Carbanion Stabilization in C,N-Dimethylnitroimidazoles

Article abstract of DOI:10.1021/jo00336a018

Kinetics of base-catalyzed exchange of C-methyl protons have been investigated for the six position isomers of C,N-dimethylnitroimidazoles (at 60 deg C in D2O-CD3OD, 1:1).Rates of exchange were based on the decrease in 1H NMR signals relative to those for the nonexchanging N-methyl groups.Values of k_OD covered a range of 5*10⁴; in 0.01 N NaOD, values of t_1/2 range from 2.8 s to 38 h.The greatest kinetic acidity was found in 1,5-dimethyl-4-nitroimidazole and the least in 1,2-dimethyl-4-nitro- and 1,4-dimethyl-2-nitroimidazole.In the latter two compounds, the methyl and nitro groups have a "meta" relationship, their resistance to exchange indicates weak resonance stabilization of the respective carbanions.The value of k_OD for 1,5-dimethyl-4-nitroimidazole (1485 M^-1 min^-1) is 4.8*10⁶ as great as that for o-nitrotoluene but only 27-fold less than that for nitromethane.The result suggests that there may be significant localization in the 4,5-double bond of N-substituted imidazoles.The order of kinetic acidities for the C-methyl groups is consistent with the order of reactivities of the same compounds in aldol condensations.