Heterocíclicos Mesoiônicos: estudo teórico da estabilidade dos derivados dos oxazóis (C3 H 2 NO-RY) e Tiazois (C3 H 2 NS-RY), com Y = O ou S e R = H, CH3, NH2 ou CN

Abstract

Mesoionics are five or six-membered heterocyclic structures with a wide variety of applications, such as synthesis of other heterocycles, transition metal complexes and biological activity and non-linear optics. This class of molecules has as characteristic a high charges separation, which gives them a high dipole moment. The present work aims to elucidate the structural stability of the azoles heterocyclic (1,3-oxazol-5-one, 1,3-oxazol-5- thione, 1,3-thiazol-5-one and 1,3-thiazol-5-thione) and their derivatives (R = CH3, NH2 e CN). DFT, MP2, and CCSD calculations were performed with the Gaussian 09 program, as well as CASSCF and MR-CISD calculations in the Columbus 7.0 program, in all cases with the augcc-pVDZ basis set. The structural stability of the mesoionic compounds was investigated from the identification of stationary points (reagent, transition state and product) relative to the ring opening process, simulating the breaking of the bond between a carbon and a heteroatom endocyclic (C-O or C-S). The stationary points were optimized and characterized by the calculation of the vibrational frequencies of the normal modes with DFT, MP2, CCSD, and MR-CISD methods, in the basis aug-cc-pVDZ. The CCSD, MRCISD and MR-CISD+Q results indicate that the sulfur structures in the endocyclic ring (1,3-thiazol-5-one and 1,3- thiazol-5-thione) are more stable than their acyclic tautomers, with oxygen endocyclics, indicate that the acyclic structures are more stable. The CCSD results confirm the multiconfigurational character of the structures (T0 > 0.02), which justifies the discrepancy between mono (DFT and MP2) and multideterminantal (MR-CISD and MR-CISD+Q) results. The MR-CISD+Q/aug-cc-pVDZ results, of great reliability due to the greater recovery of electronic correlation, predicts that the substitution of a hydrogen by a methyl (electron donor group) in the 1,3-oxazol-5-one structure slightly increases the activation energy of the ring opening process, especially in positions three and four. The MCSCF method does not correctly describe the structural and electronic parameters of the investigated systems. The characterization of the stationary points of the mesoionic structures with the MR-CISD method indicates that (i) endocyclic sulfur is much more effective in ring stabilization when compared to endocyclic oxygen and methyl substituent, and (ii) the cyclization process of these systems can occur spontaneously without kinetic impediment. A comparative study of functionals was carried out, aiming to reproduce the results obtained with ab initio methods for further investigation of the effect of the substituents. The benchmarking functionals indicate that the amine and cyano substituents on oxazoles have a more pronounced effect when they are linked at position four, increasing the stability of the heterocyclic in the isomerization process. The investigation of a possible relationship between the instability of oxazole systems with a diradical character was also carried out using the ab initio mono and multi-reference methods, in the gas phase and DMSO, considering three basis sets (6-311+G*, aug-cc-pVDZ, and aug-cc-pVTZ) and three methodologies (HOMO-LUMO gap, singlettriplet energy difference, and diradical character y0). The results indicate that the diradical character is inferior, in all cases, to 2.5%. Calculations of NBO charge and charge density made it possible to show that the investigated systems have the expected charge separation for mesoionic systems and suggest that the lower stability of oxazole systems may be related to a purely electrostatic effect.