Electrocyclic reaction cyclohexadiene-hexatriene

Why is the thermal stability of diarylethene derivatives enhanced by replacing phenyl groups with furan or thiophene groups In molecular orbitals calculation, the photochromic reaction is treated as a typical electrocyclic reaction between hexatriene and cyclohexadiene. The thermal reaction proceeds disrotatorily and the photoreaction, conrotatorily. Disrotatory cyclization of A to B requires an increase in free energy larger than 138 kJ/mol, and hence no thermal ring-closure occurs in the case of either phenyl- or furan-substituted molecules (see... 

Electrocyclic reaction (Section 30.3) A unimolecular peri-cyclic reaction in which a ring is formed or broken by a concerted reorganization of electrons through a cyclic transition state. For example, the cyciization of 1,3.5-hexatriene to yield 1,3-cyclohexadiene is an electrocyclic reaction. 

It is frequent but not invariable that where a longer conjugated system has a geometrically accessible and symmetry-allowed transition structure like that in 5.90, the longer system is used. Thus, the [8+2] and [6+4] cycloadditions on pp. 15 16, and the [14+2] cycloaddition on p. 44 take place rather than perfectly reasonable Diels Alder reactions, and the 8-electron electrocyclic reactions of 4.51 and 4.54 takes place rather than disrotatory hexatriene-to-cyclohexadiene reactions. This kind of selectivity is called periselectivity. 

Pericyclic reactions are commonly divided into three classes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. An electrocyclic reaction forms a sigma bond between the end atoms of a series of conjugated pi bonds within a molecule. The 1,3-butadiene to cyclobutene conversion is an example, as is the similar reaction of 1,3,5-hexatriene to form 1,3-cyclohexadiene ... 

Electrocyclic reactions involve the cyclization of conjugated polyenes. For example, 1,3,5-hexatriene cyclizes to 1,3-cyclohexadiene on heating. Electrocyclic reactions can occur by either conrotatory or disrotatory paths, depending on the symmetry of the terminal lobes of the tt system. Conrotatory cyclization requires that both lobes rot lte in the same direction, whereas disrotatory cyclization requires that the lobes rotate in oj )posite directions. The reaction course in a specific case can be found by looking at the symmetry of the highest occupied molecular orbital (HOMO). 

Scheme 80. A 1,3,5-hexatriene to 1,3-cyclohexadiene-type anion radical electrocyclic reaction.

Simple examples of electrocyclic reactions are the formation of cyclobutene from butadiene and cyclohexadiene from hexatriene ... 

The presence of a 1,3-cyclohexadiene or a 1,3,5-hexatriene in the starting material or the product may indicate a six-electron electrocyclic reaction. 

Electrocyclic Reactions. Electrocyclic reactions are those pericyclic reactions in which a ring is formed (or opened). Thus, cyclobutene (157), on heating, gives butadiene (158), and hexatriene (159) gives cyclohexadiene (160). 

Another, recently developed method to synthesize a broad spectrum of oligo-and polycyclic aromatics and heteroaromatics in a surprisingly simple manner is likewise based on an electrocyclic - but thermal - hexatriene-cyclohexadiene ring-closure combined with an elimination reaction. This new synthetic method and its scope will be the topic of the following report. 

In contrast to cycloadditions, which almost invariably take place with a total of (4 2) electrons, there are many examples of electrocyclic reactions taking place when the total number of electrons is a (An) number. However, those electrocyclic reactions with (An) electrons, like the butadiene-cyclobutene equilibrium, 6.50 6.51, differ strikingly in their stereochemistry from those reactions mobilising (An+2) electrons, like the hexatriene-cyclohexadiene equilibrium, 6.52 —> 6.53. This is only revealed when the parent systems are... 

Another example of an electrocyclic reaction that exhibits a high degree of stereospecificity is the ring-closure of hexatrienes to cyclohexadienes ... 

The first step is a disrotatory cyclohexadiene-hexatriene isomerization. Its product, cf5-dihydrobenzocyclooctatetraene, is less stable than the trans dimer and is known to isomerize to it, [27] the isomerization presumably taking place via an a" displacement that reduces symmetry to Ci, in which no reaction is forbidden. At the higher temperatures at which fragmentation occurs, the first product should be in equilibrium with the reactant, and its eight-membered ring is sufficiently flexible that a similar desymmetrization would allow it to serve as an unstable intermediate. The activation parameters cited above, which - for the postulated mechanism - measure the enthalpy and entropy differences between the reactant and the transition state of the second step, are not inconsistent with concerted electrocyclic rupture of both bonds via a relatively unconstrained transition state. 

Figure 3 Dependence of experimental activation energies for first three members of a series of electrocyclic reaction on the calculated value of similarity index fpp Numbering (l)-butadiene-cyclobutene, (2)-hexatriene-cyclohexadiene, (3)-oktatetraene-cyclooktatriene)...

Figure 15.17 B shows the aromatic transition state analysis of these reactions. We draw a picture of an opening pathway with the minimum number of phase changes and examine the number of nodes. The four-electron butadiene-cyclobutene system should follow the Mobius/conrotatory path, and the six-electron hexatriene-cyclohexadiene system should follow the Hiickel/disrotatory path. As such, aromatic transition state theory provides a simple analysis of electrocyclic reactions. The disrotatory motion is always of Hiickel topology, and the conrotatory motion is always of Mobius topology.

FIGURE 20.16 Another electrocyclic reaction interconverts cw-1,3,5-hexatriene and 1,3-cyclohexadiene. 

Let s now use this technique for analyzing electrocyclic reactions to make some predictions about the l,3,5-hexatriene-l,3-cyclohexadiene system (Fig. 20.16). 

The simple 1,5-hexadiene at the heart of the Cope rearrangement can be elaborated in many ways, and in fact we ve already seen one. If we add a central K bond we get back to cw-l,3,5-hexatriene (Fig. 20.56), a molecule we saw when we considered electrocyclic reactions (p. 1040). The arrow formalism of the Cope rearrangement leads to 1,3-cyclohexadiene. 

Certain polyenes and cyclic compounds can be interconverted through a pericyclic process known as an electrocyclic reaction. Examples include the 1,3-butadiene-cyclobutene and 1,3-cyclohexadiene-l,3,5-hexatriene interconversions (Figs. 20.5 and 20.16). 

Fig. 8.2 The switching process of fiilgides is driven by the electrocyclic reaction of a cyclohexadiene/all-cis hexatriene subunit. The reactive coordinates r and

---