A SPECTACULAR EXAMPLE OF THE IMPORTANCE OF ROTATIONAL BARRIERS - THE IONIZATION OF MELDRUMS ACID
A SPECTACULAR EXAMPLE OF THE IMPORTANCE OF ROTATIONAL BARRIERS - THE IONIZATION OF MELDRUMS ACID
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DOI：
10.1021/ja00237a028
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发表时间：
1987-02-04
影响因子：
15
通讯作者：
HARRELSON, JA
中科院分区：
文献类型：
--
作者：
ARNETT, EM;HARRELSON, JA
Meldrum酸（Ⅰ）的电离自由能比它的无环类似物丙二酸二甲酯的电离自由能大11.68千卡/摩尔。这种能量差一定是有机化学文献中最大的立体电子效应之一，需要仔细解释。本文通过比较不同结构对Meldrum酸的碳酸性的影响来探讨这个问题。最显著的效果来自于增加双内酯环系统的尺寸。6元环和10元环之间的酸度迅速降低，直到13元环具有与丙二酸二甲酯相同的pKa。从组合结果中得出的明显结论是，Meldrum酸与较大环双内酯或二酯的酸性之间的差异在于围绕酯键旋转的障碍。超过一半的11.68-kcal/mol效应可以通过酯的Z构象相对于E构象的3-4-kcal/mol稳定化来经验性地解释，这也见于β-二酮系列。
The free energy of ionization of Meldrum’s acid (I) is 11.68 kcal/mol more spontaneous than that of its acyclic analogue, dimethyl malonate. This energy difference must represent one of the largest stereoelectronic effects in the literature of organic chemistry and demands careful elucidation. This paper probes the problem by comparing the effects of various structural contributions to the carbon acidity of Meldrum’s acid. The most dramatic effect results from increasing the size of the bislactone ring system. There is a rapid decrease in acidity between the 6-membered and 10-membered ring until the 13-membered ring has the same pKa as dimethyl malonate. Theobvious conclusion from the assembled results is that the difference between the acidities of Meldrum’s acid and the larger ring bislactones or diesters lies in the barrier to rotation around the ester bonds. Over half of the 11.68-kcal/mol effect may be explained empirically through the 3-4-kcal/mol stabilization of the Z conformation of esters relative to the E conformation, seen also in the/3-diketone series.