Mecanismos da oxidação de álcoois por N-haloreagentes e metodologia de ciclização oxidativa de dióis à lactonas por ácido tricloroisocianúrico

Abstract

The DFT investigation of alcohol oxidation by NBS in aqueous acetic acid (30-70%) found mechanisms that differed only in the amount of explicit solvent. The alcohol was oxidized via hydride abstraction by NBS with basic acetate catalysis; acid catalysis was shown to be possible but was outside the reaction scope. The kcalc and ΔG≠ calc values in the 20 cases studied agreed fully with available kinetic data. The theoretical study of the mechanism of oxidation of alcohols by TCCA (based on a previous kinetic study) found a mechanism in which autocatalytic Cl2 abstracted hydride in an acyclic transition state in the induction zone. Alcohols with strongly electron-withdring groups, which were outside the linear free energy relationship, underwent oxidation through hydrogen abstraction by chloride from the respective benzyl hypochlorite. Furthermore, it was possible to suggest the continuity of the mechanism between aromatic and secondary alcohols at the M06-2x/6-311G(d,p) level. The ipso substitution study in the reaction with TCCA (11 cases) identified the position and leaving group as reaction limiters. The DFT study demonstrated that the higher incidence of ipso substitution in 4- methoxybenzyl alcohol at 0 °C as opposed to 25 °C occurred due to the decrease in ΔG≠ calc for ipso substitution and the increase in ΔG≠ calc for concurrent reactions. A cost-effective, safe, and convenient methodology for oxidative cyclization of diols to lactones using TCCA, H2O/Py in a 30% MeCN/CH2Cl2 solution at room temperature was proposed. Yields of 50-98% were obtained for six examples of small ring lactones and one example of a medium-sized ring lactone (decanolactone). The oxidation of 1,4-pentanediol by TCCA led to the ketone, while oxidation by TBCA/Py in acetonitrile led to γ-butyrolactone without the formation of a ketone. The mechanism study by DFT identified two concomitant mechanisms through the cyclization of acyl chloride or the oxidation of lactol with acidic medium.