Voskresenska Valentyna, Wilson R Marshall, Panov Maxim, Tarnovsky Alexander N, Krause Jeanette A, Vyas Shubham, Winter Arthur H, Hadad Christopher M
Department of Chemistry and the Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403, USA.
J Am Chem Soc. 2009 Aug 19;131(32):11535-47. doi: 10.1021/ja902224m.
Phenyl azides with powerful electron-donating substituents are known to deviate from the usual photochemical behavior of other phenyl azides. They do not undergo ring expansion but form basic nitrenes that protonate to form nitrenium ions. The photochemistry of the widely used photoaffinity labeling system 4-amino-3-nitrophenyl azide, 5, has been studied by transient absorption spectroscopy from femtosecond to microsecond time domains and from a theoretical perspective. The nitrene generation from azide 5 occurs on the S(2) surface, in violation of Kasha's rule. The resulting nitrene is a powerful base and abstracts protons extremely rapidly from a variety of sources to form a nitrenium ion. In methanol, this protonation occurs in about 5 ps, which is the fastest intermolecular protonation observed to date. Suitable proton sources include alcohols, amine salts, and even acidic C-H bonds such as acetonitrile. The resulting nitrenium ion is stabilized by the electron-donating 4-amino group to afford a diiminoquinone-like species that collapses relatively slowly to form the ultimate cross-linked product. In some cases in which the anion is a good hydride donor, cross-linking is replaced by reduction of the nitrenium ion to the corresponding amine.
已知带有强供电子取代基的苯基叠氮化物会偏离其他苯基叠氮化物的常规光化学行为。它们不会发生环扩张，而是形成碱性氮烯，该氮烯质子化形成氮鎓离子。已通过从飞秒到微秒时域的瞬态吸收光谱法并从理论角度研究了广泛使用的光亲和标记系统4-氨基-3-硝基苯基叠氮化物（5）的光化学。叠氮化物5产生氮烯发生在S(2)表面，这违反了卡沙规则。生成的氮烯是一种强碱，能极其迅速地从各种来源夺取质子以形成氮鎓离子。在甲醇中，这种质子化在约5皮秒内发生，这是迄今为止观察到的最快的分子间质子化。合适的质子源包括醇类、胺盐，甚至像乙腈这样的酸性C-H键。生成的氮鎓离子通过供电子的4-氨基得以稳定，从而产生一种类似二亚氨基醌的物种，该物种相对缓慢地分解以形成最终的交联产物。在某些情况下，若阴离子是良好的氢化物供体，则交联会被氮鎓离子还原为相应的胺所取代。
