Strained ring ethers

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. 

Another circumstance that increases leaving-group power is ring strain. Ordinary ethers do not cleave at all and protonated ethers only under strenuous conditions, but epoxides are cleaved quite easily and protonated epoxides even more easily. Aziridines and episulfides, three-membered rings containing, respectively, nitrogen and sulfur, are also easily cleaved (see p. 458). ... 

Several factors and observations support the route proposed in Scheme 10 (1) Due to steric factors, the styrenyl alkene is expected to react preferentially (versus the neighboring disubstituted cyclic olefin see below for further discussion). (2) Involvement of tetracyclic intermediates such as 43 provides a plausible rationale for the reluctance of six-membered ring ethers [46 in Eq. 4] to participate in the catalytic rearrangement and for the lack of reactivity of cyclopen-tenyl substrates [48 in Eq. 5] because of the attendant angle strain, the generation of the tetracyclic intermediate is not favored. (3) Reactions under ethylene atmosphere inhibit dimer formation, since 44 is intercepted with H2CCH2, rather than 41 [19]. 

Azatricyclo[2.2.1.02 6]hept-7-yl perchlorate, 2368 f Azetidine, 1255 Benzvalene, 2289 Bicyclo[2.1.0]pent-2-ene, 1856 2-/ert-Butyl-3-phenyloxaziridine, 3406 3 -Chloro-1,3 -diphenyleyclopropene, 3679 l-Chloro-2,3-di(2-thienyl)cyclopropenium perchlorate, 3388 Cyanocyclopropane, 1463 f Cyclopropane, 1197 f Cyclopropyl methyl ether, 1608 2,3 5,6-Dibenzobicyclo[3.3.0]hexane, 3633 3,5 -Dibromo-7-bromomethy lene-7,7a-dihy dro-1,1 -dimethyl-1H-azirino[l,2-a]indole, 3474 2.2 -Di-tert-butyl-3.3 -bioxaziridinc, 3359 Dicyclopropyldiazomethane, 2824 l,4-Dihydrodicyclopropa[ >, g]naphthalene, 3452 iV-Dimethylethyl-3,3-dinitroazetidine, 2848 Dinitrogen pentaoxide, Strained ring heterocycles, 4748 f 1,2-Epoxybutane, 1609 f Ethyl cyclopropanecarboxylate, 2437 2,2 -(l,2-Ethylenebis)3-phenyloxaziridine, 3707 f Methylcyclopropane, 1581 f Methyl cyclopropanecarboxylate, 1917 f Oxetane, 1222... 

Review articles of synthetic importance have featured eliminations involving carbon-halogen bonds and leading to highly strained rings,81 elimination and addition-elimination reactions,82 enol ether formation from unsaturated acetals,83 and the Wittig reaction and related methods.84... 

Coupling of silyl enol ethers or boron enolates with Co2(CO)6-stabilized carbocations, generated via Lewis acid treatment of the appropriate propargyl ethers or aldehydes (aldol reaction), via the Nicholas reaction has been used to obtain large, highly strained, ring ketones. 

Another circumstance that increases leaving-group power is ring strain. Ordinary ethers do not cleave at aU and protonated ethers only under... 

Tanaka [155] has attempted to make a quantitative estimate of the contribution of ring strain and basicity to reactivity of cyclic ethers in cationic copolymerization. Free energy of polymerization was used as a measure of ring strain. The relationship he derived related the logarithm of relative reactivity, l/r , of m-membered ring ethers with i substituents to n-membered ring compounds with j substituents to a linear combination of the differences in basicity, A(pXb)m,i- ,/ and in free energy, viz. 

Isolated triple bonds, on the other hand, react rather slowly with lithium dust except in strained rings. Thus we succeeded in adding lithium to the triple bond of cyclooctyne 134 even at —35 °C in diethyl ether to yield cis-1,2-dilithiocyclooctene 136 as a yellow solution containing up to 20% of the 1,4-dilithio-1,3-butadiene derivative 137, the product of dimerizing addition... 

The reaction coordinate diagrams for nucleophilic attack of hydroxide ion on ethylene oxide and on diethyl ether. The greater reactivity of the epoxide is a result of the strain (ring strain and torsional strain) in the three-membered ring, which increases its free energy. 

This chemistry is quite different from the examples we have just seen. The starting material has a bridged bicyclic structure and was made by a Diels—Alder reaction. Fragmentation is initiated by formic acid (HCO2H), which protonates the tertiary alcohol and creates a tertiary carbocation. The ether provides the push. More serious electronic interactions are needed in this fragmentation as the C—C bond being broken is not in a strained ring. 

Polymerizations of oxiranes or epoxides occur by one of three different mechanisms (1) cationic, (2) anionic, and (3) coordination. In this respect, the oxiranes differ from the rest of the cyclic ethers that can only be polymerized with the help of strong cationic initiators. It appears, though, that sometimes coordination catalysis might also be effective in polymerizations of some oxetanes. The susceptibility of oxirane compounds to anionic initiation can be explained by the fact that these are strained ring compounds. Because the rings consist of only three atoms, the electrons on the oxygen are crowded and are vulnerable to attack. ... 

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