Organobismuth chemistry is the chemistry of organometallic compounds containing a carbon to bismuthchemical bond. According to one reviewer, applications are rare even though bismuth and bismuth compounds are the least toxic among the heavy metals and are relatively cheap. The main bismuth oxidation states are Bi and Bi as in all higher group 15 elements. The energy of a bond to carbon in this group decreases in the order P > As > Sb > Bi. The first reported use of bismuth in organic chemistry was in oxidation of alcohols by Challenger in 1934. Knowledge about methylated species of bismuth in environmental and biological media is very limited. Organobismuth heterocycles are bismole and bismabenzene. For reviews, see the cited articles
Preparation of Organobismuth(III) Compounds
Organobismuth compounds can be prepared from BiCl3 by substitution with the corresponding carbon nucleophiles. Triaryl bismuth compounds are typically crystalline solids that are not air and moisture sensitive and can easily be purified by recrystallization or chromatography, thus making them attractive reagents for organic synthesis. Since formation of organobismuth complexes require the formation of highly basic coupling partners, a limited number of functional groups can be incorporated on the bismuth center through alkyl metal addition. Recently, the Gagnon group has demonstrated that chemical modification of the substituents on the bismuth center can be achieved after formation of the organobismuth center with aryl Grignards to form triaryl bismuth complexes with sensitive functionalities such as alcohols, aldehydes, and ketones.
Synthesis of Organobismuth(V) compounds
Organobismuth complexes may be accessed directly from organobismuth through oxidative addition to a halogen then displacement of the newly formed bismuth-halogen bond for a bismuth-carbon bond with an alkyl or aryl lithium or Grignard reagent. Alternatively, treatment of the organobismuth with thionyl chloride also affords the dihalo-oranobismuth complex. Bi easily forms an onium ion for example: or an ate complex for example: The thermal stability of R5M compounds decrease in the order As > Sb > Bi and aryl compounds are more stable than alkyl compounds. Me5Bi decomposes explosively at 20 °C.
Organobismuth(III) Chemistry
The most common synthetic use of Bi complexes is to transmetallate one of its substituents onto a different metal center such as palladium. One early example of this comes from the Barton group where triaryl bismuth were shown to react with acylchlorides under Pd catalysis to form a variety of phenyl ketones. More recently, the Barabe group demonstrated that tricyclopropylbismuth reagents can react with aryl halides and triflates under Pd catalysis in a similar fashion to afford a variety of aryl and heteroaryl cyclopropanes. Triphenylbismuth, unlike the related phosphorus, arsenic, and antimony compounds, undergoes mild redistribution with its trihalide to give the mixed derivatives such as diphenylbismuth chloride. Bismuth iodide is a catalyst in the Mukaiyama aldol reaction. Bi is also used in a Barbier type allylation of carbonyl compounds in combination with a reducing agent such as zinc or magnesium, possibly forming the active Bi catalyst in situ. The cyclic compound bismole, a structural analog of pyrrole, has not been isolated, but substituted bismoles are known. Triaryl bismuth compounds may also be employed in C-N bond forming transformations with an appropriate metal co-catalyst. For instance, Barton and coworkers demonstrated that amines could be N-arylated with a bismuth reagent in the presence of copper salt.
Organobismuth(V) chemistry
Bi compounds are strongly oxidizing due to the inert pair effect and relativistic effects. Oxidizing agents are Ph3Bi2, Ph3BiCO3 and 2O. Substrates for oxidation are oximes, thiols, phenols and phosphines. Compounds such as Ph5Bi and Ph3BiCl2 have been used in the arylation of arene compounds and 1,3-dicarbonyl compounds: The above transformation proceeds through in an asynchronous concerted fashion from the O-bound organobismuth reagent after loss of an aryl group. A triarylbismuth complex forms concomitantly. Regioselectivity of this transformation is guided by the directing ability of adjacent lewis basic functionalities. It is important to note that in the above arylation, a full equivalent of the pentavalent bismuth compound is required for the arylation reaction therefore leaving four ligands on bismuth inactive for further arylations. Catalytic manifolds of this chemistry are challenging due in part to the reoxidation of Bi to Bi. For more examples of bismuth mediated arylations, see the cited review. Bi compounds can be accessed through Bi compounds for example:
