17O-dynamic NMR and DFT investigation of bis(acyloxy)iodoarenes.

Bis(acetoxy)iodobenzene and related acyloxy derivs. of hypervalent I(III) were studied by variable temp. soln.-state 17O-NMR and DFT calcns. The 17O-NMR spectra reveal a dynamic process that interchanges the oxygen atoms of the acyloxy groups. For the first time, coalescence events could be detected for such compds., allowing the detn. of activation free energy data which range between 44 and 47 kJ/mol. The anal. of the 17O linewidth measured for bis(acetoxy)iodobenzene indicates that the activation entropy is negligible. DFT calcns. show that the oxygen atom exchange arises as a consequence of the [1,3]-sigmatropic shift of iodine. The calcd. activation barriers are in excellent agreement with the exptl. results. Both the 17O-NMR and DFT studies show that the solvent and chem. alterations, such as modification of the acyl groups or para-substitution of the benzene ring, hardly affect the energetics of the dynamic process. The low I-O Wiberg bond index (0.41-0.42) indicates a possible explanation of the invariance of both the energy barrier and the 17O chem. shift with para-substitution. [on SciFinder(R)]