Singh Shivendra, Mukherjee Tushar Kanti
Department of Chemistry, Indian Institute of Technology Indore Indore 453552 Madhya Pradesh India
Chem Sci. 2024 Aug 2;15(34):13949-57. doi: 10.1039/d4sc04115a.
Photosensitizer (PS)-mediated generation of singlet oxygen, O (aΔ) is a well-explored phenomenon in chemistry and biology. However, the requirement of appropriate PSs with optimum excited state properties is a prerequisite for this approach which limits its widespread application. Herein, we report the generation of O (aΔ) direct charge-transfer (CT) excitation of the solvent-O (XΣ ) collision complex without any PS and utilize it for the catalyst-free oxidative coupling of arylamines to azoaromatics under ambient conditions in aqueous medium. Electron paramagnetic resonance (EPR) spectroscopy revealed the formation of O (aΔ) upon direct excitation with 370 nm light. The present approach shows broad substrate scope, remarkably fast reaction kinetics (90 and 40 min under an open and O atm, respectively), high selectivity (100%), and excellent yields (up to 100%), and works well for both homo- and hetero-coupling of arylamines. The oxidative coupling of arylamines was found to proceed through the generation of amine radicals electron transfer (ET) from amines to O (aΔ). Notably, electron-rich amines show higher yields of azo products compared to electron-deficient amines. Detailed mechanistic investigations using various spectroscopic tools revealed the formation of hydrazobenzene as an intermediate along with superoxide radicals which subsequently transform to hydrogen peroxide. The present study is unique in the way that molecular O simultaneously acts as a light-absorbing chromophore (solvent-O complex) as well as an efficient oxidant (O (aΔ)) in the same reaction. This is the first report on the efficient, selective, and sustainable synthesis of azo compounds in aqueous medium under an ambient atmosphere without any PCs/PSs and paves the way for further in-depth understanding of the chemical reactivity of O (aΔ) generated directly CT excitation of the solvent-O complex toward various photochemical and photobiological transformations.
光敏剂（PS）介导的单线态氧O（aΔ）的产生是化学和生物学中一个已得到充分研究的现象。然而，需要具有最佳激发态性质的合适光敏剂是这种方法的先决条件，这限制了其广泛应用。在此，我们报道了在没有任何光敏剂的情况下，通过溶剂 - O（XΣ）碰撞复合物的直接电荷转移（CT）激发产生O（aΔ），并在水介质中的环境条件下将其用于芳胺到偶氮芳烃的无催化剂氧化偶联反应。电子顺磁共振（EPR）光谱显示在370 nm光直接激发下形成了O（aΔ）。本方法显示出广泛的底物范围、非常快的反应动力学（分别在开放和O气氛下90分钟和40分钟）、高选择性（100%）和优异的产率（高达100%），并且对芳胺的均相和异相偶联都有效。发现芳胺的氧化偶联是通过胺自由基从胺到O（aΔ）的电子转移（ET）而进行的。值得注意的是，富电子胺与缺电子胺相比，偶氮产物的产率更高。使用各种光谱工具进行的详细机理研究表明，形成了作为中间体的氢化偶氮苯以及超氧自由基，随后超氧自由基转化为过氧化氢。本研究的独特之处在于，分子O在同一反应中同时充当吸光发色团（溶剂 - O复合物）以及高效氧化剂（O（aΔ））。这是关于在环境气氛下的水介质中无任何光催化剂/光敏剂高效、选择性和可持续合成偶氮化合物的首次报道，并为进一步深入了解通过溶剂 - O复合物的直接CT激发产生的O（aΔ）对各种光化学和光生物学转化的化学反应性铺平了道路。
