Bismuth acetate

As described earlier one of the possible products from the AFO reaction is dihydroxyflavonols. Simpson and coworkers took advantage of this outcome in their synthesis of the flavonol rhamnocitrin (23). Chalcone 24 was subjected to the typical AFO conditions to deliver dihydroxyflavonol 25. The isolated product was further subjected to hydrogen peroxide to afford flavonol 25a in 30% yield. However, treatment of 25 with bismuth acetate, generated in situ from bismuth carbonate and acetic acid, gave 25a in 77% yield for a respectable 52% overall yield over two steps. 25a was then selectively demethylated with anilinium chloride to deliver rhamnocitrin (23). 

Initial investigations in the Mannich-type reaction of silyl enolates with benzal-dehyde and aniline employed a series of bismuth(III) salts (Scheme 9, Table 10). These results were promising because the corresponding (l-amino ketone could be obtained in moderate to good yield with bismuth halides, except bismuth fluoride (Table 10, entries 1 1). Bismuth nitrate smoothly afforded the expected product (Table 10, entry 5). While bismuth acetate gave no conversion, bismuth trifluor-oacetate provided the product in only moderate yield (Table 10, entries 6 and 7). Phenyl bismuth ditriflate and diphenyl bismuth triflate appeared to be more efficient catalysts than all those previously tested (Table 10, entries 8 and 9). Bismuth(III) triflate led to the expected product in a good yield and in a short reaction time, without any difference between the anhydrous and the hydrated form (Table 10, entries 10 and 11). 

The bismuth salt, Bi Og.GWOg.SHoO, is obtained as a white precipitate on mixing solutions of bismuth acetate and sodium tritungstate in presence of alcohol. 

Via thermolysis of bismuth sulfinates (method L) Thermolysis of bismuth tris(arenesulfinate), which can be prepared from bismuth acetate and arenesulfinic acid in glacial acetic acid, leads to the formation of triarylbismuthines [72AJC2107]. 

Solutions of benzenesulfinic acid (ca. 4 mmol) and bismuth acetate (1.0 mmol) in glacial acetic acid (50 and 100 ml, respectively) were mixed and evaporated to dryness under vacuum. The residue was washed with chloroform and dried under a stream of nitrogen to give bismuth tris(benzenesulfinate) in 82% yield [72AJC2107]. 

A similar oxidation of olefins with sodium bismuthate in acetic acid at elevated temperature (100°C) leads to a mixture of v/c-diol diacetate and carbonyl compound arising from oxidative cleavage, together with molecular oxygen and carbon dioxide [71KKZ1807]. In the absence of an olefin, sodium bismuthate works destructively on acetic acid to generate molecular oxygen, carbon dioxide, methyl acetate and a trace amount of methane, while the bismuthate is converted to bismuth acetate when the reaction is complete. The proposed mechanism is shown in Scheme 5.6. 

Scheme 5.14. Bismuth acetate-mediated oxidation of cyclohexene to 1,2-cyclohexanediol derivatives [89CC407].

Oxidation of oleflns to tra/is-l,2-diol diacetates with bismuth acetate typical procedure for dry system... 

Bismuth acetate (4.75 g, 12.3 mmol) was heated under reflux for 1 h in dry acetic acid (40 ml) containing acetic anhydride (ca. 17%). To the cooled solution, cyclohexene (3.01 g, 36.6 mmol) and iodine (9.39 g, 37.0 mmol) were added and the resulting mixture was further refluxed for 17.5 h. Usual work-up followed by distillation gave rrans-1,2-diacetoxycyclohexane (3.56 g, 80%) with no trace of the cw-isomer [89CC407]. 

Additional examples of the bismuth-promoted oxidation include the bismuth phthalocyanine catalyzed hydroxylation of phenol to hydroquinone and catechol with hydrogen peroxide [91SSSC(66)455], and bismuth acetate-mediated oxidation (modified Prevost reaction) of cyclohexenes to 1,2-cyclo-hexandiol derivatives (Scheme 5.14) [89CC407]. 

The modified Prevost reaction uses bismuth acetate instead of silver benzoate. From a mechanistic point of view, iodine is considered to be the actual oxidant, and hence the bismuth acetate may work merely as the acetoxy source. Major advantages of this method over the previously reported analogous ones are as follows (i) bismuth-containing wastes are non-toxic, (ii) bismuth acetate is inexpensive, and (iii) less metal salt is needed, since three acetoxyl groups can be transferred per one bismuth atom. 

Bismuth nitrate can be used for the nitration of aromatic hydrocarbons [88TL5909, 98S1724] and bismuth acetate for the acetylation of secondary amines [73JOC764], as shown in Scheme 5.42. Metallic bismuth and bismuth halides are employed as catalysts for the phosphorization and chlorination of... 

Oxalic acid and 20 precipitate white Bi2(C204)3, soluble in strong acids if not too dilute. Bismuth acetate is of minor importance. 

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