Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China.
J Am Chem Soc. 2012 Oct 3;134(39):16216-27. doi: 10.1021/ja305204y. Epub 2012 Sep 25.
"Synergistic effect" is prevalent in natural metalloenzymes in activating small molecules, and the success has inspired the development of artificial catalysts capable of unprecedented organic transformations. In this work, we found that the attractive π-π interaction between organic additives (as electron-donors) and the perfluorinated arenes (as electron acceptors) is effective in gold hydride catalyzed activation of C-F bonds, specifically hydrodefluorination (HDF) of perfluoroarenes catalyzed by the Sadighi's gold hydrides [(NHC)AuH] (NHC = N-heterocyclic carbene). Although a weak interaction between [(NHC)AuH] and perfluoroarenes was observed from (1)H NMR and UV-vis spectroscopies, low reactivity of [(NHC)AuH] toward HDF was found. In contrast, in the presence of p-N,N-dimethylaminopyridine (DMAP), the HDF of perfluoroarenes with silanes can be efficiently catalyzed by [(NHC)AuH], resulting in mainly the para-hydrodefluorinated products with up to 90% yield and 9 turnovers. The yield of the reaction increases with the more electron-withdrawing groups and degree of fluorination on the arenes, and the HDF reaction also tolerates different function groups (such as formyl, alkynyl, ketone, ester, and carboxylate groups), without reduction or hydrogenation of these function groups. To reveal the role of DMAP in the reactions, the possible π-π interaction between DMAP and perfluoroarenes was suggested by UV-vis spectral titrations, (1)H NMR spectroscopic studies, and DFT calculations. Moreover, (1)H and (19)F-NMR studies show that this π-π interaction promotes hydrogen transfer from [(NHC)AuH] to pyridyl N atom, resulting in C-F bond cleavage. The interpretation of π-π interaction assisted C-F activation is supported by the reduced activation barriers in the presence of DMAP (31.6 kcal/mol) than that in the absence of DMAP (40.8 kcal/mol) for this reaction. An analysis of the charge distribution and transition state geometries indicate that this HDF process is controlled by the π-π interaction between DMAP and perfluoroarenes, accompanied with the changes of partial atomic charges.
“协同效应”在激活小分子的天然金属酶中很常见，这一成功激发了人工催化剂的发展，使其能够实现前所未有的有机转化。在这项工作中，我们发现有机添加剂（作为电子供体）与全氟芳烃（作为电子受体）之间的吸引力 π-π 相互作用可有效激活金氢化物催化的 C-F 键，特别是由 Sadighi 的金氢化物[(NHC)AuH]（NHC = 氮杂环卡宾）催化的全氟芳烃的氢脱氟（HDF）。尽管从 (1)H NMR 和 UV-vis 光谱中观察到 [(NHC)AuH] 和全氟芳烃之间存在弱相互作用，但发现 [(NHC)AuH] 对 HDF 的反应性较低。相比之下，在 4-N,N-二甲基氨基吡啶 (DMAP) 的存在下，硅烷与全氟芳烃的 HDF 可被 [(NHC)AuH] 高效催化，主要生成对位氢脱氟产物，产率高达 90%，转化率高达 9 次。反应产率随芳烃上的吸电子基团和氟取代度的增加而增加，HDF 反应还能耐受不同的官能团（如甲酰基、炔基、酮、酯和羧酸盐基团），这些官能团不会被还原或氢化。为了揭示 DMAP 在反应中的作用，通过紫外可见光谱滴定、(1)H NMR 光谱研究和 DFT 计算，提出了 DMAP 与全氟芳烃之间可能存在的 π-π 相互作用。此外，(1)H 和 (19)F-NMR 研究表明，这种 π-π 相互作用促进了 [(NHC)AuH] 向吡啶 N 原子的氢转移，导致 C-F 键断裂。在存在 DMAP（31.6 kcal/mol）时，该反应的活化能垒低于不存在 DMAP（40.8 kcal/mol）时的活化能垒，这支持了 π-π 相互作用辅助 C-F 活化的解释。电荷分布和过渡态几何结构的分析表明，这种 HDF 过程受 DMAP 与全氟芳烃之间的 π-π 相互作用控制，同时伴随着部分原子电荷的变化。
