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The reaction of 2-chloro-5-nitro­pyridine with two equivalents of base produces the title carbanion as an inter­mediate in a ring-opening/ring-closing reaction. The crystal structures of the tetra-n-butyl­ammonium salts of the inter­mediates, C16H36N+·C5H3N2O3-, revealed that pseudo-cis and pseudo-trans isomers are possible. One crystal structure displayed a mixture of the two isomers with approximately 90% pseudo-cis geometry and confirms the structure predicted by the SN(ANRORC) mechanism. The pseudo-cis inter­mediate undergoes a slow isomerization over a period of months to the pseudo-trans isomer, which does not have the appropriate geometry for the subsequent ring-closing reaction. The structure of the pure pseudo-trans isomer is also reported. In both isomers, the negative charge is highly delocalized, but relatively small differences in C-C bond distances indicate a system of conjugated double bonds with the nitro group bearing the negative charge. The packing of the two unit cells is very similar and largely determined by the inter­actions between the planar carbanion and the bulky tetra­hedral cation.
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