A Theoretical Comparison of Two Competing Diradical Cyclizations in Enyne-Allenes:  The Myers−Saito and the Novel C²−C⁶ Cyclization

Abstract

The reaction mechanism and the energetics of the C2−C7 and of the C2−C6 cyclization of (Z)-1,2,4-heptatriene-6-yne are studied employing large scale MR-CI calculations. In addition the influence of vibrational motions and of temperature corrections on the barrier height of both reactions is estimated. For both products the first excited electronic states are computed to estimate possible switches from diradical to zwitterionic reaction mechanism.

Two Novel Thermal Biradical Cyclizations of Enyne-Ketenimines: Theory, Experiment, and Synthetic Potential

“…In contrast to our recent calculations on the thermal cyclization of enyne-allenes (A CH, B CH 2 ) [8,9] it was necessary in the present study to include the annulated benzene ring, because otherwise the biradical intermediates are found to have a different electronic structure. [10] A correct description of the biradical intermediates requires a multireference approach.…”

“…[10] A correct description of the biradical intermediates requires a multireference approach. [11] However, because the biradical nature of the wave function develops only beyond the transition state (TS)Ðthat is, in the density function theory (DFT) calculations [S 2 ] 0 for all the transition states [8] Ðthe influence of substituents on the activation energy DE = in both processes can be determined from DFT calculations. Consequently, activation energies were studied with a density functional approach in combination with a 6-31G* AO basis set, whereas a multireference configuration interaction (MR-CI) approach was used to compute the reaction energies (DE r ).…”

Carbon‐rich Compounds: Computational Considerations

“…Of course, a proper treatment would utilize multireference methods, but these are very time consuming and limited to model structures only [36,88]. Furthermore, dynamic electron correlation is also very important in these reactions so that, for instance, simple CASSCF (complete active space SCF) computations are meaningless, as they mostly include static correlation.…”

Zwei neue thermische Diradikal- Cyclisierungen von Eninketeniminen: Theorie, Experiment und Synthesepotential

“…Im Unterschied zu unseren Rechnungen zur thermischen Cyclisierung von Eninallenen (A CH, B CH 2 ) [8,9] muûten wir in diesem Fall den anellierten Benzolring in die Studien einbeziehen, da die Diradikalzwischenstufen sonst eine andere elektronische Struktur erhalten hätten. [10] Die korrekte Beschreibung von Diradikalzwischenstufen erfordert einen Multi-Referenz-Ansatz.…”

“…[10] Die korrekte Beschreibung von Diradikalzwischenstufen erfordert einen Multi-Referenz-Ansatz. [11] Da sich jedoch der diradikalische Charakter der Wellenfunktion erst nach dem Übergangszustand (ÜZ) entwickelt und in der Dichtefunktionaltheorie(DFT)-Rechnung somit für alle Übergangszustände [S 2 ] 0 gefunden wird, [8] kann der Substituenteneinfluû auf die Aktivierungsenergie DE = beider Prozesse aus DFT-Rechnungen bestimmt werden. Daher wurden die Aktivierungsenergien über den Dichtefunktionalansatz in Verbindung mit einem 6-31G*-AO-Basissatz bestimmt, während für die Berechnung der Reaktionsenergien DE r ein Multireference-Configuration-Interaction(MR-CI)-Ansatz verwendet wurde.…”