Substituent and temperature controlled tautomerism of 2-phenacylpyridine: the hydrogen bond as a configurational lock of (Z )-2-(2-hydroxy-2-phenylvinyl)pyridine

Abstract

2-Phenacylpyridines substituted in the benzene ring are in equilibrium with (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines when dissolved in chloroform. The substituent affects significantly the tautomeric equilibrium [the amount of the enolimine form stabilized by the intramolecular hydrogen bond is 1 and 92% for R = p-N(CH 2 ) 4 and p-NO 2 , respectively]. The negative logarithm of the tautomeric equilibrium constant, K T , is linearly dependent on the Hammett σ substituent constants. The dependence of K T vs. temperature is exponential in character: the more electronwithdrawing is the substituent, the more distinct is the influence of temperature. Unexpectedly, the tautomer present in the crystalline state is not the same for all compounds studied (it is the ketimine one for those carrying strong electron-donor groups). Among the different ab initio methods used to calculate the enthalpy of the proton transfer in chloroform solution, MP2/6-31G** gives the best results.

A convenient access to 1-substituted-2-azinyl-1-ethanones via acylation of alkylated azines with N-acylbenzotriazoles

“…Although 1 H NMR spectra showed a tautomeric mixture of about 1:1 for 10k, 10b was observed exclusively in the K form. This finding is in accord with the facts that hydrogenbonding reinforces tautomeric effects 31 and tautomer ratio is crucially dependent on both steric and polar effects of the substituents attached to the carbonyl group 30 (Scheme 3). However, the condensation products 10l−p derived from 4-benzylpyridine and 4-methylquinoline exist predominantly in the ketonic form.…”

Identity Double‐Proton Transfer in (3Z)‐3‐Hydroxy‐1,4‐di(quinolin‐2‐yl)but‐3‐en‐2‐one

“…Table 3. RHF/6-31G** optimized dihedral angles [8] in different tautomers 2 (initial angles in parentheses).…”

“…[11] Indeed, it is intermediate between the 15 N chemical shifts seen in the NMR spectra of enaminones [11] and enolimines. [8] The experimental 13 C NMR chemical shifts of 2 are also averaged typical signal positions of the enaminone [11] and enolimine forms. [8] This shows that, in solution, a fast proton exchange may take place between two tautomers that contain the O and E moieties, for example, 2 OK > 2 EO or 2 OE > 2 EE.…”

“…[8] The experimental 13 C NMR chemical shifts of 2 are also averaged typical signal positions of the enaminone [11] and enolimine forms. [8] This shows that, in solution, a fast proton exchange may take place between two tautomers that contain the O and E moieties, for example, 2 OK > 2 EO or 2 OE > 2 EE. Double-proton transfer is required to transform 2 OO and 2 EE into each other.…”

“…[8] GIAO-RHF/DFT calculations afford additional support, especially if one is going to see which conformer predominates in the tautomeric mixture. [41] Such data for some tautomers 2 are shown in Table 1.…”