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- W4252022113 abstract "The Bergman cyclization of (Z)-hex-3-ene-1,5-diynes (1, enediynes), which produces pharmacologically important DNA-cleaving biradicals (1,4-benzyne, 2), was studied by using Hartree-Fock (HF) and density-functional theory (DFT) based valence bond (VB) methods (VB-HF and VB-DFT respectively). We found that only three VB configurations are needed to arrive at results not too far from complete active space {CASSCF(6×6)} computations, while the quality of VB-DTF utilizing the same three configurations improves upon CASSCF(6×6) analogous to CASPT2. The dominant VB configuration in 1 contributes little to 2, while the most important biradical configuration in 2 plays a negligible role in 1. The avoided crossing of the energy curves of these two configurations along the reaction coordinate leads to the transition state (TS). As a consequence of the shape and position of the crossing section, the changes in geometry and in the electronic wavefunction along the reaction coordinate are non-synchronous; the TS is geometrically ≈80 % product-like and electronically ≈70 % reactant-like. While the π resonance in the TS is very small, it is large (64.4 kcal mol−1) for 2 (cf. benzene=61.5 kcal mol−1). As a consequence, substituents operating on the σ electrons should be much more effective in changing the Bergman reaction cyclization barrier. Furthermore, additional σ resonance in 2 results in unusually high values for the nucleus-independent chemical shift (NICS, a direct measure for aromaticity). Similarly, the high NICS value of the TS is due mostly to σ resonance to which the NICS procedure is relatively sensitive. Die Bergman-Cyclisierung von (Z)-Hex-3-en-1,5-diin (1, Endiin), die pharmakologisch aktive, DNA-spaltende Biradikale (1, 4-Benzin, 2) liefert, wurde mit Hilfe der Valenzbindungstheorie (VB-Theorie) auf dem Hartree-Fock- (HF) und Dichtefunktionaltheorie- (DFT) Niveau (VB-HF bzw. VB-DFT) untersucht. Man benötigt nur drei VB-Konfigurationen um Ergebnisse zu erzielen, die denen von „complete active space“ {CASSCF(6×6)} Rechnungen nur wenig nachstehen. Die dominante VB-Konfiguration in 1 trägt nur wenig zu 2 bei. Umgekehrt spielt die Biradikalkonfiguration von 2 in 1 eine untergeordnete Rolle. Das „avoided crossing“ der Energiekurven dieser beiden Konfigurationen entlang der Reaktionskoordinate führt zum Übergangszustand (TS). Bedingt durch die Form und Lage der Überlappungsregion sind die Änderungen der Geometrie und der elektronischen Wellenfunktion nicht synchron; der TS ist geometrisch ≈80 % Produkt- elektronisch aber ≈70 % Edukt-artig. Während die π-Resonanz im TS sehr klein ist, ist sie in 2 groß (64.4 kcal mol−1; zum Vergleich: Benzol=61.5 kcal mol−1). Folglich sollten Substituenten, welche auf die σ-Elektronen wirken, die Cyclisierungsbarriere der Bergman-Reaktion stärker beeinflussen als π-aktive Substituenten. Bedingt durch die erhöhte σ-Resonanz in 2 ergeben sich ungewöhnlich hohe „nucleus-independent chemical shift-“ Werte (NICS, ein direktes Maß für Aromatizität). In ähnlicher Weise ist der hohe NICS-Wert des TS hauptsächlich auf σ-Resonanz zurückzuführen, worauf das NICS-Verfahren besonders empfindlich anspricht." @default.
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- W4252022113 date "2000-04-17" @default.
- W4252022113 modified "2023-10-18" @default.
- W4252022113 title "A Valence Bond Study of the Bergman Cyclization: Geometric Features, Resonance Energy, and Nucleus-Independent Chemical Shift (NICS) Values" @default.
- W4252022113 doi "https://doi.org/10.1002/(sici)1521-3765(20000417)6:8<1446::aid-chem1446>3.3.co;2-9" @default.
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