Strain in Three-Membered Rings

5 Ingraham, L. L., in Steric Effects in Organic Chemistry, Edited by Newman, M. S., John Wiley, New York, 1956, Chapter 11. 

7 Beaven, G. H., Steric Effects in Conjugated Systems, Edited by Gray, G. W., Butterworths, London, 1958, Chapter 3. 

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Strain in three-membered rings such as thiirane and disilathiirane ((H2Si)2S) was examined in terms of a- and 7t-delocalization <2003PCA2860>. Those terms denote the relief of strain by electron delocalization from a doubly... 

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... 

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 (106 kJ mol-1) by -11 kJ mol-1 to oxelan-2-one (95 kJ mol-1) (corrected for electronic effects) and 4-methyleneoxetan-2-one (95 kJ mol" ). In contrast, an increase of 10 kJ mol 1 over the value for cyclobutane (111 kJ mol-1) is observed on going to both methylenecyclobutane and l,3-bis(methylene)cyclobutane. 

Formation of two single bonds from a carbon atom is also well known for building up carbon skeletons. In diis process, a three-membered ring is formed by reaction of a difunctional carbon atom widi an olefin. Because of die strain of three-membered rings, their synthesis is not trivial and a small number of reactions which effectively append three-membered rings to molecules are important and widely used. 

The ring strain inherent in three-membered rings makes them susceptible to C—C bond cleavage. Reaction of a Ni(0) complex with 3,3-dimethylcyclopropene in ether solution at —78 C gives a dark green, slightly air-sensitive material ... 

We have pointed out earlier that the C-C bonds in strained hydrocarbons have negative electrostatic potentials near their midpoints [39,53]. (This is not observed for more typical C-C bonds, such as those in ethane and propane.) In general, these potentials are more negative, and the calculated strain in the individual bonds is greater, in three-membered rings than in four-membered rings [53-58]. (We measure bond strain by the extent to which the actual bond path differs from a reference path [53,54,57].)... 

These factors - Is,min. electrostatic potential, and bond strain - are all consistent with a relatively high degree of reactivity toward electrophiles on the part of C-C bonds in three-membered rings. The ethylene molecule also has both a negative potential and an Is,min near the midpoint of its double bond [20,59], and indeed it has long been recognized that cyclopropane and bicyclobutane have some olefin-like properties [60-64]. 

A sigmatropic rearrangement involving a three-membered ring. The o-bonds in three-membered rings are strained and more reactive than normal o-bonds and take part readily in peri cyclic reactions. 

Torsional strain is the second factor in cyclopropane s large ring strain. The three-membered ring is planar, and all the bonds are eclipsed. A Newman projection of one of the carbon-carbon bonds (Fig. 3-16) shows that the conformation resembles the totally eclipsed conformation of butane. The torsional strain in cyclopropane is not as great as its angle strain, but it helps to account for the large total ring strain. 

The formation of p-trifluoromethyl ketones has been achieved by exploiting the ring strains of three-membered rings [47]. As illustrated in Scheme 15, cyclopropyl alcohols react readily with a small excess of reagent 2 in the presence of a catalytic amount of Cu to furnish the desired p-trifluoromethyl ketones. 

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