Olefins torsionally distorted

Structural features of olefins with distorted double bonds have been discussed within the deformation space defined by the eight bond angle deformations. The out-of-plane bond angle distortions are of particular interest because they are involved in addition reactions of the double bond. The symmetrical Blg-type deformation is related to concerted anti-additions, whereas the J3lu-type distortion (cf. Table 1) is appropriate for concerted syn-addition and those reactions that involve three-center intermediates and the formation of transition metal complexes. Twist or torsion is due to the Alu-type oop distortion and may be related to addition reactions, which in principle would lead—in the extreme case of a 90° twist angle—to an eclipsed rather than a staggered arrangement. 

Using the CFF method, a torsional angle of 75° has been found, which upon improvement of the torsional potential by Ermer decreased to 43.3° (11). The latter value is very similar to the MM 1 and MM2 results, which gave 45.2-44° (76,77). Nevertheless, the parameterization of force fields for highly distorted olefins such as 64-66 would become more reliable if the elusive olefin 64 could be prepared and its structure elucidated. 

Apart from in-plane distortions, bridgehead olefins of type 44 (Table 4) show torsion and oop bending. Typical examples and the precursors from which they are prepared are given in Scheme 7. 

It was mentioned earlier that the strain of a molecule is distributed over various degrees of freedom. Nevertheless, it is possible to distinguish olefins with preferential if not exclusive in-plane distortions from those with out-of-plane bending and torsional deformations. 

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. 

Rondan, N. G., Paddon-Row, M. N., CarameUa, R, Houk, K. A. (1981). Nonplanar Alkenes and Carbonyls A Molecular Distortion which Parallels Addition Steroselectivity. J. Am. Chem. Soc., 103,2436. Ess, D. H. Houk, K. N. (2007). Distortion/Interaction Energy Control of 1,3-Dipolar Cycloaddition Reactivity. J. Am Chem. Soc., 129, 10646-10647. Lopez, S. A., Houk, K. N. (2013). Alkene Distortion Energies and Torsional Effects Control Reactivities, and Stereoselectivities of Azide Cycloadditions to Norbomene and Substituted Norbomenes. J. Org. Chem., 78(5), 1778-1783. Hong, X., Liang, Y, Griffith, A. K., et al. (2013). Distortion-Accelerated Cycloadditions and Strain-Release-Promoted Cycloreversions in the Organocatalytic Carbonyl-Olefin Metathesis. Chem. Sci., 5(2), 471-475. 

---