Alkenes torsional effect

Addition reactions of the 4-tm-butylcyclohexyl radical have been studied with a variety of alkenes and also in atom-abstraction reactions (see Section D.2.2.)3 58. While hydrogen or halide abstraction reactions yield preferentially the axial product, unselective addition occurs with terminally unsubstituted alkenes. In addition reactions of alkenes substituted by alkyl groups at the attacked olefinic center, the preference for axial attack decreases further and the equatorial addition product is formed. This influence of the size of the reaction partner on the selectivity is rationalized on the basis of the simultaneous presence of steric effects (hindering axial attack) and torsional effects (hindering equatorial attack), very similar to those discussed for nucleophilic addition reactions to cyclic ketones59. 

Some examples of 1,3-dioxinones that employ a tether to control the regiochemistry and stereochemistry of the photochemical [2 + 2] include work by Sato [115] and Haddad [116]. Sato found that the length of the tether had a profound effect on the regioselectivity of the reaction (Scheme 37). The shorter the tether, the less selective the reaction, reportedly due to kinetic and torsional effects. When the compound 115c, where the alkene moiety is tethered to the enone by a 4-methylene chain, was irradiated, a 90% yield of product 116c was obtained, showing that the cycloaddition occurred preferentially from the less hindered side of the molecule. In the same vein, Haddad found that when = 1 in compounds 117, the approach of the olefin to the enone... 

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. 

The effect of introducing -hybridized atoms into open-chain molecules was discussed earlier, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are noted when sp centers are incorporated into six-membered rings. Whereas the fiee-energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecyclohexane and to 4.9 kcal/mol in cyclohexanone. ... 

Wiberg et al. [171] gained information related to the origin of the change in optical rotation as functional of torsional angle, and thus the vibrational/rotational effects on their OR, by exploring a set of six achiral hydrocarbons, alkanes and alkenes from C2 to C4 at the B3LYP/6-31++G(d,p) level of theory. 

If other elements of chirality are present in the molecule, the descriptors should be arranged in the order central > carbocationic > axial > planar > torsional (note that Sokolov [6] uses a different order for the descriptors). The effects caused by such other types of chirality are discussed elsewhere in some detail [41], e.g., S-cis/S-trans-isomerism in ferrocenyl ketones and alkenes, as well as the chemistry of biferrocenyl (containing a direct bond between two ferrocenes). 

The rate of oxymercuration is not increased when the ground-state strain energy of the alkene is increased. Steric effects and torsional or twist-strain effects in the transition state determine the reactivity . 

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 physical interpretation has been ascribed to each of the three terms in the MM2 torsional expansion from an analysis of ab initio calculations on simple fluorinated hydrocarbons. The first, onefold term corresponds to interactions between bond dipoles, which are due to differences in electronegativity between bonded atoms. The twofold term is due to the effects of hyperconjugation (in alkanes) and conjugation effects (in alkenes), which provide double bond character to the bond. The threefold term corresponds to steric interactions between the 1,4 atoms. It was found that the additional terms in the torsional potential were especially important for systems containing heteroatoms, such as the halogenated hydrocarbons and molecules containing CCOC and CCNC fragments. 

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