Electrochemical Azidooxygenation of Alkenes Mediated by a TEMPO–N3 Charge-Transfer Complex

We report a mild and efficient electrochemical protocol to access a variety of vicinally C–O and C–N difunctionalized compounds from simple alkenes. Detailed mechanistic studies revealed a distinct reaction pathway from those previously reported for TEMPO-mediated reactions. In this mechanism, electrochemically generated oxoammonium ion facilitates the formation of azidyl radical via a charge-transfer complex with azide, TEMPO–N3. DFT calculations together with spectroscopic characterization provided a tentative structural assignment of this charge-transfer complex. Kinetic and kinetic isotopic effect studies revealed that reversible dissociation of TEMPO–N3 into TEMPO• and azidyl precedes the addition of these radicals across the alkene in the rate-determining step. The resulting azidooxygenated product could then be easily manipulated for further synthetic elaborations. The discovery of this new reaction pathway mediated by the TEMPO+/TEMPO• redox couple may expand the scope of aminoxyl radical chemistry in synthetic contexts.本研究报道了一种温和高效的电化学合成方案，可由简单烯烃制备多种邻位碳-氧与碳-氮双官能化化合物。详细的机理研究揭示了一条与此前报道的TEMPO（2,2,6,6-四甲基哌啶氮氧自由基）介导反应截然不同的反应路径。在该反应机理中，电化学生成的氧鎓离子可通过与叠氮化物形成电荷转移复合物TEMPO–N3，进而促进叠氮自由基的生成。密度泛函理论（DFT，Density Functional Theory）计算与光谱表征手段，为该电荷转移复合物的初步结构指认提供了支撑。动力学与动力学同位素效应研究表明，TEMPO–N3可逆解离为TEMPO自由基与叠氮自由基的步骤为反应决速步，随后这两种自由基才会加成至烯烃双键上。所得的叠氮氧化官能化产物可便捷地进行后续合成衍生化操作。这种由TEMPO+/TEMPO•氧化还原对介导的全新反应路径的发现，有望拓展氮氧自由基化学在合成领域的应用范畴。