A chlorine-free microwave-assisted, ionic liquid-catalyzed esterification of arylsulfonic acids with alcohols: an experimental and theoretical study.

In the area of esterification of heteroatomic acids, after the microwave-assisted ionic liquid-catalyzed esterification of phosphinic acids, the esterification of arylsulfonic acids was also developed applying a 14-fold excess of alcohols at 200 °C in the presence of 10% butyl-methylimidazolium hexafluorophosphate as an additive. The esterifications were optimized, and the effect of the substituents in the aromatic ring was evaluated. At the same time, a similar procedure described by Mandal et al. using only one equivalent of alcohol at 120 °C for 5 min in toluene was refuted. The mechanism and energetics of the reaction of benzenesulfonic acid and butyl alcohol were determined at the B3LYPD3/def2TZVP[PCM(BuOH)] level of theory using the explicit-implicit solvent model, and, as a comparison, the implicit solvent model. Three possible reaction pathways were explored: the direct esterification of benzenesulfonic acid through an S_N2 protocol including the nucleophilic addition of butyl alcohol to the SO function of the sulfonic acid via an intermediate with a hexavalent-pentacoordinated S atom (Route I), via protonation of the alcohol by the arenesulfonic acid followed by the recombination of the sulfonate anion and the alkyl cation formed by dehydration (Route II), and an S_N1 route involving the initial formation of a sulfonium cation by dehydration of the protonated sulfonic acid followed by the nucleophilic attack of the alcohol (Route III). Judging from the energetics of the three potential pathways, the alkylating esterification (Route II) seems to be the predominant route. Microwave irradiation may overcome the enthalpy of activation of 132 kJ mol^-1 required for this protocol. The addition-elimination (S_N2) sequence (Route I) may also be operative as a minor reaction component.