4,6-DINITRO-7-METHYLBENZOFUROXAN - A STRONG CARBON ACID OF VERY LOW INTRINSIC REACTIVITY

Rates of ionization of 4,6-dinitro-7-methylbenzofuroxan (3) by water, hydroxide ion, and various general bases have been measured in aqueous solution at t = 25-degrees-C. The solvent pathway is the major process contributing to the formation of the resulting carbanion (C-3) in solutions of pH less-than-or-equal-to 8. The very high thermodynamic stability of C-3 (pK(a)CH = 2.50), the exocyclic carbon of which is essentially olefinic in nature, is shown to derive for the most part from an extensive delocalization of the negative charge through the two nitro groups and the annelated furoxan ring. In agreement with the idea that considerable molecular, electronic, and solvational reorganization is required to produce a highly delocalized carbanion, the deprotonation of 3 is associated with one of the highest intrinsic energy barriers for proton transfer so far measured in aqueous solution (DELTA-G0(double dagger) = 85.5 kJ mol-1). The strong acidity of 3 also emphasizes the exceptional activating effect of a 4,6-dinitrobenzofuroxan moiety.