Hexatriene-cyclohexadiene interconversion

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 ... 

There are numerous examples of interconversion of 1,3,5-trienes and 1,3-cyclohexadiene systems by the electrocyclic mechanism. An especially interesting case of hexatriene-cyclohexadiene interconversion is the equilibrium between cyclo-heptatrienes and bicyclo[4.1.0]hepta-2,4-dienes. ... 

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

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. 

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. 

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. 

Can these observations be generalized Let us look at the stereochemistry of the cis-1,3.5-hexatriene-cyclohexadiene interconversion. Surprisingly, the six-membered ring is formed thermally by the disrotatory mode, as can be shown by using derivatives. For example, heated trans,cis,trans-2,4,6-oclatriene gives cii-5,6-dimethyl-l,3-cyclohexadiene, and cw,cw,trfl 5 -2,4,6-octatriene converts into irfl i-5,6-dimethyl-l,3-cyclohexadiene, both disrotatory closures. 

Fig 4.2. Thermal reaction is feasible by disrotatory mode in 1,3,5-hexatriene cyclohexadiene interconversion. 

Predictions for 1, 3, 5-Hexatriene cyclohexadiene interconversions These predictions can be made on the similar grounds as for 1, 3-butadiene cyclobutene interconversion. Photochemical reaction is feasible by conrotatory mode whereas thermal reaction follow disrotatory mode of ring closure as is explainable by Fig. 4.1. and Fig. 4.2, respectively. 

In 1, 3, 5-hexatriene cyclohexadiene interconversion contrary to above transformation thermal ring-closure is disrotatory and photochemical... 

C) Six-membered Rings Hexatriene cyclohexadiene Interconversion [(4n+2)]C system] involves conrotatory mode under photochemical conditions and disrotatory process occurs under thermal conditions. Always sterically less hindered product is major product. 

The hexatriene-cyclohexadiene interconversion is the prototype reaction for this category. The thermochemical cydizations, for example Equations (6.21)-(6.23), confirm the predicted [ 2s + or [ 2a + 4J pathway. The last example illustrates the operation of both of the possible disrotatory modes. 

Figure 15 Computed Sj/Sq conical intersection structure for the problem of cyclohexadiene/hexatriene photochemical interconversion. The relevant geometrical parameters are in angstrom units.

P. Celani, S. Ottani, M. Olivucci, F. Bernardi, and M. A. Robb,/. Am. Cbem. Soc., 116, 10141 (1994). What Happens During the Picoseconds Lifetime of 2Aj Cyclohexa-1,3-diene A CAS-SCF Study of the Cyclohexadiene/Hexatriene Photochemical Interconversion. 

M. Garavelli, P, Celani, M. Fato, M. Olivucci, and M.A. Robb,/. Phys. Chem. A, 101,2023 (1997). Relaxation Paths from a Conical Intersection The Mechanism of Product-Formation in Cyclohexadiene/Hexatriene Photochemical Interconversion. 

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). 

The cyclohexadiene-hexatriene photochemical interconversion is predicted by orbital symmetry considerations to involve conrotatory motion. Cyclohexadiene derivatives undergo photochemical electrocyclic ring opening. The photostationary state... 

GaraveUi M, Celani P, Fato M, et al. Relaxation paths from a conical intersection the mechanism of product formation in the cyclohexadiene/hexatriene photochemical interconversion. J Pkys Chem A. 1997 101 2023-2032. 

Zilberg S, Haas Y. The photochemistry of 1,4-cyclohexadiene in solution and in the gas phase conical intersections and the origin of the helicopter-type motion of H-2 photo-generated in the isolated molecule. Phys Chem Chem Phys. 2002 4 34-42. Tamura H, Nanbu S, Nakamura H, Ishida T. A theoretical study of cyclohexadiene/ hexatriene photochemical interconversion multireference configuration interaction potential energy surfaces and transition probabilities for the radiationless decays. Chem Phys Lett. 2005 401 487-491. 

Celani P, Bemardi F, Robb MA, OUvucd M (1996) Do photochemical ring-openings occur in the spectroscopic state B2 pathways for the cyclohexadiene/hexatriene photochemical interconversion. J Phys Chem 100 19364... 

Predict reaction conditions for conrotatory and disrotatory interconversion of 1, 3, 5-hexatriene cyclohexadiene through correlation diagram method. Also write which symmetry is being conserved under which process. 

Correlation diagrams can be constructed in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclohexadiene. They lead to the prediction that the disrotatory mode is an allowed process whereas the conrotatory reaction is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclizations can be analyzed by the same method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases in which the substitution pattern does not correspond in symmetiy to the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

The correlation of the conrotatory diagram of interconversion of cyclohexadiene-hexatriene is as follows where C2 symmetry is maintained. 

Various substituted 1,3-cyclohexadienes and their open-chain isomers, the respective 1,3,5-hexatrienes, have been studied by El mass spectrometry with special regard to the stereospecificity of the mutual pericyclic interconversion. A brief discussion including the parent systems, ionized 1,3-cyclohexadiene and 1,3,5-hexatriene has been provided by Dass in his review on pericyclic reactions of radical cations4. McLafferty and coworkers119 have shown that the two parent isomers are (almost) indistinguishable... 

Photochromic materials based on cyclohexadiene/hexatriene interconversions... 

The third mechanism of isomerization, photoinduced rearrangements of radical cations, has been pursued in a variety of systems. Matrix isolated radical cations have been noted to undergo some rigorous reorganizations as well as subtle ones. For example, the ring opening of cyclohexadiene to hexatriene radical cation and the interconversion of its different rotamers have been achieved by irradiation with UV or visible light [173-174]. 

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