Formation and reactions of heteroaromatic anions in the gas phase
Formation and reactions of heteroaromatic anions in the gas phase
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DOI：
10.1021/jo00254a001
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发表时间：
1988-09
影响因子：
3.6
通讯作者：
C. DePuy;S. Kass;G. P. Bean
中科院分区：
文献类型：
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作者：
C. DePuy;S. Kass;G. P. Bean
用流动余辉装置测定了呋喃、噻吩、吡啶、1-甲基吡咯及其甲基衍生物的气相酸度。母体杂环的AG酸（kcal/mol）和去质子化位点如下：呋喃380±3（2位），噻吩373±3（2位），吡啶384±3（3位和/或4位），1-甲基吡咯386±3（甲基）。对于所研究的甲基化物质，酸度为2-甲基呋喃377±3、2-甲基噻吩373 ± 3、3-甲基噻吩373 ±3、3-甲基吡啶371±3、2-甲基吡啶370±3和4-甲基吡啶368±3。通过氢-氘交换研究和其他化学方法估算了这些分子中其他位置氢的相对酸度。使来自这些分子的阴离子（M- 1）与N2 O、O2、COS和CS2反应，并测定离子产物。气相酸度与半经验方法AMI和MNDO的计算值进行了比较，发现在溶液中，许多五元和六元杂环化合物具有足够的酸性，它们的共轭碱很容易通过夺氢反应生成。1例如，2-呋喃锂由呋喃与正丁基锂2反应生成，吡啶锂由吡啶和正丁基叔丁醇锂钾反应生成。3在每种情况下，所得到的负碳离子可以用于合成用途，以良好至优异的产率形成取代的杂芳族化合物。合成和机理研究都表明，由氢提取产生的离子的特定异构形式是实验条件的函数。例如，Verbeek和Brandsma 3发现，形成的2-、3-和4-锂吡啶的相对量“强烈取决于向碱中添加吡啶和添加锂之间的时间长度”。Zoltewicz，Grahe和Smith 4表明，2-，3-，吡啶的4-位从CH_3 OD-CH_3 ONa溶液变为ND_3-NaND_2溶液。事实上，对于吡啶溶液，2，6-位的氢在动力学上是最具反应性的，而4-位的氢在动力学上是最具酸性的。[3]从这些数据和其他数据看来，至少在溶液中，许多化合物的动力学酸度和热力学酸度是不一样的。
The gas-phase acidities of furan, thiophene, pyridine, 1-methylpyrrole, and a number of their methyl derivatives have been determined in a flowing afterglow apparatus. The AGacid (kcal/mol) and site of deprotonation of the parent heterocycles are as follows: furan 380±3 (2-position), thiophene 373±3 (2-position), pyridine 384±3 (3-and/or 4-position), 1-methylpyrrole 386±3 (methyl group). For the methylated species studied the acidities are 2-methylfuran 377±3, 2-methylthiophene 373±3, 3-methylthiophene 373±3, 3-methylpyridine 371±3, 2-methylpyridine 370±3, and 4-methylpyridine 368±3. The relative acidities of the hydrogens in the other sites in these molecules have been estimated by hydrogen-deuterium exchange studies and byother chemical methods. Anions (M- 1) from these molecules have been allowed to react with N20, 02, COS, and CS2 and the ionic products determined. The gas-phase acidities have been compared with those calculated by the semiempirical methods AMI and MNDO.In solution many five-and six-membered heteroaromatic compounds are sufficiently acidic that their conjugate bases can readily be formed by hydrogen abstraction re-actions. 1 For example, 2-lithiofuran results from reaction of furan with n-butyllithium2 and lithiopyridines from pyridine and n-butyllithium-potassium tert-butoxide. 3 In each case, the resulting carbanion can be put to synthetic use to form substituted heteroaromatics in good to ex-cellent yield. Both synthetic and mechanistic investiga-tions have shown that the particular isomeric form of the ion produced by hydrogen abstraction is a function of the experimental conditions. For example, Verbeek and Brandsma3 found that the relative amounts of 2-, 3-, and 4-lithiopyridine formed “depends strongly upon the length of the time between addition of pyridine to the base and addition of the (quenching) reagent”, and Zoltewicz, Grahe, and Smith4 showed that the relative rates of H, D exchange among the 2-, 3-, and 4-positions of pyridine changes in going from a solution of CH3OD-CH3ONa to ND3-NaND2. Indeed, it appears that for pyridinein solution, hydrogens at the 2, 6-positions are kinetically the most reactive, whereas the 4-hydrogen is the most acidic thermodynam-ically. 3 From these and other data it appears likely that, at least in solution, the kinetic acidity and the thermodynamic acidity are not the same in many of these com-pounds.