Octatetraenes, cyclization

The predicted conrotatory cyclization of octatetraenes was confirmed for the case of the methyl-substituted compounds, which above 16 °C readily formed the cyclooctatrienes shown in equations 13 and 14)14. We conclude this section with an electrocyclic reaction involving ten TT-electrons, that is, the formation of azulene (17) when the fulvene 16 is heated (equation 15)15,16. 

If a stepwise cyclization mechanism is assumed—for example, for /j-Cg—two octatriene intermediates may be formed, viz. 1,3,5-octatriene would lead to ethylbenzene, and 2,4,6-octatriene to o-xylene (Scheme II). The dehydrogenation of the latter would give octatetraene, which, in turn, gives styrene via vinylcyclohexadiene. Dehydrogenation and cyclization of octatriene were reported to compete over chromia and molybdena catalysts (67) with less hydrogen present (e.g., in a pulse system with in helium carrier gas), styrene formation is enhanced. 

The transformation of cyclooctatetraene 315 to 316 has been achieved and is quoted387" as an example of a [ a + 7i2a] cycloaddition, and the alternative transposition 317 - 318 is also known.3876 For polyazacyclo-octatetraenes (319, 321, 323, 324), this type of cyclization can lead to heteroethylenic structures 320 or aromatic systems (322, 262, 15). 

Sol 14. (a) The c/i-orientation of hydrogens in the product indicates that cyclization involves disrotatory mode. Under thermal conditions, cyclo-octatetraene preferentially undergoes a hrr-electron disrotatory electrocyclization forming only the cw-isomer. It should be noted that cyclooctatetraene does not undergo the thermally allowed 4TT-electron conrotatory electrocyclization as the frani-fused bicyclic structure is highly strained. 

Gold chloride forms an unstable complex with cyclo-octatetraene at low temperatures. This decomposes at — 20°C to l,2-dichlorocyclo-octa-3,5,7-triene, which gradually cyclizes to (562). In an attempt to form pentalene-metal complexes, cyclo-octatetraene was reduced to a mixture of trienes and bicyclo [4,2,0] octadiene, which was then treated with substituent ruthenium carbonyls, affording a variety of complexes, including (563). °... 

Extension to the cyclization of higher homologs is straightforward The lower 1 tt orbitals of a polyene with N conjugated double bonds will correlate in C2v with all of the tt MOs of its cyclization product, leaving the Nth out of correlation with cr(ai). If N is even (A = 2, where q = 0,1,2...), xn will be fl2 and a conrotation will be required in order to induce a correspondence with (j if N is odd N = 2q + 1), xn will be bi and a disrotation will be called for. Since all N orbitals are doubly-occupied, the number of electrons involved k) is 2N] the W.-H. Rules for thermal cyclization [1, pp. 38-45] follow directly. Thus, for example, all-czs-octatetraene cyclyzes readily to cyclooctatriene via a conrotatory pathway. [9]... 

The course of the intramolecular photocycloadditions of the isoelectronic ketene and allene of structure (372 X = O or CH2) is quite different the ketene cyclizes to (373), whereas the allene gives (374). Similar behaviour is found for other cyclo-hexenones, and opposite charge distribution in the ground states of the allenes and ketenes is thought to be responsible for the differences in regioselectivity. Deuteriumlabelling studies show that the isomerization of the benzobarrelene (375) into the benzocyclo-octatetraene (376) occurs by way of initial [2 + 2] photo-addition between the aliphatic C=C bonds. This is not the pathway favoured by previous investigators or by the Epiotis polar pericyclic theory. The photochemical addition... 

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