Ring strain driving rearrangement

The enol ether double bond is epoxidized by the peracid. Relief of the epoxide ring strain drives the rearrangement with migration of the silyl group to give the silylated a-hydroxy ketone product. 

Somfai enhanced the driving force of some amide enolate aza-Claisen rearrangements by choosing vinylaziridines as reactants [24]. The additional loss of ring strain offered the advantage of running most of the reactions at room temperature to synthesize unsaturated chiral azepinones. Various substitution... 

A number of complexes, mainly of Ag(l) but also Rh(l) and Pd(ll), can catalyze the rearrangement and degradation of a wide variety of highly strained polycyclics, which may not react in the absence of catalysts. The products vary with the catalyst, the concentration of which features in the rate law. Again, the relief of ring strain which is a driving force for the reaction, appears aided by metal complexing. " ... 

All-carbon ene reactions can go in reverse when ring-strain is released 6.24. This reaction is curious, because it could also be considered as a homologue of a [1,53-sigmatropic rearrangement—it is quite common to think of the chemistry of cyclopropanes as similar to that of alkenes. With heteroatoms in the chain it is possible to drive such reactions in reverse without having to release strain. Thus esters such as acetates and benzoates undergo a cyclic (3-elimination on pyrolysis. This type of elimination is known to be syn... 

In either case, lessening of TMt-repulsion and release of ring strain is supposed to be the driving force of the rearrangement. 

This undergoes a vQtro-Claisen rearrangement under the reaction conditions. The mechanism of the [3,3]-sigmatropic rearrangement is shown above. The concerted reaction proceeds via a six-membered, chair-like transition state (36). The driving force of the reaction in this case is the reduced ring strain of the product molecule 24. 

The ring size also influences the course of the reaction. For example, both the cis- and /ranj-divinylcy-clopentanols lead to a single ( )-nonenone, while for the divinyldodecanols the trans isomer produces only the ( )-enone, whereas the cis isomer produces mostly the (Z)-enone (Scheme 46)." Relief of ring strain can provide a considerable driving force for the rearrangement, such as in the example shown in Scheme 47, where rearrangement of the carbinol occurs at only 50 C." ... 

Ring strain makes trimethylsilylcyclopropenes susceptible to attack by Ar3P such that further reaction with aldehyde is realized. A Brook-type rearrangement drives the reaction to completion by expulsion of the ArsP from the adducts. In practice, tris(2,4,6-trimethoxy-phenyl)phosphine is used instead of Ph3P (much better yields). ... 

Some examples of Type II dyotropic rearrangements by double C—H transfer are given in Scheme 6.9. The driving force for the reaction is provided by the proximity of the interacting orbitals and the rigid carbon framework. Furthermore, the reaction is accelerated by the release of the ring strain at the receptor 7t-bond and aromatization of the diene ring. 

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