Double bond torsion, substituent effect

In 1972, Mock considered double-bond reactivity and its relationship to torsional strain, by which he understood the strain imposed on a double bond in medium-ring fra 5-cycloalkenes or by steric compression of large cis substituents [28]. He argued that the loss of 7t overlap due to a torsion about the double bond can be partially compensated by rehybridization in these two situations, leading, respectively, to syn and anti pyramidalization of the double bond consequently, such bonds will favor different modes of addition (cis and trans). The proposition was supported by examples of X-ray structures of strained olefins, STO-3G energy calculations for the twisted and pyramidalized ethylene geometries, and by analysis of the out-of-plane vibrational frequencies of ethylene. Mock concluded that small ground-state distortions may produce sizable effects in the transition states. 

The effect of introducing /j -hybridized atoms into acyclic molecules was discussed in Section 2.2.1, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are seen when sp centers are incorporated into six-membered rings. Whereas the energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecy-clohexane ° and to 4.9 kcal/mol in cyclohexanone. The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffraction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. The C(l)—C(2) bond length is 1.335 A, and the C(l)-C(2)-C(3) bond angle is 123°. The substituents at C(3) and C(6) are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial. 

A theoretical study of the rotatory strength of truus-cyclo-octene and other twisted olefins suggests that the Cotton effect may be dominated by the torsional effect when the torsional angle is ca. 10° or larger, whereas dissymmetrically disposed substituents are probably more important when the olefin is only slightly twisted. The possible role of a torsional term for some steroidal olefins clearly has to be considered, although the double bond in cholest-5-ene is known from early X-ray data to be essentially non-twisted. ... 

The antibacterial action of this family is exerted on the synthesis of new cell wall, and by acylating the same enzyme site attacked by the penicillins (Abraham, 1962). The double-bond in the six-membered ring of cephalosporin-derivatives activates the p-lactam group in the same way as the torsional effect of the 5-membered ring in penicillin does (Sweet and Dahl, 1970). The sulphur atom of cephalosporins may be replaced by oxygen or methylene substituents without loss of antibacterial activity (Gama and Christensen, 1974). 

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