Some substitution reactions of aryl sulfides and aryl ethers with aliphatic amines in DMSO : the mechanism of base catalysis
The reactions of several nitro-aromatic substrates with the aliphatic amines, n- butylamine, pyrrolidine and piperidine in DMSO have been studied. Reaction of the amines with trinitroaromatic compounds, such as ethyl thiopicrate, phenyl thiopicrate and phenyl 2,4,6-trinitrophenyl ether, was shown to occur in two well separated processes. A rapid reaction, occuring at an unsubstituted ring position, resulted in the formation of a a-adduct; this was followed by a slower substitution reaction, resulting in the displacement of the 1-substituent to give the amino-substituted compound. Kinetic results show that in the formation of the σ-adduct, the rate determining step changes from nucleophilic attack by amine with n-butylamine to proton transfer from the zwitterionic intermediate with piperidine; with pyrrolidine the proton transfer step is partially rate limiting. The substitution reaction involving n-butylamine is not base catalysed indicating nucleophilic attack, the k(_1) step, is rate determining. However, the reactions with pyrrolidine and piperidine are subject to general base catalysis showing proton transfer is involved in the slow step. The rate limiting step is thought to be the proton transfer from the zwitterionic intermediate to base. A single substitution reaction was seen for the reaction of the less activated 1-substituted 2,4-dinitrophenyl and 1-substituted 2,4-dinitronaphthyl compounds. The rate limiting step again changes from nucleophilic attack with n-butylamine to proton transfer with pyrrolidine and piperidine. The uncatalysed, k(_2), pathway was detected during the reaction of the 1-substituted 2,4-dinitrophenyl compounds. Those were the least activated substrates studied and as the effectiveness of the base catalysed step relative to the uncatalysed decomposition of the intermediate decreases with decreasing activation of the substrate, detection of the uncatalysed step became possible. Kinetic and equilibrium studies were also made on the reaction of morpholine with several phenyl ethers. Results were compared with those for piperidine with the same substrates, the values of the kinetic and equilibrium constants were as expected considering morpholine has the same steric requirements as piperidine but is considerably less basic.