How does the Diels-Alder reaction work?
The Diels-Alder reaction is a [4+2] cycloaddition where a conjugated diene, with four π electrons, reacts with a dienophile, which has two π electrons, to form a cyclohexene ring. It is a concerted, pericyclic process, meaning all bond-breaking and bond-forming occur simultaneously in a single step. The diene and dienophile approach each other in a parallel plane, and the electron flow is governed by the frontier molecular orbitals—the highest occupied molecular orbital of the diene interacts with the lowest unoccupied molecular orbital of the dienophile. The endo rule dictates the stereochemical outcome: the dienophile's substituents preferentially orient toward the diene's π system during the transition state, leading to endo products. This mechanism explains the high regio- and stereoselectivity we observed in our experiments, such as in the synthesis of adducts from cyclopentadiene and maleic anhydride.
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