Microwave Technique: A Potential Approach in Organic Synthesis and Drug Discovery Processes
Organic synthesis with microwaves has fascinated a considerable amount of attention in recent years. In academia and industry, microwave radiations have been used to accelerate the rate of chemical reaction with shorter reaction time in the synthesis of organic molecules and find its utility for drug discovery and drug development processes.
Gedye and Giguere/ Majetich in 1986 firstly reported the use of microwave heating to speed up organic chemical transformations. Although most of the early revolutionary experiments performed in domestic, kitchen microwave ovens, the existing scenario is to use dedicated instruments i.e. synthetic microwave which has only become available in the last few years for chemical synthesis of organic substances. Therefore, many academic and industrial research groups have been using microwave assisted organic synthesis for rapid optimization of reactions, for the efficient synthesis of new chemical entities, and discovering of new chemical entities.
Two types of microwave effects i.e. specific microwave effects and non-thermal microwave effects have been observed in microwave chemistry. The specific microwave effect requires the transfer of microwave energy into thermal energy. The effect includes selective heating of specific reaction components, rapid heating rates and temperature gradients, elimination of wall effects and superheating of solvents etc. whereas non-thermal microwave effects explain anomalous observations in microwave chemistry and found to be controversial.
The basic mechanism observed with microwave techniques in relation to heating is by either dipolar polarization or ionic conduction. The electromagnetic waves when passing through dipoles or ions of the sample resulting in the oscillation of molecules of the sample, which leads to loss of energy in the form of heat. As microwave radiation is introduced into the reaction system distantly without direct physical contact with the reaction mixture, this can lead to a rapid increase in temperature throughout the sample causing the formation of fewer by-products or decomposition products. On the other hand, conventional heating of organic reaction mixtures such as heating mantles, oil baths or sand baths are rather slow and produce an inward temperature gradient resulting in localized overheating as well as reagent decomposition when heated for a long period of time. Thus, microwave synthesis has emerged as a potential “lead” in organic synthetic chemistry and provided the momentum for many organic chemists to switch on from traditional heating method to microwave assisted chemistry.
Advantages of microwave techniques
Microwave-assisted organic synthesis has been found to be an invaluable approach in the field of academia and pharmaceutical industry since it can reduce the reaction times from days or hours to minutes or even seconds. It provides advantages, over the conventional heating methods including uniform heating observed throughout the material, increased process speed, provides energy-efficient internal heating, unwanted side reaction and formation of by-products are greatly reduced, very less reaction time observed, greatly reduced exposure to hazardous chemicals, environmental heat loss can be avoided, reproducibility improved, observed pure compounds with higher yield of final products and less expensive techniques etc.
Applications of microwave techniques
- Synthesis of silver nanoparticles containing starch as reductant cum stabilizing agent from silver nitrate has been done under direct, controlled heating and microwave irradiation. The microwave irradiation was found better for reduction of silver ions to silver nanoparticles. It also afforded smaller particle sizes and distribution of particle size with an average size of 12 nm.
- As the increase rate of coupling reactions should lead to shorter cycle times, higher repetitive yields and eventually purer peptides, the application of microwave technique is particularly beneficial in solid-phase peptide synthesis.
- Cystine rich peptides synthesis has been carried out in the presence of microwave radiation.
- The other chief industrial application of microwave includes the formation of hydrogen cyanide, drying of pharmaceutical powders and pasteurization of food products etc.
Thus microwave assisted organic synthesis is found to be the reliable and potential approach in terms of uniform heating throughout the material, shortage reaction time, reduced exposure to hazardous chemicals and reduced formation of by-products etc.