Theoretical Study on the Aromaticity of Transition States in Pericyclic Reactions

Abstract

The aromaticity of transition states in pericyclic reactions such as electrocyclic reactions, cycloaddition reactions, and sigmatropic shifts was studied by the IDA (index of deviation from aromaticity) on the basis of a CASSCF wave function. The aromaticity defined by the IDA classified the allowed and forbidden transition states of pericyclic reactions treated here. The order of the aromaticity levels corresponds to that of the energy barriers of some reactions. The difference between the aromaticity defined by the IDA and that by the magnetic properties as a NICS is also discussed.

Molecular Rearrangements: Part 1. Pericyclic Reactions

“…In the disubstituted series, rearrangement barriers have been shown to be lowered by π -donor groups, and the position of equilibrium was found to correlate with the BCO stabilization effects of the substituents. 5 A DFT investigation of the Cope rearrangement of the cyclobiphenalenyl (1) has shown it to occur via the unusual diradical (2), stabilized by π -dimerization, and having an unusually long inter-phenalenyl distance (comparable to the inter-allyl distance in a classical Cope rearrangement) of ∼2.8Å. 6 (1) (2) Calculations of the interactions between matured catalytic antibody AZ28 and the different conformers and enantiomers of 2,5-diaryl-hexa-1,5-dien-3-ol, allowing for the flexibility of the antibody, have reproduced, qualitatively only, the observed selectivity of the antibody for the S,equatorial substrate.…”

“…5 A DFT investigation of the Cope rearrangement of the cyclobiphenalenyl (1) has shown it to occur via the unusual diradical (2), stabilized by π -dimerization, and having an unusually long inter-phenalenyl distance (comparable to the inter-allyl distance in a classical Cope rearrangement) of ∼2.8Å. 6 (1) (2) Calculations of the interactions between matured catalytic antibody AZ28 and the different conformers and enantiomers of 2,5-diaryl-hexa-1,5-dien-3-ol, allowing for the flexibility of the antibody, have reproduced, qualitatively only, the observed selectivity of the antibody for the S,equatorial substrate. 7 Doubly α,β-unsaturated N -arylimines (3) have been shown to react with α,β-unsaturated ketenes to give azocinones (4), …”

“…In view of the occasional observation of allene by-products, and of various regioisomeric byproducts, a mechanism has been proposed involving Au coordination to the alkyne, rearrangement to an allene by acetate migration, and hydroarylation of the gold-coordinated allene. The acetate migration may formally occur by either two [1,2]-migrations with a vinyl acetate intermediate or a single [1,3]-or [3,3]rearrangement. 21 The synthesis of N -arylindoles from N ,N -diaryl-N -trifluoroacetyl enehydrazines, a process analogous to the Fischer indole synthesis, has been studied in various solvents, including water, and in solvent-free conditions.…”

“…A possible catalytic cycle and transition-state model have been proposed, in which the sigmatropic rearrangement occurs from an enamine, with hydrogen bonding from the ether to the methanolic solvent playing an important rôle in determining transition-state geometry. 30 Vinyl epoxide derivatives have been shown to undergo ring expansion to 3,6-dihydropyran-2-carboxylates on treatment with diazoacetate esters and a Cu(II) catalyst, presumably by [2,3]-sigmatropic rearrangement of the resulting oxonium ylids. 1,2-trans-Disubstituted epoxides have been found to give exclusively 2,6-transdisubstituted dihydropyrans, but it has been shown that only 1,2-trans-divinylsubstituted epoxides gave useful yields of the dihydropyran products; with other substituents, deoxygenation to penta-1,3-dienes was found to compete.…”

Aromaticity changes along the reaction coordinate connecting the cyclobutadiene dimer to cubane and the benzene dimer to hexaprismane

“…With this analysis we intend to get an insight into the relationship between reactivity and aromaticity in pericyclic reactions. In previous works, it has Scheme 1 Schematic representation of reactants and product of the cycloaddition reaction between two cyclobutadiene molecules Scheme 2 Schematic representation of reactants and product of the cycloaddition reaction between two benzene molecules been shown that usually thermally allowed cycloadditions take place through aromatic transition states (TSs) [33], while TSs of thermally forbidden reactions are less aromatic or antiaromatic [34][35][36]. As recommended by several authors [20,[37][38][39][40][41], in the present work the local aromaticity of the 4-and 6-MRs has been measured using a set of differently based aromaticity criteria, to take into account the multidimensional character of aromaticity [42][43][44][45].…”