Estudo sobre o uso do NbCl5 como ácido de Lewis em reações de Diels-Alder no anel furano

• Autor(a) principal: Santos, Deborah Araujo dos
• Data de Publicação: 2013
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: Repositório Institucional da Universidade Federal do Espírito Santo (riUfes)
• Texto Completo: http://repositorio.ufes.br/handle/10/4697
• topic: Niobium penthacloride; Furan; Pentacloreto de nióbio; Reação de Diels-Alder; Furanos; Química; 54

The present study aims at evaluating the use of niobium pentachloride (NbCl5) as catalyst in the Diels-Alder reactions between furan ring and some reactive dienophiles. The influence of reaction parameters such as temperature, time and ratio of catalyst were varied in order to find the best reaction conditions for each diene/dienophile pair.Niobium compounds present Lewis acidity and it wasverified by several studies. In particular, NbCl5hasshown good activity as catalyst in many organic reactions. Since its discovery in 1928, the [4+2] cycloaddition, or the Diels-Alder reaction is widely studied and used to construct six-membered rings. The great interest in these reactions is due to the possibility of obtaining molecules with high structural complexity in only one reaction step. The Diels-Alder reaction of furan ring asdiene provides the 7-oxabicyclo[2.2.1]hept-5-ene with an intermediate for thesynthesis of variousnatural products, such as derivate of shikimic acid, terpenes and prostaglandins. An issue associatedwith these reactions is thelow reactivityof furan, thereby a variety of studies have proposed the use of Lewis acids catalyst or high pressures in order to increase reaction yieldand selectivity.O+EOEH7-oxabicyclo[2.2.1]hept-5-eneNbCl5In reactions in which furan was used asdiene and the methyl acrylate was thedienophile, theyields were betterthan those described in the literature, especially whenlow temperatureswere studied.The NbCl5was not active in any reactionalconditionsusing dimethyl maleate. In the reactions in which acrylonitrile participated as dienophile,yields were low andthe formation of byproductswas observed, due to polymerization. The reaction with 2-chloro-acrylonitrile also showed lowyieldsbutwith inversion of selectivity compared to that observed in the other adducts. Based ontheoretical studiesreported in the literature, there was an attempt to perform a hetero Diels-Alder reaction with acetaldehyde as heterodienophile.However,there wasevidencesofaproduct resulted from an aromatic substitution. When the 2-methyl-furan was used asdiene,reactions showed onlyby-products, withno adduct formation.