Structure Elucidation of In Situ Generated Chiral Hypervalent Iodine Complexes via Vibrational Circular Dichroism (VCD)

Abstract

The structure of in situ generated chiral arylλ 3 -iodanes obtained under oxidative reaction conditions was not yet observable with experimental techniques and their proposed structures are purely based on DFT calculations. Herein, we establish vibrational circular dichroism (VCD) spectroscopy as an experimental technique to verify DFT-calculated chiral iodane structures. Based on a chiral triazole-substituted iodoarene catalyst, we were able to elucidate a yet undescribed cationic chiral iodane as the most populated intermediate under oxidative conditions with a significant intramolecular NÀ I-interaction and no significant interactions with tosylate or m-chlorobenzoic acid as potential anionic ligands. Instead, aggregation of these substrates was found, which resulted in the formation of a noncoordinating anionic hydrogen bonded complex. The importance of VCD as a crucial experimental observable is further highlighted by the fact that our initial structural proposal, that was purely based on DFT calculations, could be falsified.

Modelling solute–solvent interactions in VCD spectra analysis with the micro-solvation approach

“…Likewise, any intermolecular interactions that alter conformational preferences or that affect the VCD signature of single conformer must also be considered explicitly in the spectra calculations. Hence, although we extensively utilized this sensitivity, for instance, to study substrate-catalyst interactions in asymmetric catalysis 8–12 and other stereochemical aspects, 13–17 it can quickly turn into a major obstacle in routine AC determinations due to strong interactions solvent molecules.…”

Matrix effects in MI-VCD spectra of two chiral oxiranes and their potential microscopic origin

“…Hence, besides being an established tool for the determination of absolute configurations (AC), 5–7 VCD spectroscopy has revealed a unique sensitivity to conformational changes and intermolecular interactions. While in particular solvent effects were found to influence VCD spectra, 8–13 intermolecular interactions in catalysis, 14–17 supramolecular chemistry, 18–26 and biomolecules 27–29 have also been investigated.…”

VCD spectroscopy distinguishes the enamine and iminium ion of a 1,1′-binaphthyl azepine

“…Our proof-of-principle VCD studies on asymmetric catalysts have taken advantage of this conformational sensitivity and provided new insights into a variety of catalyst systems. 4 Besides examples from asymmetric counterion directed catalysis (ACDC), 9 thiourea-based hydrogen bonding organocatalysis 10 and iodane catalysis, 11 we have particularly focussed on representative examples from enamine and iminium ion catalysis: 12 In a case study on Jørgensen-Hayashi-type prolinol ether organocatalysts, 12b,13 we demonstrated that VCD spectroscopy can be used to characterize the conformational preferences of the corresponding in situ generated enamines by direct comparison of experimental and computed spectra. Likewise, for MacMillan's imidazolidinone catalyst, 14 the major conformer of the corresponding iminium ion 15 could be determined.…”