Generation of 4,5-Diazacyclopentane-1,3-diyl Radical Cations by Chemical Electron Transfer (CET) Oxidation of Urazole-Bridged Bicyclic Housanes (Bicyclo[2.1.0]pentanes) and Their Chemical Transformations.
4,5-Diazacyclopentane-1,3-diyl radical cations 3(*)(+)() were generated from urazole-bridged bicyclic housanes 3 through chemical oxidation by using tris(4-bromophenyl)aminium hexachloroantimonate as oxidant to afford the two olefinic products 4 and 5. Product studies establish that the bisolefins 5 are the result of double oxidation of the housanes 3, whereas the monoolefins 4 are formed by acid-catalyzed rearrangement, which can be suppressed by excess of base (2,6-di-tert-butylpyridine). In the case of dibenzyl substitution (3c), disproportionation of two monoradical species 5(H)c(*) serves as an alternative pathway to the corresponding olefins 4 and 5 because higher amounts of double oxidation product were isolated in the absence of base than expected if only a stoichiometric reaction were operating. Semiempirical MO calculations suggest that ionization takes place from one of the nitrogen lone pairs rather than from the strained central C-C bond as implied by the significantly lower (by ca. 0.5 eV) ionization potential. Furthermore, in the initially puckered radical cation, the positive charge is mainly located at the two nitrogen atoms, while after relaxation to the planar geometry, the charge shifts essentially entirely to the radical cation carbon atoms. The trapping reaction with methanol leads to the hemiaminal-type products 6 and 7, which establish the involvement of the cationic intermediates 3(H)(+)() and 5(H)(+)(). In addition, (13)C NMR spectroscopy confirmed these cationic intermediates [3(H)(+)() and 5(H)(+)()] by detection of the characteristic signals below delta 250 for carbenium ions. Unquestionably, the urazole ring significantly influences the radical cation reactivity of the housanes 3. Thus, in contrast to the corresponding homocyclic tricyclooctane derivatives, stoichiometric instead of catalytic amounts of CET oxidant are needed, the two nitrogen atoms of the hydrazino bridge stabilize the radical cation 3(*)(+)() by conjugation, and the carbonyl groups of the urazole moiety assist the deprotonation to the exocyclic double bonds to prevent 1,2 alkyl migration.
Adam
Waldemar
W
Institute of Organic Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany.
Kammel
Thomas
T