Selective cis-trans isomerization of a tetraene via an allene intermediate: Geometric isomerism via an sp2 to sp2 sigmatropic hydrogen shift and kinetic isotope effect studies

A kinetic investigation in the unlabeled and deuterium-labeled series of the (1,5) sigmatropic hydrogen shift of vinylallene 5 to trienes 7 plus 8 and the (1,7) sigmatropic hydrogen shift of dienallene 6 to tetraenes 9 plus 10 has been completed. The results lead to the proposal that the pathway for geometric isomerization of the terminal double bond of tetraene 9 involves as an intermediate the allene 6. The allene 6 could in fact be isolated in the case of the isomerization of 9, and moreover, at long reaction times 6 and 9 are transformed to 10. For the isomerization of 5 to 7 plus 8, it was shown that 7 and 8 interconvert with one another, but it is unclear whether allene 5 is an intermediate in their interconversion. The temperature dependencies of the primary deuterium kinetic isotope effects observed for the rearrangement of 5 (via a (1,5) sigmatropic hydrogen shift) and 6 (via the corresponding (1,7)-shift) suggest a tunneling contribution