ESTUDO CINÉTICO DE PROCESSOS DE TRANSIÇÃO DE FASE. INFLUÊNCIA DA SIMETRIA, FLEXIBILIDADE E ROTAÇÃO INTERNA MOLECULARES

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Chromone derivatives as novel dienes in Diels-Alder reactions

Publication . Albuquerque, Hélio; Santos, Clementina M.M.; Lima, C.F.R.A.C.; Santos, L.M.N.B.F.; Cavaleiro, José; Silva, Artur

Chromones are a group of heterocyclic compounds widespread in Nature and an interesting scaffold because they have been involved in a range of chemical transformations [1]. Following our previous studies on Diels‒Alder (DA) reactions of chromone derivatives [2], novel 2-[(1E,3E)-4-arylbuta-1,3-dien- 1-yl]-4H-chromen-4-ones 1 were synthesized to further study the reactivity of their two diene systems (α,β:γ,δ versus 2,3:α,β) in DA reactions (Fig.1). A series of dienophiles were reacted with chromones 1 under microwave-assisted conditions. The best yields of adducts 2 were achieved using N-methylmaleimide as dienophile and Sc(OTf)3 as Lewis acid in solvent-free conditions. The crucial role of Sc(OTf)3 relies on the chelation with carbonyl group of adducts 2 avoiding both the occurrence of a second DA reaction and the formation of bisadducts 6. The α,β:γ,δ-diunsaturated system of chromones 1 showed to be the most reactive diene for maleimides, dicarboxylates and azo-dicarboxylates, affording adducts 2, 4 and 5, respectively. Adducts 2 were dehydrogenated to the corresponding flavone derivatives 3 using DDQ as oxidizing agent (Fig.1). Computational calculations confirmed the experimental findings on DA reactions and helped to explain the unexpected lack of reactivity of some dienophiles. All the experimental procedures and extensive 1D and 2D NMR characterization of the final adducts and intermediates will be presented and discussed.

2016Conference object

Open access

2-[(1E,3E)-4-Arylbuta-1,3-dien-1-yl]-4H-chromen-4-ones as dienes in Diels–Alder reactions: experimental and computational studies

Publication . Albuquerque, Hélio; Santos, Clementina M.M.; Lima, Carlos F.R.A.C.; Santos, Luís M.N.B.F.; Cavaleiro, José; Silva, Artur

The synthesis and reactivity of 2-[(1 £,3£)-4-arylbuta-1 ,3-dien-l-yl]-4H-chromen-4-ones 1 as dienes in Diets-Alder (DA) reactions towards several electron-poor and electron-rich dienophiles under microwave irradiation was studied. The optimized reaction conditions were achieved by using N-methylmaleimide2 as dienophile and Sc(0Tf)3 as Lewis acid under microwave-assisted and solvent-free conditions. The Lewis acid improved the reaction yields since it prevented the adducts obtained to undergo a second DA reaction, thus avoiding formation of a bisadduct. The a,~:y ,o-diene of the starting chromones showed to be the most reactive and the computational results confirmed the experimental findings. Theoretical calculations also allowed explaining some unexpected lack of reactivity by some dienophiles. The adducts prepared were dehydrogenated by using DDQ, however, the aza-adducts showed to be sensitive to the high energetic reaction conditions necessary to perform the aromatization.

2016Conference object

Open access

2-[(1E,3E)-4-Arylbuta-1,3-dien-1-yl]-4H-chromen-4-ones as dienes in Diels-Alder reactions - experimental and computational studies

Publication . Albuquerque, Hélio; Santos, Clementina M.M.; Lima, Carlos F.R.A.C.; Santos, Luís M.N.B.F.; Cavaleiro, José; Silva, Artur

The synthesis and reactivity of 2-[(1E,3E)-4-arylbuta-1,3-dien-1-yl]-4H-chromen-4-ones as dienes in Diels–Alder (DA) reactions with several electron-poor and electron-rich dienophiles under microwave irradiation was studied. The optimized reaction conditions were achieved with N-methylmaleimide as the dienophile and Sc(OTf )3 (OTf = triflate) as a Lewis acid under microwave-assisted and solvent-free conditions. The Lewis acid improved the reaction yields as it prevented the adducts obtained from undergoing a second DA reaction; thus, the formation of a bisadduct was avoided. The α, :γ,δ-diene of the starting chromones was the most reactive, and the computational results confirmed the experimental findings. Theoretical calculations also provided a rationale for the unexpected lack of reactivity shown by some dienophiles. The adducts prepared were dehydrogenated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ); however, the aza adducts were sensitive to the highly energetic reaction conditions necessary for the aromatization.

2017Journal article

Open access