A boronium ion with exceptional electrophilicity.
A boronium ion with exceptional electrophilicity.
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DOI：
10.1002/anie.201005663
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发表时间：
2011-02-25
影响因子：
16.6
通讯作者：
Vedejs, Edwin
中科院分区：
文献类型：
--
作者：
Prokofjevs, Aleksandrs;Kampf, Jeff W.;Vedejs, Edwin
在我们对芳族硼基化的研究中，[1]我们考虑了高度亲电的R2 BNTf 2试剂与碱的组合，该碱将中和硼基化的HNTf 2副产物而不使亲电试剂失活。原则上，这些要求可以通过1，8-双（二甲基氨基）萘（1）来满足，1，8-双（二甲基氨基）萘（1）是一种受阻的和异常碱性的苯胺，由于其传说中的亲核性的缺乏，其作为碱性催化剂或试剂而具有许多应用。[2，3]强亲电体与胺氮的相互作用很弱，如果有的话，并且很少有已知的例子可以在1和大于氢的亲电基团之间形成稳定的氮键。[2，4-7]在这些例外情况中，环状硼鎓结构2和3相对稳定，因为亚基BH 2和BF 2具有最小的空间要求。[5]然而，更受阻的BMe 2衍生物4尚未被检测到，并且没有类似的BR 2结构是已知的。[5a鉴于这一悠久的历史，我们有些惊讶地发现，尽管9-BBN核的跨环空间需求和需要与硼以及氮形成相邻的季键，但在将1与9-BBN双三氟甲磺酰亚胺试剂5a混合时容易形成加合物。[9，10]该加合物的显著结构特征和不寻常的反应性是以下通信的主题。以前未报道的5a在甲苯中加热时容易地由市售的9-BBN二聚体和双（三氟甲磺酰基）酰亚胺制备。在室温下，大体积硼试剂5a与化学计量量的1在CD 2Cl 2中的组合形成深红色溶液，其在混合试剂的数秒内变为无色。通过11B NMR光谱分析所得溶液显示δ 16.2 ppm处的信号，表明产物中存在单个四配位[11]硼原子。19 F NMR光谱在δ-79.4 ppm处显示出单峰，这是bistriflimide阴离子的特征[12]，因此含硼片段被鉴定为阳离子。1H NMR光谱表明，在室温下，阳离子的溶液结构在NMR时间尺度上是高度对称的。只有四组对应于二胺亚基1的质子被观察到，包括一个尖锐的单峰的所有四个甲基，和一个良好的解决（在500 MHz）AMX系统的芳香族质子。1H和13 C NMR光谱中的其他峰也与阳离子的对称时间平均结构一致（例如，δ 57.1 ppm处的单个13 C甲基峰，以及9-BBN笼碳的仅3个峰）。由于共价加合物6（X= NTf 2）由双三氟甲磺酰亚胺阴离子[12 b]的观察结果决定，并且三配位阳离子硼鎓结构7（X= NTf 2）与观察到的11B NMR化学位移不一致，因此由1和5a形成的物质的最合理的结构是异常受阻的硼鎓盐8a。8a的形成可以遵循方案1中的逻辑顺序，但不排除替代机制。[13]形成> 95%的8a取决于
During our studies on aromatic borylation,[1] we considered the combination of a highly electrophilic R2BNTf2 reagent with a base that would neutralize the HNTf2 byproduct of borylation without deactivating the electrophile. In principle, these requirements might be satisfied by 1, 8-bis (dimethylamino) naphthalene (1), a hindered and exceptionally basic aniline that finds numerous applications as a basic catalyst or reagent due to its legendary lack of nucleophilicity.[2, 3] Strong electrophiles interact weakly, if at all, with the amine nitrogens, and very few examples are known where stable bonds to nitrogen can be formed between 1 and electrophilic groups larger than hydrogen.[2, 4-7] Among these exceptional cases, cyclic boronium structures 2 and 3 are relatively stable because the subunits BH2 and BF2 have minimal steric requirements.[5] However, the more hindered BMe2 derivative 4 has not been detected and no analogous BR2 structures are known.[5a, 8] In view of this long history, we were somewhat surprised to find that an adduct is readily formed simply upon mixing 1 with the 9-BBN bistriflimide reagent 5a despite the transannular steric demands of the 9-BBN core and the need to form adjacent quaternary bonds to boron as well as nitrogen.[9, 10] The remarkable structural features and unusual reactivity of this adduct are the subject of the following communication.The previously unreported 5a was easily prepared from the commercially available 9-BBN dimer and bis (trifluoromethanesulfonyl) imide upon heating in toluene. Combination of the bulky boron reagent 5a with a stoichiometric amount of 1 in CD2Cl2 at room temperature formed a deep red solution that turned colorless within seconds of mixing the reagents. Analysis of the resulting solution by 11B NMR spectroscopy revealed a signal at δ 16.2 ppm, suggesting that a single tetracoordinate [11] boron atom is present in the product. The 19F NMR spectrum showed a single peak at δ-79.4 ppm, which is characteristic of bistriflimide anion,[12] so the boron-containing fragment was thus identified to be a cation. The 1H NMR spectrum suggested that the solution structure of the cation is highly symmetrical on the NMR timescale at room temperature. Only four groups of protons corresponding to the diamine subunit 1 were observed, including one sharp singlet for all four methyl groups, and a well-resolved (at 500 MHz) AMX system for the aromatic protons. Other peaks in the 1H and 13C NMR spectra were also consistent with a symmetrical time-averaged structure for the cation (for example, a single 13C methyl peak at δ 57.1 ppm, and only 3 peaks for the 9-BBN cage carbons). Since the covalent adduct 6 (X= NTf2) is ruled by observation of the bistriflimide anion,[12b] and the tricoordinate cationic borenium structure 7 (X= NTf2) is not consistent with the observed 11B NMR chemical shift, the most plausible structure for the species formed from 1 and 5a is the exceptionally hindered boronium salt 8a. Formation of 8a may follow the logical sequence in Scheme 1, but alternative mechanisms have not been excluded.[13] Formation of> 95% 8a depends on
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