Three-membered rings, ring strain

What accounts for the much faster reaction of the trans-substituted compound In this reaction, the thio substituent is an intramolecular nucleophilic catalyst. It displaces the chloro substituent by attacking the back side of the carbon to which the chloro substituent is attached. Back-side attack requires both substituents to be in axial positions, and only the trans isomer can have both of its substituents in axial positions (Section 2.14). Subsequent attack by water or ethanol on the sulfonium ion is rapid because the positively charged sulfur is an excellent leaving group and breaking the three-membered ring releases strain. 

In the sequence shown, 64 requires Fe(II), dioxygen ascorbate, and CO2. The first step is a radical reaction that transfers one electron to form a radical cation (65) and a reduced form of iron, iron (III) from iron (IV). The three-membered ring is strained (Chapter 8, Section 8.5.3) and opens to form radical 66. This radical reacts with the Fe(III) species to generate an iron intermediate (67), and an internal acid-base reaction generates the OH group on iron and an imine unit (C=NH) in 68. 

Figure 7-2. Strain energies [kj/mol] of three-membered ring systems.

Disubstituted cyclopropanes exemplify one of the simplest cases involving stabil ity differences between stereoisomers A three membered ring has no conformational mobility so the ring cannot therefore reduce the van der Waals strain between cis sub stituents on adjacent carbons without introducing other strain The situation is different m disubstituted derivatives of cyclohexane... 

The chief reason why ethylenebromonium ion m spite of its strained three membered ring IS more stable than 2 bromoethyl cation is that both carbons and bromine have octets of electrons whereas one carbon has only six electrons m the carbocation... 

Ethylene oxide is a highly reactive compound, and so is used iudustriaHy as an iatermediate for many chemical products. The three-membered ring is opened iu most of its reactions. These reactions are very exothermic because of the tremendous ring strain iu ethylene oxide, which has been calculated (39). Reviews of ethylene oxide reactions are given iu References 40 and 41. 

Exocyclic unsaturation can stabilize small ring heterocycles. In three-membered rings it is difficult to separate the contributions from increased angle strain and from electronic interactions between the unsaturation and the heteroatom. In four-membered rings such separation has been done 74PMH(6)199, p. 235). The CRSEs change from oxetane... 

The three-membered epoxy ring is highly strained and is reactive to many substances, particularly by with proton donors, so that reactions of the following schematic form can occur ... 

Compound (223), the 6a,7a-methano homolog of dienone (174), is extraordinarily stable on irradiation in dioxane, while the 6, 7 -isomer (224) isomerizes readily to the bicyclohexenone (225). The apparent failure of (223) to react may be attributed to the high steric strain which would be introduced by the two adjacent three-membered rings on ring B upon 1,5-bridging. This could cause reversal of primary product formation [cf. (226)] rather than ensuing formation of the corresponding zwitterion and... 

Thus epoxides, like cyclopropanes, have significant angle strain. They tend to undergo reactions that open the three-membered ring by cleaving one of the car bon-oxygen bonds. 

Three-membered rings with one hetero atom were known in the 19th century. Today they are of great interest, both scientifically and technically. Because of ring strain, they are energy-rich compounds comparable with cyclopropane. 

Unlike other ethers, epoxide rings con be cleaved by base as well as by acid. Although an ether oxygen is normally a poor leaving group in an reaction (Section 11.3), the strain of the three-membered ring causes epoxides to react with hydroxide ion at elevated temperatures. 

An initial attack of a lithium reagent on the sulfur atom of 16, leading to alkenes, has been discussed in the previous section. The similarity in the chemical consequences of the electrophilicity of both the sulfone and sulfoxide functional groups in strained three-membered ring systems is thus established. 

Presumably, the heterolytic scission of the carbon-sulfur bond in the oxide is assisted by the hydrogen bonding, in addition to the inherent strain of the three-membered ring. 

The unique characteristics of three-membered ring sulfoxides and sulfones raise the question Are the major features observed in the three-membered ring series extended into the still small and strained four-membered ring series, or will the latter be more reminiscent of the larger ring and acyclic sulfoxides and sulfones ... 

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