Some kinetic and equilibrium studies of σ-adduct formation and proton transfer in the reactions of aromatic nitro-compounds with bases
Comparison of kinetic and equilibrium data for the cyclisation of 1-(2,2-dimethyl-3-hydroxypropoxy)-2,4-dinitro- naphthalene in alkaline media with those for the cyclisation of 1-(3-hydroxypropoxy)-2,4-dinitronaphthalene indicates the absence of a marked gem-dimethyl effect. (^1)H n.m.r. and visible spectral measurements show that alkoxide addition to 2,2’4,4’,6,6’-hexanitrobibenzyl (HNBB) and 2,2’,4,4’,6,6’-hexanitrostilbene (HNS) gives σ-adducts. Formation of the 3-(3’-) adduct is kinetically preferred but the 1-(1’-) adduct is thermodynamically more stable. In media of high basicity the 1:2 adduct with alkoxide addition at the 1- and 1’- positions is observed. For HNS a third interaction occurs which may be alkoxide addition at the olefinic bond. Kinetic and equilibrium data are reported for the reactions with methoxide ions in methanol and ethoxide ions in ethanol and compared with data for related compounds. The interactions of aliphatic amines with 2,4,6-trinitro- benzyl chloride (TNBCl), HNBB, HNS and 2,4,6-trinitrophenetole (TNP) in dimethyl sulphoxide have been investigated using visible and (^1)H n.m.r. spectroscopic methods. Kinetic and equilibrium data are reported for the various processes observed. The reversible reactions of TNBCl with primary amines are found to be: rapid formation of the 3-adduct, followed by isomerisation to the thermodynamically more stable 1-adduct, followed by equilibration with the conjugate base formed by transfer of a side-chain proton. With the secondary amines piperidine and pyrrolidine, σ-adduct formation at the 1-position is not observed because the presence of two bulky groups at the 1-position is sterically unfavourable. The σ-adduct forming reactions occur via zwitterionic intermediates and it is shown that proton transfers between these species and amines may be kinetically significant. The reactions of HNBB and HNS with amine also involve the initial formation of 3-adducts and 1-adducts. At high amine concentrations di-adducts may be formed by reaction of the 1- and 1’- or 3- and 3’- positions. A slow reaction of HNBB with amines gives a blue species which is shown to be a dianion formed by loss of two methylene protons. The slow step-in this reaction is shown to be rate limiting proton transfer from the substrate or from l:lσ-adducts. TNP reacts with primary and secondary amines to give isomeric σ-adducts at the 3-position and 1-position. Nucleophilic substitution involves general acid catalysed expulsion of the ethoxy group and yields N-substituted picramides. Data are also reported for the reactions of TNBCl with hydroxide ions in water and 30:70 (v/v) DMSO-water, of TNBCl with hydroxide ions in mixed (methanol-water-tetrahydrofuran) solvents, of HNBB with sulphite ions and of 1,3,5-trinitrobenzene with thioglycollic acid in water.