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

Bis(acetoxy)iodobenzene and related acyloxy derivatives of hypervalent I(III) were studied by variable temperature solution-state ¹⁷O-NMR and DFT calculations. 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 compounds, allowing the determination of activation free energy data which are found to range between 44 and 47 kJ/mol. The analysis of the ¹⁷O linewidth measured for bis(acetoxy)iodobenzene indicates that the activation entropy is negligible. DFT calculations show that the oxygen atom exchange arises as a consequence of the [1,3]-sigmatropic shift of iodine. The calculated activation barriers are in excellent agreement with the experimental results. Both the ¹⁷O-NMR and DFT studies show that the solvent and chemical 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 chemical shift with para-substitution.