Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019, USA.
Dalton Trans. 2013 Nov 21;42(43):15399-410. doi: 10.1039/c3dt52152d.
Sodium salts of two new tris(pyrazolyl)borates HB(4-Cl-3,5-(CF3)2Pz)3 and HB(4-(NO2)-3,5-(CF3)2Pz)3, which are not only highly fluorinated, but also loaded with additional electron-withdrawing substituents, have been synthesized by reacting 4-Cl-3,5-(CF3)2PzH or 4-(NO2)-3,5-(CF3)2PzH with NaBH4 under nitrogen in a solventless process, and isolated after a work up in tetrahydrofuran (THF) or diethyl ether (Et2O), as their THF or Et2O adducts. Metathesis of these sodium salts with AgOTf in THF leads to [HB(4-Cl-3,5-(CF3)2Pz)3]Ag(THF) and [HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(THF). The corresponding cis-cyclooctene (c-COE) complexes [HB(4-Cl-3,5-(CF3)2Pz)3]Ag(c-COE) and [HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE) were obtained by displacing THF with cis-cyclooctene. The related [HB(3,5-(CF3)2Pz)3]Ag(c-COE) can also be obtained via a similar process. X-ray crystal structures show that [HB(4-(R)-3,5-(CF3)2Pz)3]Ag(c-COE) (R = H, Cl, NO2) feature pseudo-tetrahedral silver sites supported by κ(3)-bound tris(pyrazolyl)borate ligands. [HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE) displays the smallest upfield shift of the alkene carbon peak (versus free alkene) followed by [HB(4-Cl-3,5-(CF3)2Pz)3]Ag(c-COE) and [HB(3,5-(CF3)2Pz)3]Ag(c-COE). Experimental and computational data indicate very electron poor silver(I) sites in all three [HB(4-(R)-3,5-(CF3)2Pz)3]Ag(c-COE) adducts with minimal Ag → (cis-cyclooctene) backbonding. Although the impact of R (H, Cl, NO2) on the alkene carbon chemical shift of these adducts and the olefin π/π* populations is small, the (13)C NMR chemical shifts and NBO analysis suggest that [HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE) possesses the most electrophilic metal site, which correlates with the pKa values of the free pyrazoles. [HB(4-(R)-3,5-(CF3)2Pz)3]Ag(c-COE) adducts effectively catalyze the insertion of the carbene moiety of ethyl diazoacetate into C-H bonds of 2,3-dimethylbutane. The [HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE) catalyst shows a higher selectivity for primary C-H bonds compared to the reactions catalyzed by [HB(3,5-(CF3)2Pz)3]Ag(c-COE).
两种新的三（吡唑基）硼酸盐HB(4-Cl-3,5-(CF3)2Pz)3和HB(4-(NO2)-3,5-(CF3)2Pz)3的钠盐已通过在氮气下无溶剂反应，由 4-Cl-3,5-(CF3)2PzH 或 4-(NO2)-3,5-(CF3)2PzH 与 NaBH4 反应合成，然后在四氢呋喃（THF）或乙醚（Et2O）中进行后处理，得到它们的 THF 或 Et2O 加合物。这些钠盐与 AgOTf 在 THF 中的复分解反应导致[HB(4-Cl-3,5-(CF3)2Pz)3]Ag(THF)和[HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(THF)。通过用顺式-环辛烯（c-COE）取代 THF，得到相应的顺式-环辛烯（c-COE）配合物[HB(4-Cl-3,5-(CF3)2Pz)3]Ag(c-COE)和[HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE)。通过类似的过程也可以得到[HB(3,5-(CF3)2Pz)3]Ag(c-COE)。X 射线晶体结构表明，[HB(4-(R)-3,5-(CF3)2Pz)3]Ag(c-COE)（R = H，Cl，NO2）具有由κ(3)-键合的三（吡唑基）硼酸盐配体支撑的伪四面体银位点。[HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE)显示出最小的烯烃碳峰（相对于游离烯烃）的上磁场位移，其次是[HB(4-Cl-3,5-(CF3)2Pz)3]Ag(c-COE)和[HB(3,5-(CF3)2Pz)3]Ag(c-COE)。实验和计算数据表明，所有三种[HB(4-(R)-3,5-(CF3)2Pz)3]Ag(c-COE)加合物中的银（I）位点都非常缺电子，与顺式-环辛烯的回键作用最小。尽管 R（H，Cl，NO2）对这些加合物的烯烃碳化学位移和烯烃π/π* 电子密度的影响很小，但 13C NMR 化学位移和 NBO 分析表明，[HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE)具有最具亲电性的金属位点，这与游离吡唑的 pKa 值相关。[HB(4-(R)-3,5-(CF3)2Pz)3]Ag(c-COE)加合物有效地催化乙基重氮乙酸酯的卡宾部分插入 2,3-二甲基丁烷的 C-H 键中。[HB(4-(NO2)-3,5-(CF3)2Pz)3]Ag(c-COE)催化剂对伯 C-H 键的选择性高于[HB(3,5-(CF3)2Pz)3]Ag(c-COE)催化的反应。
