Some studies of the reactions of aromatic nitro compounds with nitrogen and carbon nucleophiles

Abstract

Kinetic and equilibrium studies have been made of the reactions of a series of ring-substituted anilines with l,3,5-trinitrobenzene (TNB) and with 4- nitrobenzofuroxan (4-NBF) in dimethyl sulfoxide (DMSO). There is evidence from (^1)H NMR spectroscopy that in the presence of Dabco these reactions yield anionic σ-adducts. Kinetic studies are compatible with a two-step process involving initial nucleophilic attack by amine to give a zwitterionic intermediate which can transfer an acidic proton to Dabco. In the case of TNB the proton transfer step is rate-limiting due to the high rate of reversion of the zwitterion to reactants. Measurement of substituent effects shows that values of the equilibrium constant K(_1) for the initial zwitterions formation decrease by a factor of ca 1000 as R, the ring substituent in the anilines, is changed from 4-OMe to 3-CN (p=-3.82), while values of K(_Dabco) for the proton transfer equilibrium increase by a similar factor (p=+3.62). Hence overall values of K(_1)Doabco show only a small variation with the nature of R. Related studies with 4-NBF indicate that there is a balance between nucleophilic attack and proton transfer as the rate limiting step. This is due to the lower value for the rate of reversion of the zwitterion to reactants in the case of 4- NBF, relative to TNB. Values of K(_Doabco) are found to be larger for the 4-NBF reaction than for the TNB reaction, indicating greater electron withdrawing ability for the 4- NBF system than for TNB .(_pKa) values in DMSO for the nine aniline derivatives used were measured using the proton transfer equilibrium with 2,4-dimtrophenol. (^1)H NMR studies in d(_6)-DMSO show that the carbanions of nitromethane, nitroethane and 2-nitropropane add to 4-nitrobenzofiirazan derivatives to give carbon- bonded σ-adducts. With time there is evidence for the base-catalysed elimination of nitrous acid from the adducts to yield anionic alkene derivatives. Kinetic and equilibrium results for the corresponding reactions in methanol have been measured. Three ring-substituted benzyl triflones were prepared and reactions of the carbanions formed from them with 4,6-dinitrobenzofuroxan, 1,3,5-trinitrobenzene and benzofurazan derivatives were examined. There is evidence for the initial formation of σ-adducts followed by rapid elimination of HSO2CF3. Rate constants for these reactions in methanol are reported.