Effective modulation of the donor properties of N-heterocyclic carbene Ligands by "Through-Space" communication within a planar chiral scaffold
Effective modulation of the donor properties of N-heterocyclic carbene Ligands by "Through-Space" communication within a planar chiral scaffold
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DOI：
10.1021/ja076028t
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发表时间：
2007-10-24
影响因子：
15
通讯作者：
Lehmann, Christian W.
中科院分区：
文献类型：
--
作者：
Fuerstner, Alois;Alcarazo, Manuel;Lehmann, Christian W.
介绍了一种高产率合成平面手性卡宾配体的方法，其中咪唑并[1，5-a]吡啶-3-亚基单元嵌入到环蕃骨架中。从作为该家族的母体成员的相应铑络合物18的IR数据可以明显看出，这些新的配体被证明是异常强的电子给体，与迄今已知的其他二氨基稳定的五元N-杂环卡宾（NHC）相媲美或甚至优于它们。然而，如果环蕃的远端环被四个氟原子取代，则给体容量会因四氟苯部分与下面的卡宾实体的贯穿空间相互作用而显著降低。由于X-射线数据表明，氟化和非氟化的环番-2-亚基的空间需求几乎是相同的，这些结果表明，NHC配体的电子性质可以在很宽的范围内调谐，而不需要改变其结构或空间属性。导致用作直接碳烯前体的平面手性咪唑并吡啶鎓盐的合成路线涉及在吡啶-N-氧化物12暴露于N，N -二甲基-氨基甲酰氯和TMSCN时的先前未识别的6 π-四电环开环反应，如从所得氮杂三烯加合物13的晶体结构分析中明显的。
A high yielding approach to planar chiral carbene ligands is described, in which an imidazo[1,5-a]pyridine-3-ylidene unit is embedded into a cyclophane scaffold. As evident from the IR data of the corresponding rhodium complex 18 as the parent member of this family, these new ligands turned out to be exceptionally strong electron donors, rivaling or even outperforming the other diaminostabilized five-membered N-heterocyclic carbenes (NHC) known to date. If the remote ring of the cyclophane is substituted by four fluorine atoms, however, the donor capacity is significantly reduced by the through-space interaction of the tetrafluorobenzene moiety with the underneath carbene entity. Since X-ray data suggest that the steric demand of the fluorinated and the nonfluorinated cyclophane-2-ylidenes are virtually identical, these results illustrate that the electronic properties of an NHC ligand can be tuned over a wide range without the need to change its constitution or steric attributes. The synthesis route leading to the planar chiral imidazopyridinium salts serving as the immediate carbene precursors involves a previously unrecognized 6 pi-Telectrocyclic ring opening reaction upon exposure of the pyridine-N -oxide 12 to N N -dimethyl-carbamoyl chloride and TMSCN, as evident from the crystal structure analysis of the resulting azatriene adduct 13.