Polar Diels-Alder Reactions Under Microwave Irradiation Employing Different Heterocyclic Compounds as Electrophiles

Abstract

During the last twenty years, our research group has been working with aromatic nitrosubstituted compounds acting as electrophiles in Polar Diels-Alder (P-DA) reactions with different dienes of diverse nucleophilicity. In this type of reaction, after the cycloaddition reaction, the nitrated compounds obtained as the [4+2] cycloadducts suffer cis-extrusion with the loss of nitrous acid and a subsequent aromatization. In this form, the reaction results are irreversible. On the other hand, the microwave-assisted controlled heating become a powerful tool in organic synthesis as it makes the reaction mixture undergo heating by a combination of thermal effects, dipolar polarization and ionic conduction. As the Diels-Alder (D-A) reaction is one of the most important process in organic synthesis, the microwave (MW) irradiation was applied instead of conventional heating, and this resulted in better yields and shorter reaction times. Several substituted heterocyclic compounds were used as electrophiles and different dienes as nucleophiles. Two experimental situations are involved: one in the presence of Protic Ionic Liquids (PILs) as solvent and the other under solvent-free conditions. The analysis is based on experimental data and theoretical calculations.

Supramolecular Diversity of Oxabicyclo[2.2.2]octenes Formed between Substituted 2H-Pyran-2-ones and Vinyl-Moiety-Containing Dienophiles

“…Additionally, a single crystal of cyclohexadiene product 4a was also prepared; however, in contrast to the high-pressure conditions appropriate for the preparation of 3, here thermal activation with microwaves was necessary. Microwave irradiation as a way to facilitate organic reactions has during the last decades found some surprisingly widespread applications, as recently vividly described by Kappe [48], including acceleration of Diels-Alder reactions [2,49], Kabachnik-Fields reaction [50] and many others. It is worth noting that product endo-3e was isolated by precipitation upon slow evaporation of the volatile components from the mother liquor (which remained after the precipitation of exo-3e and its removal by vacuum filtration) containing an appreciable amount of the starting dienophile 6a (which was initially applied in a large excess, also as a co-solvent, in molar ratio 1e:6a = 1:27).…”

“…The Diels-Alder reaction, mechanistically classified as a [4+2] cycloaddition belonging to the larger group of pericyclic reactions, is one of the crucial synthetic tools for the construction of novel C-C bonds [1,2], gaining importance in all its varieties, including (organo)catalytic and enantioselective versions [3,4]. Cycloadditions are highly versatile reactions as a broad range of starting dienes and dienophiles (alkenes or alkynes) can be used.…”