Hexatriene thermal

Hexatrienes undergo disrotatory ring closure by thermal activation to afford cyclohex-adienes in agreement with the Woodward-Hoffmann rule (delocalization band in Scheme 8) [41 3]. Photo-irradiation of hexatrienes is known to give bicylic products in a stereospecific [4n +2nJ manner (delocalization band in Scheme 8) [40] in contrast to this rule. 

According to this scheme, the catalyst serves primarily to promote dehydrogenation. Cyclization of the hexatriene was shown years ago (JJ.) to occur thermally in the gas phase at temperatures well below these dehydrocyclization conditions. Thus, the overall reaction is projected to be the combination of several catalytic dehydrogenation steps and a non-catalytic cyclization step. This projection implies that the design of the catalytic reactor may be important in order to optimize the ratio of void space for cyclization and catalyst space for dehydrogenation. 

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... 

FIGURE 4. Molecular structures of 1,3-butadiene and trans-l, 3,5-hexatriene presentation with thermal probability plots of 50%... 

The thermal disrotatory cyclization of hexatrienes leads to cyclohexadienes (equations 11 and 12)11"13. 

The ground state bonding orbitals of cyclohexadiene is correlated with the ground state bonding orbitals of hexatriene and so it will be a thermally allowed reaction. 

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... 

These processes may be designated conrotatory (16) and disrotatory (17). In practice the isomerization of the appropriately substituted cyclobutenes follow a conrotatory pathway. Thus ci5-3,4-dimethylcyclobutene yields only cia-irans-2,4 hexadiene, and iraws-3,4-dimethyleyclobutene yields only transition state suggested previously, the conrotatory process is in fact the one to be expected. However, the situation is not quite as simple as here implied. By similar arguments the thermal cyclization of hexatrienes would also be expected to be conrotatory, whereas in fact it is disrotatory, viz. ... 

The process is much more rapid over platinum. Dautzenberg and Plat-teeuw (23) assumed the formation and thermal cyclization of hexatriene [similarly to the earlier suggestion with respect to oxides (22)]. However, it is not likely that such an extremely unstable intermediate would leave the catalyst surface just in order to cyclize and then rapidly readsorb to complete aromatization. Still, thermal cyclization cannot be a priori excluded at high temperatures where the equilibrium concentration of triene is higher and its adsorptivity lower, but its appearance may be rather exceptional. We suggest, instead, a surface cyclization step of dj-l,3,5-hexatriene. 

Essentially the same triple peak system also appeared with Pt-black, but the entire spectrum was shifted toward lower temperatures (r ,a,j = 155°, 175°, and 195°C, respectively). 7>a s-hexatriene gave the same spectrum, but with cw-triene a fourth, low-temperature peak appeared at 140°C. This should correspond to the facile cyclization of cij-triene. The fact that a peak of similar appears when benzene is desorbed from the catalyst is another strong argument against the purely thermal (noncatalytic) character of cyclization of c/s-l,3,5-hexatriene. 

A further example of a concerted thermal elimination reaction of a thiepane derivative was the formation of c/s-hexatriene and the extrusion of sulfur dioxide from heating 2,7-dihydrothiepin 1,1-dioxide (116) (67JA1281). That this reaction was under orbital symmetry control was deduced from the results obtained by heating cis- (117) and trans-... 

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. 

The heavily annulated benzo- and furanothia[ll]annulenes depicted in 6182 and 6283 were synthesized by condensation of dialdehyde (47) with properly structured Wittig components. Further, treatment of 62 with maleic anhydride produced a mixture of endo and exo cycloadducts of general formula 63 whose hexatriene segment that was not directly bound to the heteroatom was found to be thermally labile, readily... 

For the thermal and photochemical cyclization of hexatriene, the controlling HOMO S are Vo and respectively. As shown in Fig. 3.5.9, the allowed... 

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