Intermediates in Rearrangements

Carbanions also appear as intermediates in rearrangement processes. In some cases, this involves the rearrangement from one carbanion to another, but in other cases. 

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Fig. 5.11. Contrasting potential energy diagrams for stable and unstable bridged norbomyl cation. (A) Bridged ion is a transition state for rearrangement between classical structures. (B) Bridged ion is an intermediate in rearrangement of one classical structure to the other. (C) Bridged nonclassical ion is the only stable structure.

The present review is comprehensive Chemical Abstracts has been searched by indexes up to mid 1981 and by a computer on-line substructure search up to Issue 26 of Volume 96. A few more recent references are included directly from the commoner international journals. This review covers 3//-pyrazoles that have been isolated or characterized spectroscopically, although some examples that are only transient intermediates in rearrangement reactions are also mentioned. Compounds having exocyclic double bonds and the benz-fused derivatives, the 3//-indazoles, are considered to be outside its scope. 

Stohrer and Hoffmann (1972) were the first to suggest a square-pyramidal structure [242] as a possible intermediate in rearrangements. They... 

Protonated cyclopropanes are also presumed intermediates in rearrangements during the solvolysis of various compounds in which primary cations would otherwise be formed. Thus deamination of 3.3.3-trideuterio-l-aminopropane 28) leads to cyclopropane in which over half of the molecules still contain three deuterium atoms 34) dearly this... 

Norcaradienes have been trapped by addition of a variety of dienophiles and 1,3-dipoles to cycloheptatrienes and have been proposed as reaction intermediates in rearrangements and condensations leading to cycloheptatrienes. ... 

SCHEME 1.147 Anionic intermediates in rearrangements of O-vinyl oximes to 4//-2-hydroxy-2,3-dihydropyrroles. 

Substituted thiiran radicals were discussed as possible intermediates in rearrangements of j8-halogeno-sulphides on treatment with reducing metals such as zinc. The loss of stereochemistry in the formation of olefins by treatment of j8-phenylthioethyl bromides with tributyltin radicals does not favour the intermediacy of an 5-phenylthiiran radical. ... 

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