Abstract
This paper presents a study of solvent-free reaction conditions using microwave irradiation (MW) to obtain 4,5-dihydro-1H-pyrazoles and dehydrated pyrazoles by the cyclocondensation reaction of 4-alkoxy-1,1,1-trifluoro-3-alken-2-ones [CF3C(O)CH=C(R¹)OR, where R/R¹ = Et/H, Me/Me and Me/Ph] with hydrazines [NH2NH-R², where R² = CO2Me, Ph, CH2CH2OH]. Some reactions were performed under the same reaction conditions using methanol as solvent. The results obtained using MW equipment for synthesis under solvent-free conditions were also compared with those described in literature for conventional thermal heating and heating with a domestic MW oven. In general, the products furnished by reaction in MW equipment for synthesis presented better yields and shorter reaction times. In addition, it was demonstrated that the reaction temperature altered the formation of products for each hydrazine showing that MW equipment for synthesis is efficient for reacting hydrazines and 4-alkoxy-1,1,1-trifluoro-3-alken-2-ones to procedure the products 4,5-dihydro-1H-pyrazoles and dehydrated pyrazoles.
Highlights
Removing organic solvents in chemical synthesis is important in the drive towards benign chemical technologies
The aims of this work are: (i) to investigate the use of microwave irradiation to promote and accelerate the cyclocondensation reaction between enones (4-alkoxy1,1,1-trifluoro-3-alken-2-one) and three hydrazines in MW equipment for synthesis; (ii) to determinate the reaction conditions to obtain 4,5-dihydro-1H-pyrazoles and dehydrated pyrazoles in MW for synthesis under solventfree conditions and in MeOH; (iii) to compare the reaction conditions with the results obtained under conventional thermal heating and domestic MW oven in order to evaluate the effect of the MW equipment for synthesis in these reactions (Scheme 1)
In order to evaluate the cyclocondensation reaction conditions to obtain 4,5-dihydro-1H-pyrazoles or pyrazoles from the reaction of enones and hydrazines using MW equipment for synthesis, a series of experiments was performed. These experiments were organized into three groups and in each group, three enones 1a-c were reacted with hydrazines 2, 5 and 9 under solvent-free conditions (Schemes 2, 3 and 4)
Summary
Sustainability is an increasingly important issue in the wider context of population, health, the environment, energy, technology, renewable resources, and, in the sciences, as an integral part of the rapidly emerging field called Green Chemistry.[1,2,3] Removing organic solvents in chemical synthesis is important in the drive towards benign chemical technologies. In this context, the use of microwave-assisted organic synthesis (MAOS) has emerged as an alternative and efficient tool, especially in heterocyclic synthesis.[4] Many reviews have been published recently that. Heating organic solvents in open vessels in a microwave oven can lead to violent explosions induced by
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