Dimerizations of dienes by cycloaddition reactions
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص887
2025-07-22
435
Dimerizations of dienes by cycloaddition reactions
Because dienes have relatively high-energy HOMOs and low-energy LUMOs they should be able to take part in cycloadditions with themselves. And indeed, dienes do dimerize, by a Diels–Alder reaction. One molecule of the diene plays the role of the dienophile. The symmetry is correct for the interaction shown, and we call such reactions (like all the Diels–Alder reactions in this chapter) ‘[4 + 2] cycloadditions’—the numbers referring to the number of atoms of each component taking part in the reaction.
What dienes cannot do is form an eight-membered ring in one step in a [4 + 4] cycloaddition (although this is possible photochemically or with transition metal catalysis, as we shall see later).
You should have expected this failure because the ends of the required orbitals must again have the wrong symmetry, just as they had when we tried the alkene dimerization.
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