Computational note on the tautomeric preferences of 2,7-di(pyridin-2-yl)-and 2,7-di(quinolin-2-yl)hexahydronaphthalene-1,8-diones

Abstract

2011 Elsevier B.V. All rights reserved. Although simple b-diketones equilibriate only with their ketoenol tautomers, instable pyridin-2-yl-b-diketones may compete with more than three different prototropic forms [1,2]. There are many interesting compounds carrying the methylcarbonylpyridin-2-yl moiety in their molecules. In this note we report the results of quantum-chemical calculations concerning of 2,7di(pyridin-2-yl)hexahydronaphthalene-1,8-dione (1PKKP, Scheme 1) and 2,7-di(quinolin-2-yl)hexahydronaphthalene-1,8-dione (2PKKP). Multiple basic (nitrogen and oxygen atoms) and acidic (methylene hydrogen atoms) centers present in these molecules enable these compounds to exist in 12 tautomeric forms (Scheme 1). Full geometry optimizations for all studied systems have been carried out using Gaussian software package [3]. Computations for the isolated molecules were performed using B3LYP functional and 6-31+G(d,p) basic set. The frequencies were calculated at the same level (no imaginary frequencies). The DFT calculated energies (Table 1) show that the most stable tautomers of 1 are the enaminone 1EKOP2 and dienol 1POOP2. It was found that the benzene ring in derivates 2 completely changes tautomeric preferences (2EKKE is the most stable tautomer). Comparison with 1,5-bis(pyridin-2-yl)and 1,5-bis(quinolin-2yl)pentane-2,4-diones (1POOP1, 1EOKP and 2EKKE, 2EKOP2, respectively were their most stable tautomers [1]) shows that although stiffening of the molecule affects the studied tautomeric preferences, the same forms are still among the most stable. The studied tautomers can be stabilized by three different hydrogen bonds, i.e. by N–H O, O–H N and O–H O. The HOMA (Harmonic Oscillator Model of Aromaticity) [4] values (Table S1 in Supplementary Materials) show that N–H O interaction in 1EKOP2 and 2EKKE is of RAHB (Resonance Assisted Hydrogen Bond) type [5–8]. The Gaussian procedure of geometry optimization precludes calculations to be done for such labile forms as 1EOKP, 2EOKP and 2EOOP. Spontaneous proton transfer in these extremely unstable species transforms them automatically into more stable systems. Thus, 1EOKP was converted to 1EKOP2, 2EOKP to 2EKOP2 and 2EOOP to 2EOKE.