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

Abstract

Reaction of beta-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 A; C1C2 1.388 A; HO...O=C(OMe) distance 2.479 A) 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 delta(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 A, O...O distance 2.513 A); delta(OH) = 12.04-13.22 ppm in CDCl3, THF-d8, and CD3CN, and Kenol = > or = 100, 7.7, and 3.4 respectively. In DMSO-d6 enol 15 ionizes to its conjugate base. Substantial upfield shifts of the apparent delta("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.

The Photoelectric Response of Gold‐Nanoparticle Monolayers

“…In DMSO‐d 6 , 13a shows only a negligible concentration effect on the OH chemical shift (Table 2). Based on the behavior of other systems27, 29, 41 where OH ionization followed by interaction of the formed proton with H 2 O, is reflected by large upfield chemical shifts, this is interpreted as evidence that 13a does not significantly ionize. Moreover, it strengthens the absence of exchange of the OH with traces of water in the DMSO‐d 6 .…”

Enols of 2‐nitro‐ and related 2‐substituted malonamides

“…In DMSO-d 6 , 13a shows only a negligible concentration effect on the OH chemical shift ( Table 2). Based on the behavior of other systems [27,29,41] where OH ionization followed by interaction of the formed proton with H 2 O, is reflected by large upfield chemical shifts, this is interpreted as evidence that 13a does not significantly ionize. Moreover, it strengthens the absence of exchange of the OH…”

Synthesis and Crystal Structures of Methyl 3-(Benzoylamino)-6-methyl-2-oxo-2H-pyran-5-carboxylate and N-[5-(3,4-Dimethoxyphenyl)-6-methyl-2-oxo-2H-pyran-3-yl]benzamide

“…Such small deviations of carboxylic group (either as an acid or ester functionality) from the planarity of 2H-pyran-2-one [28][29][30][31][32][33][34] or benzene ring [35][36][37][38][39][40][41][42][43] are often observed. When a bulky substituent is present in the ortho position to the carboxylic group much larger twist angles ranging from *20°up to *80°are observed in the cases of 2H-pyran-2-one [44][45][46][47][48] or benzene ring [49][50][51][52][53][54][55][56][57][58].…”