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

Nico Santschi,René Verel,Cody Ross Pitts,Benson J Jelier

doi:10.3762/bjoc.14.203

Abstract

Cyclic benziodoxole systems have become a premier scaffold for the design of electrophilic transfer reagents. A particularly intriguing aspect is the fundamental II–IIII tautomerism about the hypervalent bond, which has led in certain cases to a surprising re-evaluation of the classic hypervalent structure. Thus, through a combination of 17O NMR spectroscopy at natural abundance with DFT calculations, we establish a convenient method to provide solution-phase structural insights for this class of ubiquitous reagents. In particular, we confirm that Shen’s revised, electrophilic SCF3-transfer reagent also adopts an "acyclic" thioperoxide tautomeric form in solution. After calibration, the approach described herein likely provides a more general and direct method to distinguish between cyclic and acyclic structural features based on a single experimental 17O NMR spectrum and a computationally-derived isotropic shift value. Furthermore, we apply this structural elucidation technique to predict the constitution of an electrophilic iodine-based cyano-transfer reagent as an NC–I–O motif and study the acid-mediated activation of Togni's trifluoromethylation reagent.

Highlights

The remarkable stability and reactivity of Togni's hypervalent iodine-based trifluoromethylation reagents (e.g., 4a) [1] have inspired the development of analogous compounds, including a well-known SCF3-transfer reagent 5 in 2013 by Shen and co-workers [2,3]
The structural reassignment was prompted by a series of remarkable, detailed inspections of 1H NMR spectra of precursors and congeners
Acyclic isomer 5b is predicted to be thermodynamically favored over the cyclic form 5a by more than 10 kcal/mol by DFT calculations [5]

Summary

Introduction

The remarkable stability and reactivity of Togni's hypervalent iodine-based trifluoromethylation reagents (e.g., 4a) [1] have inspired the development of analogous compounds, including a well-known SCF3-transfer reagent 5 in 2013 by Shen and co-workers [2,3]. While at the time the proposed cyclic hypervalent iodine structure 5a appeared reasonable in analogy to other well-established transfer reagents, it was Beilstein J. Establishing a reliable way to differentiate cyclic (a) from acyclic (b) isomers in solution would facilitate future structure determination of similar iodine-based group-transfer reagents and provide greater mechanistic insight into reactivity of these reagents (Figure 1).

Results

Conclusion