Aspects of tautomerism. Part V. Solvent, substituent, and steric effects on the ring–chain tautomerism of o-benzoylbenzamides

Abstract

Ring-chain tautomeric equilibria of o-benzoylbenzamides in 95% ethanol, chloroform, dioxan, and acetonitrile have been estimated using U.V. spectroscopy. Unlike the case of acids, solvent polarity has only a small effect. In ethanol the cyclic form is favoured. Electron-withdrawing groups in the amide-bearing ring disfavour the cyclic form. Substitution of methyl, ethyl, and phenyl groups on the nitrogen atom of the amide function results in increase of the proportion of the cyclic form in the first two cases and decrease in the last.GRAEBE and ULLNIAN treated the acid chloride of o-benzoylbenzoic acid (I) with ammonia and obtained an amide which was designated as (11). The i.r. spectrum of the amide in chloroform revealed that it exists in the cyclic form (III).2 We have found that it exists in the open form in the solid state, but is completely in the cyclic form in s ~l u t i o n . ~ Reaction of (I) with aniline resulted in the formation of two anilides [(A), m.p. 221" (75%) and (B), m.p. 195" (12.5%)]. Compound (A) was originally assigned structures (IV) or (V) and (B) structure (VI).4 On the basis of i.r. spectra (A) and (B) have now been assigned structures (VI) and (VII) respectively.6

Intramolecular Reversible Addition Reactions to the C=O Group

“…One may suppose that the ring form is stabilized by intermolecular hydrogen bonds ROH···N (imidazole) thus increasing the electron withdrawing effect of the imidazole group. An analogous solvent shift of the equilibrium in ethanol is observed for N -unsubstituted, Nmethyl, N -ethyl, and N -phenyl-2-( 4-dimethylaminobenzoyl)benzamides [191].…”

“…In protic solvents the interconversions of the keto ami des usually take place at a measurable rate. Equilibria were obtained in methanol-d4 [190] (for rate constants see Table 13) and ethanol [191]. The reaction takes place more rapidly in aqueous pyridine and a considerable acceleration may be achieved by adding sodium bicarbonate to the solution [192] (for rate constants see Table 10).…”

“…Ring-chain equilibria are described for N -unsubstituted, N -methyl, N -ethyl, and N -phenyl-2-( 4-dimethylaminobenzoyl)benzami des [191], and also for N-(t-alkyl)-2-(2-imidazolylcarbonyl)benzamides [194] (see Table 14). …”

“…The hydrogen bonded C=O group bands of crystalline 2benzoylbenzamides, strictly established as cyclic [33,80,146,170,205], have sometimes been erroneously attributed to the overlapping bands (IJc=o diarylketone + amide-I) of open isomers (3A). This led to an incorrect conclusion [151,191,206] about the structure of these ami des in solid state, the incorrectness of which is confirmed [207] by the absence of an amide-II band and the presence of a broad OH band in the spectra. For this reason the IR spectra of 2-phenylacetylbenzamides [208][209][210] and 2-a-(N-alkylamino )phenylacetylbenzamides [208] were interpreted erroneously as indicating an open structure.…”

Pyridazine, XXI. Synthese, Struktur und Reaktivität funktioneller Derivate von 5‐Aroyl‐4‐pyridazincarbonsäuren

“…-'H-NMR:6 = 3.7O(s,3H,CH3),7.5-7.9(m,5H,Phenyl-H), 9.60,9.75 (AB-System, J = 1.5 Hz; 2H, 3-H, 6-H). B) 2.28 g (10 mmol) l a werden in 100 ml trockenem Dichlormethan suspendiert und unter Ruhren rnit 2.10 g (13 Ethyl-5-benzoyl-4-pyridazincarboxylat (3): 2.28 g (10 mmol) 1 a werden wie bei der Darstellung von 2 (Methode B) beschrieben rnit 2.10 g (13 mmol) l,l'-Carb~nyldiimidazol und 10 ml itbsol. Ethanol, das 0.2 mmol Natriurn-ethanolat enthalt, umgesetzt.…”