Counterion Dependence on the Synthetic Viability of NHC-stabilized Dichloroborenium Cations
Counterion Dependence on the Synthetic Viability of NHC-stabilized Dichloroborenium Cations
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DOI：
10.1021/om400541q
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发表时间：
2013-11-25
期刊：
影响因子：
2.8
通讯作者：
Vidovic, Dragoslav
中科院分区：
文献类型：
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作者：
Muthaiah, Senthilkumar;Dinh Cao Huan Do;Vidovic, Dragoslav
几种n -杂环碳稳定二氯硼离子[NHC中心点BCl2](+) (NHC = (RC)(2)(NR')(2)C的合成活力1、R = R' = Me；2、R = H， R' = R -i；3、R = Me， R' = R -i；4、R = H， R' = but -t；研究了Cr-、AlCl4-、OTf- (Tf = O2SCF3)、NTf2-和[BAr4Cl]（-) （Ar-Cl = 3,5- cl -2- c6h3）存在下，R = H， R' = 2,6- pr -i(2)- c6h3）的反应。在Cl-存在的情况下，所有的靶位都不能合成，只观察到中性的NHC中心点BCl3化合物。另一方面，在AlCl4-存在下，所有目标阳离子都是合成可行的，这并不奇怪，但从B-11核磁共振实验中可以明显看出不同程度的相互作用。在AlCl4-反离子存在下，对[1中心点BCl2]、[2中心点BCl2]（+）和[3中心点BCl2]（+）的x射线分析证实了这一点，因为正负离子相互作用的程度取决于相应NHCs的位阻。除[4中心点BCl2]（+）外，以OTf-和NTf2-为反离子时，没有合成可行的硼配位。最后，我们能够证明，在制备[4中心点BCl2][BAr4Cl]时，只有[4中心点BCl2]（+）是可行的，而没有反离子稳定效应(s)。尽管检测到[3中心点BCl2] [BAr4Cl]的存在，但该化合物似乎并不热稳定，因为它在48 h后就在溶液中分解了。[4中心点BCl2]（+）的热稳定性和[3中心点BCl2]（+）在[BAr4Cl]（-）存在下的不稳定性分别归因于这两种硼的存在和不存在非常弱的内相互作用。
The synthetic viability of several N-heterocyclic carbene stabilized dichloroborenium cations [NHC center dot BCl2](+) (NHC = (RC)(2)(NR')(2)C; 1, R = R' = Me; 2, R = H, R' = Pr-i; 3, R = Me, R' = Pr-i; 4, R = H, R' = Bu-t; 5, R = H, R' = 2,6-Pr-i(2)-C6H3) in the presence of Cr-, AlCl4-, OTf- (Tf = O2SCF3), NTf2-, and [BAr4Cl](-) (Ar-Cl = 3,5-Cl-2-C6H3) was investigated. None of the target borocations could be synthesized in the presence of Cl-, as only neutral NHC center dot BCl3 compounds were observed. On the other hand, it was not surprising that all targeted cations were synthetically viable in the presence of AlCl4- but a different degree of interion interaction was evident from B-11 NMR experiments. This was confirmed by X-ray analyses of [1 center dot BCl2], [2 center dot BCl2](+), and [3 center dot BCl2](+) in the presence of AlCl4- counterions, as the degree of cation-anion interaction was dependent on the steric encumbrance of the corresponding NHCs. Apart from [4 center dot BCl2](+), no borocation was synthetically viable when OTf- and NTf2- were used as the counterions. Finally, we were able to show that only [4 center dot BCl2](+) could be synthetically viable without the counterion stabilization effect(s) as the preparation of [4 center dot BCl2][BAr4Cl] was achieved. Even though the presence of [3 center dot BCl2] [BAr4Cl] was detected, this compound appeared not to be thermally stable, as it decomposed in solution after 48 h. The thermal stability of [4 center dot BCl2](+) and instability of [3 center dot BCl2](+) in the presence of [BAr4Cl](-) was attributed to the presence and absence, respectively, of very weak intraion (agostic) interactions in these two borocations.