Cis-trans isomerization about double

Some derivatives of triafulvene undergo rotation about the carbon-carbon double bond even at room temperature. Given that cis-trans isomerization about double bonds is normally very difficult (see Chapter 7, Problem 1), how would you rationalize this Examine the electrostatic potential map for perpendicular hexaphenyltriafulvene (the rotational transition state).Would polar solvents tend to lower or raise the rotation barrier Explain. 

It is generally agreed that the physical property differences between organic-type pol3nners can be attributed to differences in crystallinity, to side group steric or polar effects, and occasionally to the existence of cis-trans isomerism about double bonds in the skeleton. To what extent are the properties of polyphosphazenes determined by the same factors ... 

Leigh and Arnold (1981) investigated cis-trans isomerizations about C—C bonds in substituted tetraphenylethylenes [42]. This isomerization involves a biradical or biradical-like transition structure which is attained by a 90° rotation about the double bond. The predominant action of the substituent is assumed to become evident mainly at the orthogonal stage of... 

It is interesting to note that although cis-trans isomerism about the double bond is theoretically possible in cinnamic acid, the Perkin reaction gives rise only to the traits form, m.p. 133°, the cis form, m.p. 68° (termed aWo-cinnamic acid) being unstable and easily converted into the trans acid. 

If this interaction is not possible, the shift does not occur.37 The ultraviolet spectra have been studied in connection with the structure of aliphatic 3 and cyclic enamines,1, 28,37, 43 cis-trans isomerism about their double bond,27 and the structures of enamines with additional functional groups.17, 20, 31,32 Ultraviolet spectral data have been used also as a general method to determine the structure of alkaloids and in the study of some reaction mechanisms. 

Both the chirality of an atom and the cis-trans isomerism about a double bond can be determined by using a set of rules developed by and adopted by lUPAC [43]. These rules govern the sequencing of substituents about the chiral atom or double bond. Once the substituents are assigned a priority, simple geometric algorithms determine which type of isomerism is present These same rules are used to determine the prochirality of an atom. 

Finally, the recent study confirmed that the last stage of Qa formation from ACMS is indeed a spontaneous (nonenzymatic) pericyclic reaction that proceeds via ACMS cis-trans isomerization about C2-C3 and C4-C5 double bonds followed by the electrocyclization of the enol tautomer. ... 

Considerable effort was expended to insure that no artistic talent is required in structural input. A reasonable amount of inaccuracy is permitted in pointing to an atom or bond. LHASA determines what was intended and acts accordingly. There is no need to worry about consistency or precision of bonds or angles. With the one exception of indicating cis-trans isomerism around double bonds, it makes no difference how distorted the structure is input. 

We know that the rotation about a double bond is impossible without disturbing the n bond, which requires a large amount of energy (about 60 K cals/mole). This restricted rotation gives rise to cis-trans isomerism in olefines and their derivatives. 

An important controlling factor is the conformation of the triene about the two single bonds that link the three double bonds (2.21) an s-c/s-s-cis conformation is required for 6-electron ring-closure, but 4-electron closure can occur also in an s-cis-s-trans conformation. The different conformations interconvert rapidly in acyclic trienes, but because each has different absorption characteristics, the wavelength of irradiation can influence the course of reaction dramatically. 2,5-Dimethylhexa-1,3,5-triene undergoes efficient cis trans isomerization at 254 nm (where the major absorbing species is the s-trans-s-frans conformation), but it cyclizes to a cyclohexadiene (2.22) using 313 nm radiation. 

---