Pericyclic Reactions in Stereochemistry
It is well established that the diene can only inter in a cycloaddition reaction with a dienophile in the cisoid form and the rate of the reaction with the open chain dienes depends on the equilibrium proportions of the cisoid/transoid conformers. Thus substituents in the diene can significantly affect the rate of the reaction not only by their electronic character but by their influence on the conformers proportions.
Thus for example cis I -substituted butadiene I is less reactive than its trans isomer II since a bulky R disfavors the cisoid conformation. Bulky 2-substituents in the diene favor the cisoid conformation more than the transoid and thus the diene in this case is more reactive.
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