Isomerization cope-type

Most of these hydrocarbons have been prepared and their chemical behavior has been investigated. Many of them undergo mechanistically and preparatively interesting Cope-type isomerizations (see below). 

The skipped enallene 28 is easily prepared by gas-phase pyrolysis of l-hexen-5-yne (232), which under high-temperature conditions undergoes a Cope-type isomerization via transition state 233 (Scheme 5.35) [41, 92]. 

The initial isomerization product is now the (trimethylallyl)allene 30 which subsequently participates in a Cope-type rearrangement leading to the enyne 31. 

Three fundamental types of isomerization may be distinguished. The first is double-bond migration, as in the isomerization of pent-l-ene to (Z)-pent-2-ene. The second involves skeletal rearrangements, such as the conversion of bicyclo[3.1.0]hex-2-ene derivatives to the corresponding cyclohexadienes. The third, and possibly most useful type, is the catalysis of group migrations, particularly in the Cope-and Claisen rearrangements. 

In the context of the potential Cope rearrangement of hexa-1,5-diene radical cations (Section 2.4.1), we mentioned the triplet recombination of radical ion pairs generating a biradical [202, 203]. Because of continuing interest in this type of reaction we briefly mention two additional examples involving radical cationic systems discussed in this review, viz., the isomeric 1,2-diphenylcyclopropane radical cations, cis- and trans- 3 , and norbornadiene radical cation, 91 +. 

There are other types of proton shift tautomers, such as phenol/dienone, nitroso/oxime and imine/enamine, but these are less often encountered. There are also valence tautomers that exhibit fluxional structures, which undergo rapid sigmatropic rearrangements. Molecules that exhibit this type of isomerism are very interesting, but are not often encountered in undergraduate courses. An example of a valence tautomer is illustrated by the Cope system. 

Irradiation (quartz filtered) of the oxazolone derivative (199) in acetonitrile results in decarbonylation and the formation of the imine (200). When this imine is formed in the presence of allyl alcohols, trapping (a thermal reaction) results in the formation of the ethers which are also photochemically reactive and are transformed by a Norrish type II hydrogen abstraction process into the isomeric compounds (201). Oxy-Cope rearrangement of (201) yields the second product (202) isolated from the initial irradiation. 

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