研究室工作進展, Mar. 19th, 2015
Dianions as Formal Oxidants: Synthesis and Characterization of Aromatic Dilithionickeloles from 1,4-Dilithio-1,3-butadienes and Ni(COD)2
Junnian Wei, Wen-Xiong Zhang, and Zhenfeng Xi*
Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201411009
Organolithium compounds can behave as reductants but never as oxidants in redox reactions. Here we report that 1,4-dilithio-1,3-butadienes reacted with Ni(COD)2 to offer dilithionickeloles. Single-crystal X-ray structural analysis revealed a coplanar structure of dilithionickeloles with averaging of bond lengths. XPS data confirmed the oxidation state of Ni in dilithionickeloles was Ni2+. 7Li NMR spectra of dilithionickeloles and theoretical calculations revealed a considerable aromatic character. In this redox reaction, the dilithio dianionic compounds behaved as formal oxidants, oxidizing the Ni0 atom to Ni2+. These results demonstrated that organolithium compounds with π-conjugation could be used as oxidants and could continue to gain extra electrons.
亮點介紹
由于碳負離子的富電本質，在氧化還原反應中碳負離子（如有機锂試劑）可以作為還原劑失去電子，但是從不作為氧化劑得到電子。本工作報道雙锂試劑的丁二烯基雙碳負離子共轭體系内部軌道的協同效應，與低價金屬如零價鎳反應時，獲得金屬鎳d軌道的電子從而将零價鎳氧化成二價，打破了傳統所認為的碳負離子不能夠作為氧化劑的認知，為共轭體系碳負離子化學的進一步發展和應用提供了一個全新的思路。該工作再次顯示了本組雙锂試劑的獨特化學性質。
關于該反應産物中鎳的氧化态，僅從傳統離子型無機化合物的金屬氧化态（表觀氧化态）概念理解是不合适的。該産物鎳雜環構成一個芳香性整體，鎳d軌道的電子離域至丁二烯的π*軌道形成6-π芳香體系。傳統金屬氧化态（表觀氧化态）概念适合于描述離子型無機化合物，但是在描述金屬有機化合物中金屬的氧化态時存在缺陷，需要延伸傳統概念或建立新概念。有關金屬有機化合物中金屬氧化态描述的延伸閱讀：M. L. H. Green, J. Organomet. Chem. 1995, 500, 127; A. F. Hill, Organometallics 2006, 25, 4741; G. Parkin, Organometallics 2006, 25, 4744; http://www.columbia.edu/cu/chemistry/groups/parkin/cbc.htm