Decisive Steps of the Hydrodefluorination of Fluoroaromatics using [Ni(NHC)2]
Decisive Steps of the Hydrodefluorination of Fluoroaromatics using [Ni(NHC)2]
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DOI：
10.1021/om2009815
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发表时间：
2012-02-27
期刊：
影响因子：
2.8
通讯作者：
Radius, Udo
中科院分区：
文献类型：
--
作者：
Fischer, Peter;Goetz, Kathrin;Radius, Udo
本文报道了以[Ni-2（（i）Pr（2）Im）（4）（COD）]（1;（i）Pr（2）Im = 1，3-双（异丙基）咪唑啉-2-亚基）为催化剂的全氟芳烃的加氢脱氢反应，并通过化学计量变换阐明了该反应的决定性步骤。六氟苯与5当量三苯基硅烷在5mol%1存在下的反应在60 ℃下48小时后得到1，2，4，5-四氟苯，在80 ℃下96小时后得到1，4-二氟苯;全氟甲苯和5当量Et 3SiH在80 ℃下反应4天导致选择性形成1-（CF 3）-2，3，5，6-C6 F4 H。配合物cis-[Ni（（i）Pr（2）Im）（2）（H）（SiPh 3）]和cis-[N-i（（i）Pr（2）Im）（2）（H）（SiMePh 2）]与六氟苯在室温下的化学计量转化导致形成trans-[Ni（（i）Pr（2）Im）（2）（F）-（C6 F5）]（2）和trans-[Ni（（i）Pr（2）Im）（2）（H）（C6 F5）]（4）。同时消除相应的硅烷或氟硅烷。C-F活化产物trans-[Ni（（i）Pr（2）Im）（2）（F）-（C6 F5）]（2）和trans-[Ni（（i）Pr（2）Im）（2）（F）（4-（CF 3）C6 F4）]（3）分别与PhSiH 3和Ph 2SiH 2反应，得到氢化物trans-[Ni（（i）Pr（2）Im）（2）（H）（C6 F5）]（4）和trans-[Ni（（i）Pr（2）Im）（2）（H）（4-（CF 3）C6 F4）]（5），其分别转化为化合物反式-[Ni（（i）Pr（2）Im）（2）（F）（2，3，5，6-C6 F4 H）]（7）、反式-[Ni（（i）Pr（2）Im）（2）（F）（3-（CF 3）-2，4，5-C6 F3 H）]（9a）和反式-[Ni-（（i）Pr（2）Im）（2）（F）（2-（CF 3）-3，4，6-C6 F3 H）]（9 b）。在反式-[Ni（（i）Pr（1）Im）（2）（H）（4-（CF 3）C6 F4）]（5）重排的情况下，检测到中间体[Ni（（i）Pr（2）Im）（2）（eta（2）-C，G-（CF 3）C6 F4 H）]（8）。8与全氟甲苯反应得到C-F活化产物trans-[Ni（（i）Pr（2）Im）（2）（F）（4-（CF_3）C_6F_4）]（3）。所有这些实验发现都指出了[Ni（（i）Pr（2）Im）（2）]通过“氟化物途径”进行HDF的机理，该机理涉及多氟芳烃的C-F活化，所得氟化镍的H/F交换，多氟芳基氢化镍还原消除成具有η（2）-C，C-配位芳烃配体的中间体，随后与更高级氟化多氟芳烃的配体交换，和多氟芳烃的重新GF活化。在没有额外试剂的情况下，[Ni（（i）Pr（2）Im）（2）（eta（2）-C，C-（CF 3）C6 F4 H）]（8）重排为异构体反式-[Ni（（i）Pr（2）Im）（2）（F）（3-（CF 3）-2，4，5-C6 F3 H）]（9a;主要）和反式-[Ni（（i）Pr（2）Im）（2）（F）（2-（CF 3）-3，4，6-C6 F3 H）]（9 h;次要），比例为80：20.对反式-[Ni（（i）Pr（2）Im）（2）（H）（4-（CF 3）C6 F4）] 5转化成两种产物反式-[Ni（（i）Pr（2）Im）（2）（F）（3-（CF 3）-2，4，5-C6 F3 H）] 9a和反式-[Ni（（i）Pr（2）Im）（2）（F）（2-（CF 3）-3，4，6-C_6F_3H）]（9 b）预测9 b为主要产物。因此，我们认为这种反应机理对于[Ni（NHC）（2）]介导的具有特殊取代模式的部分氟化芳烃的C-F活化是无效的。
The hydrodefluorination reaction of perfluorinated arenes using [Ni-2((i)Pr(2)Im)(4)(COD)] (1; (i)Pr(2)Im = 1,3-bis(isopropyl)imidazolin-2-ylidene) as a catalyst as well as stoichiometric transformations to elucidate the decisive steps for this reaction are reported. The reaction of hexafluorobenzene with 5 equiv of triphenylsilane in the presence of 5 mol % of 1 affords 1,2,4,5-tetrafluorobenzene after 48 h at 60 degrees C and 1,4-difluorobenzene after 96 h at 80 degrees C; the reaction of perfluorotoluene and 5 equiv of Et3SiH for 4 days at 80 degrees C results in the selective formation of 1-(CF3)-2,3,5,6-C6F4H. Stoichiometric transformations of the complexes cis-[Ni((i)Pr(2)Im)(2)(H)(SiPh3)] and cis-[N-i((i)Pr(2)Im)(2)(H)(SiMePh2)] with hexafluorobenzene at room temperature lead to the formation of trans-[Ni((i)Pr(2)Im)(2)(F)-(C6F5)] (2) and trans-[Ni((i)Pr(2)Im)(2)(H)(C6F5)] (4) with elimination of the corresponding silane or fluorosilane. The reactions of the C-F activation products trans-[Ni((i)Pr(2)Im)(2)(F)-(C6F5)] (2) and trans-[Ni((i)Pr(2)Im)(2)(F)(4-(CF3)C6F4)] (3) with PhSiH3 and Ph2SiH2 afford the hydride complexes trans-[Ni((i)Pr(2)Im)(2)(H)(C6F5)] (4) and trans-[Ni((i)Pr(2)Im)(2)(H)(4-(CF3)C6F4)] (5), which convert into the compounds trans-[Ni((i)Pr(2)Im)(2)(F)(2,3,5,6-C6F4H)] (7), trans-[Ni((i)Pr(2)Im)(2)(F)(3-(CF3)-2,4,5-C6F3H)] (9a), and trans-[Ni-((i)Pr(2)Im)(2)(F)(2-(CF3)-3,4,6-C6F3H)] (9b), respectively. In the case of the rearrangement of trans-[Ni((i)Pr(1)Im)(2)(H)(4-(CF3)C6F4)] (5) the intermediate [Ni((i)Pr(2)Im)(2)(eta(2)-C,G-(CF3)C6F4H)] (8) was detected. Reaction of 8 with perfluorotoluene gave the C-F activation product trans-[Ni((i)Pr(2)Im)(2)(F)(4-(CF3)C6F4)] (3). All these experimental findings point to a mechanism for the HDF by [Ni((i)Pr(2)Im)(2)] via the "fluoride route" involving C-F activation of the polyfluoroarene, H/F exchange of the resulting nickel fluoride, reductive elimination of the polyfluoroaryl nickel hydride to an intermediate with a eta(2)-C,C-coordinated arene ligand, subsequent ligand exchange with the higher fluorinated polyfluoroarene, and renewed G F activation of the polyfluoroarene. Without additional reagents, [Ni((i)Pr(2)Im)(2)(eta(2)-C,C-(CF3)C6F4H)] (8) rearranges to the isomers trans-[Ni((i)Pr(2)Im)(2)(F)(3-(CF3)-2,4,5-C6F3H)] (9a; major) and trans-[Ni((i)Pr(2)Im)(2)(F)(2-(CF3)-3,4,6-C6F3H)] (9h; minor) in a ratio of 80:20.DFT calculations performed on conversion of trans-[Ni((i)Pr(2)Im)(2)(H)(4-(CF3)C6F4)] 5 into the two products trans-[Ni((i)Pr(2)Im)(2)(F)(3-(CF3)-2,4,5-C6F3H)] 9a and trans-[Ni((i)Pr(2)Im)(2)(F)(2-(CF3)-3,4,6-C6F3H)] (9b) using the commonly accepted intramolecular concerted pathway via re-C,F-a-bound transition states predict 9b to be the major product. We thus propose that this reaction mechanism is not valid for the [Ni(NHC)(2)]-mediated C-F activation of partially fluorinated arenes with special substitution patterns.