Role of hydrogen bonding in the oxidation of thianthrene 5-oxide with peroxy acids

The oxidation of thianthrene 5-oxide, i.e., a mechanistic probe for the ass essment of the electronic character of various oxidants, with peroxybenzoic acids in various oxygen bases as solvents was investigated. The nucleophil icity (X-SO) of peroxy acids was increasing with increasing basicity of the oxygen base. A good linear correlation was observed by plotting X-SO value s vs either the Kamlet-Taft beta values or the OOH H-1 NMR chemical shifts of m-chloroperoxybenzoic acid (m-CPBA) in solvents of various basicity. The se observations, together with the results of IR and 1H NMR spectroscopic s tudies of peroxybenzoic acids, and DFT (B3LYP/6-311++G**) studies of the in tramolecular hydrogen bonding in peroxyformic, peroxyacetic, and m-CPBA, as well as the intermolecular hydrogen bonding in the complexes of the these peroxy acids with dimethyl ether as a model oxygen base, support the involv ement of the peroxy acid-oxygen base complexes in the transition states of these reactions. The increased nucleophilicity (X-SO) of peroxy acids in ba sic solvents is most likely due to the increased negative charge on the ter minal "electrophilic" peroxycarboxylic oxygen atom (OH), and/or the increas ed LUMO and HOMO energies of the peroxy acid in the complexes, as compared to those parameters in the intramolecularly hydrogen-bonded form of peroxy acids, believed to be operative in inert solvents.