Journal of Photochemistry and Photobiology C: Photochemistry Reviews
ReviewMicrowave-assisted solvent-free heterocyclic synthesis
Introduction
In the recent decades, microwave heating has taken an incontestable place in analytical and organic laboratories practice as a very effective and non-polluting method of activation. Examples of this technology in digestion and organic synthesis are numerous [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. In the electromagnetic spectrum, the microwave radiation area is located between infrared radiation and radio waves. Microwaves have wavelength in the range of 1 mm to 1 m, corresponding to frequencies between 30 and 3 GHz. Telecommunications and microwave radar equipments occupy many of the band frequencies in this region. Hence, the microwave frequencies for industrial and scientific purposes are imposed by international convention being the 2.45 GHz (wavelength of 12.2 cm) the most routinely used.
The total of publications on microwave-assisted organic chemistry is increasing rapidly with approximately more than 2000 publications on print since the pioneering work of Gedye et al. [17] in 1986 (Fig. 1), the percentage of reviews is quite high and several articles are well worth reading [8], [9], [10], [11], [12], [13], [14], [15]. All synthesis techniques described earlier are represented in the material, the solvent-free technique being the most popular. It is of interest to note that the country in which the technique seems to be most developed, according to the number of publications, is India. The main reasons for this increase include the availability of commercial microwave equipment devoted for organic chemistry and the development of solvent-free techniques, which have improved the safety aspects, but are mostly due to an increased interest in shorter reactions times.
The majority of these publications describes important accelerations for a wide range of organic reactions especially when carried out under solvent-free reactions conditions and microwave irradiation leads to large reductions in reaction times, enhancements in conversions, and sometimes [11], [14] in selectivity with several advantages of the environmental approach, termed green chemistry. These solvent-free MW-assisted reactions [9] have gained popularity as they provide potentialities to work open vessels and enhanced possibility of up-scaling the reactions on preparative scale. Three types of solvent-free procedures can be coupled with microwave activation [9], [18]:
- (i)
Reactions between neat reactants, needing at least one liquid polar molecule [19], as liquid–liquid or liquid–solid systems. In the absence of solvent the radiation is absorbed directly by the reagents, so the effect of microwaves is more marked.
- (ii)
Reactions between supported reagents on solid mineral supports in “dry media” by impregnation of compounds on alumina, silicas or clays [20].
- (iii)
Phase transfer catalysis (PTC) conditions in the absence of organic solvent when a liquid reagent acts both as a reactant and an organic phase [10], [21].
Section snippets
Origin of microwave heating
Microwave consists in an electric and magnetic field and thus represents electromagnetic energy. This energy can act as a non-ionising radiation that causes molecular motions of ions and rotation of the dipoles, but does not affect molecular structure. The rotation of the dipoles in an alternating field causes friction, which produces heat, up to 10 °C/s (Fig. 2). More exactly, the applied microwave field causes the molecules, on average to temporarily spend slightly more time orienting
Specific microwave effects
Microwave effects result from material–wave interactions and, due to the dipolar polarization phenomenon, the greater the polarity of a molecule the more pronounced the microwave effect when the rise in temperature [22] is considered. In terms of reactivity, the specific effect has therefore to be considered according to the reaction mechanism and particularly with regard to how the polarity of the system is altered during the progress of the reaction [1], [13].
Specific microwave effects can be
Microwave equipments used in organic synthesis
The most popular and cheap equipment in organic synthesis is the domestic oven (with limited power at 800–1000 W) (Fig. 4a). The distribution of electric field is heterogeneous. This distribution is complex and possibly unstable in the time and their use for synthetic purposes requires a previous cartography to determine the hot spots of high energy using a filter paper sheet impregnated with a solution of cobalt chloride [34a]. The power is not tenable and in fact the sample is always subjected
Safety and cautions relating to the use of microwave apparatus
Although manufacturers of microwave-heating apparatus have directed their research to make microwaves a safe source of heating, uncontrolled reaction conditions may result in undesirable results, for example, chemical reactions involving volatile reactants under superheated conditions may result in explosive conditions. Moreover, improper use of microwave heating for rate enhancement of chemical reactions involving radioisotopes may result in uncontrolled radioactive decay.
Certain problems,
Coupling microwave activation with solvent-free heterocyclic reactions
Due to benefit of pollution and economy, solvent-free methods are of large interest in order to develop conventional procedures making them more clean, safe and easy to perform. Reactions on solid mineral supports: reactants are impregnated as neat liquids on solid supports such as aluminas, silicas, zeolites and clays or using their solutions in adequate organic solvent and further solvent elimination in the case of solids, reaction in “dry media” is performed between individually impregnated
Applications in heterocyclic chemistry
In this short review, we have focused our efforts on the application of microwave irradiation under solvent-free conditions in heterocyclic synthesis essentially via cycloaddition and cyclocondensation reactions and other heterocyclic reactions. More than 300 papers related to heterocyclic synthesis and reactivity under microwaves has been published. We also present in our review a template reaction in combinatorial chemistry and examples of “one-pot” heterocyclic reactions.
It is clear that the
Conclusions
This eco-friendly solvent-free approch using microwave irradiation opens numerous possibilities for conducting rapid heterocyclic synthesis via cycloaddition and cyclocondensation reactions using a variety of supported reagents on mineral oxides and phase transfer catalysis conditions. The use of multimode oven, monomode reactor and conventional glass apparatus, demonstrates the numerous practical applications in laboratory scale experiments. Furthermore, there are different advantages of these
Acknowledgments
Financial support for this work from the Moroccan Government and CNRST-Morocco under Grant of the Programme Thématique d’Appui à la Recherche Scientifique (PROTARSIII, D13/57) is gratefully acknowledged. We thank Dr. A. Petit (Univ. Paris-sud, Orsay, France), Pr. T. Besson (Univ. Rochelle, La Rochelle, France); Pr. A. De la Hoz (Univ. Castilla-La Mancha, Ciudad Real, Spain); Pr. F. Langa (Facultad de Ciencias del Medio Ambiente, Toledo, Spain) for very helpful discussions and for their
Khalid Bougrin received his diploma of the studies superior (DES) in 1995 and PhD degree in 2001 from Mohammed V-Agdal University in Faculty of Sciences at Rabat city, Morocco, under the direction of Prof. Soufiaoui on heterocyclic synthesis in solvent-free microwaves conditions. He has been awarded by the first chemistry Junior Prize of the professor Jean Marie Lehn in 1995 (Chemistry Nobel Prize in 1987). After a postdoctoral stay in 1996–1997 with Dr. A.Loupy concerning heterocyclic
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Khalid Bougrin received his diploma of the studies superior (DES) in 1995 and PhD degree in 2001 from Mohammed V-Agdal University in Faculty of Sciences at Rabat city, Morocco, under the direction of Prof. Soufiaoui on heterocyclic synthesis in solvent-free microwaves conditions. He has been awarded by the first chemistry Junior Prize of the professor Jean Marie Lehn in 1995 (Chemistry Nobel Prize in 1987). After a postdoctoral stay in 1996–1997 with Dr. A.Loupy concerning heterocyclic synthesis in solvent-free conditions using focused microwaves, he moved to Faculty of Sciences as an assistant professor. In 1997, he integrated the laboratory of Prof. Soufiaoui where he is now ‘professeur Habilité’and he started his collaboration with the Dr. Loupy's group. His current research interests focus on heterocyclic chemistry including heterogeneous and homogeneous media effects, solvent-free microwave conditions, and now he develops his activity around the question ‘coupling microwaves-sonochemistry and green chemistry: common future?’
André Loupy received his PhD in 1975 from Paris-South University under the direction of Dr. Jacqueline Seyden-Penne in the centre National de la Recherche Scientifique (C.N.R.S.) in Thiais on the solvent effects in competitive β-elimination/SN2 reactions. His current research interests focus on medium effects in organic synthesis including solvent and salt effects (he is a co-author with Bianca Tchoubar of a book edited by VCH on this subject), solvent-free conditions with special interest for supported reactions and phase transfer catalysis. Since 1987, he has been concerned with microwave activation especially when coupled to solvent-free procedures. Author or co-author of roughly 250 publications, he was a recipient, together with professor Jack Hamelin, of the M.J. Collins Award in 1998 for ‘creative work in microwave chemistry’. He is editor of “ Microwaves in Organic Synthesis”, the first book dedicated to the use of microwaves in organic chemistry in Wiley-VCH edition (November 2002).
Mohamed Soufiaoui received his diploma of engineering's, doctor's degrees from Besançon University, France, in 1973 and 1976, respectively. He received his PhD degree in 1978 from Besançon University under the direction of Prof. Laude, on the synthesis and stereochemistry in heterocyclic cycloaddition reactions. In 1976, he was appointed as assistant professor at Mohammed V-Agdal University in Faculty of Sciences at Rabat city, Morocco and he participated at the creation of the ‘Laboratoire de Chimie des Plantes et de Synthèse Organique et Bio-organique’. He was promoted to full professor in 1982. Presently, he is the head of this laboratory. He was President of the Chemistry Moroccan Society and ex-President of the Arabian chemists union. His current research interests focus on medium effects in heterocyclic chemistry including solvent and solid catalysts effects, solvent-free microwave conditions. He is author or co-author of about 130 publications and one book dealing with ‘Exercices et Problèmes de Chimie Organique et Spectroscopie’.