SolutionsPericyclic Reactions
1. Cis or trans?
Which stereoisomer of 5,6-dimethyl-1,3-cyclohexadiene is obtained thermally from (2E,4Z,6Z)-2,4,6-octatriene?
The trans isomer. The thermal pathway with 6 electrons (4n + 2) is disrotatory; since one of the end methyl groups now has the other arrangement (Z), disrotatory closure leaves them on opposite sides.
2. Cyclobutenes
cis-3,4-Dimethylcyclobutene opens on heating. Which isomer of 2,4-hexadiene is obtained?
(2E,4Z)-2,4-Hexadiene. With 4 electrons the thermal opening is conrotatory: the two methyl groups, which were cis, rotate in the same direction, and one ends up pointing outwards and the other inwards.
3. The rules
Complete: an 8-electron electrocyclic reaction is ______ thermally and ______ photochemically.
Conrotatory thermally and disrotatory photochemically, because 8 = 4n with n = 2.