Article focuses on microwave-induced bismuth salts-mediated synthesis of useful organic compounds

Bismuth salts are used as medicinally active ingredients for the past many years due to their minimum toxicity and side effects. It is important to note that lead and antimony salts are much more toxic than bismuth salts. Reactions with less toxic reagents and catalysts are actively pursuing by many researchers following green protocols and environmentally friendly methods. Researchers around the globe are developing and maintaining chemical reactions environmentally safe and sustainable. Many procedures that utilize toxic materials are not acceptable for business development. Despite these improvements, a few hazardous processes are continuing because of lack of other green methods and as a result, the cost to manufacture useful compounds remains very high.

Bismuth salts-mediated reactions were reported by us 19 years ago. The preparation of organic compounds with bismuth salts was not routinely performed at that time and therefore, it was difficult to convince scientists that these salts may prove to be efficient catalysts or reagents in organic and medicinal chemistry. Nevertheless, we realized that numerous organic molecules and their immediate precursors can be synthesized using bismuth salts-mediated processes. Our continued efforts have culminated in the development of many new methods as well as the improvement of existing methods. Since then, the application of bismuth salts in the synthesis of organic compounds is increasing because of a number of factors. Bismuth salts possess excellent catalytic power and they can produce compounds following two distinct mechanistic routes. For example, they are able to generate mineral acids in the reaction systems and they are also good substrates for effective coordination with electronegative atoms and functional groups. Bismuth is heavier and bigger in atomic radius in comparison to group I and II metals. Due to these facts, the attraction of bismuth to numerous anions is much less. This property is further strengthened because of the presence of vacant d-orbitals in bismuth (III) salts. This vacant orbital is used by electronegative atoms of other reagents through their coordination activities and facilitates simultaneous nucleophilic reactions to occur.

The principal aim of this article is to describe the development of bismuth salts-induced reactions for the synthesis of useful organic compounds that we are pursuing for the past 19 years. Our endeavors in this area are proved to be attractive and these also uncover bismuth nitrate pentahydrate as an extremely active catalyst. During the course of this investigation, it is anticipated that microwave irradiation method coupled with bismuth salts are the choice for many effective chemical transformations. Since the discovery of microwave-induced chemistry, this method has received highly significant application in many areas. It is known that microwaves are non-ionizing radiations and because of dielectric heating exerted by microwave, successful reactions are performed within a few minutes instead of several hours. Many scientists believe microwave exposures alter conformation and configuration of reactant molecules instantly and activate polar solvents very quickly. Therefore, non polar solvents with low dipole moment are not suitable in microwave-induced chemistry. Polar solvents are able to activate non-polar reactants provided sufficient microwave energy is applied to the reaction vessels. Our group has reported numerous methods for the preparation of organic molecules effectively using polar solvents and in the absence of any solvents. It has been found that bismuth salts are highly compatible to microwave irradiation and they are efficient promoters for the synthesis of milligram to several hundred grams of compounds. For this reasons, bismuth salts-mediated reactions under microwave-induced methods remains one of the attractive strategies that we have been developing for many years.

Numerous articles have demonstrated the superiority of microwave-induced method over conventional heating in accelerating the rate of diverse reactions. In contrast, a few scientists argue that microwaves are only heating instruments and according to them radiation has no effects on chemical reactions. Our studies on microwave-induced bismuth salts-catalyzed reactions have become significant in the synthesis of organic compounds. The products obtained by this method are very important. Some of the products are also transformed to medicinally active compounds by chemical manipulations. For example, we have demonstrated synthesis of hormones, anticancer drug candidates, antibacterials and compounds for rare diseases. Notably, a number of expeditious methods are developed following this procedure. On the basis of our findings, this review is written to inform professionals working in academic institutions and industries.