Use of Furan Derivatives Acting as Electrophilic Dienophiles: An Experimental and Theoretical Analysis of Polar Cycloaddition Reactions

Abstract

Furans have, fundamentally, biological and chemical uses. For several years, we have been working with substituted furans as eletrophilic dienophiles joint to different nucleophilic dienes in Polar Diels-Alder Reactions (P-DA). The principal objective of this analysis was the preparation of benzofurans and dibenzofurans. To develop these reactions, we used conventional thermal conditions and microwave irradiation. The solvents employed were organic ones and protic ionic liquids (PILs). Also, we worked in free solvent conditions. We demonstrated that substituted furans result good electrophiles in cycloaddition processes. The best experimental condition was the combination of microwave irradiation in presence of PILs like ethylamonium nitrate (NEA) and N-methylimidazolium tetrafluoroborate ([HMIM][BF4]). When the nitro group is used as substituent in the electrophile, the process is irreversible due to the loss of nitrous acid. Then this substituent is convenient for this transformation. Moreover, a computational theoretical study of furan reactivity as dienophile in Polar Diels-Alder reactions (P-DA) was performed. For this purpose, the Density Functional Theory (DFT) method was employed. The principal aim of this theoretical study was to analyze the reactivity, regioselectivity, solvent effect (organics and ionic liquids) and reaction mechanisms of these reactions. Gaussian 09 is the software used to develop the theoretical calculations. The functional applied was B3LYP and the basis set 6-31G(d). The structures of reactants, products and transition states were optimized and validated through the calculation of the vibrational frequencies. For the analysis of reactivity and regioselectivity, the nucleophilic and electrophilic global and local indexes were used, respectively. The solvent effect was considered employing two different solvatation models: the Polarizable Continuum Model (PCM) and the Supermolecular Approach. For the mechanistic analysis, the stationary points were located in the Potential Energy Surface (PES) and then optimized and validated. The activation energy values and reactions paths were analyzed for each reaction.