Hexatriene photochemical

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... 

Robb M, Ohvucci M. Photochemical processes potential energy surface topology and rationalization using VB arguments. J Photochem Photobiol A. 2001 144 237—243. Celani P, Ottani S, Ohvucci M, Bemardi F, Robb MA. What happens during the picosecond hfetime of2a(l) cyclohexa-1,3-diene — a CAS-SCF smdy of the cydohex-adiene hexatriene photochemical interconversion. J Am Chem Soc. 1994 116 10141-10151. 

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... 

The cyclohexadiene-hexatriene system seems to be less complicated than the cyclobutene-butadiene system. Cyclohexadiene undergoes photochemical electrocyclic ring opening ... 

The hexatriene is polarized by unsymmetrical substitution with the C=0 group, and further activated by coordination with Lewis acid. The catalyzed reaction is polar. The similarity between the catalyzed and the photochemical reactions can be understood if polar reactions belong to the pseudoexcitation band as has been proposed in Sect 1. 

The alternate approach of Dewar and Zimmerman can be illustrated by an examination of the 1,3,5-hexatriene system.<81,92> The disrotatory closure has no sign discontinuity (Hiickel system) and has 4n + 2 (where n = 1) ir electrons, so that the transition state for the thermal reaction is aromatic and the reaction is thermally allowed. For the conrotatory closure there is one sign discontinuity (Mobius system) and there are 4u + 2 (n = 1) ir electrons, so that the transition state for the thermal reaction is antiaromatic and forbidden but the transition state for the photochemical reaction is aromatic or allowed (see Chapter 8 and Table 9.8). If we reexamine the butadiene... 

Photochemical d.s jraw.v-confonnational interconversion is also known to occur in larger polyenes. Brouwer and Jacobs have reported the results of irradiation of E- and Z-2,5-dimethyl-l,3,5-hexatriene (14) in argon matrices at 10 K108. Irradiation of the -isomer gives rise to various retainers, while irradiation of the Z-isomer results only in ,Z-isomerization. Photochemical translcis conformer interconversion has also been observed for , -1,3,5,7-octatricnc in matrices at temperatures below 10 K32. 

Cyclohexadiene itself undergoes smooth photochemical ring opening to Z-l,3,5-hexatriene in both the gas phase (d> = 0.13)176 and in solution (d> = 0.41)71,177. As is almost always the case, extended irradiation in solution leads to the formation of a variety of isomeric products due to secondary irradiation of the Z-triene and its E-isomer (vide infra)11. 

It should be noted that products like 443 and 447 are the normal products of photochemical reactions of acyclic 1,3,5-hexatrienes, as well as of divinyl aromatics, but are quite unusual for thermal transformations of such substrates. Presumably, the electrostatic repulsion between CF2 groups prevents the formation of conformation 450 which is necessary for the electrocyclic ring closure (i.e. 438 — 439 and 445 -> 446). Instead, it leads to conformation 451 which is favorable to generate the diradical and then the fused vinyl-cyclopropane intermediates 452 (equation 170). Note that the rearrangement 452 —> 453... 

By contrast with the photochemical cyclobutene-to-butadiene conversion, the photochemical conversion of 1,3-cyclohexadiene has been shown to proceed in a highly stereospecific conrotatory manner to yield the ds-hexatriene [306]. 

If we analyze the case of hexatriene to cyclohexadiene conversion, the situation is just the reverse. The thermal reaction should be disrotatory and the photochemical reaction conrotatory. The butadiene belongs to (4ri)n system and hexadiene to (4n +2)rc system, and a generalization of the systems may be attempted. 

It should also be noted that Leach and Migirdician37 have produced adducts of hexatriene in mixtures of benzene with certain other molecules in glassy matrices at low temperatures. Thus, along with benzene isomers which retain the empirical formula of the monomer, there may be formed photochemically other products which result from a rupture of the ring. Just possibly some of the cuprenelike polymer formed at very short wavelengths20 may be made in this way. We will return to this point in later sections. 

---