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Photochemistry of Crystalline Chlorodiazirines: The Influence of Conformational Disorder and Intermolecular Cl···NN Interactions on the Solid-State Reactivity of Singlet Chlorocarbenes†
journal contribution
posted on 2003-03-27, 00:00 authored by Carlos N. Sanrame, Christopher P. Suhrada, Hung Dang, Miguel A. Garcia-GaribayA photochemical study was carried out with 3-substituted 3-chlorodiazirines with 4-biphenyl- (4a), (4-biphenyl)methyl- (4b), 2-(4-biphenyl)ethyl- (4c), and 1,1-dimethyl-1-(4-biphenyl)methyl (4d) substituents. The
chlorodiazirines were prepared from the corresponding amidines (7) by a procedure involving oxidation with
tert-butyl hypochlorite under phase-transfer catalysis. The crystalline nature of 4a−d was established by
differential scanning calorimetric analysis, which revealed melting endotherms prior to thermal decomposition.
Photochemical results in crystalline solids were analogous to those observed in solution, and the products
were analyzed in terms of the corresponding singlet-state chlorocarbene intermediates (5a−d). Irradiation of
4a in solution and in crystals resulted in formation of azine 9a by reaction of carbene 5a with its precursor.
Equally selective, diazirine 4d gave alkene 6d as the only product by a 1,2-Ph migration from carbene 5d.
In contrast, irradiation of compounds 4b and 4c resulted in formation of two alkenes by 1,2-H shifts and
formation of azines by reactions of the carbenes with their precursors. The low selectivity of 4b was rationalized
in terms of structural data from single-crystal X-ray diffraction analysis, which revealed two disordered diazirine
conformers and close Cl···N contacts between adjacent molecules. Rapid conformational equilibration in the
solid state was also suggested by solid-state 13C CPMAS NMR. Similar structural effects are also postulated
to account for the solid-state reactivity of 4c.