Alain-Pierre Chatrousse

Transition state imbalance in the ionization of nitroaromatic hydrocarbons: 2,2′,4,4′-tetranitrodiphenylmethane and 2,4,4′-trinitrodiphenylmethane in aqueous dimethyl sulphoxide solutions

Rates of reversible proton abstraction from 2,2′,4,4′-tetranitrodiphenylmethane and 2,4,4′-trinitrodiphenylmethane by a variety of bases in 50% water–50% dimethyl sulphoxide have been measured at 25 °C. Bronsted αCH and βB values have been determined and show a charge imbalance in the transition states for these reactions, in accord with findings for similar carbon acids involving extensive charge delocalization on ionization. Intrinsic rate constants (in the Marcus sense), ko′ have been obtained for these systems, and are shown to vary considerably with choice of catalyst type. As expected, high intrinsic energy barriers for these ionizations are predicted, arising from the extensive molecular and solvent reorganization required to produce planar, delocalized carbanions, Data for the former compound have also been obtained in water, which allow a direct estimation of its pKaH2O value to be made; pKaH2O= 13.16.

ELECTROPHILIC HETEROAROMATIC SUBSTITUTIONS : THE COUPLING OF 4,6-DINITROBENZOFUROXAN WITH 3-METHOXYTHIOPHENE IN H2O-ME2SO MIXTURES

Rate constants have been determined in three aqueous Me2SO mixtures for the SEAr substitution of 3-methoxythiophene (5) by super-electrophilic 4,6-dinitrobenzofuroxan (DNBF). The absence of a significant dependence of the rates of reactions on the hydrogen or deuterium labelling at Cα of the thiophene ring indicates that electrophilic addition (k1DNBF) of DNBF at this position is the rate-limiting step of the substitution process. A notable increase in the k1DNBF rate constant with increasing water content of the solvent mixtures is observed. This trend is consistent with the idea that the addition step proceeds through a highly dipolar transition state where the development of negative charge in the DNBF moiety is concomitant with that of a partial positive charge in the thiophene ring. Assuming that the behaviour of 5 fits the rate-equilibrium, Bronsted-type relationship previously established for the coupling of DNBF with nitrogen heterocycles, a pKa H2O value of –6.5 may be estimated for the protonation of 5 in aqueous solution. Interestingly, a similar estimate is obtained when the reactivity of 5 is compared with that of an homogeneous series of enol methyl ethers. Despite a low carbon basicity, comparable to that of a rather weakly basic enol ether, 5 appears to be one of the most reactive thiophene compounds toward electrophilic reagents.

Proton abstraction from bis-(2,4-dinitrophenyl)methane by methoxide ion

Rate constfnts for hydrogen or deuterium abstraction from bis-(2,4-dinitrophenyl)methane by methoxide ion have been determined in methanol, methanol–dimethyl sulphoxide, and deuteriated methanol solutions, together with the temperature variation of proton abstraction and carbanion protonation for the hydrogen-containing substrate in methanol. Isotope and solvent effects confirm the rate-limiting proton transfer.