Why do two π-electron four-membered Hückel rings pucker?

• Verfasst von: McKee, William C. [VerfasserIn]
• Verfasst von: Hofmann, Matthias [VerfasserIn]
• Titel: Why do two π-electron four-membered Hückel rings pucker?
• Verf.angabe: William C. McKee, Judy I. Wu, Matthias Hofmann, Armin Berndt, and Paul von Ragué Schleyer
• Umfang: 4 S.
• Fussnoten: Gesehen am 15.11.2018
• Titel Quelle: Enthalten in: Organic letters
• Jahr Quelle: 2012
• Band/Heft Quelle: 14(2012), 22, S. 5712-5715
• ISSN Quelle: 1523-7052
• DOI: doi:10.1021/ol302726c
• Datenträger: Online-Ressource
• Sprache: eng
• K10plus-PPN: 1583744991

Abstract

Notwithstanding their two (i.e., 4n + 2) π electrons, four-membered ring systems, 1-4, favor puckered geometries (1a-4a) despite the reduction in vicinal π overlap and in the ring atom bond angles. This nonplanar preference is due to σ → π* hyperconjugative interactions across the ring (A) rather than to partial 1,3-bonding (B). Electronegative substituents (e.g., F in C4F42+) reduce the σ → π* electron delocalization, and planar geometries result. In contrast, electropositive groups (e.g., SiH3 in C4(SiH3)42+) enhance hyperconjugation and increase the ring inversion barriers substantially.