Strain release

Recovery is the strain response that occurs upon the removal of a stress or strain. The mechanics of the recovery process are illustrated in Fig. 2-34, using an idealized viscoelastic model. The extent of recovery is a function of the load s duration and time after load or strain release. In the example of recovery behavior shown in Fig. 2-34 for a polycarbonate at 23°C (73°F), samples were held under sustained stress for 1,000 hours, and then the stress was removed for the same amount of time. The creep and recovery strain measured for the duration of the test provided several significant points. 

Hetero substituted 2-cyclopropylideneacetates are ring-strain activated acrylates, highly reactive dienophiles in Diels-Alder reactions, but also powerful Michael acceptors. The reactivity of these compounds is enhanced by the same strain release in the Diels-Alder cycloadditions as well as in the 1,4-additions, and indeed the borderline between tandem Michael-cyclization and Diels-Alder-type cycloaddition is not well defined in many cases. 

While the haloform reaction normally only cleaves methyl ketones because of the structural requirements for the a,a,a-tribromomethyl ketone to induce fragmentation, the strain release that accompanies cleavage of a cyclobutanone permits extension... 

It is noteworthy that Br2 addition to 3 in aprotic and protic solvents gives exclusively the anti 1,2-addition product. For diene 55, the intramolecular nucleophilic attack of the Z-double bond on the cationic center is exclusive, even in the presence of Et3N-3HF. This has been ascribed to a large strain release in the formation of the d.v-dccalin system from the highly strained medium-sized system. 

The lactonization of a crowded hydrocoumarin (78), or more generally reactions driven primarily by strain release, can be regarded... 

Next, enantiopure silicon allylation reagent will be presented, which already inherits Lewis acidity. It is accepted that Lewis acidity of silicon, as well as its high tendency to expand valence shell, increases [59, 60] if it is tetravalent and incorporated into strained four- or five-membered ring systems (strain-release Lewis acidity) [61]. This corresponds to smaller energy gaps between sp and dsp orbitals of a strained system as compared to an acyclic species. 

Scheme 5.23) [37]. This unusual spontaneous aldehyde allylsilylahon in the absence of a Lewis acid promoter/catalyst was attributed to strain-release Lewis acidity. 

Page, M. I., and Webster, P. (1990), The hydrolysis of azetidinyl amidinium salts. Part 1. The unimportance of strain release in the four-membered ring, J. Chem. Soc. Perkin Trans., 2, 805-811. 

One of the primary advantages to the use of SCB reagents is the ease by which they undergo ring-opening processes. These strain-release events can trigger many important processes in organic synthesis. 

The larger the population in which the bulky face of the isopropyl group is directed toward the sulfur, the larger is the strain release in the transition state during alkylation (74JCS(P2)1304 76JA2847). 

Angle bending is the next step in energy minimization. It occurs when strain release by rotation of substituents is not enough to accommodate all the steric strain or when rotation is not possible, as with spherical substituents. Quantitative results are scarce an interesting example is the methylation of a series of 3,5-dialkylimidazo(l,2-a)pyridines (81T83 81T91) (Scheme 114). 

---