Octatetraenes. rearrangement

Much experimental and theoretical work has been performed with the two allenes 1,2,6-heptatriene (32) and 1,2,6,7-octatetraene (34). Thermal isomerization of 32 leads to 3-methylene-l,5-hexadiene (346), a process that at first sight looks like a typical Cope rearrangement. However, trapping experiments with either oxygen or sulfur dioxide have shown that at least half of the rearrangement passes through the diradical 345 (Scheme 5.52) [144],... 

This thermal rearrangement, which is the formal result of a orbital symmetry forbidden. Rather, an homolytic cleavage of the cyclopropane bond, giving rise to a trimethylenemethane, would be involved. This rearrangement was the first reported intramolecular trapping of a trimethylenemethane by a double bond (for a related intermolecular trapping of trimethylenemethane by butadiene see Ref 49). On further heating to 600°C, the main product obtained was isopropylcyclo-octatetraene (29) (equation 22f. ... 

Of related interest is the fact that cw-2-allyl-l-ethynylcyclopropane rearranges to 1,2,5,7-octatetraene with log k=. 2 - 25 100/23RT (Scheme 7.52). Apparently the allyl group sufficiently weakens the bond to the hydrogen that must be transferred to lower the energetic demands so that the subsequent isomerizations do not occur at the lower temperatures needed to accomplish the hydrogen shift in this case. 

Charge-driven dm conrotatory opening to coiled 1,3,5,7-octatetraene followed by in recyclization for antz-adduct, and straightforward oxy-Cope rearrangement for sjn-adduct... 

The mechanism of the thermal rearrangement of [n,4,2]propellatrienes (100) to cyclo-octatetraenes (101) involves an intramolecular Diels-Alder reaction followed by a ( 2s + 25 + 2s) cyclo-reversion. Rearrangement of propellatrienes (100 n = 4 or 5) to cyclo-octatetraenes (101 n = 4 or 5) is facilitated by molybdenum hexacarbonyl, and under these conditions, at least formally, involves a [l,5]shift under milder conditions the molybdenum complexes (102 n = 4 or 5) have been isolated. 

Tetramethylenesemibullvalene (133) rearranges to 1,2-tetramethylenecyclo-octatetraene on pyrolysis at 460 °C, and to the tricyclic system (134) when treated with activated alumina. " Cyclo-octatetraene was cleanly formed by pyrolysis of the tetracyclic (135). Vinyl-vinyl bonding dominates in the photochemical conversion of 2-cyanobenzobarrelene into cyclo-octatetraenes. °... 

Dimethylcyclo-octatetraenes are interconverted by heating at elevated temperatures, e.g. heating 1,2-dimethylcyclo-octatetraene at 390—590 °C causes isomerization, first to the 1,4-isomer, and then on further heating, to the 1,3- and 1,5-isomers. The specificity of these rearrangements has been interpreted in terms of an intramolecular Diels-Alder reaction of the bicyclic valence isomer of the cyclo-octatetra-ene, followed by a reverse ( 2g + 4- 2j) cycloaddition. However, pyrolysis of... 

Ring-contraction of cyclo-octatetraene to phenyl acetaldehyde is catalysed by copper(i), platinum(ii), palladium(ii), and silver(i) complexes. Cycloheptatriene alde--hyde was detected during the copper(i) rearrangement. ... 

The bicyclo[4,2,0]octa-2,4,7-trienyl iron carbonyl complexes (285) and (286) are stable, even in refluxing benzene, in marked contrast to the rapid valence isomerization of free bicyclo[4,2,0]octa-2,4,7-triene to cyclo-octatetraene. This stability is due to the loss of the inert gas configuration on rearrangement to a diTiapto-cyclo-octatetra-ene-Fe(CO)3 complex. The inert gas structure could be retained if the cyclobutene ring of the complexes opened via a forbidden disrotatory opening, but this does not occur because the metal is not in a position to help sterically. ... 

Octamethylbicyclo[4,2,0]octa-2,4,7-triene has been prepared from octamethyl-cyclo-octatetraene via its dimethyl azocarboxylate adduct. The rearrangement of octamethylbicyclo[4,2,0]octa-2,4,7-triene back to the monocyclic octamethylcyclo-octatetraene was studied and the Diels-Alder adducts with TCNE and maleic anhydride were prepared. Cyclo-octatetraene and vinylene carbonate react at 180°C to give the polycyclic adduct (150 50%) mechanisms for this reaction were discussed. 

Preparation of bicyclo[3,2,l]oct-2-en-8-one (547 57%) has been achieved by successive treatment of tricarbonyl(cycloheptatriene)iron with tetrafluoroboric acid and sodium borohydride. Similarly, tricarbonyl(cyclo-octatetraene)iron gave bicyclo-[3,3,l]nona-2,6-dien-9-one (62%). Thermolysis of (—)-(548) at 195°C (1 h) gave (— )-(550) by the fast Cope rearrangment of the one-centre epimerization intermediate (549). The alternative two-centre epimerization pathway by way of intermediate... 

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