The Source of the endo Rule in the Diels−Alder Reaction: Are Secondary Orbital Interactions Really Necessary?

Abstract

The endo preference in Diels−Alder reactions is usually attributed to the occurrence of attractive Secondary Orbital Interactions (SOI), whereas other interaction mechanisms (primary interactions, closed-shell repulsions, electrostatics) are assumed to be identical for both endo and exo approaches. However, analysis of the parallel approximation between strans butadiene and fumaronitrile shows that SOI is overcome by closed-shell repulsions. Furthermore, the study of several reactions (cyclopentadiene + maleic anhydride, cy-

Comparative Analyses of a Family of Potential Self‐Replicators: The Subtle Interplay between Molecular Structure and the Efficacy of Self‐Replication

“…Additionally, utilisation of symmetrical dienophiles 3 and 4 automatically eradicated any regioselectivity issues. Under such conditions, the furans engage their complementary counterparts, the maleimides, in pericyclic [21] unions via intermediacy of two transition states, endo and exo, on the basis of relative orientations of the furan and maleimide nuclei in the pretransition-state phase. This furnishes pairs of racemic, diastereoisomeric adducts endo and exo (Scheme 2), distinguished by the spacers connecting the hydrogen-bonding sites to the cycloadduct core.…”

A Theoretical Study of the Stereoselectivities of the Diels–Alder Addition of Cyclopentadiene to Ethyl‐(S)‐lactyl Acrylate Catalyzed by Aluminium Chloride

“…[38,49,50] Our own attempts to understand these phenomena brought us to the conclusion that it is still necessary to carry out more in-depth studies on a variety of simpler systems if we want to understand complex ones as that discussed in this paper. Electronic, electrostatic and steric effects are all important factors and it is our understanding that it would not be correct to attempt to justify this kind of selectivity based only on one of these factors.…”

Inverse‐Electron‐Demand Diels–Alder Reactions: Principles and Applications

“…[16] Af avorables econdary orbitali nteraction is the commone xplanation, [17] althought his has been disputed by somer esearchers. [18] However,i ns ome cases, the endo selectivity may be reversed.…”

“…The selectivity of the iEDDA reaction generally follows that of the normal Diels–Alder reaction in that the endo adduct is usually the favored product . A favorable secondary orbital interaction is the common explanation, although this has been disputed by some researchers . However, in some cases, the endo selectivity may be reversed.…”