The 2-Oxocyclobutanecarboxylic Acid Keto−Enol System in Aqueous Solution: A Remarkable Acid-Strengthening Effect of the Cyclobutane Ring
作者:J. A. Chang、Y. Chiang、J. R. Keeffe、A. J. Kresge、V. A. Nikolaev、V. V. Popik
DOI:10.1021/jo060253w
日期:2006.6.1
carbocyclic ring. The acidity constant of the enol group of 2-oxocyclobutanecarboxylate ion, on the other hand, is greater, by 4 orders of magnitude, than that of the corresponding enol in the cyclopentyl system. This remarkable increase in acidity with diminishing ring size is consistent with the enhanced s character of the orbitals used to make the exocyclic bonds of the smaller cyclobutane ring.
水溶液中2-重氮环戊烷-1,3-二酮的快速光解产生了2-氧代环丁烯酮,将其水合成2-氧代环丁烷羧酸的烯醇。然后将烯醇异构化成该酸的酮形式。在酸,碱,测定乙烯酮和烯醇的反应速率,并且在整个酸度范围的缓冲溶液[H + ] = 10 - 1 -10 - 13M和对这些数据的分析,以及通过溴清除确定的2-氧代环丁烷羧酸的酮形式的烯醇化速率,给出了酮-烯醇平衡常数以及酮和烯醇形式的酸度常数。事实证明,酮-烯醇平衡常数比先前报道的下一个更高的同系物2-氧代环戊烷羧酸的常数小2个数量级,反映出难以将碳-碳双键插入小的应变碳环中的困难。另一方面,2-氧代环丁烷羧酸根离子的烯醇基的酸度常数比环戊基体系中相应的烯醇的酸度常数大4个数量级。