Abstract
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.
Highlights
Hypervalent iodine chemistry is a field of great interest as shown by the large body of literature dealing with its various aspects [1,2,3,4,5,6]
In contrast, distinctive 17O-NMR signals were observed at 25 °C for the O1 and O2 atoms of the carboxylic group not involved in the iodoxolone ring
Our previous studies showed that the I-O bond of either I(III) or I(V) acyloxy derivatives is fluxional as a consequence of a [1,3]-sigmatropic shift mechanism
Summary
Hypervalent iodine chemistry is a field of great interest as shown by the large body of literature dealing with its various aspects [1,2,3,4,5,6]. Using acetoxysilanes as model compounds, we showed that currently available NMR instrumentations and pulse sequences allow for quantitative 17O-DNMR studies to be carried out without enrichment being needed [10] This opened the perspective to precisely determine ΔG# values for the [1,3]-sigmatropic shift of iodine in I(III) acyloxy derivatives and, to determine the activation enthalpy (ΔH#) and the activation entropy (ΔS#) of this dynamic process. The present work reports on several bis(acyloxy)iodoarenes: bis(acetoxy)iodobenzene (1), bis(trifluoroacetoxy)iodobenzene (2), para-methoxy-bis(acetoxy)iodobenzene (3) and para-nitrobis(acetoxy)iodobenzene (4) (Figure 3) These λ3 iodanes were studied by 17O-DNMR with the primary goal of investigating the effect of electron-withdrawing and electron-donating groups on the energetics of the [1,3]-sigmatropic shift of iodine. The experimental data are compared to the results of DFT calculations
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