Hexa-1,2,5-triene rearrangement

Unlike the well-known chemistry of the vinylcyclopropane-cyclopentene rearrangement, there is no general method for the rearrangement of alkynyl-cyclopropane to cyclopentene derivatives. One specific example is the pyrolysis of l-ethynyl-2-methylcyclopropane to methylenecyclopentene and other compounds [5]. At 530°C, l-ethynyl-2-methylcyclopropane (1) undergoes a [1,5]-hydrogen shift to give hexa-l,2,5-triene (2), which further isomerizes to methy-lenecyclopentenes 3 and 4 in 38 and 29% yield, respectively (Scheme 1). 

Although one can envisage dehydrogenation and rearrangement of the coordinated trienes in 149 and 150 to yield 114, no such process has actually been observed to take place. Because both triene complexes are derived from the trans ligand, and not the cis isomer, it is possible that it is the ds-hexa-l,3,5-triene ligand that produces the benzene ring in cluster... 

Whereas the hexa-1,3,5-triene/cyclohexa-l,3-diene rearrangement serves as a model reaction in the hydrocarbon series, the perfluoro analog behaves differently, as presented in Section 5.3.2.1. Photolysis of perfluorohexa-l,3,5-triene (8) in the gas phase yields per-fluorobicyclo[2.2.0]hex-2-ene (10) as the major product, presumably via perfluorocyclohexa-1,3-diene (9).4 Perfluoro(3-vinylcyclobutene) is formed as the minor product. Thermolysis of hexa-tricnc 8 gives cyclohexadiene 9.4 Photolysis of 9 provides bicyclohexene 10, which reverts to 9 thermally.5... 

Theoretical studies have been carried out on the conrotatory and disrotatory reaction pathways of hexa-l,3,5-triene to cyclohexadiene, and the effect of solvent and salt effects on the rates of the electrocyclic ring closure of (IZ, 3Z, 5 )-l,2,6-triphenylhexa-l,3,5-triene has been determined. An ab initio study of the electro-cyclization of (Z)-hexa-l,3,5-triene and its hetero-substituted analogues has been undertaken.The involvement of a lone pair on the nitrogen or oxygen atom appears to facilitate the interaction between the terminal atoms that bond to each other to close the ring. It has been reported that the intramoiecular aza-Diels-Alder reaction of an a./S-unsaturated hydrazones to a quinone, as in (197), is followed by an unprecedented rearrangement in which the aminoaryi moiety formally undergoes a 1,2-shift to yield benzo- or pyrido-[fc]acridine-6,11-dienes (198). 

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