Double bonds twisted

Nakazaki, M., Yamamoto, K., and Naemura, K. Stereochemistry of Twisted Double Bond Systems, 125, 1-25 (1984). 

Low torsional barriers in combination with strong steric interactions between donor and acceptor groups in push-pull ethylenes have in several cases been demonstrated to cause permanently twisted double bonds, in which a planar arrangement of substituents at the double bond may represent an energy maximum. 

The interest in twisted double bonds has also generated a considerable activity in the theoretical field, and interesting stable twisted structures have been proposed, which, however, still await experimental confirmation. 

A review of the older literature on compounds with a stereogenic axis is available22, as are reviews on planar chiral molecular structures 23, on the stereochemistry of twisted double bond systems 24, on helical molecules in organic chemistry 25, and on the synthesis and stereochemistry of chiral organic molecules with high symmetry 26. 

Another group of compounds that have a twisted double bond are the bicyclic compounds with bridgehead double bonds such as 1,2-norbomene (9) and 1,7-norbornene (10). " It has been found that many compounds, such as 11, which is based on trawi-cyclooctene, may be isolated whereas those based on smaller trauj -cycloalkenes are usually quite unstable. Some evidence for the formation of 9 has been obtained by trapping the product of the dehalogenation of 1,2-diha-lonorbornanes." Here, the simplest view is that the two p orbitals that form the double bond in 9 and 10 are roughly perpendicular to each other. However, pyr-amidalization and rehybridization also are involved. One indication is the reduced localized 7i-orbital population found in the NBO analysis. Whereas normal alkenes have 71 populations of 1.96 e, for 9 with OS = 57 kcal/mol, it is 1.921, and for 10 with OS = 86 kcal/mol, it is 1.896. With 9, the deviations of the a and n orbitals from the line of centers are 24° and 19°, respectively, and with 10, the deviations are 34° and 29°. 

Since discussions in the preceding sections are limited solely to the twisted double bond systems whose unsaturated centers are constrained within the ring, this last section will give some example of systems in which the double bonds are twisted by crowdedness around these unsaturated centers. 

Contents Stereochemistry of twisted double bond systems/M Nakazaki, K Yamamoto, 1C Naemura — Planarchial molecular structures K Schldgel — Carbohehcenes and heterohelicenes/W H Laarhoven, W J C. Pnnsen — [etc.]... 

Aromatic amines are not the only type of molecule to undergo a spontaneous twist in the excited state, but DMABN is the best-studied example. Other such compounds are stilbene-type molecules, where the double bond twists in the excited state. The electronic structure for both cases, twisting double bonds and twisting single or partly double bonds of 7r-donor linked to 7r-acceptor (TICT molecules), can be related to each other and characterized in the framework of quantum-chemical treatment as described in Section III. 

The reverse migrations, i.e. the thermal isomerization of olefins to carbenes, have been well established to occur if the system involves the significantly twisted double bonds. Although no evidence of this isomerization was found for acyclic vinylsilanes, Conlin and coworkers first observed the isomerization of a-methylenesilacyclobutane 8 to a mixture... 

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