Hexatriene-cyclohexadiene

Analysis of the hexatriene-cyclohexadiene system leads to the conclusion that the disrotatory process will be favored. The basis set orbitals for the conrotatory and disrotatory transition states are shown below. 

An especially interesting case of the hexatriene-cyclohexadiene type interconversion is the rapid equilibrium between cycloheptatrienes and bicyclo[4.1.0]hepta-2,4-dienes ... 

Draw the orbitals for the hexatriene-cyclohexadiene reaction 4.47 —>4.48 and its reverse, in the style 4.40, 4.41 and 4,42 used for the corresponding cyclobutene-butadiene reaction, identify the developing overlap, and hence show that the reaction is disrotatory in both directions. 

A priori, we would expect disrotatory reactions to show poorer torquoselectivity than conrotatory reactions for two reasons. Consider, for example, the hexatriene cyclohexadiene interconversion. On the one hand, the overlap between R and the distal carbon C6 is similar for the in and out pathways, as in the in mode, the major lobe at C6 is oriented away from R ... 

Electrocyclic ring closures are particularly important in the formation of six-membered rings many are hetero analogues of the hexatriene-cyclohexadiene transformation 4—>5. As discussed in Section 3.2.1.6.1, they are frequently involved in ring interconversions initiated by nucleophilic attack on a six-membered ring. Further examples are discussed in Sections 4.2.3.6 (preparation of seven-membered rings) and 4.4.8.2.2.2 (formation of bicyclic 6,6 ring systems). 

Measurement of activation energy and entropy has not distinguished between antara-antara Cope rearrangement and other concerted processes in the photochemical rearrangement of y-thujaplicin O-methyl ether to the bicyclohep-tadienones (157) and (158).255 The hexatriene-cyclohexadiene disrotatory elec-... 

The hexatriene/cyclohexadiene isomerization has been extensively studied and has been the topic of numerous reviews and monographs this section will attempt to deal only with applications of these reactions to synthesis, and in particular the use of these reactions for the synthesis of natural products. Much of the early work in this area was done by Havinga and coworkers during the course of their detailed work on the stereochemical consequences of the thermal and photochemical conversions in the vitamin D field this work provided much of the impetus for the development and elaboration of the Woodward-Hoffman mles (Scheme 7). The reversible photochemical ring opening of provitamin 30 to precalciferol (31) and the photochemical ring closure of 31 to lumisterol 32 can be explained by consecutive photochemically allowed conrotatory processes. ... 

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... 

The pattern that emerges from the experimental and computational studies of the conjugated dienes and trienes is the involvement of CIs having certain features in common. The singlet CI for the dienes appears to be a tetraradicaloid with the potential for several re-bonding schemes. In the absence of steric problems, it is structurally compact. The hexatriene-cyclohexadiene system also appears to involve a tetraradicaloid structure, one component of which is an allylic system. These structures can account for the major product types in both systems. 

Similarly, we can analyze the hexatriene-cyclohexadiene system having (4n + 2) 7T-electrons (Figure 2.10). In this case, a disrotatory mode of ring closure leads to a Hiickel array, which is aromatic with (4n + 2) rr-electrons. Therefore, the disrotatory mode of reaction now becomes thermally allowed. However, a conrotatory mode of ring closure uses a Mobius array, which is antiaromatic with (4n + 2) 7r-electrons. Therefore, the reaction is thermally forbidden in this mode. 

FIGURE 2,10 PMO approach to disrotatoiy and conrotatory [nxx esses Fot the hexatriene-cyclohexadiene system. 

Similarly, in the hexatriene-cyclohexadiene transformation, the HOMOs of the open-chain partner under thermal and photochemical conditions are W3 and W4, respectively. As may be expected, the reaction proceeds by disrotation on heating and by conrotation under photochemical conditions (Figure 2.12). 

FIGURE 2.12 Hexatriene-cyclohexadiene interconversion on the basis of FMO approach. 

The aroylation of l-aIkyl-3,4-dihydroisoquinolines produces enamides which readily undergo photocyclization to oxoberberine derivatives. The reaction is analogous to the hexatriene-cyclohexadiene rearrangement and proceeds most favorably (50-807o yield) when the irradiations are performed on degassed solutions ... 

The terms correlation and correspondence that were so prominent in the preceding discussion, are not quite synonymous. The distinction is illustrated by a comparison of Fig. 5.2 with Fig. 5.3, the correlation diagram actually calculated for the hexatriene-cyclohexadiene interconversion in the simple HMO approximation. 

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)...

By now you should anticipate that exactly the opposite conclusions are reached for the hexatriene-cyclohexadiene interconversion. Indeed, a full orbital symmetry analysis, given as an Exercise at the end of the chapter, leads to the conclusion that the disrotatory process is orbital symmetry allowed, whereas the conrotatory process is forbidden. Furthermore, a state correlation analysis constructed along the lines of Figures 15.4 and 15.5 supports the conclusions of the orbital symmetry analysis. Again, we leave this as an Exercise at the end of the chapter. 

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.

Develop an orbital symmetry correlation diagram for the hexatriene-cyclohexadiene interconversion of Eq. 15.18. 

Develop state correlation diagrams for the conrotatory and disrotatory hexatriene-cyclohexadiene interconversion and discuss their implications. 

FIGURE 20.18 For the hexatriene-cyclohexadiene system, in order to produce a bonding interaction, the thermal reactions must take place in a disrotatory fashion and the photochemical reactions in a conrotatory way. 

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