The Stereoselection Rule for Electrocyclic Interactions

Abstract

A general stereoselection rule governing the electrocyclic interactions has been proposed with a molecular orbital rationale. The reactions discussed from a unified point of view cover the ring-closure of polyenes, the Diels-Alder additions, 1,4-dipolar additions, the Cope and the Claisen rearrangements, the hydrogen and the proton migrations, 1,3-dipolar additions, the ring-opening of cyclic olefins, the deamination of cyclic unsaturated imines, 1,2-noncycloadditions, 1,2-eliminations, the nucleophilic displacement with allylic rearrangements, and other cyclic intermediate or complex formations. The relation to the molecular orbital symmetry has been also discussed.The molecular orbital (MO) theory, which has early proved itself suitable for interpreting the chemical reactivity, is exhibiting its usefulness also in the problem of the stereoselectivity. In particular, the symmetry property of MO has been intimately connected to the steric course of chemical reactions. One of the present authors first pointed out the importance of the symmetry of the highest occupied (HO) MO of dienes and the lowest vacant (LV) MO of dienophiles for the occurrence of the Diels-Alder reaction.1) The MO symmetry was correlated with the stereoselectivity in the ringclosure of linear polyenes and in the ring-opening of cyclic olefins first by Woodward and Hoffmann,2) and later by Longuet-Higgins and Abrahamson,3) and by Fukui.4) Woodward and Hoffmann2) considered the symmetry of HO only, while Fukui1,4) HO and LV for discussing the interaction of two systems.These particular orbitals were coincident with those which played an important role in the MO

Enamines I. Vinyl Amine, a Theoretical Study of its Structure, electrostatic potential, and proton affinity

“…In fact, whereas electrostatic potential minima indicate an 8 kcaVmol preference for exo over endo attack of the double bond, vertical proton affinities favor endo over exo protonation by about 2 kcaVmo1. The inference that without nuclear relaxation in the non-planar equilibrium structure of vinyl amine proton addition to the nitrogen and the P-carbon atom should occur preferentially from opposite sides of the molecule is of particular interest in view of continuing discussions concerning similar alternations in the stereochemical mode of substitution and elimination reactions involving allylic systems [4] [26] [27].…”

Regio- and Stereochemistry of Gas-Phase Acid-Induced Nucleophilic Substitutions on Chiral Allylic Alcohols1

“…Carrion and Dewar suggest that the predominance in solution of the S N 2 mechanism over the S N 2‘ one is primarily due to energy-demanding ion desolvation in the S N 2‘ transition structure, which mostly contributes to the building up of the relevant activation barrier, whereas the more favored S N 2 barrier is essentially determined by electronic factors. In light of these modern concepts, these and other authors conclude that there is no real reason concerted S N 2‘ reaction should not be feasible in the gas phase. − This indication has been followed in a recent gas-phase study of acid-induced nucleophilic substitution on some allylic alcohols that showed that the concerted S N 2‘ reaction actually competes with the classical S N 2 pathway in the absence of solvation and ion-pairing factors. − The same modern concepts consider restrictive any theoretical rationale of the S N 2‘ stereochemistry simply based on stereoelectronic factors, − since an important role may be played by Coulombic interactions between NuH and LG and among these and the reaction medium.…”

Governing organic reactions through secondary orbital interactions. Semiempirical and density functional theory study of catalyzed cycloaddition reactions between pyrrole and ether dienophiles

“…Let us first explore the relative aromatic (resonance) 22 character of pyrrole and its derivatives through bond order uniformity 23 and the frontier molecular orbital (FMO) 24 energy gap 25 computed with the AM1 semiempirical method (Table 1). According to the bond order uniformity approach, 23 the ring systems that have the most uniform bond order distributions are the most stable ones.…”