Desaminierungsreaktionen, 38. Zur Bildung von 2-Norbornylkationen durch Ringerweiterung

2-Norbornylkationen wurden durch Ringerweiterung aus Bicyclo[2.1.1]hex-5-ylmethandiazonium-Ionen (29, 37) erzeugt. Das exo-Isomere 37 lieferte ausschlieslich exo-2-Norbornanol (30), wahrend das endo-Isomere 29 ein exo/endo-Verhaltnis (30/31) von 6 ergab. In beiden Fallen entstand 30 aus einem symmetrischen (verbruckten oder rasch aquilibrierenden) 2-Norbornylkation, wie durch die Verteilung einer Deuterium-Markierung gezeigt wurde. Die Ringerweiterungsreaktionen der 1-(Bicyclo[2.1.1]hex-5-yl)ethandiazonium-Ionen (50, 58) verliefen stereoselektiv; 50 fuhrte zu exo-3-Methyl-2-norbornylkationen (52, 54) 58 zu endo-3-Methyl-2-norbornylkationen (60). Dies wird durch die Vorzugskonformationen der Bicyclo[2.1.1]hexanderivate erklart. Die endo-Vorstufe 50 ergab einen hohen Anteil an exo-3-Methyl-endo-2-norbornanol, wahrend die Ringerweiterung von 58 mit 3,2-H-Verschiebung in ungewohnlichem Ausmas verbunden war. 1-(Bicyclo[2.1.1]hex-5-yl)-1-methylethylkationen (68, 72) zeigten keine Umlagerung zu 3,3-Dimethyl-2-norbornylkationen (69). Die ungewohnlichen Reaktionen der 2-Norbornylkationen fuhren wir auf verdrillte Konformationen zuruck, welche die Uberlappung zwischen dem leeren p-Orbital und sonst ungunstig angeordneten σ-Bindungen verbessern. Die verdrillten 2-Norbornylkationen werden im Zuge der Ringerweiterungsreaktionen durchlaufen, doch durften ihnen keine Potentialminima zuzuordnen sein. Deamination Reactions, 38. The Ring Expansion Route to 2-Norbornyl Cations 2-Norbornyl cations have been generated by ring expansion reactions of bicyclo[2.1.1]hex-5-ylmethanediazonium ions (29, 37). The exo isomer 37 yielded exclusively exo-2-norbornanol (30) while the endo isomer 29 produced an exo: endo (30:31) ratio of 6. In both cases 30 was derived from a symmetrical (bridged or rapidly equilibrating) 2-norbornyl cation, as shown by scrambling of a deuterium label. The ring expansion reactions of 1-(bicyclo[2.1.1]hex-5-yl)ethanediazonium ions (50, 58) proceeded stereoselectively, 50 giving rise to exo-3-methyl-2-norbornyl cations (52, 54), and 58 to endo-3-methyl-2-norbornyl cations (60). These observations are explained in terms of conformational preferences of the bicyclo[2.1.1]hexane derivatives. The endo precursor 50 afforded a large fraction of exo-3-methyl-endo-2-norbornanol while the ring expansion of 58 was associated with an exceptional extent of 3,2-hydride shift. 1-(Bicyclo[2.1.1]hex-5-yl)-1-methylethyl cations (68, 72) did not rearrange with formation of 3,3-dimethyl-2-norbornyl cations (69). The unconventional reactions of the 2-norbornyl cations are attributed to distorted conformations which improve overlap of the vacant p orbital with otherwise unfavorably oriented σ bonds. The distorted 2-norbornyl cations intervene in the course of ring expansion but are not thought to represent potential minima.

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