The First Solid Enols of Anhydrides. Structure, Properties, and Enol/Anhydride Equilibria1

详细信息    查看全文

• 作者：Jinhua Song ; Yi Xiong Lei ; Zvi Rappoport
• 刊名：Journal of Organic Chemistry
• 出版年：2007
• 出版时间：November 23, 2007
• 年：2007
• 卷：72
• 期：24
• 页码：9152 - 9162
• 全文大小：209K
• 年卷期：v.72,no.24(November 23, 2007)
• ISSN：1520-6904

文摘

Reaction of -methylglutaconic anhydride with NaOMe followed by reaction with methyl or phenylchloroformate gave the corresponding O-methoxy (and O-phenoxy) carbonylation derivatives. Reactionof the anhydride with MgCl₂/pyridine, followed by methyl chloroformate gave C-methoxycarbonylationat C3 of the anhydride. The product (4) was previously suggested by calculation to be the enol of theanhydride 5 and this is confirmed by X-ray crystallography (bond lengths: C-OH, 1.297 Å; C1C21.388 Å; HO···O=C(OMe) distance 2.479 Å) making it the first solid enol of an anhydride. In CDCl₃,CD₃CN, or C₆D₆ solution it displays the OH as a broad signal at ca. 15 ppm, suggesting a hydrogenbond with the CO₂Me group. NICS calculations indicate that 4 is nonaromatic. With D₂O in CDCl₃ boththe OH and the C5H protons exchange rapidly the H for D. An isomeric anhydride 5a of 5 is formed inequilibrium with 4 in polar solvents. In solution, anhydride(s)/enol equilibria are rapidly established withK_enol of 6.40 (C₆D₆, 298 K), 0.52 (CD₃CN, 298 K), 9.8 (CDCl₃, 298 K), 22.8 (CDCl₃, 240 K), anddecreasing K_enol in CDCl₃:CD₃CN mixtures with the increase in percent of CD₃CN. The percentage ofthe rearranged anhydride in CDCl₃:(CD₃)₂CO increases with the increased percent of (CD₃)₂CO. In DMSO-d₆ and DMF-d₇ the observed species are mainly the conjugated base 4^- and 5a. Deuterium effects on the(¹³C) values were determined. An analogous C2-OH enol of anhydride 15 substituted by 3-CO₂Me and4-OCO₂Me groups was prepared. Its structure was confirmed by X-ray crystallography (CO bond length1.298 Å, O···O distance 2.513 Å); (OH) = 12.04-13.22 ppm in CDCl₃, THF-d₈, and CD₃CN, andK_enol = 100, 7.7, and 3.4 respectively. In DMSO-d₆ enol 15 ionizes to its conjugate base. Substantialupfield shifts of the apparent ("OH") proton on diluting the enol solutions are ascribed to the interactionof the H⁺ formed with the traces of water in the solvent to give H₃O⁺, which gives the alleged "OHproton" signal.