SolutionsPericyclic Reactions
1. Claisen rearrangement
Which product does allyl vinyl ether (3-vinyloxypropene, CH2=CH–O–CH2–CH=CH2) give on heating? And allyl phenyl ether?
Allyl vinyl ether gives 4-pentenal, a γ,δ-unsaturated aldehyde: the allylic O–CH2 bond breaks, a C–C bond forms between the ends and the oxygen ends up as C=O. Allyl phenyl ether first gives 6-allylcyclohexa-2,4-dienone, which tautomerizes to the aromatic 2-allylphenol.
2. Oxy-Cope rearrangement
Which carbonyl compound is obtained when 1,5-hexadien-3-ol is heated?
The [3,3] reorganization breaks the C3–C4 bond and forms a new one between C1 and C6. The oxygen ends up in an enol (CH=CH–OH), which tautomerizes to the aldehyde: 5-hexenal is obtained.
In both reactions the transition state is cyclic, with six atoms, and bonds break and form at the same time.