Structural strain

This interpretation is supported by results on the acetolysis of the bicyclic tosylates 9 and 10. With 9, after three months in acetic acid at 150°C, 90% of the starting material was recovered. This means that both ionization to a cyclopropyl cation and a concerted ring opening must be extremely slow. The preferred disrotatory ring-opening process would lead to an impossibly strained structure, the /ran -cyclohexenyl cation. In contrast, the stereoisomer 10 reacts at least 2x10 more rapidly because it can proceed to a stable cis-cyclohexenyl cation ... 

There is some increase in selectivity with functionally substituted carbenes, but it is still not high enough to prevent formation of mixtures. Phenylchlorocarbene gives a relative reactivity ratio of 2.1 1 0.09 in insertion reactions with i-propylbenzene, ethylbenzene, and toluene.212 For cycloalkanes, tertiary positions are about 15 times more reactive than secondary positions toward phenylchlorocarbene.213 Carbethoxycarbene inserts at tertiary C—H bonds about three times as fast as at primary C—H bonds in simple alkanes.214 Owing to low selectivity, intermolecular insertion reactions are seldom useful in syntheses. Intramolecular insertion reactions are of considerably more value. Intramolecular insertion reactions usually occur at the C—H bond that is closest to the carbene and good yields can frequently be achieved. Intramolecular insertion reactions can provide routes to highly strained structures that would be difficult to obtain in other ways. 

Adsorption of caesium at a Cu(110)-O surface exhibits different structural features (Figure 6.7). First there are well-ordered c(2 x 4) domains, but second, and somewhat unusual, they coexist with features which have no simple relationship with either the Cu(110) substrate atoms or the c(2 x 4) structure.16 The rows are bent , suggestive of strained structures associated with the (2 x 1)0 rows modified by caesium adsorption. 

On the other hand, the ladder polysilanes containing a syn moiety have more strained structures (Fig. 8). The Si-Si bond lengths of syn-2... 

Since the reverse reaction is thermally disallowed, the products may be unexpectedly stable. Thus cyclopentadiene can be partially converted photochemically into bicyclo [2,1,0] pentene, which in spite of having a strained structure has a half life of about two hours at room temperature. 

The computed transition state also rationalizes the observed stereochemistry. For the current reactions, a transition structure represented in Figure 31 appears to be appropriate. This structure indicates that the three substituents R and R are all equatorial and give rise to the anti product. The corresponding syn product would require a highly strained structure in which one R group is axial. 

As expected, the metathesis polymerization of more strained cycloalkenes, such as cyclobutene, occurs more rapidly than less strained structures such as cyclopentene. 

A) in the D3d conformation. The conformations can be described in terms of the torsion angles around the crown ether ring. A common notation involves a consideration of the R—O—CH2—CH2—O—R unit and whether the torsion angles are primarily synclinal (gauche, g) or antiperiplanar (a). In the D3d conformation B the (aga ag a aga) pattern is found while in conformation A the (aga ag g aaa ag a) pattern is adopted. Conformation B represents a minimum torsional strain structure,... 

Even though it is a trisubstituted alkene, owing to its strained structure, 1-methylcyclobutene reacts under mild metathesis conditions 122... 

Aside from the feet that II is a strained structure and therefore less stable than I, it will not contribute to the resonance of NzO because the bond angle is 180s in I and 60 in II. For any intermediate hybrid, the contribution of either I Or II would be unfavorable because of the high energy cost when 1 is bent or when 11 is opened up. 

Alkyl and cycloalkyl substituents seem to have little effects on the geometry of the cyclopropane ring. There are hardly any reliable data on ring distortion in simple molecules. Recent studies have concentrated on highly strained structures (Table 2). 

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