A strained aminoazoniatricyclene species: Result of an intramolecular stabilization of a cyclopropaniminium ion by an amino moiety

Ab initio calculations of the cationic intermediates of a cyclopropane-forming process of 1,4-diamino-6-chlorocyclohexene precursors predict the 3 alpha-aminobicyclohexane-6-iminium ion 6c to be more stable than its 3 beta-isomer 7c. The latter, however, stabilizes itself by intramolecular cyclization leading to aminoazoniatricyclene species 9c. The aminocyclohexenyl cation 8c proved to have no minimum on both the MP2 and the MP4(SDTQ) potential energy surfaces. Optimization of species 8c causes a ring closure reaction to give iminium analogues 6c or 7c. The potential energy curve of this transformation 6c --> 8c --> 7c was calculated. Experimentally, solvolysis of chloro enamine isomers 4a and 5a was studied as a simple access to these cationic intermediates: 3 alpha-amino 6-iminium ion 6a leading to 3 alpha-amino 6-semiaminal 12 is generated exclusively from trans chloro enamine 4a and as main intermediate (67%) from cis chloro enamine 5a. In the latter case, aminoazonia compound 9a is formed as a byproduct (33%) which decomposes in water to give 3 beta-amino 6-semiaminal 13 via 3 beta-amino 6-iminium ion 7a. Aminoazoniatricyclene species 9a is obtained as stable tetrafluoroborate in acetonitrile or dichloromethane by reaction of chloro enamine isomers 4a or 5a with AgBF4-. H-1 and C-13 NMR spectral and X-ray structural data are given for this strained tricyclic system 9a.