Kinetics of the deprotonation of methylnitroacetate by amines: unusually high intrinsic rate constants for a nitroalkane.

A kinetic study of the reversible deprotonation of methylnitroacetate (4H) by primary aliphatic amines, secondary alicyclic amines, hydroxide ion, and water in water at 25 degrees C and in 50% DMSO/50% water (v/v) at 20 degrees C is reported. Intrinsic rate constants, k0, determined by extrapolation or interpolation of Brønsted plots have been determined. In comparison to proton transfers involving other nitroalkanes, the intrinsic rate constants for 4H are exceptionally high; for example, log k0 for the reaction of 4H with secondary alicyclic amines in water (1.22) is 1.81 log units higher than log k0 for nitromethane (-0.59), while in 50% DMSO/50% water, log k0 for 4H (2.44) is 1.71 log units higher than that for nitromethane (0.73). A general discussion of the factors affecting intrinsic rate constants of proton transfer from nitroalkanes is presented; it provides the context for an understanding as to why k0 is so high for the proton transfers from 4H. The correlation between intrinsic rate constants for the addition of nucleophiles to alkenes of the type R'R' 'C=CXY and the intrinsic rate constants of proton transfers from carbon acids of the type H2CXY is also discussed as a general proposition as well as with specific reference to the Ph(SMe)C=C(NO2)CO2Me/H2C(NO2)CO2Me pair.