Double-bond torsions

Intramolecular allylzinc aldehyde additions lead, in one synthetic step, to bicyelic a-methylene-y-lactones41. Interestingly, the (Z)-isomer gave only the r/5-fused bicyclic products, which arises from a. vyw-selectivc addition, while the ( >isomeric bromide furnished a cisjtrans mixture (yield 56%, cisjtrans 72 28)41 indicating that the double-bond torsion and the intramolecular carbonyl addition have similar rates. A similar sequence was the key step in a confertin synthesis28. 

The angle 6 is defined according to Scheme 2, the exact definitions of the angles in the last column, describing the double bond torsion, being given in Reference 38. Here we note only that 0 ° torsion means a planar olefin. The values are reported only for dimethyl-substituted double bonds. 

A. Importance of double-bond torsion in photochemical and photophysical processes of styrene and stilbene derivatives... 

A. Importance of Double-Bond Torsion in Photochemical and Photophysical Processes of Styrene and Stilbene Derivatives... 

Both the single- and the double-bond torsions in NVET. 

Figure 4.13 Potential energy surfaces for c s-stilbene along (a) the double-bond torsion coordinate ( ) and (b) the single-bond torsion coordinate (p) [46], (Reproduced with permission from Elsevier.)...

Stationary point geometries on Bi So and Tj surfaces are presented in Figure 4.14. The crystal structure of t-Bi shows it to be strictly planar, eliminating phenyl-vinyl torsion toward planarity as a crucial NVET reaction coordinate. The authors emphasized that while experiment and theory support the initial proposition that double-bond torsion is the key reaction coordinate enabling NVET to stilbenes, as multidimensional surfaces are involved, other vibrations, including bond stretching, will also contribute. 

Torsional strain (Section 3 1) Decreased stability of a mole cule associated with eclipsed bonds trans (Section 3 11) Stereochemical prefix indicating that two substituents are on opposite sides of a ring or a double bond (Contrast with the prefix cis )... 

An sp3-sp2 or sp -sp - single bond where the atom con-nected to the central sp (sp - ) atom is another sp (sp - ) atom, as in the H-C-C-double bond 0 torsion of acetic acid, is described by the MM-t parameters of acetic acid, Vl=-0.167 kcal/mol and V3=-0.1 kcal/mol. 

Table 6.7 gives a few other examples of torsional barrier heights. That for ethylene is high, typical of a double bond, but its value is uncertain. The barriers for methyl alcohol and ethane are three-fold, which can be confirmed using molecular models, and fhose of toluene and nifromefhane are six-fold. The decrease in barrier heighf on going fo a higher-fold barrier is fypical. Rofafion abouf fhe C—C bond in toluene and fhe C—N bond in nifromefhane is very nearly free. 

Fig. 13.9. Schematic reaction profile for the EZ isomerization of stilbene. The reaction coordinate 6 is the torsion angle about the double bond. [From FI. Meier, Angew. Chem. Int. Ed. Engl. 31 1399 (1992)]. Reproduced by permission of Wiley-VCH.

The natural bond length varies between 1.503 A and 1.337 A for bond orders between 0 and 1, these are the values for pure single and double bonds between two sp -carbons. Similarly the force constant varies between the values used for isolated single and double bonds. The rotational barrier for an isolated double bond is 60kcal/mol, since there are four torsional contributions for a double bond. 

In agreement with this mechanism is the fact that the stereochemistry of addition is syn.44 The rate of reaction with diimide is influenced by torsional and angle strain in the alkene. More strained double bonds react at accelerated rates.45 For example, the more strained trans double bond is selectively reduced in Z,K-1,5-cyclodecadiene. 

The energetical description of rotations around bonds with high torsional barriers (e.g. the C=C double bond) demands the evaluation of the influence of higher cosine terms. Rotations around single bonds with sixfold symmetric torsional potentials have very low barriers (18) they occur in alkylsubstituted aromatic compounds (e.g. toluene), in nitro-alkanes and in radicals, for example. 

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