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

Reaction of β-methylglutaconic anhydride with NaOMe followed by reaction with methyl or phenyl chloroformate gave the corresponding O-methoxy (and O-phenoxy) carbonylation derivatives. Reaction of the anhydride with MgCl2/pyridine, followed by methyl chloroformate gave C-methoxycarbonylation at C3 of the anhydride. The product (4) was previously suggested by calculation to be the enol of the anhydride 5 and this is confirmed by X-ray crystallography (bond lengths: C−OH, 1.297 Å; C1C2 1.388 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 hydrogen bond with the CO2Me group. NICS calculations indicate that 4 is nonaromatic. With D2O in CDCl3 both the OH and the C5H protons exchange rapidly the H for D. An isomeric anhydride 5a of 5 is formed in equilibrium with 4 in polar solvents. In solution, anhydride(s)/enol equilibria are rapidly established with Kenol of 6.40 (C6D6, 298 K), 0.52 (CD3CN, 298 K), 9.8 (CDCl3, 298 K), 22.8 (CDCl3, 240 K), and decreasing Kenol in CDCl3:CD3CN mixtures with the increase in percent of CD3CN. The percentage of the 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 δ(13C) values were determined. An analogous C2-OH enol of anhydride 15 substituted by 3-CO2Me and 4-OCO2Me groups was prepared. Its structure was confirmed by X-ray crystallography (CO bond length 1.298 Å, O···O distance 2.513 Å); δ(OH) = 12.04−13.22 ppm in CDCl3, THF-d8, and CD3CN, and Kenol = ≥100, 7.7, and 3.4 respectively. In DMSO-d6 enol 15 ionizes to its conjugate base. Substantial upfield shifts of the apparent δ(“OH”) proton on diluting the enol solutions are ascribed to the interaction of the H+ formed with the traces of water in the solvent to give H3O+, which gives the alleged “OH proton” signal.