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4.3.15 Bismuth Compounds (Update 2018)
A. Gagnon, E. Benoit, and A. Le Roch General Introduction
Bismuth, the 83rd element of the periodic table, is a metal with the electronic configuration [Xe]4f145d106s26p3. Bismuth is the heaviest of the main p-block group 15 elements called the pnictogens, which include nitrogen, phosphorus, arsenic, and antimony. Despite having a high atomic weight and being surrounded in the periodic table by highly toxic elements such as lead, tin, antimony, tellurium, and polonium, bismuth shows surprisingly low toxicity.[1] Due to relativistic effects, the lone pair of electrons in trivalent organobismuthines is located in a 6s-orbital.[2,3] As a consequence, the organic groups in trivalent organobismuthines (R13Bi) are connected at 90° angles to the bismuth via its 6px,y,z-orbitals. Bismuth compounds have been used in medicinal chemistry,[4–12] as radiopaque agents for orthopaedic bone cements,[13] as reagents for organic transformations (vide infra), as catalysts for applications in green chemistry,[14–16] and as initiators for polymerization reactions.[17–21] Organobismuth compounds are a class of organometallic reagents that contain a C—Bi bond. These compounds show unique reactivities which are dictated by the oxidation state of the bismuth center (+3, +5), the net charge of the species (+2, +1, 0, -1), the number of coordinated groups (3 to 6), the ability of bismuth to form hypercoordinating interactions, the dual behavior of bismuth compounds as Lewis bases and soft Lewis acids, its borderline character between a metal and a non-metal, and the high functional-group tolerance of organobismuth compounds. Standard enthalpies of formation of 489.7 and 600.6 kJ·mol–1 have been calculated for triphenylbismuthine in the crystalline and gas phases, respectively.[22] An oxidation potential of 1.60 V relative to a saturated sodium calomel electrode was measured for triphenylbismuthine using a platinum electrode and tetraethylammonium 4-toluenesulfonate as the electrolyte.[23] Inversion barriers ranging from 33.7 to 68.7 kcal·mol–1 have been calculated for trivalent organobismuthines (R13Bi; R1= H, Me, t-Bu, vinyl, Ph) using a semiempirical PM3 method.[24] Inversion barriers of 14.6 to 20.5 kcal·mol–1 have been measured by NMR spectroscopy for a diarylbismuth alkoxide where one aryl group bears a (dimethylamino)methyl arm that forms an intramolecular N?Bi interaction with the bismuth center.[25] The most recent review on the synthesis, properties, and use of organobismuth compounds in organic synthesis was released in 2017.[26] Two other reviews on this class of compounds have appeared in the literature since the publication of the previous Science of Synthesis review.[27,28] A monograph on the chemistry of organobismuth compounds is also available.[29] This update describes methods for the synthesis of organobismuth compounds and their applications in organic synthesis. It mainly focuses on publications reported between 2001 and 2017, but also includes some methods and compounds covered in the previous Science of Synthesis review by Suzuki and Ikegami in 2002 (Section 4.3). This review does not cover the synthesis of inorganic bismuth compounds or their use in chemistry; reviews on this topic can be found elsewhere.[14,30–34] SAFETY: Although inorganic bismuth-derived compounds usually show low toxicity,[1] little is known about the toxicity of organobismuth compounds. Therefore, all organobismuth compounds described in this review should be manipulated with great care while wearing proper personal protective equipment. Waste should be disposed of according to local hazardous material and environmental regulations. 4.3.15.1 Synthesis of Bismuth Compounds
4.3.15.1.1 Alkyl- and Arylbismuthines
Alkyl- and arylbismuthines follow the general formula R13Bi and Ar13Bi, respectively. Alkylbismuthines, also called trialkylbismuthines or tertiary alkylbismuthines, are sensitive to air and must therefore be manipulated under inert atmosphere. Low-molecular-weight alkylbismuthines are usually liquids that can be purified by distillation, whereas higher-molecular-weight alkylbismuthines are purified by crystallization under inert atmosphere. Arylbismuthines, also called triarylbismuthines or tertiary arylbismuthines, are inert to air and water. Arylbismuthines are usually solids that can be purified either by crystallization under air or by column chromatography. Triarylbismuthines adopt a distorted trigonal pyramidal structure, where the C—Bi—C bond angles are around 90°. This deviation from the ideal value of 109.5° for sp3-hybridized atoms can be explained by relativistic effects that place the lone pair of the bismuth in a 6s-orbital, leaving three valence electrons in 6p-orbitals for Bi—C s-bond formation. Because many of them are liquid and because they are pyrophoric, very few trialkylbismuthine structures have been reported. The structures of trimethylbismuthine and triisopropylbismuthine were established by growing crystals directly on a diffractometer using an IR-laser-assisted technique in a closed quartz glass capillary under an inert argon atmosphere.[35] X-ray analysis showed average Bi—C bond lengths of 2.259 and 2.287 Å for trimethylbismuthine and triisopropylbismuthine, respectively. The presence of Bi?Bi intermolecular interactions was also observed, as suggested by a distance of 3.899 Å between both atoms, a value which is significantly shorter than the sum of their van der Waals radii. Alkyl- and arylbismuthines are not basic, because their lone pair of electrons are located in a 6s atomic orbital.[36] Even though they are weaker ligands than their lighter phosphine or stibine analogues, triorganobismuthines can act as s-donor ligands with group 13 elements[37] and with transition metals (see ? Section 4.3.15.1.5). Arylbismuthines have been used as arylating agents in copper-catalyzed C-, N-, O-, S-, and Se-arylation reactions (see ? Section 4.3.15.2.1). They have also been abundantly used in palladium- and rhodium-catalyzed reactions (see Sections 4.3.15.2.2 and 4.3.15.2.3). Because of their sensitivity to air, alkylbismuthines have found fewer applications in organic synthesis. Nonetheless, a few examples of copper-catalyzed N-alkylation reactions and palladium-catalyzed cross-coupling reactions involving trialkylbismuthines have been reported (see Sections 4.3.15.2.1 and 4.3.15.2.2). 4.3.15.1.1.1 Method 1: Synthesis from Grignard and Organolithium Reagents Symmetrical triorganobismuthines are routinely prepared by adding an organomagnesium or organolithium reagent to an inorganic bismuth salt, usually bismuth(III) chloride. The reaction is normally performed at low temperature in an aprotic polar solvent such as tetrahydrofuran or diethyl ether. The required organomagnesium reagents are most commonly prepared by refluxing the corresponding aryl, hetaryl, or alkyl halide in tetrahydrofuran or diethyl ether in the presence of metallic magnesium, whereas the organolithium reagents are prepared either by lithium–halogen exchange or by direct lithiation of the aryl, hetaryl, or alkyl precursor. Using these approaches, a wide range of triaryl-, trihetaryl-, and trialkylbismuthines have been prepared. Triaryl- and trihetarylbismuthines bearing substituents at the ortho, meta, and para position are accessible via these routes. Functional groups that are resistant to the highly reactive magnesium or lithium species such as fluorides, ethers, acetals, trifluoroalkyl groups, and dialkylamino groups can be introduced on the triorganobismuth species using these methods. Triarylbismuthine 2 bearing diethyl acetal moieties at the meta positions is prepared by reacting [3-(diethoxymethyl)phenyl]magnesium bromide (1) with bismuth(III) chloride at low temperature (? Scheme 1).[38] Using the diethyl acetal as a handle for functional-group transformation, triarylbismuthine 2 was then used to prepare a wide variety of highly functionalized triarylbismuthines (see ? Scheme 21). Scheme 1 Synthesis of a Symmetrical Triarylbismuthine from a Grignard Reagent[38] Tris[4-(1H-pyrrolo[2,3-b]pyridin-1-yl)phenyl]bismuthine (4), a blue phosphorescent triarylbismuthine, is synthesized by the addition of the corresponding aryllithium reagent, prepared by lithium–halogen exchange between 1-(4-bromophenyl)-1H-pyrrolo[2,3-b]pyridine (3) and butyllithium at -78°C, to bismuth(III) chloride (? Scheme 2).[39] Phosphorescence was observed for compound 4 with a ?max = 478 nm. Compound 4 is air-stable and crystallizes in a trigonal pyramidal geometry where the average C—Bi—C angle is 94.1°and the C—Bi bond lengths lie between 2.220 and 2.293 Å. Compound 4 forms a propeller-like molecular assembly structure with approximate C3 symmetry. Scheme 2 Synthesis of a Symmetrical Triorganobismuthine from an Organolithium...
