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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.


Energy profile for the reaction between 4-halonitrobenzene (12) and piperidine (13).
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sch06: Energy profile for the reaction between 4-halonitrobenzene (12) and piperidine (13).

Mentions: The computational study was performed at the B3LYP(PCM)/6-31G*+ΔZPVE level using DMSO as solvent (Table 5). The reaction paths are collected in Scheme 6. In all processes, very polar intermediates are involved (Table 5). The activation energy in all three cases (X=F, Cl, Br) is not very high and, as expected, is lower when the substituent (X) is a fluoro group. It is interesting to note that in this reaction a large increase in the polarity occurs due to the high polarity of both the transition state and the intermediate. On the basis of these outcomes, it can be concluded that in this process, the theory proposed by Loupy is valid: “Specific microwave effects can be expected for the polar mechanism, when the polarity is increased during the reaction on going from the reactants to the transition structures”.8a The stabilization of the transition state is more effective than that of ground state, which results in an enhancement in the reactivity through a decrease in the activation energy. The conclusion in this case is that in this process the acceleration effect is a consequence of the strong absorption of microwave irradiation by the intermediates present in the reaction mixture.


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)

Energy profile for the reaction between 4-halonitrobenzene (12) and piperidine (13).
© Copyright Policy - open-access
Related In: Results  -  Collection

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

sch06: Energy profile for the reaction between 4-halonitrobenzene (12) and piperidine (13).
Mentions: The computational study was performed at the B3LYP(PCM)/6-31G*+ΔZPVE level using DMSO as solvent (Table 5). The reaction paths are collected in Scheme 6. In all processes, very polar intermediates are involved (Table 5). The activation energy in all three cases (X=F, Cl, Br) is not very high and, as expected, is lower when the substituent (X) is a fluoro group. It is interesting to note that in this reaction a large increase in the polarity occurs due to the high polarity of both the transition state and the intermediate. On the basis of these outcomes, it can be concluded that in this process, the theory proposed by Loupy is valid: “Specific microwave effects can be expected for the polar mechanism, when the polarity is increased during the reaction on going from the reactants to the transition structures”.8a The stabilization of the transition state is more effective than that of ground state, which results in an enhancement in the reactivity through a decrease in the activation energy. The conclusion in this case is that in this process the acceleration effect is a consequence of the strong absorption of microwave irradiation by the intermediates present in the reaction mixture.

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.