The Synthesis and Properties of Dimethyl 1,2-Diazetine-1,2-dicarboxylate, a Potentially Aromatic Molecule

The cyclobutene n-bond of dimethyl 2,3-diazabicyclo[2,2,0]hex-5-ene-cis-2,3-dicarboxylate (15) has been employed as a dienophile in the Diels-Alder reaction with 2,5-dimethyl-3,4-diphenylcyclopenta- 2,4-dienone (17). The reaction occurred with high stereoselectivity and led, almost exclusively, to the formation of the exo-fused adduct (18). A similar cycloaddition was performed between the dienone (17) and the bicyclo[2,2,0]hex-5-ene-cis-2,3-dicarboxylic anhydride (5) to yield a 90 : 10 mixture of adducts (6). These several adducts were used as precursors for the preparation of the related bicyclo[4,2,0]octa-2,4-diene derivatives (7) and (20), themselves used as photosubstrates for the 1,2-photoaromatization reaction. In this way the carbocyclic precursor (7) yielded cyclobut-3- ene-cis-1,2-dicarboxylic anhydride (8), and the hetero analogue (20) yielded the title diazetine (21). Hydrogenation of this heterocycle yielded the related 1,2-diazetidine (22) which was fully characterized. The reactivity of the diazetine, which formally contains six delocalized electrons, is considered especially as it relates to its potential aromatic character. In practice the facile ring- opening of the diazetine to the 1,4-diazabuta-1,3-diene (23) fairly reflects the lack of aromaticity of this ring system. An INDO MO SCF method has been used to evaluate the energetics of the nitrogen inversion process in the unsubstituted 1,2-diazetine ring. The results indicate that the planar form lies at a maximum on the energy surface and that there is little electron delocalization. The planar form is best described as being antiaromatic. PMO arguments are presented which support this conclusion.