Relief of ring strain

The relative stabilities of the species involved appear to be responsible for the stereochemical outcomes. Relief of ring strain must play a role in determining the course of the reaction. An explanation for the different reaction paths on using different Grignard reagents must wait further experimentation. 

There is evidence that relief of ring strain is not the only factor responsible for the high rates of ring-opening of three-membered rings Di Vona, M.L. Illuminati, G. Lillocci, C. J. Chem. Soc., Perkin Trans. 2,1985, 1943 Bury, A. Earl, H.A. Stirling, C.J.M. J. Chem. 

On the contrary, rearrangements of cyclobutylhalocarbenes to the corresponding halocyclopentenes could be anticipated to have lower energy barriers, because of greater relief of ring strain and less hyperconjugative stabilization in the carbenes... 

Cyclobutanones were found to be much more reactive under these conditions, presumably due to relief of ring strain (131). Racemic cyclobutanone (192) is oxidized under the conditions described above to provide lactones 193 and 194 in a ratio of 55 45, Eq. 111. The expected lactone product 193 is formed in 67% ee while the abnormal product 194 is formed in 92% ee. The major enantiomers of the two products are complementary, resulting from enantiomeric ketones. 

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. " ... 

The tertiary carbocation contains a strained four-membered ring, and an alkyl shift allows relief of ring strain, generating five-membered rings and a secondary carbocation. It would appear that the relief of ring strain more than compensates for the loss of tertiary character in the carbocation. Thus, it is the secondary carbocation that interacts with a nucleophile. In this case, the nucleophile is water, the major component of the aqueous HCl. The product is thus isoborneol. 

Concerning steric factors, 43 is attacked in the most hindered position ( inverse effect of substitution ) likewise, 39 is attacked at the most hindered carbon. Obviously, the transition states for the formation of 44 or 50 show limited sensitivity to the degree of substitution, and the relief of ring strain is a more significant factor than the steric hindrance in the transition state. On the other hand, steric factors are important in systems such as P-phellandrene radical cation 40 which is attacked at the xo-methylene carbon (most easily accessible), or the tricyclane radical cation 56 which is attacked at the less hindered 3° carbon further removed from the dimethyl-substituted bridge (approach a). Both reactions also benefit Irom the formation of the most highly substituted, hyperconjugatively stabilized free radicals. 

ROMP is generally an irreversible process, and relief of ring strain is the driving force for the forward reaction. Reversibility (with the formation of macrocyclic oligomers) is possible for less strained cycloalkenes like cyclooctene when the lifetimes for propagating chains are high. 

Strained rings may be opened by a ruthenium carbene-catalyzed reaction with a second alkene following the mechanism of the Cross Metathesis. The driving force is the relief of ring strain. As the products contain terminal vinyl groups, further reactions of the Cross Metathesis variety may occur. Therefore, the reaction conditions must be optimized to favour the desired product. 

Among the earliest examples of symmetrical bifunctional radical cations, the distonic trimethylene species (103) invoked by Williams and coworkers [293, 296, 297] are stabilized solely by hyperconjugation. The main rationale for their formation would be the relief of ring strain. On the other hand, the non-vertical radical cations 137 derived from cyclopentadiene dimers [386-389] are favored by two elements of allylic stabilization. This radical cation has three eonformat-... 

However, factors such as ring strain or conjugation can make one product predominate at equilibrium. For example, the cyclopropane derivative shown in die following equation is stable at — 20°C but rearranges completely to 1.4-cycloheptadiene at room temperature. The equilibrium is driven toward the product by relief of ring strain. 

---