ChemInform Abstract: Bismuth(V) Reagents in Organic Synthesis

Abstract

The concept that the valency change from BiV to Bi could be harnessed to produce a family of selective oxidants as shown to be correct. However, completely unexpected was the efficiency of Bi reagents in carrying out arylations on oxygen, carbon and nitrogen. The new arylation procedures permit the synthesis under very mild conditions of compounds only available otherwise by indirect routes. In general, these arylation reactions do not involve radical intermediate, but are rather examples of reductive elimination. The Chemistry of Natural Products has always been associated with major advances in Organic Chemistry from its origin up to the present time. Major objectives of Natural Product Chemistry have been first the determination of structure, and then synthesis. Due to the development of powerful physical methods, the structural analysis of natural products is now a relatively trivial matter. The synthesis of natural products however has remained an active field, always requiring new synthetic methods. As we argued some years ago, the invention of new synthetic methods takes place through conception, or misconception, or by accident (ref. 1). We will examine some of our work which resulted from the combination of the first and third concept, i.e. application of a conceptually devised reaction to the Chemistry of Natural Products which led to an accident and the development of a family of unusual arylation reactions. Mild and selective oxidation of polyfunctional substrates remains a major goal for organic chemists. Metal and metalloid oxides have been used for a long time. However, reactions can be more or less selective, and recycling of the oxidant, thus giving a catalytic system, is rarely seen. The reactivity of metalloids like selenium, tellurium or bismuth is improved through using covalently bonded substituted derivatives. Thus the chemistry of compounds (ref. 2) has grown into an active field. Some such examples can be found in the chemistry of selenium (ref. 3), tellurium (ref. 4), iodine (ref. 5) to name only a few. Among borderline organo—metalloid derivatives, in the group VA, organobismuth derivative?4ha15 ben3known for a long time (ref. 6). With an electronic configuration [Xe] 4p 5d 6s 6p , bismuth has all its lower—energy orbitals filled, leaving five electrons in non—equivalent outer arbitals. The participation of the two s electrons leads to two possible valencies : Bi(III) and Bi(V). As expected, two series of organic compounds exist : the trivalent derivatives with a pair of non—bonded s electrons, and the pentavalent derivatives. The Bi(V) —> Bi(III) change led to the conception of an oxidation reaction (Scheme 1), which might moreover be developed into a catalytic reaction, as in the group VI elements. Scheme 1. Conception of the Oxidation Mechanism ,.H Ph3 =0