Relief of strain

Fragmentation reactions are particularly common in small rings. Relief of strain and the gain in stability in forming certain common fragments (such as N2, CO2), as felt in the... 

It appears that the -cycloalkenes can be protonated exceptionally easily, because of the enormous relief of strain that accompanies protonation . The -isomers of cyclopen-... 

The driving force for ring expansion of 17-hydroxy-20-keto steroids probably comes from relief of strain accompanying conversion of trans-fused hydrindanes to tra/75-fused decalins. The greater susceptibility to rearrangement of 17/ -hydroxy-20-ketones as compared to the 17a-hydroxy-20-... 

Epoxides, in contrast to ethers, readily undergo nucleophilic attack, resulting in ring opening and relief of strain. Ring opening proceeds by a different mechanism, and may lead to different products, depending on reaction conditions. 

The Ag-C bonds tend to be asymmetric study of silver cycloalkene complexes shows their stability to decrease in the order C5 > C6 > C7 > C8, corresponding to relief of strain in the cyclic molecules consequent upon the lengthening of the double bond on coordination. 

The mechanisms of these reactions are not completely understood, although relief of strain undoubtedly supplies the driving force. The reactions are thermally forbidden by the orbital-symmetry rules, and the role of the catalyst is to provide low-energy pathways so that the reactions can take place. The type 1 reactions are the reverse of the catalyzed [2 + 2] ring closures discussed at 15-61. The following... 

The cage-like phosphonium salt (17) with phenyl-lithium in THF gave the phosphorane (18) which probably owes its great stability to the relief of strain in the ring structure on changing the bond angle at phosphorus to 90°. For the photolysis of (18) see Chapter 10, Section 1. 

The thermolysis of acyclic- and/or six- and larger ring sulfoxides to yield olefins and sulfenic acids is well documented . The formation of allylic sulfenic acids and thiosulfinates in the thermolysis of thiirane oxides containing hydrogen on the a-carbon of the ring substituent (which is syn to the S—O bond) has been discussed previously in terms of /i-elimination of hydrogen, which is facilitated by relief of strain in the three-membered ring (Section llI.C.l). 

This reflects the relative ease with which the C—H bond in the alkane precursor will undergo homolytic fission, and more particularly, decreasing stabilisation, by hyperconjugation or other means, as the series is traversed. There will also be decreasing relief of strain (when R is large) on going from sp3 hybridised precursor to essentially sp2 hybridised radical, as the series is traversed. The relative difference in stability is, however, very much less than with the corresponding carbocations. 

The marked shifts observed when going from the seven-ring cyclopropenone 39 to the six-ring cyclopropenone 40 have been interpreted as an indication of a change in electron distribution for 40 a stronger contribution of resonance structures b might lead to relief of strain and thus stabilize the fiised-ring system. 

The di-tert. butyl substituted calicene 230 was calculated to possess considerable non-bonding (steric) interactions in the planar geometry184. The relief of strain when going to a perpendicular transition state is reflected by the low coalescence temperature of tert. butyl signals found on temperature-dependent H-NMR spectroscopy187. ... 

Radialene 52 has been envisaged as a precursor to hexaethynyl[3]radialene, a potential building block for carbon networks. However, desilylation under very mild conditions led to an unstable product of so far unknown identity. In this context, it should be mentioned that according to thermochemical calculations, the still unknown hexaethynyl[3]radialene has an increased conjugation energy with respect to vinylacetylene, probably because of partial relief of strain in the radialene corc26b. A remarkable aspect of 52 is its color the crystals are deep-red and a hexane solution has a purple color. In this respect, 52 differs from the yellow radialenes 50 and 51 and resembles [3]radialenes such as 27 and 38, which have much more extended --systems. 

The situation is somewhat more favorable for the rearrangement of olefin 27 to give carbene 28. Here, the energy difference of E(28) — E(27) is computed as only 17.3 kcal/mol, indicating the relief of strain energy of 27. 

Whereas lactone annulation invokes a relief of strain of the four membered ring by migration of the ring bond to an electron deficient oxygen, a similar migration to an election deficient carbon creates a cyclopentanone synthesis (Eq. 73). The release of approximately 84 kJ/mole (20 kcal/mole) provides a strong driving force. Thus, the 1,1-cyclobutanone annulation of ketones translates into a 1,1-cyclopentanone annulation. 

The effect of substituents and the relief of strain is also visible in certain cyclic ethers. The presence of adjacent oxygen in the following cyclic ether assists ring opening smoothly at 60°C and... 

Lactams are generally more reactive toward nucleophiles than are normal amides. The ease of the nucleophilic attack on the lactam carbonyl group is usually attributed to either relief of strain upon opening the ring [68], or to a reduction in the usual amide resonance due to nonplanarity of the bicyclic system [69]. However, the evidence to support unusual strain in the ring or reduced amide resonance in /3-lactam antibiotics is ambiguous. 

A carbon labelling study has elucidated the rearrangement mechanism for formation of chalcone (97) which accompanies formation of (91) by the expected vicinyl elimination of trimethylsilyl and benzotriazolyl groups from 2-benzotriazolyl-2-aryl-3-ketopropylsilanes, on reaction with fluoride ion in DMF. ° Thus, it has been possible to distinguish between the two alternative mechanisms depicted in Scheme 11 (via intermediates (93) or (95), respectively, by determining the fate of the labelled quaternary carbon of substrate (89). The results are consistent with the formation of a cyclopropane intermediate (95) which subsequently ring opens, with relief of strain, to form delocalized carbanion (96), from which the chalcone (97) is obtained (labelled... 

General acid catalysis by glutamic acid-35 represents at present the mechanism for lysozyme best able to explain the kinetic and structural data. For it to occur, however, distortion of the hexose ring in subsite D to a half-chair must take place so that relief of strain in the transition state will make bond breaking sufficiently easy. A question that must be answered for this picture to be tenable is whether relief of strain can so greatly facilitate bond breaking when the carbonium ion is a glycosyl ion [see also the discussion in Fife (1972) and Atkinson and Bruice (1974)]. 

Because the clays have a basic layered structure of aluminosilicate sheets the expectation was a platy habit. The creation of tubular forms was studied by Bates (1959). He postulated that the curved forms, observed by electron microscopic investigation of halloysite and chrysotile, originated through the relief of strain between sheets of unequal dimensions. 

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