17O-Dynamic NMR and DFT Investigation of Bis(acyloxy)iodoarenes

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.

An oxygen-17 dynamic NMR study of the Pr–DOTA complex

“…6D and Table 4, as well as Fig. S1-c and Table S3, of the work of Mayer et al 7 ); (iii) similarly, for non-metallic diamagnetic compounds such as acetoxysilanes and bis(acetoxy)iodobenzene, which undergo oxygen atom interchange via the [1,3] sigmatropic shift mechanism, 11,12 it is also the carbonylic O 1 signal which is the largest and the linewidths of both the O 1 and O 2 resonances exhibit the same temperature dependence. This strongly suggests that, at low temperature, the linewidth of the O 1 signal of the Pr-DOTA complex is primarily due to quadrupole relaxation while the O 2 signal is significantly affected by additional broadening which, actually, is not due to PRE effects but is a consequence of the TSAP-SAP exchange process.…”

Determining the predominant tautomeric structure of iodine-based group-transfer reagents by 17O NMR spectroscopy

“…While natural abundance 17 O NMR has been employed previously, including the analysis of hypervalent iodine compounds [ 8 – 10 ], this spectroscopic method has not yet found its entry into the organic chemist's standard NMR toolbox. This, in large part, may be attributed to the extremely low natural abundance of the 17 O isotope (<0.04%) [ 11 – 12 ].…”

“…Geometry optimizations of both cyclic and acyclic isomers were followed by calculation of oxygen isotropic shift values (δ iso ) using the GIAO method ( Table 1 ) [ 8 – 10 ]. Furthermore, these computed isotropic shift values (δ iso ) were not referenced, for example to water, since they were directly correlated to experimentally determined 17 O NMR shifts (vide infra).…”

An 17O NMR study of diamagnetic and paramagnetic lanthanide‐tris(oxydiacetate) complexes in aqueous solution

“…[25] Our investigations by 17 O NMR began with a study of the dynamic behaviour of a series of organic diamagnetic molecules characterised by carboxylate shifts, which might be expected to dramatically affect the line width of the 17 O resonance lines. [26][27][28] Diamagnetic and paramagnetic Ln complexes of DTPA and DOTA ligands were investigated next, obtaining dynamic information about some complexes with light elements of the Ln series. [29,30] Then, DOTA-based complexes with all stable lanthanides were studied, and the bound-state signals of the carboxylate groups were observed.…”