Cyclopenten-3-yl cation

In Scheme 1, the radical cations of the linear hexadienes and some cyclic isomers are contrasted. The heats of formation, AHr, as determined from the heats of formation of the species involved, as well as the heats of formation of the isomeric radical cations themselves clearly reveal the favourable stability of the cyclic isomers and/or fragment ions. Thus, instead of the linear pentadienyl cation (3), the cyclopenten-3-yl cation (2) is eventually formed during the loss of a methyl radical from ionized 1,3-hexadiene (1). Since 1,2-H+ shifts usually have low energy requirements (5-12 kcalmol-1), interconversion of the linear isomers, e.g., 4, and subsequent formation of the cyclic isomers, in particular of the ionized methylcyclopentenes 5 and 6, can take place easily on the level of the... 

The Cs 1-methyl-l,3-pentadien-5-yl cation has one C-H bond of the methyl group eclipsed with the r-system, analogously to the 1-methylallyl cation. Rearrangements of this cation to cyclopenten-3-yl derivatives have long been known experimentally, The 1-methyl-l,3-pentadien-5-yl cation can... 

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