Methylation of Ylidene-Triazenes: Insight and Guidance for 1,3-Dipolar Cycloaddition Reactions
Methylation of Ylidene-Triazenes: Insight and Guidance for 1,3-Dipolar Cycloaddition Reactions
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亚叉三氮烯的甲基化：1,3-偶极环加成反应的见解和指导
DOI：
10.1002/ejoc.201000939
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发表时间：
2010
影响因子：
2.8
通讯作者：
C. Bielawski
中科院分区：
文献类型：
--
作者：
Andrew G. Tennyson;Eric J. Moorhead;B. Madison;J. A. Er;V. Lynch;C. Bielawski
1,3维咪唑-三氮杂烯(1)与对官能化苯叠氮化物2 （a: H, b: OCH 3, c: no2）反应，得到了收率较高的咪唑-三氮杂烯3（65- 99%）。随后用碘化甲酯处理产生相应的甲基化产物4，产量接近定量（99%）。NOESY - 1D NMR和单晶x射线衍射分析表明，甲基化反应发生在离杂环最远的氮原子上，具有区域选择性。与离子盐的形成一致，4中咪唑质子的1 H NMR信号比1向下移动了> 0.5 ppm，表明正电荷积累。同样，4的λ max值比3的λ max值的范围窄得多（分别为16 nm和77 nm），这表明前者的前沿轨道能量主要由库仑效应（即获得正电荷）主导，而不是由芳基叠氮化物的供电子或吸电子基的电子效应主导。计算分析表明，甲基化反应的区域选择性可以用热力学来解释，动力学因素（如立体位阻）也可能是重要的因素，并且由于活化能较低，使得一个反应途径更容易接近。
Reaction of 1,3-dimesitylimidazolylidene (1) with p-functionalized phenyl azides 2 (a: H, b: OCH 3 , c: NO 2 ) afforded the respective imidazolylidene-triazenes 3 in good yields (65-99 %). Subsequent treatment with methyl iodide produced the corresponding methylated products 4 in near-quantitative yields (99 %). Analysis by NOESY 1D NMR spectroscopy and single-crystal X-ray diffraction revealed that the methylation reaction was regioselective and occurred at the nitrogen atom most distal from the heterocycle. Consistent with the formation of ionic salts, the 1 H NMR signals for the imidazole protons in 4 were shifted > 0.5 ppm downfield compared to 1, indicating the accumulation of positive charge. Similarly, the λ max values recorded for 4 exhibited a much narrower range than those for 3 (16 vs. 77 nm, respectively), suggesting that the frontier orbital energies within the former were dominated by Coulombic effects (i.e., the acquisition of positive charge) as opposed to electronic effects from the electron-donating or withdrawing groups of the aryl azide. Whereas computational analyses revealed that the observed regioselectivity of the methylation reaction may be explained by thermodynamics, kinetic factors (e.g., sterics) may also be important contributors and render one reaction pathway significantly more accessible due to a lower activation energy.
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