Rings strain and reactivity

Introduction of additional unsaturation in cyclooctyne leads to increasing ring strain and reactivity depending on the relative position and the number of double bonds, as is shown in the following scheme106,144). 

The intermolecular enyne cross metathesis, and consecutive RCM, between a terminal alkyne and 1,5-hexadiene produces cyclohexadienes, by cascade CM-RCM reaction, and trienes, formed during the sole CM step. Studies of various parameters of the reaction conditions did not show any improvement of the ratio of desired cyclohexadiene product [25] (Scheme 12). The reaction with cyclopentene instead of hexadiene as the alkene leads to 2-substituted-l,3-cycloheptadienes [26]. After the first cyclopentene ROM, the enyne metathesis is favored rather than ROMP by an appropriate balance between cycloalkene ring strain and reactivity of the alkyne. 

These data indicate that the rate constants of reactions between various monomers and model active species, both ionic and covalent are the function of monomer basicity on the other hand no correlation between ring strain and reactivity was found (44). 

All the peaks are somewhat upfield of the aromatic region, suggesting polyene character. This stmcture would also be consistent with the observed reactivity since the polyene has a. quinodimethane structure (see Section 11.3). The implication of a nonaromatic stmcture is that the combination of ring strain and the antiaromaticity associated with the four--nembered ring results in a localized system. ... 

Recently, 2,6-dioxabicyclo[2.2.1 Jheptane 33 and 2,7-dioxabicyclo[2.2.1 Jheptane 34 were synthesized, and their rates of acidic solvolysis were compared293. Table 1 summarizes the data, along with those for relevant bicydic acetals determined in a similar manner. Tremendous differences in reactivity among these compounds are noteworthy. The approximate relative reactivities under these conditions spun more than five powers of ten. These differences are attributable to ring strain and anomeric... 

The highly strained and reactive 2iT-azirines have been extensively studied for various synthetic purposes, such as ring expansion reactions, cycloaddition reactions, preparation of functionalized amines and substituted aziridines. The older literature on azirines in synthesis has extensively been reviewed [69]. Concerning azirines with defined chirality only scarce information is available. Practically all reactions of azirines take place at the activated imine bond. Reduction with sodium borohydride leads to cz5-substituted aziridines as is shown in Scheme 48 [26,28]. 

The preparation illustrates the utility of the reaction in preparing the highly strained and reactive nortricyclanone, a compound of current interest in studies of strained ring systems. 

Epoxides are much more reactive than simple ethers due to ring strain, and are useful intermediates because of their chemical versatility. They undergo nucleophilic substitution reactions with both acids and bases to produce alcohols (see Sections 4.3.7 and 5.5.4). 

The enhanced reactivity of SCB-derived enol ethers is attributed to the combination of ring strain and the potential for silicon to expand its coordination number form penta- to hexacoordinate compounds. Specifically, for SCBs, the reaction with nucleophiles allows for relief of the strain energy via rehybridization of the geometry at silicon from tetrahedral to trigonal bipyramidal (tbp) upon formation of a pentacoordinate species. 

Like cyclopropane, epoxides have a large amount of ring strain and are much more reactive than normal ethers. Because of this ring strain, one carbon-oxygen bond of an... 

With the initial synthesis of cyclopropane in 1882 , and the report of its thermal structural isomerization to propene in 1896, this simplest of cyclic hydrocarbons began its extraordinarily fruitful stimulation of fresh insights on fundamental problems in organic chemistry, ranging from basic concepts of ring strain and structural isomerism to questions of thermochemistry and reactivity and of a aromaticity And from the beginning there was controversy, extending a few years before suitably authoritative commentators confirmed the fact that cyclopropane is indeed converted thermally to propene. ... 

Tanaka [198] made an attempt to determine the contribution of ring strain and basicity to the reactivity of cyclic ethers in copolymerization by means of Gibbs polymerization energy. He derived a relation between the relative reactivities of an m-membered ring with i substituents to that of an n-membered cyclic ether with j substituents. He included a linear combination of their basicity differences, A(pKb)m and of Gibbs energy, A(AG)m., in his relation (a, b, and c are constants)... 

The double bond in thiirene, thiirene-1-oxide, and thiirene-1,1-dioxide is expected to increase ring strain and lead, in general, to enhanced reactivity in any reactions that would lead to strain relief. Such relief would result from ringopening reactions, extrusion of S, SO, or SO2, or additions to the double bond that convert sp - to sp -carbon. Moreover, conjugation between the 7t-bond and sulfur, with its substituents, might be expected to lead to absorption of light and photochemical reactions. 

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