The 1,3-dipolar cycloaddition reaction is a powerful tool for the cycloaddition of nitrile oxides to olefins, and this reaction is of considerable interest to obtain isoxazolines. Density functional theory (DFT) was used to study the 1,3-dipolar cycloaddition reaction mechanism that initially occurs between benzonitrile oxide and vinylacetic acid to yield a bicyclo, from successive cycloadditions. PBE1PBE, B3LYP and CAM-B3LYP functionals were used together with 6-311+G(2d,p) basis set. CCSD(T)/6-311+G(2d,p) calculations were done to compare the DFT energy barriers. The solvent effects were included using polarizable continuum model (PCM), with three different solvents. In the first cycloaddition, only the 3,5-regioisomer is expected. In the gas phase, the β face attack, that originates the trans-bicyclo, is slightly favored, but the cis-bicyclo is considerably more stable. However, the α face attack was favored with solvent effects. The PBE1PBE functional gives the closest activation energies and reaction energies to CCSD(T). The inclusion of solvent effects changes the preferential rotamer in each cycloaddition.

Uma série de calcogeno ésteres, 1a, 6a e 6b foi obtida em uma rota sintética conveniente. A análise de microscopia óptica de luz polarizada (POM) e calorimetria diferencial de varredura (DSC) mostrou que estes calcogeno ésteres apresentam uma ampla faixa de mesofase nemática. Além disso, espectros de absorção na região do UV-Vis em solução (ca.10-5 mol L-1) também foram obtidos de maneira a se obter a diferença de energia entre os orbitais moleculares HOMO-LUMO, para posterior comparação com os resultados teóricos. Os valores calculados para o gap de HOMO-LUMO estão de acordo com os valores experimentais obtidos, onde a eletronegatividade do heteroátomo (O, S e Se) deve ser o principal fator para a localização do máximo de absorção. Um estudo teórico foi realizado utilizando cálculos de DFT/B3LYP com método híbrido de Becke aplicando uma correlação funcional de Lee-Yang-Parr (B3LYP) com a base DGDZVP a fim de avaliar algumas propriedades físico-químicas relacionadas com o mesomorfismo e as propriedades eletrônicas espectrais. Através da síntese destes compostos, a influência do átomo de calcogênio nas propriedades fotofísicas e mesomórficas pôde ser avaliada de uma forma mais precisa.

A series of chalcogeno esters 1a, 6a and 6b have been synthesized in a straightforward manner. Polarized-light optical microscopy (POM) and differential scanning calorimetry (DSC) showed that these chalcogeno esters displayed a wide nematic range of mesophase. In addition, UV-Vis in solution (ca.10-5 mol L-1) was applied in order to obtain the HOMO-LUMO gap for further comparison with the theoretical results. The calculated values to the HOMO-LUMO gap are in excellent agreement with the experimental results, where the electronegativity of the heteroatom (O, S and Se) seems to play a fundamental role in the absorption maxima location. A theoretical study was performed using a DFT/B3LYP calculations with Becke's three-parameter hybrid method applying the Lee-Yang-Parr functional correlation (B3LYP) with DGDZVP basis set in order to evaluate some physical chemistry data related with mesomorphic and spectral eletronic properties. By synthesizing these compounds we were able to get a more precise description about the influence of the chalcogen atom on the mesomorphic and photophysical properties.

A síntese de uma plataforma molecular para materiais líquido-cristalinos derivados do ácido 3-arilisoxazolil-5-carboxílico (1) e do 5-(hidroximetil)-3-aril-2-isoxazolina (2) é descrita. Os intermediários 1 e 2 são obtidos através da reação de cicloadição [3+2] 1,3-dipolar entre arilóxidos de nitrilas e os dipolarófilos ácido acrílico e álcool alílico, respectivamente. Os compostos cristais líquidos são sintetizados através de uma estratégia de alongamento molecular do núcleo primitivo isoxazolínico pela conexão de subunidades arilacetilênicas, as quais foram obtidas da reação de Sonogashira. Sob essas condições, séries de cristais líquidos 5a-c, 6, 7a-g e 8a-d têm sido sintetizadas com rendimentos de médios para bons. Os compostos finais apresentam propriedades líquido-cristalinas nematogênica e esmectogênica. Um cálculo DFT no nível B3LYP/6-31G(d,p) é também apresentado e alguns parâmetros estruturais obtidos são analisados.

The synthesis of the molecular platform for liquid-crystalline materials based on 3-arylisoxazolyl-5-carboxylic acid (1) and 5-(hydroxymethyl)-3-aryl-2-isoxazoline (2) is described. The key intermediates 1 and 2 are obtained by [3+2] 1,3-dipolar cycloaddition reaction between an arylnitrile oxide and an acrylic acid and allylic alcohol as the dipolarophile. The liquid crystals (LC) compounds are synthesized through a "molecular elongation strategy" from the initial isoxazolinic core by connecting the arylacetylene moiety obtained from the Sonogashira reaction. Under these conditions, the series of liquid crystals 5a-c, 6, 7a-g and 8a-d have been successfully synthesized in fair to good yields. The final compounds display nematic and smectic liquid-crystalline properties. The structural properties of the series of the liquid crystals has been studied using DFT methods at level B3LYP/6-31G(d,p). The equilibrium geometries in the gas phase are presented and analyzed.