Angle Strain and Its Effect on Reactivity

The more strain relieved by the bond rupture, the more reactive is the molecule. The substantially increased stabflity of [l.l.ljpropellane is due to the fact that not as much strain is relieved at the diradical stage because the diradical remains highly strained.  

Alkenes exhibit large strain energy when molecular geometry does not permit all the bonds to the two 5 -hybridized carbons to be coplanar. An example that illustrates this point is -cycloheptene  

With only five methylene units available to bridge the tmns double bond, the molecule is highly strained and very reactive. Isolation of. E-cycloheptene has not been possible, but evidence for its formation has been obtained by trapping experiments. The alkene is generated in the presence of a reagent expected to react rapidly with it— in this case, the very reactive Diels-Alder diene 2,5-diphenyl-3,4-isobenzofuran. The adduct that is isolated has the structure and stereochemistry anticipated for that derived from E-cycloheptene. The lifetime of -cycloheptene has been measured after generation by photoisomerization of the Z-isomer. The activation energy for isomerization to Z-cycloheptene is about 17 kcal/mol. The lifetime in pentane is on the order of minutes at 

Although -cyclohexene has been postulated as a reactive intermediate, it has not been observed directly. MO calculations at the 6-31G level predict it to be 56 kcal/mol less stable than the Z-isomer and yield a value for the barrier to isomerization of about 15 kcal/mol. 

The geometry of bicychc rings can also cause distortion of the alkene bond from coplanarity. An example is bicyclo[2.2.1]hept-l-ene  

Bicyclo[1.1.0]butane is an example of a molecule in which severe angle strain results in decreased stability and greatly enhanced reactivity. The bicyclo[1.1.0]butane ring has a strain energy of 63.9kcal/mol, and the central bond is associated with a relatively high 

HozmThe Chemistry of the Cyclopropyl Group,Vait 2, S. PataiandZ. Rspf ip rt,eds., Wiley, Chichester, UK., 1987, pp. 1121-1192 M. Christl, Adv. Strain Org. Chem. 4 163 (1995). 

Another manifestation of the relatively small release of energy associated with breaking the central bond comes from MP4/6-31G calculations on the reverse ring closure.  

Many of the conformational effects on reactivity can be described and analyzed in terms of the difference between van der Waals interactions in the ground state and the transition state. Some cyclic molecules contain another type of strain, known as angle strain, resulting from distortion of bond angles from optimal values. We would now like to consider how such distortions might effect reactivity. 

The molecule tetrahedrane represents still one more increment of angle strain. 

Kinetic and product structure studies of the reaction of cyclopropanes and bicyclic compounds containing three-membered rings have shown that the protonation of the cyclopropane ring is followed by addition of the nucleophile at the most substituted carbon. The product composition is determined by the ability of the more highly substituted carbons to sustain more of the positive charge. The substitution at the incipient carbocation is the most important factor in determining the degree of reactivity. The relative rates of cyclopropane and its methyl, 1,1- 

The parent compound has not yet been synthesized but the tetra-t-butyl derivative is known. Molecular orbital calculations estimate that the breaking of a C—C bond in tetrahedrane would require only about 10 kcal/mol, indicating that the molecule would have only a short existence even at quite low temperatures. The tetra-t-butyl derivative, however, is stable up to 100 C.  

Cycloalkene H (trans cis) (kcal/mol) Ref. SECTION 3.8. ANGLE STRAIN AND ITS EFFECT ON 

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