Isolation and X-ray Crystal Structure of an Electrogenerated TEMPO–N3 Charge-Transfer Complex

Abstract

Recent advances in radical-based catalytic reactions have created an increasing demand for the understanding of their mechanistic underpinnings. Structural elucidation of transient reactive intermediates via diffraction techniques, though rarely possible, is one of the most decisive ways to support such mechanistic hypotheses. Here we present the isolation, structural elucidation, and theoretical analysis of an electrochemically generated and catalytically relevant charge-transfer species formed between the azidyl radical and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). The unusual bent N–N–N angle and the pancake bonding between these two fragments highlight the weak bonding interactions present in this complex. This X-ray structure validates computational predictions as well as mechanistic proposals of TEMPO-mediated radical azidation reactions.

Eight-Fold Intensification of Electrochemical Azidooxygenation with a Flow-Through Electrode

“…Both azidyl and TEMPO radicals were then successively added onto the alkene to form the final product (Figure S3 in the Supporting Information). 37,55 For an electro-organic reaction, the applied potential should be high enough to ensure a high rate of charge transfer while being low enough to avoid unwanted reactions. To determine the ideal potential window, we first performed azidooxygenation with cell potentials between 2.2 V and 3.4 V, and with two different flow rates (0.5 and 2.0 mL/min).…”

New Redox Strategies in Organic Synthesis by Means of Electrochemistry and Photochemistry

“… Using cyclic voltammetry, Lin observed a Nernstian dependence of the peak potential of TEMPO oxidation on azide concentration, which enabled the stoichiometry of the CTC formed between TEMPO and azide to be determined. Recently, the structure of this CTC has also been unambiguously elucidated by X-ray crystallography, which shows an unusual pancake bonding between N 3 and the N-O motif of TEMPO that resembles a [3 + 2] cycloaddition transition state . Insights into the azidooxygenation mechanism and discovery of the CTC informed the discovery of a metal-free aminoxyl radical-catalyzed alkene diazidation reaction (Figure D) …”

“…Recently, the structure of this CTC has also been unambiguously elucidated by X-ray crystallography, which shows an unusual pancake bonding between N 3 and the N-O motif of TEMPO that resembles a [3 + 2] cycloaddition transition state. 53 Insights into the azidooxygenation mechanism and discovery of the CTC informed the discovery of a metal-free aminoxyl radical-catalyzed alkene diazidation reaction ( Figure 4 D). 54 …”

Electron Diffraction of 3D Molecular Crystals

“…18 Whenever possible, XRD remains the technique of choice for small-molecule structure determination. 240 Nevertheless, if X-ray-scale crystals prove impossible to obtain despite rigorous effort, ED is a powerful alternative which can match or surpass the resolution achieved by XRD. As the technique continues to mature, the development of specialized hardware engineered exclusively for ED will undoubtedly alleviate many current issues with data quality.…”