Bismuth triflate acylation

Scheme 11.4 shows some other representative Friedel-Crafts acylation reactions. Entries 1 and 2 show typical Friedel-Crafts acylation reactions using A1C13. Entries 3 and 4 are similar, but include some functionality in the acylating reagents. Entry 5 involves formation of a mixed trifluoroacetic anhydride, followed by acylation in 85% H3PO4. The reaction was conducted on a kilogram scale and provides a starting material for the synthesis of tamoxifen. Entry 6 illustrates the use of bismuth triflate as... 

Carrigan MC, Eash KJ, Oswald MC, Mohan RS (2001) An efficient method for the chemoselective synthesis of acylals from aromatic aldehydes using bismuth triflate. Tetrahedron Lett 42 8133-8135... 

Bismuth(III) triflate is also a powerful acylation catalyst that catalyzes reactions with acetic anhydride and other less reactive anhydrides such as benzoic and pivalic anhydrides.113 Good results are achieved with tertiary and hindered secondary alcohols, as well as with alcohols containing acid- and base-sensitive functional groups. 

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... 

Bismuth tra-tri lluoromcthancsulfonate, Bi(OTf)3, and BiCh were found to be effective catalysts for the Friedel-Crafts acylation of both activated and deactivated benzene derivatives such as fluorobenzene.19 Ga(III) triflate is also effective for Friedel-Crafts alkylation and acylation in alcohols and can tolerate water.20 This catalyst is water-stable... 

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. 

In most instances these catalysts were not satisfactory in terms of yield, range of substrates and turnover numbers of the catalyst. Additionally, they were mostly efficient only in the case of activated aromatics. In order to develop also Friedel-Crafts acylations of less activated or even deactivated arenes, a new generation of catalysts had to be developed. Here, particularly bismuth(III) triflate [33] and hafnium(IV) triflate in the presence of lithium perchlorate [34] or triflic acid [35] are effective, alternative catalysts actually acylating benzene, toluene and halobenzenes. 

Le Roux, C., Dubac, J. Bismuth(lll) chloride and triflate novel catalysts for acylation and sulfonylation reactions. Survey and mechanistic aspects. Synlett2002, 181-200. 

Use of bismuth(III) triflate allows achieved acylation of both activated and deactivated aromatic compounds with anhydrides and acyl chlorides. Thus, the acylation of aromatics such as trifluoromethoxybenzene, toluene, benzene, fluorobenzene, and chlorobenzene can be achieved in high... 

Desmurs, J. R., LabrouiUere, M., Le Roux, C., Gaspard, H., Laporterie, A., and Dubac, J. 1997. Surprising catalytic activity of bismuth (III) triflate in the Friedel-Crafts acylation reaction. Tetrahedron Lett. 38 8871-8874. 

Matsushita, Y.-L, Sugamoto, K., and Matsui, T. 2004. The Eriedel-Crafts acylation of aromatic compounds with carboxylic acids by the combined use of perfluoroalkanoic anhydride and bismuth or scandium triflate. Tetrahedron Lett. 45 4723-4727. 

Water-stable ionic liquids were later used for Friedel-Crafts acylations, using a metal triflate catalyst. Cu(0Tf)2 proved to be the most efficient catalyst for this transformation, and acylation of anisole by benzoyl chloride in [bmim][BF4] gave almost exclusively the para adduct 68 (Scheme 24). This reaction can also be performed in organic solvents, but an accelerated rate is observed in ionic liquids.Catalyst loading can be decreased (up to 1 mol%) using bismuth(m) salts as catalysts. ... 

Finally, it s worth emphasizing that trivalent bismuth salts such as the chloride, nitrate, and trifluoromethanesulfonate (triflate) are extremely attractive as Lewis acid catalysts. Applications include Friedel-Crafts acylations, Diels-Alder reactions, esterifications. 

Metal triflates that can be easily prepared from metal halides and triflic acid at -78°C [14] show several unique properties compared with the corresponding metal halides. The use of bismuth(III) triflate allows for acylation of both activated and deactivated aromatic compounds with anhydrides and acyl chlorides [15]. Thus, the acylation of deactivated aromatics such as trifluoromethoxyben-zene, fluorobenzene, and chlorobenzene can be achieved in high yields with benzoyl chloride in the presence of bismuth(III) triflate (10% mol) without solvent. The />ara-acylation product is the most abundant in all cases (trifluoromethoxybenzene 87% yield, ortho.para 4 96 fluorobenzene 86% yield, orthoipara 0 100 chlorobenzene 89% yield, ortho para 13 87). 

Acylations were also catalyzed by bismuth(III) triflate. This property has been well demonstrated in the case of the benzoylation of benzene, toluene, and chlorobenzene [34], The mechanistic investigations confirmed BzOTf as active species of the benzoylation. Again, the activity of Bi(OTf>3 was much higher than that of other metallic triflates, and was comparable with that of TfOH. It has also the advantage that the triflate moieties are more easily recoverable. Other acylations of activated or deactivated benzenes such as fluorobenzene also proceeded in high yields in the presence of cataljdic amounts of Bi(OTf)3 [35]. 

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