Hexadienes rearrangement

For example, EPR evidence showed that cyclohexane-1,4-diyl, generated by radiolysis of hexadiene, rearranged to cyclohexene radical cation. Similarly, ant/-5-methylbicyclo[2.1.0]-pentane radical cation (33) rearranged to 1-methylcy-clopentene radical cation (34) via a 1,2-shift of the syn-5-hydrogen. ... 

Examples given in Section V,C,2 show that the l,6-diphospha-l,5-hexadienes rearrange irreversibly at low temperatures to 3,4-diphospha-1,5-hexadienes, transforming the double-coordinated phosphorus into the preferred triple-coordinated atom. An experiment to convert 3,4-diphosphahexadiene via Cope rearrangement to 1,6-diphosphahexa-l,5-diene should thus be undertaken with the intention... 

In a finding of greater practical significance. Overman and coworicers showed that the reactions could be carried out with catalytic amounts of the palladium(II) complex, and that the catalytic effect was broadly applicable to acyclic 1,5-dienes as well7 In a typical example (equation 32), 2-methyl-3-phe-nyl-l,5-hexadiene rearranges in 1 h at room temperature in 87% yield in the presence of 0.06 equiv. of bis(benzonitrile)palladium dichloride, in contrast to the thermal rearrangement which has t n = 13 h at 177 °C. The cat yst thus provides an estimated rate acceleration of about 10 °. The product is a 93 7 mixture of ( )- and (Z)-isomers, corresponding to the equilibrium ratio. Palladium acetate and tetra-kis(triphenylphosphine) were ineffective as catalysts. One serious limitation is that the catalyzed reaction occurs only with those 1,5-dienes which possess an alkyl or aryl substituent at C-2 or C-5 (but not both). 

Thus, the case for a non-concerted 3,3-shift via a cyclohexane-1,4-diyl is weak. Nonetheless, substituent effects on the rate of the 3,3-shift were intially interpreted in terms of the diyl species. In particular, Dewar found the 2-phenyl and 2,5,-diphenyl-l,5-hexadiene rearrange 40 and 1600 times, respectively, more rapidly than that of the parent diene. Further, semi-empirical MINDO/3 calculations supported the proposition that even the parent species proceeded via the chair-like cyclohexane-1,4-diyl. These observations and calculations provided stimulus for a substantial effort in the subsequent years to address the question of transition state structure in and the energy surface for the 3,3-sigmatropic shift of 1,5-hexadiene. 

The rearrangement of the simplest possible case, 1,5-hexadiene, has been studied using deuterium labeling. The activation enthalpy is 33.5kcal/mol, and the entropy of activation is — 13.8eu. The substantially negative entropy reflects the formation of the cyclic transition state. 

In 1966, a photochemical rearrangement by ultraviolet (UV) irradiation of hexafluorobenzene to hexafluorobicyclo[2 2 0]hexa-2,5-diene was achieved Since then, many reactions analogous to the valence tautomerism of benzene and bicy clo[2 2 0]hexadiene (1 e, Dewar benzene), as well as of fluonnated benzvalene and... 

Step through the sequence of stmctures depicting Cope rearrangement of 1,5-hexadiene. Plot energy (vertical axis) vs. the length of either the carbon-carbon bond being formed or that being broken (horizontal axis). Locate the transition state. Measure all CC bond distances at the transition state, and draw a structural formula for it... 

The rearrangement proceeds from the Si-state of the 1,4-diene 1. The Ti-state would allow for different reactions like double bond isomerization. Rigid systems like cyclic dienes, where EfZ -isomerization of a double bond is hindered for steric reasons, can react through the Ti-state. When the rearrangement proceeds from the Si-state, it proves to be stereospecific at C-1 and C-5 no -isomerization is observed. Z-l,l-Diphenyl-3,3-dimethyl-l,4-hexadiene 5 rearranges to the Z-configured vinylcyclopropane 6. In this case the reaction also is regiospecific. Only the vinylcyclopropane 6 is formed, but not the alternative product 7. ... 

Two other important sigmatropic reactions are the Claisen rearrangement of an allyl aryl ether discussed in Section 18.4 and the Cope rearrangement of a 1,5-hexadiene. These two, along with the Diels-Alder reaction, are the most useful pericyclic reactions for organic synthesis many thousands of examples of all three are known. Note that the Claisen rearrangement occurs with both allylic aryl ethers and allylic vinylic ethers. 

Cope rearrangement (Section 30.8) The sigmatropic rearrangement of a 1,5-hexadiene. 

These reactions, called electrocyclic rearrangements, take place by pericyclic mechanisms. The evidence comes from stereochemical studies, which show a remarkable stereospecificity whose direction depends on whether the reaction is induced by heat or light. For example, it was found for the thermal reaction that cis-3,4-dimethylcyclobutene gave only cw,tran5-2,4-hexadiene, while the trans isomer gave only the trans-trans diene... 

As we have indicated with our arrows, the mechanism of the uncatalyzed Cope rearrangement is a simple six-centered pericyclic process. Since the mechanism is so simple, it has been possible to study some rather subtle points, among them the question of whether the six-membered transition state is in the boat or the chair form. ° For the case of 3,4-dimethyl-l,5-hexadiene it was demonstrated conclusively that the transition state is in the chair form. This was shown by the stereospecific nature of the reaction The meso isomer gave the cis-trans product, while the ( ) compound gave the trans-trans diene. If the transition state is in the chair form (e.g., taking the meso isomer), one methyl must be axial and the other equatorial and the product must be the cis-trans alkene ... 

It was pointed out earlier that a Cope rearrangement of 1,5-hexadiene gives 1,5-hexadiene. This is a degenerate Cope rearrangement (p. 1380). Another molecule that undergoes it is bicyclo[5.1.0]octadiene (105). At room temperature the NMR... 

Fig. 19. Two examples of degenerate Cope rearrangement, a) 1,5-hexadiene b) semibullvalene.

The next homolog, 1,5-hexadiene (1,5-HD), is of special chemical interest because the molecule is capable of undergoing the so-called Cope rearrangement. A GED study of 1,5-HD was also recently reported6. Because of the increased conformational complexity of this molecule compared to that of 1,4-PD, the structural details of the various con-formers could not be resolved and only averaged structure parameters were determined from the gas phase. Molecules in the solid state are frozen, mostly in only one conformation, which may but must not represent the conformational ground state. Therefore, conformational isomerization is usually not discussed with X-ray structures presented in the literature. 

SCHEME 18. van der Waals volume of activation AV and volume of activation calculated for degenerate Cope rearrangement of 1,5-hexadiene... 

SCHEME 19. Activation volumes of Cope and Claisen rearrangements in polar 1,5-hexadiene systems... 

B. Kinetic Isotope Effects in the Thermal Rearrangement of 3-Oxa-1,5-hexadienes... 

The numbering is written by the order i, j written in a bracket. The letter / and j denote the number of atoms across which the o bond migrates. Let us take the case of cope-rearrangement of 1, 5 hexadiene. 

There are also instances where a system undergoes Cope rearrangement through different possibilities. This is afforded by the examples of hexadiene. The alternatives are of two types. 

A diradical intermediate appears to be formed in the rearrangement of 2, 5 diphenyl-1, 5-hexadiene probably due to the phenyl groups stabilizing the radical centres. 

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