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. File list (3) download file view on ChemRxiv 02_SI_updated.pdf (4.34 MiB) download file view on ChemRxiv 01_MS.pdf (1.81 MiB)

Chemical versatility of azide radical: journey from a transient species to synthetic accessibility in organic transformations

“…in ACN solution at room temperature. 73 Apart from the nucleophilic sources, the successful and rapid use of electrophilic azide-containing hypervalent iodine(III) reagents are also familiar as the precursors of electrophilic azides as well as azido radicals. Acyclic azidoiodinanes, PhI(N 3 )X (X = -N 3 , -OAc, -OCOCF 3 , -OTMS, etc.)…”

“…11). 73 In the case of a redox-active low-valent metal ion (M n+ ), the in situ formed N 3 can bind directly to the metal-center, or initially, the metal ion can coordinate with N 3 À through the ligand exchange event, followed by oxidation to form the [M n+1 ]-N 3 complex. 1c, 82,223 Now, this persistent and open-shell complex delivers the radical species to a C-centered radical or olefin, forming the desired C-N 3 bond.…”

“…Lin et al demonstrated that the electrochemically produced oxoammonium ion facilitates the formation of N 3 via inner sphere ET with N 3 À . 73 Though the complex formation process is reversible, it is still quantitative (association constant, K a 4 1), and follows uphill fragmentation to provide transient N 3 along with corresponding persistent aminoxyl radical. Due to the reversible nature, the concentration of N 3 becomes low in the reaction medium, which drives the addition reaction with olefins in a slow and rate-determining step (RDS), thus more favorably diminishing the decay process.…”

“…), in which the azide radical temporarily resides in a ''resting state'' through complexation. 73 Apart from all these techniques, several approaches have also been established where redox-active metal catalysts, such as Cu, Fe, Mn, Ce, and Ag, can serve as efficient initiators to generate azide radicals in the presence of NaN 3 , TMSN 3 , or azidoiodine(III) oxidant [IBA-N 3 ] as an azide source. 70,71 It is also believed that the azide radicals (N 3…”

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

“…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 4D). 54 2.8.…”