5-exo or 6-endo? Exploring Transition State Structures in Cyclizations of 4-Penten-1-oxyl Radicals†‡
Corresponding Author
Dr. Jens Hartung
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]Search for more papers by this authorDipl.-Chem. Ralf Stowasser
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]
Search for more papers by this authorDipl.-Chem. Daniel Vitt
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]
Search for more papers by this authorCorresponding Author
Prof. Dr. Gerhard Bringmann
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]Search for more papers by this authorCorresponding Author
Dr. Jens Hartung
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]Search for more papers by this authorDipl.-Chem. Ralf Stowasser
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]
Search for more papers by this authorDipl.-Chem. Daniel Vitt
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]
Search for more papers by this authorCorresponding Author
Prof. Dr. Gerhard Bringmann
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]
Institut für Organische Chemie der Universität, Am Hubland, D-97074 Würzburg (Germany), Fax: Int. code +(931)888-4755, e-mail: [email protected] e-mail: [email protected]Search for more papers by this authorThis work was supported by the Deutsche Forschungsgemeinschaft (SFB 347 “Selektive Reaktionen Metall-aktivierter Moleküle” and “Normalverfahren”) and by the Fonds der Chemischen Industrie (fellowships for J. H. and R. S. as well as equipment). We thank Dipl.-Chem. Martin Stahl for helpful discussions.
Dedicated to Professor Siegfried Hünig on the occasion of his 75th birthday
Graphical Abstract
An exo:endo ratio of 98:2! Thermochemistry would have predicted the opposite, but in the rearrangements of 5-hexenyl radicals the exo pathway is preferred due to kinetic reaction control. Selected ab initio methods are able to properly describe cyclizations of the 4-penten-1-oxyl radical (see sketch).
References
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- 4 The semiempirical calculations were carried out using unrestricted Hartree–Fock wave functions. The general term UHF in this communication refers to ab initio calculations. The following abbreviations have been used throughout the paper: DFT: density functional theory; DZVP: double-zeta valence split polarization; UBP: unrestricted Becke–Perdew; UQCISD: unrestricted quadric configuration interaction including singles and doubles substitutions.
- 5 UHF/6-31G* geometries: (a) Selected bond angles in 2a: O-C2-C3 = 107.7°, C2-C3-C4 = 104.1°, C3-C4-C5 = 107.9°, C4-C5-C6 = 121.1°. Selected torsion angles in 2a: O-C2-C3-C4 = −48.55°, C2-C3-C4-C5 = 41.06°, C3-C4-C5-C6 = −131.7°. Selected bond angles in 3a: O-C2-C3 = 106.5°, C2-C3-C4 = 101.8°, C3-C4-C5 = 101.4°, C4-C5-C6 = 115.7°. Selected torsion angles in 3a: O-C2-C3-C4 = −24.50°, C2-C3-C4-C5 = 35.61°, C3-C4-C5-C6 = −156.0°, H6syn-C6-C5-H5 = −20.39°, H5-C5-C4-H4syn = −44.61°; (b) Selected bond angles in 2b: C1-C2-C3 = 108.3°, C2-C3-C4 = 107.3°, C3-C4-C5 = 110.6°, C4-C5-C6 = 121.7°. Selected torsion angles in 2b: C1-C2-C3-C4 = −50.00°, C2-C3-C4-C5 = 50.07°, C3-C4-C5-C6 = −142.6°. Selected bond angles in 3b: C1-C2-C3 = 105.8°, C2-C3-C4 = 105.8°, C3-C4-C5 = 104.4°, C4-C5-C6 = 114.9°. Selected torsion angles in 3b: C1-C2-C3-C4 = −0.05°, C2-C3-C4-C5 = 24.83°, C3-C4-C5-C6 = −165.1°.
- 6 UHF/6-31G* geometries: (a) Selected bond angles in 4a: O-C2-C3 = 113.6°, C2-C3-C4 = 111.2°, C3-C4-C5 = 108.4°, C4-C5-C6 = 118.2°. Selected torsion angles in 4a: O-C2-C3-C4 = −61.48°, C2-C3-C4-C5 = 53.28, C3-C4-C5-C6 = −65.55°, H5-C5-C4-H4syn = −32.78°, H6syn-C6-C5-H5 = 15.38°. Selected bond angles in 5a: O-C2-C3 = 111.0°, C2-C3-C4 = 110.2°, C3-C4-C5 = 110.4°, C4-C5-C6 = 117.9°, H5-C5-C4 = 119.4°, C6-C5-H5 = 117.9°. Selected torsion angles in 5a: O-C2-C3-C4 = −58.56°, C2-C3-C4-C5 = 45.43°, C3-C4-C5-C6 = −38.79°; (b) Selected bond angles in 4b: C1-C2-C3 = 114.5°, C2-C3-C4 = 113.5°, C3-C4-C5 = 110.5°, C4-C5-C6 = 119.7°. Selected torsion angles in 4b: C1-C2-C3-C4 = −64.33°, C2-C3-C4-C5 = 59.20, C3-C4-C5-C6 = −70.86°. Selected bond angles in 5b: C1-C2-C3 = 111.3°, C2-C3-C4 = 111.5°, C3-C4-C5 = 111.7°, C4-C5-C6 = 118.5°. Selected torsion angles in 5b: C1-C2-C3-C4 = −57.28°, C2-C3-C4-C5 = 49.59°, C3-C4-C5-C6 = −45.05°.
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– ΔH
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D. Vitt,
G. Bringmann, unpublished results). However, MNDO is generally not as well suited as AM1 [7b] or PM3 [7c] for the description of radical reactions, and this result will not be discussed further;
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