The first kinetically stabilized azete, the title compound 1, is sensitive toward oxidation (→ formation of the dioxetane 2) and undergoes hydrolysis to the β‐imino ketone 3. Sterically nondemanding cycloaddition partners such as cyclopentadiene or diazomethane (CH2N2) (→ formation of the adduct 4) attack at C‐2/C‐3, whereas those with bulky groups such as acetylenedicarboxylate esters or tert‐butylphosphaacetylene (PCtBu) (→ formation of the bicyclic compound 5) react at N‐1/C‐4.
Fig. 1). For vinylcyclopropanes, 1 has an energetically high-lying "Walsh orbital" in the three-membered ring, which, together with the 71-system of the double bond has suitable symmetry for interaction with the LUMO of Z.1141 The endo-addition is preferred over the exo-addition, presumably on steric grounds (hindrance by the methoxycar-bony1 group in 1).
Im Kristall liegt das antiaromatische 2,3‐Di‐tert‐butyl‐4‐mesityl‐azet 1 nur als Valenzisomer 1A vor. Der Vierring hat verzerrte Rechteckstruktur, und aromatischer und antiaromatischer Ring weisen einen Interplanarwinkel von 70° auf. In Lösung beteiligt sich das Valenzisomer 1B bestenfalls mit 5% an einem Gleichgewicht (13C‐NMR). Im Azet‐Cobaltkomplex 2 sind dagegen aufgrund der Elektronendelokalisierung die Bindungslängen im Vierring fast gleich. magnified image
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