Lewis bismuth

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

The carbonyl group can be deprotected by acid-catalyzed hydrolysis by the general mechanism for acetal hydrolysis (see Part A, Section 7.1). A number of Lewis acids have also been used to remove acetal protective groups. Hydrolysis is promoted by LiBF4 in acetonitrile.249 Bismuth triflate promotes hydrolysis of dimethoxy, diethoxy, and dioxolane acetals.250 The dimethyl and diethyl acetals are cleaved by 0.1-1.0 mol % of catalyst in aqueous THF at room temperature, whereas dioxolanes require reflux. Bismuth nitrate also catalyzes acetal hydrolysis.251... 

Friedel-Crafts acylation generally involves reaction of an acyl halide and Lewis acid such as A1C13, SbF5, or BF3. Bismuth(III) triflate is also a very active acylation catalyst.46 Acid anhydrides can also be used in some cases. For example, a combination... 

Bertrand and co-workers synthesized compound 310 in good yield (90%) by the treatment of a toluene solution of 311 with BiCls at — 78 °C. The coordination geometry around the bismuth center was found to be nearly trigonal bipyr-amidal. The reactivity of compound 310 toward various Lewis acids was studied. Transmetallation occurs with both... 

Diels-Alder reactions are one of the most famous examples which are accderated by a Lewis acid. Various water-stable Lewis adds such as Ln(OTf)3,1371 methylrhenium trioxide,1381 copper nitrate,1391 copper bis(dodecyl sulfate) (4b),1401 indium chloride,1411 and bismuth triflate1421 have been used for Diels-Alder and aza-Diels-Alder reactions in water. Furthermore, a catalytic asymmetric Dids-Alder reaction in water using a copper complex of an amino... 

Abstract Several bismuth-catalyzed synthetic reactions, which proceed well in aqueous media, are discussed. Due to increasing demand of water as a solvent in organic synthesis, catalysts that can be used in aqueous media are becoming more and more important. Although bismuth Lewis acids are not very stable in water, it has been revealed that they can be stabilized by basic ligands. Chiral amine and related basic ligands combined with bismuth Lewis acids are particularly useful in asymmetric catalysis in aqueous media. On the other hand, bismuth hydroxide is stable and works as an efficient catalyst for carbon-carbon bond-forming reactions in water. 

Discovery of water-compatible Lewis acids has greatly expanded the use of Lewis acids in organic synthesis in aqueous media. However, conventional Lewis acids such as Alm, Tifv, Snfv, etc. still cannot be used in aqueous media under standard conditions. Bismuth triflate, Bi(OTf>3, is reported to exist in water as an equilibrium mixture of Bi(OTf)3 with bismuth hydroxide and triflic acid [18]. 

To elucidate the reaction pathway, deuterium-labeled allenyl pinacol boronate 10 was prepared, and the addition reaction with hydrazonoester 6 was conducted in the presence of Bi(OH)3 and Cu(OH)2 (Scheme 4). In both Bi- and Cu-catalyzed cases, the reactions proceeded smoothly (in quantitative yields in both cases). In the Bi(OH)3-catalyzed reaction, a major product was allenyl compound 11, in which the internal position was deuterized. It was assumed that a propargyl bismuth was formed via transmetalation from boron to bismuth, followed by addition to hydrazonoester via y-addition to afford allenyl compound 11. Thus, two y-additions could selectively provide a-addition products [75, 76, 105, 106]. It was confirmed that isomerization of 10 did not occur. Recently, we reported Ag20-catalyzed anti-selective a-addition of a-substituted allyltributyltin with aldehydes in aqueous media [107], On the other hand, in the Cu(OH)2-catalyzed reaction, a major product was propargyl compound 12, in which the terminal position was deuterized. A possible mechanism is that Cu(OH)2 worked as a Lewis acid catalyst to activate hydrazonoester 6 and that allenyl boronate 10 [83-85] reacted with activated 6 via y-addition to afford 12. 

We have previously reported that when the rearrangement of trans-stilbene oxide was carried out with CF3SO3H, the solution turned red and the product diphenylacetaldehyde was less pure than that obtained with bismuth triflate. This observation points to the role of bismuth(III) triflate as a Lewis acid in the rearrangement of epoxides and not to protic acid catalysis by triflic acid released by hydrolysis of bismuth triflate. 

On the basis of these initial results, various rare earth metal triflates, including Sc(OTf)3, Hf(OTf)4 and Yb(OTf)3 were applied as catalysts [27-29]. Recently Beller and coworkers developed efficient Friedel-Crafts alkylations with catalytic amounts of Rh, W, Pd, Pt and Ir complexes [30] or FeCl3 [31-34] as Lewis acid catalysts. However, in the latter cases high catalyst loadings had to be applied. To overcome these major drawbacks, we decided to develop a Bi(III)-catalyzed Friedel-Crafts alkylation of arenes with benzyl alcohols. Although bismuth-catalyzed Friedel-Crafts acylations were well known at this time, Friedel-Crafts alkylations using benzyl alcohols had not been reported. 

Scheme 6 Loss of stereoinformation during the Bi(OTf)3-catalyzed Friedel-Crafts-alkylation implies a carbocationic intermediate. Mechanism A TfOH generated in situ from Bi(OTf)3 is thought to be the catalytic active species. Mechanism B Bismuth(III) acts as a Lewis acid. TfOH only regenerates Bi(OTf)3 from its less reactive monohydroxide...

This overview impressively demonstrates that Bi(III) salts are not only versatile Lewis acid catalysts for the activation of cr-donors, including benzyl and propargyl alcohols, but also efficient catalysts for the activation of Ji-donors such as styrenes or alkynes. In recent years, various environmentally benign bismuth-catalyzed methods have been developed for the alkylation of arenes, heteroarenes,... 

Gaspard-Iloughmane H, Le Roux C (2008) Bismuth(III) Lewis acids. In Yamamoto H, Ishihara K (eds) Acid catalysis in modem organic synthesis. Wiley, New York, p 551... 

Keywords Bismuth(IH) salts Catalysis Green chemistry Lewis acidity Moisture tolerance Non-toxicity Organic transformations... 

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