5 and C0-CMS6 have also been tested for the air oxidation of a-pinene. In both the cases air oxidation commences quickly with the formation of a-pinene oxide, verbenol and verbenone as the main products along with other rearranged products as observed in the reactions described above. A conversion of 50.6% and 48.0% are observed for Co-CMSS and C0-CMS6 respectively after 24h (Table 6). 

Concerning their structure and reactions, organic radical cations have been the focus of much interest. Among bimolecular reactions, the addition to alkenes and their nucleophilic capture by alcohols, which lead to C—C and C—O bond formation, respectively have been investigated in detail. Unimolecular reactions like geometric isomerization and several other rearrangements have also attracted attention. 

Rearrangements of hydroxylamines, oximes and hydroxamic acids IX. OTHER REARRANGEMENTS... 

A detailed study of the rearrangement of the cyclopropanaphthalene 21 has identified the cyclobutenes 22 (44%) and 23 (13%, GC on the basis of consumed 21), in addition to other rearranged products. The reaction was more rapid in nonpolar solvents, such as cyclohexane, than in polar solvents.79,80... 

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