Benzoin, formation

In an attempt to use an acyl anion equivalent to open an aziridine, Wu and co-workers isolated an unexpected ring opened product 316 (Eq. 31) [158], The authors found that the presence of oxygen was the determining factor between benzoin formation and ester formation. No desired ketones were ever formed. Various aromatic substituted aldehydes were treated under standard reaction conditions to afford esters in good yields. 4-Methoxybenzaldehyde provided product in only 40% yield, presumably due to the ease of aldehyde oxidation. 

As to the mechanism of benzoin formation, cyanide ion adds to the aldehyde to form 12. This anion is in equilibrium with 13, wherein the negative charge can be delocalized over the phenyl and nitrile groups. A subsequent aldol-type addition of 13 to the carbonyl carbon of a second aldehyde molecule gives the addition product 14, and loss of HCN from 14 leads to the benzoin ... 

Condensed Polymerization. A polymerigation in which atomic displacement occurs to some extent, as in aldol polymerization and benzoin formation... 

It has been suggested that cyanhydrin formation is an intermediate stage in this class of condensation, and benzoin formation is represented by the following scheme ... 

Quite often, the bisulfite product is isolated and purified before the treatment with alkali cyanide, particularly in the conversion of aromatic aldehydes since their bisulfite compounds are easily manipulated. The preparation of aromatic cyanohydrins from their bisulfite products is advantageous since benzoin formation, which is catalyzed by alkali cyanides, is largely avoided. Furthermore, because of the basic environment, hydrogen cyanide fumes are curtailed. 

Staudinger, H. The autooxidation of organic compounds connection between autooxidation and benzoin formation. Ber. Dtsch. Chem. Ges. 1913,46, 3535-3538. 

Chen, Y.-T., Barletta, G. L., Haghjoo, K., Cheng, J. T., Jordan, F. Reactions of Benzaldehyde with Thiazolium Salts in Me2SO Evidence for Initial Formation of 2-( -Hydroxybenzyl)thiazolium by Nucleophilic Addition, and for Dramatic Solvent Effects on Benzoin Formation. J. Org. Chem. 1994, 59, 7714-7722. 

Quinoxaline-2-carboxaldehyde has been converted into the 2-carboxylic acid by oxidation with potassium permanganate in acetone and reduced to the 2-hydroxymethyl compound by treatment with formalin and potassium hydroxide. It also undergoes other typical reactions of aromatic aldehydes such as benzoin formation on reaction with potassium cyanide - and condensation reactions with malonic acid and its diethyl ester and Schiff base formation. Acid-catalyzed reaction of quinoxaline-2-carboxaldehyde with ethylene glycol gives the cyclic acetal the diethylacetal has been prepared by reaction of 2-dibromomethylquinoxaline with sodium ethoxide. " An indirect preparation of the oxime 11 is achieved by treatment of 2-nitromethyl-quinoxaline (10) with diazomethane followed by thermolysis of the resulting nitronic ester. 

Use of the Electron-Withdrawal Catalyst (benzoin reaction). The specificity of the basic cyanide ion used as a catalyst in the formation of benzoin has been considered puzzling, and apparently no adequate explanation has yet been suggested. Hydrogen cyanide, mercuric cyanide, and sodium hydroxide have no effect on the rate of benzoin formation, whereas sodium cyanide, potassium cyanide, and barium cyanide have a powerful catalytic effect. ... 

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