Homodienyl shifts rearrangement

This rearrangement, which has been called an enolene rearrangement, a homodienyl [1,5] sigmatropic hydrogen shift (see 18-29), and a [1,5] homo sigmatropic... 

The thermal rearrangements of vinylcyclopropanes to form cyclopentenes as well as 1,4-hexadienes by homodienyl [l,5]-shift are well-known16,49-51 and even described in textbooks (see, e.g., Chapter 18 in Reference 4). However, the heteroanalogous transformations are less known. 

Whereas the parent difluoro-vinylcyclopropane isomerizes to difluorocyclopentene under pyrolysis conditions, the corresponding alkyl compounds also lead to acyclic dienes. The activation energy for the difluoro-vinylcyclopropane isomerization is practically identical with that observed for the unsubstituted hydrocarbon [211, 212], If the alkyl group is oriented cis to the vinyl substituent, only dienes are isolated, and the process occurs at much lower, temperatures. Presumably these stereoisomers rearrange by a different mechanism (a 1,5-homodienyl hydrogen shift [213]). When the dichlorocyclopropane XVII is subjected to flash vacuum pyrolysis it isomerizes to 9,9-dichloro-bicyclo[5.3.0]dec-l(7)-ene [214],... 

Also reported was the subsequent conversion of the isopropylidene tetracyclic material to isopropylidenecyclooctatetraene. A homodienyl hydrogen shift to give an isopropylsemibullvalene, which could rearrange to the monocyclic product (see Chapter 9, Section 2.1), was envisioned to account for the reaction (Scheme 10.38). 

It would appear that the synthesis was successful, but the product rearranged via a homodienyl 1,5-hydrogen shift (see Chapter 7, Section 4). 

Claisen rearrangement. It arises when the allyl group has an alkyl substituent as indicated in Equation (6.114). The first-formed normal rearrangement product is transformed by two consecutive [1,5] homodienyl hydrogen shifts into the abnormal product, (6.114). 

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