A DFT Study on the Vinylcyclopropanecarbaldehyde‐to‐2,5‐Dihydrooxepin Hetero‐Cope‐type Rearrangement and on Related Reactions

Abstract

The prototypical 1,2-cis-vinylcyclopropanecarbaldehyde-to-2,5-dihydrooxepin hetero-Cope-type rearrangement was studied by "exact" first-principle methods. The reaction pathway was examined. The reaction, as well activation energies, was calculated for the unimolecular transformation of vinylcyclopropanecarbaldehyde and various derivatives. The derivatives differ from vinylcyclopropanecarbaldehyde by replacement of the formyl (CH=O) by the thioformyl (CH=S) or formiminyl (CH=NH) group and, in part, by replacement of hydrogen atoms at the substituted carbon atoms of the cyclopropane ring by hydroxyl or formyl substituents. The experimental reaction parameters of vinylcyclopropanecarbaldehyde are surprisingly well reproduced by B3LYP/6-31G * density functional and MP2/6-31G * a b initio quantum theoretical calculations. Reactant and product are nearly isoenergetic, while the activation energy amounts to about 25 kcal/mol. In the case of the nitrogen and sulfur containing compounds the isomeric seven-membered ring structures are considerably favoured over the cyclopropanes. Due to a low calculated activation energy a rapid formation of the 2,5-dihydrothiepin is expected. Substitution of the hydrogen at the substituted ring carbon atoms in vinylcyclopropanecarbaldehyde by OH and CH=O also lowers the barrier and increases, in general, the exothermicity of the reaction. As shown by the reaction energies of isodesmic reactions both reactants and products are stabilized by substitution. However, the seven-memberd ring compounds are more strongly stabilized than the cyclopropanes. The OH group exerts a different effect depending on whether the linkage is geminal or vicinal to the C=X group. The difference is caused by hydrogen bond formation in geminal arrangements. The substituent effects in the cyclopropane series parallel those for the prototypical Cope-type and Claisen-type series. The particular feature of the cyclopropane series is the lower stability of the cyclopropanes relative to the corresponding open chain congeners. This is obviously due to the ring strain which overcompensates for attractive interactions between cyclopropane and the substituents. In consequence, the formation of seven-membered ring compounds proceeds more easily than the formation of the corresponding compounds in related Cope-type rearrangements.The [3,3]-sigmatropic Cope-type and Claisen-type rearrangements are useful and versatile reactions that have attracted paramount interest in the past years [']. The prototypical degenerate Cope rearrangement of 1,Shexadiene to 1,5-hexadiene, and the Claisen rearrangement of allylvinylether to 4-pentenal (eq. 1 in Scheme l), have been studied theoretically at a high level of ab initio quantum theoryc21 and, more recently, at the level of density functional the0ry[~1 [~]. The pericyclic reactions proceed over delocalized ("aromatic") transition structures in a concerted manner.According to eq. 2 of Scheme 1 the divinylcyclopropaneto-cycloheptadiene and vinylcyclopropanecarbaldehyde-todihydrooxepin rear...

Transition Structures, Energetics, and Nucleus-Independent Chemical Shifts for Divinylcyclobutene-to-Cyclooctatriene Rearrangement: A DFT Study

“…17 To the best of our knowledge, cyclooctatrienes 9cct and 9tct are unknown compounds. Notably, the rearrangement of divinylcyclobutene 7 is more exothermic than those of divinylcyclopropane 1 15,16 and divinylcyclobutane 4 16 by 10.1 and 12.9 kcal/mol, respectively, at the same level of theory, which might be attributed to higher ring strain in 7. It is not clear to exactly what the ring strain affects the reaction exothermicity but the ring strain influences the exothermicity of a reaction more than its activation barrier, as seen in the rearrangements of 1, 4, and 7.…”

“…6c,14 Very recently, (U)B3LYP/6-31G* calculations have been successfully used in the calculation of TSs and reaction parameters for the Cope rearrangements of divinylcyclopropanes, 15,16 divinylcyclobutanes, 16 and fickle hexadienes. 17 Notably, the calculated activation barriers of 19.7 15,16 and 25.0 16 kcal/mol for Cope rearrangements of 1 and 4 are in good agreement with their experimentally derived values of 19.0-20.0 18 and 24.0 19 kcal/mol, respectively.…”

Asymmetric Catalytic Formal 1,4‐Allylation of β,γ‐Unsaturated α‐Ketoesters: Allylboration/Oxy‐Cope Rearrangement

“…Four isomers of 3aa [(2R,3S)-3aa,( 2 S,3S)-3aa,( 2 R,3R)-3aa and (2S,3R)-3aa]were optimized, and the corresponding transition states in the rearrangement reactions were located (for details,see the Supporting Information). Additionally,t he process of the oxy-Cope rearrangement is irreversible because of the tautomerization of the productive diene to am ore stable ketone ester 4aa.C onsequently,t he reaction pathway via the boatlike transition state is more favorable for (2R,3S)-3aa, [25][26] (for details,s ee the Supporting Information). For( 2 R,3S)-3aa,t he chairlike transition state (2R,3S-TS-C) associated with (R,E)-4aa is energetically less favorable than the boatlike transition state 2R,3S-TS-B associated with (S,E)-4aa by 0.8 kJ mol À1 (see Figure 1).…”

Stereoselective Lewis Acid Mediated [1,3] Ring Contraction of 2,5‐Dihydrooxepins as a Route to Polysubstituted Cyclopentenes

“…[8,9] An equilibrium between 1 and 2 has been predicted computationally, [10] and may be shifted towards the 2,5-dihydrooxepin with p-stabilizing substituents [Eq. (2); EDG = electrondonating group, EWG = electron-withdrawing group].…”