Ring opening of pyridines: the pseudo-cis and pseudo-trans isomers of tetra-n-butylammonium 4-nitro-5-oxo-2-pentenenitrilate.
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, C(16)H(36)N(+).C(5)H(3)N(2)O(3)(-), 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 S(N)(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.
Zeller
Matthias
M
Department of Chemistry, Youngstown State University, One University Plaza, Youngstown, OH 44555-3663, USA.
Pett
Virginia B
VB
Haynes
LeRoy W
LW