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Influence of Polarity and Activation Energy in Microwave-Assisted Organic Synthesis (MAOS).

Rodríguez AM, Prieto P, de la Hoz A, Díaz-Ortiz Á, Martín DR, García JI - ChemistryOpen (2015)

Bottom Line: The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves.This comprises six types of reactions.The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Química Orgánica, Universidad de Castilla-La Mancha, Facultad de Ciencias y Tecnologías Químicas 13071, Ciudad Real, Spain.

ABSTRACT
The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20-30 kcal mol(-1) and a polarity (μ) between 7-20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation.

No MeSH data available.


Cycloaddition reaction of azidomethylphosphonate and functionalized enamines. Reaction conditions: a) conventional conditions: 90 °C, 6 h, 86 % (5/6 ratio =100:0); microwave irradiation: 90 °C (120 W), 5–20 min, 55 % (5/6 ratio =85:15). Result: modification of the selectivity.
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sch01: Cycloaddition reaction of azidomethylphosphonate and functionalized enamines. Reaction conditions: a) conventional conditions: 90 °C, 6 h, 86 % (5/6 ratio =100:0); microwave irradiation: 90 °C (120 W), 5–20 min, 55 % (5/6 ratio =85:15). Result: modification of the selectivity.

Mentions: Type I reactions are exothermic reactions (ΔH<0 kcal mol−1) with low activation energies (Ea<20 kcal mol−1). Loupy and co-workers20 performed the cycloaddition reaction of azidomethylphosphonate and functionalized enamines under solvent-free conditions and under microwave irradiation (Scheme 1). Experimentally, triazole 5 was obtained in good yields (70–86 %) under conventional heating in solvent-free conditions at 100 °C but very long reaction times were required (6 h). Under these conditions, the reaction is 100 % regioselective. For shorter times (20 min), there was no reaction under conventional heating, whereas the reaction proceeded with a 55 % yield under microwave irradiation under comparable conditions but with lower regioselectivity (5/6 ratio, 85:15).


Influence of Polarity and Activation Energy in Microwave-Assisted Organic Synthesis (MAOS).

Rodríguez AM, Prieto P, de la Hoz A, Díaz-Ortiz Á, Martín DR, García JI - ChemistryOpen (2015)

Cycloaddition reaction of azidomethylphosphonate and functionalized enamines. Reaction conditions: a) conventional conditions: 90 °C, 6 h, 86 % (5/6 ratio =100:0); microwave irradiation: 90 °C (120 W), 5–20 min, 55 % (5/6 ratio =85:15). Result: modification of the selectivity.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4522181&req=5

sch01: Cycloaddition reaction of azidomethylphosphonate and functionalized enamines. Reaction conditions: a) conventional conditions: 90 °C, 6 h, 86 % (5/6 ratio =100:0); microwave irradiation: 90 °C (120 W), 5–20 min, 55 % (5/6 ratio =85:15). Result: modification of the selectivity.
Mentions: Type I reactions are exothermic reactions (ΔH<0 kcal mol−1) with low activation energies (Ea<20 kcal mol−1). Loupy and co-workers20 performed the cycloaddition reaction of azidomethylphosphonate and functionalized enamines under solvent-free conditions and under microwave irradiation (Scheme 1). Experimentally, triazole 5 was obtained in good yields (70–86 %) under conventional heating in solvent-free conditions at 100 °C but very long reaction times were required (6 h). Under these conditions, the reaction is 100 % regioselective. For shorter times (20 min), there was no reaction under conventional heating, whereas the reaction proceeded with a 55 % yield under microwave irradiation under comparable conditions but with lower regioselectivity (5/6 ratio, 85:15).

Bottom Line: The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves.This comprises six types of reactions.The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate.

View Article: PubMed Central - PubMed

Affiliation: Departamento de Química Orgánica, Universidad de Castilla-La Mancha, Facultad de Ciencias y Tecnologías Químicas 13071, Ciudad Real, Spain.

ABSTRACT
The aim of this work was to determine the parameters that have decisive roles in microwave-assisted reactions and to develop a model, using computational chemistry, to predict a priori the type of reactions that can be improved under microwaves. For this purpose, a computational study was carried out on a variety of reactions, which have been reported to be improved under microwave irradiation. This comprises six types of reactions. The outcomes obtained in this study indicate that the most influential parameters are activation energy, enthalpy, and the polarity of all the species that participate. In addition to this, in most cases, slower reacting systems observe a much greater improvement under microwave irradiation. Furthermore, for these reactions, the presence of a polar component in the reaction (solvent, reagent, susceptor, etc.) is necessary for strong coupling with the electromagnetic radiation. We also quantified that an activation energy of 20-30 kcal mol(-1) and a polarity (μ) between 7-20 D of the species involved in the process is required to obtain significant improvements under microwave irradiation.

No MeSH data available.