α位氧取代氮烯中间体的产生、光谱、结构与反应机理

Nitrene and carbene intermediates play important role in organic synthesis, bio-photolabeling, advanced surface functionalization, combustion and astrophysical chemistry. In contrast to the well established and also broadly used carbene analogues, the fundamental knowledge of nitrenes is rather limited due to the occurrence of complex secondary reactions during their formation from the photolysis of corresponding precursors. Therefore, as an important part of nitrene chemistry, it's highly desirable to find out relatively more stable nitrene species, so as to fully disclose their spectroscopic and structural properties, and also, their application for the synthesis of some long-sought synthetically challenging small molecules which bear novel bonding and structural properties. This project will mainly focus on three types of synthetically useful α-oxo nitrenes, i.e., carbonyl nitrene RC(O)N, phosphoryl nitrene R2P(O)N, and sulfonyl nitrene RSO2N. By introducing an oxygen atom at the α position of the conventional nitrene RN, the relative stability of the highly reactive lowest singlet state will be significantly improved because of electronic effect, and most importantly the intramolecular O→N(nitrene) interactions. Such stabilizing interaction will greatly promote the rapid intersystem crossing (ISC) from the initially formed singlet state to the lower-energy triplet ground state through facile spin-orbital coupling, which may allow a first-time experimental characterization of some highly interesting nitrene intermediates. By using homemade low-temperature glass-vacuum system, series of prototype metastable covalent azide precursors (RC(O)N3, R2P(O)N3, and RSO2N3) will be synthesized and fully characterized. Photo- and thermal-decomposition reactions of these azides will be studied in both gas phase and solid matrices (Ar, Ne, N2) using matrix isolation techniques (< 30 K). The identification of the involving intermediates and reaction mechanism will be analyzed with the aid of isotope labeling and extensive quantum chemical calculations. The photo- and thermal behaviors of nitrenes including rearrangement will be investigated in both solid matrices and solutions, and reactions with small molecules such as O2, CO, CO2, N2, NO, H2 will be explored by identification of the key intermediates. Later on, the stability of these α-oxo nitrenes will be evaluated using flash vacuum pyrolysis experiments as combined with low-temperature Raman spectroscopy, the stable ones will be used as precursors to synthesize some simple but yet unknown main-group molecules like 2H-azirinone (cyclo-HC(O)CN), formyl isocyanate (HC(O)NCO), phosphoryl isocyanate (OPNCO), phosphoryl cyanate (OPCN), sulfonyliminyl radical (O2SN), and their respective isomers, most of which have already been computationally predicted to be observable candidates with rich chemistry in spectroscopy, structure, and reactivity.

双自由基物种氮烯与等电子卡宾是有机合成、生物光化学、材料表面改性、以及燃烧与星际化学过程中的一类重要反应中间体。与已经获得广泛应用的卡宾相比，氮烯的基础实验研究比较缺乏。本课题通过搭建多级低温冷阱合成、分离与在线表征平台,选取以中心原子为碳、磷、硫的α位氧取代叠氮为氮烯前体，利用电子与结构效应诱导分子内O→N相互作用，调节氮烯物种RC(O)N、R2P(O)N、RSO2N的单重态与三重态基态之间的能级差ΔEST。促进系间窜越ISC，从初始条件下的单重态过渡到反应活性相对较低而稳定性更高的基态三重态，以此获取一类相对较稳定的α位氧取代氮烯物种.尤其是在热分解条件下可稳定存在的氮烯分子，作为研究模型结合各类低温光谱手段和量子化学理论揭示这类特殊反应中间体的分子光谱、分子结构、反应特性和反应机理。并利用此类氮烯的异构化反应,低温合成与表征一系列具有重要基础研究价值的新颖主族元素小分子。

本项目主要针对四类重要的α位氧取代氮烯中间体即羰基氮烯RC(O)N、磺酰基氮烯RS(O)2N、亚磺酰基氮烯RS(O)N、以及磷酰基氮烯R1R2P(O)N等，开展低温产生、原位指纹光谱探测、分子与电子结构表征、光化学反应过程跟踪、以及微观反应机制等方面的系统性研究。借助于课题组研制的反射式2.8K低温基质隔离光谱系统，结合深紫外激光光解与高温闪光热分解，成功获取四类氮烯中间体的原型分子的指纹红外与顺磁共振光谱数据；通过对氮烯在低温基质隔离条件下反应的精准光化学调控，实现一系列通过常规合成方法难以制备的新颖小分子物种。借助高精度量子化学计算，解析氮烯以及新颖小分子的电子结构与化学键特性。通过实验结合理论计算直观揭示不同α位氧取代氮烯的异构化反应机制。代表性的工作包括: (1) 实现简单羰基氮烯CF3C(O)N的直接光谱探测，在2.8K的基质中发现了在首例羰基氮烯在经典Curtius重排过程中重原子量子隧道效应QMT，并通过改变微观化学环境达到对QMT过程的调节；(2) 直接获取三重态磺酰基氮烯MeOS(O)2N，利用其高温闪光热分解同时诱导分子内协同氢迁移得到新颖的SO3等电子分子HNSO2，不仅获得其完整的指纹光谱数据，而且借助于同位素标记证实其光致异构化HONSO的反应通道；(3) 以亚稳态叠氮CF3S(O)N3为前体，低温分解得到具有共轭稳定性的单重态亚磺酰基氮烯CF3S(O)N，然后借助其高温裂解获得新颖拟卤素自由基OSN存在的直接证据，在获得其指纹光谱数据的同时发现了其光异构化产生SNO的反应过程；(4) 结合低温基质隔离和选择性光解跟踪FP(O)(N3)2分解历程，获取极不稳定的磷酰基氮烯F(N3)P(O)N，并结合理论解析其完整的分解势能曲线；(5)通过构建分子内S-N相互作用，在低温基质隔离条件下制备了基态为单重态的新颖氮烯F2P(S)N，同样以选择性光诱导实现分子内重排的可逆异构。在本项目的大力资助下，到目前为止，项目负责人以第一通讯作者发表科研论文（标注本项目基金资助）30篇，其中包括6篇Angew. Chem. Int. Ed. (2篇封面)、2篇J. Am. Chem. Soc.。本项目所涉及的系统性研究工作推动了氮烯中间体化学的基础物理化学研究，为活性氮烯中间体的广泛应用提供了重要参考。