Cyclohexadiene - hexatriene interconversions

Photochromic materials based on cyclohexadiene/hexatriene interconversions... 

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 ... 

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

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. 

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... 

A mechanistic model based on minimization of steric repulsion in transition structures can rationalize results of these butadiene-cyclobutene interconversions, but that explanation cannot explain the results of analogous cyclohexadiene-hexatriene reactions. As shown in equation 11.5, frflMS,ds,trajis-2,4,6-octatriene (13) is converted only to 14, so this process must take place through rotation of the two methyl groups in different directions (analogous to paths (c) or (d) in Figure 11.2). On the other hand. 

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. 

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). 

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... 

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. 

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... 

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]. 

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). 

To depict these symmetry requirements leading to the stereospecificity of the interconversion of (1) to (2), consider the orbital correlation diagram involving the six rr-orbitals of the hexatriene and the four tt-orbitals and two cr-orbitals of cyclohexadiene (Figure 3). In the thermal disrotatory process, which maintains a mirror plane of symmetry (or o-symmetry), ground state reactant (1) orbitals (TTa TTb TTc ) correlate with the corresponding product (2) orbitals This suggests that the disrotatory ther-... 

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