The Diels-Alder reaction of para-benzoquinones under Lewis acid catalysis conditions and microwaves the effect

Abstract

In this work it was done the studies of the Diels-Alder reactions of the parabenzoquinones 2,5-dimethyl-para-benzoquinone (A1), 2,6-dimethyl-parabenzoquinone (A2) and thymoquinone (A3) with the dienes cyclopentadiene (B1), 2,3- dimethyl-1,3-butadiene (B2), isoprene (B3), trans-piperylene (B4) and 1- vinylcyclohexene (B5) (Figure I) under catalysis conditions employing as catalyst a mixture of Aerosil® silica and iron chloride (III). The amount of silica used was equimolar with respect to para-benzoquinones, and the iron chloride was employed in catalytic amounts of 4 mol% and 10 mol%. Under these catalysis conditions it was observed the reduction of the reaction times and a significant increase in yield for all the Diels-Alder reactions studied when compared to results obtained when the same reactions were carried out at room temperature and absence of catalysis. In the case of the Diels-Alder reaction of 2,6-dimethyl-para-benzoquinone (A2) with unsymmetrical dienes isoprene (B3), trans-piperylene (B4) and 1-vinylcyclohexene (B5), under catalytic conditions it was observed the reversal of the regioselectivity of these reactions when compared to the regioselectivities obtained for these reactions performed in the absence of catalysis... (continue)Financiadora de Estudos e ProjetosNo presente trabalho foram feitos estudos das reações de Diels-Alder 2,5- dimetil-para-benzoquinona (A1), 2,6-dimetil-para-benzoquinona (A2) e timoquinona (A3) com o ciclopentadieno (B1), 2,3-dimetil-1,3-butadieno (B2), isopreno (B3), transpiperileno (B4) e 1-vinilcicloexeno (B5) (Figura I) nas condições de catálise com a mistura de sílica aerosil® (SiO2 Aerosil®) e cloreto de ferro (III) (FeCl3) catalítico (4 mol% e 10 mol%). Foi observada a redução dos tempos reacionais e o aumento significativo dos rendimentos de todas as reações, quando comparados aos resultados obtidos para estas reações realizadas em temperatura ambiente e ausência de catálise em diclorometano (CH2Cl2). No caso das reações de Diels-Alder da 2,6-dimetil-para-benzoquinona (A2) com os dienos não simétricos B3, B4 e B5 verificou-se que houve a inversão da regiosseletividade destas reações quando comparadas as regiosseletividades obtidas para estas reações na ausência de catálise... (continua