Benzaldehydes conversion into benzoin

Mix 10 grammes of benzaldehyde with 20 grammes of alcohol and treat the mixture with a solution of 2 grammes of potassium cyanide and 5 c.c. of water. Boil on the water-bath for one hour (reflux condenser). The hot solution is poured into a beaker and allowed to cool slowly the crystals separating out are filtered off, washed with alcohol, and dried on the water-bath. For conversion into benzil (see next preparation), they need not be recrystallised. In order to obtain perfectly pure benzoin, a small portion of the crude product is recrystallised from a little alcohol in a test-tube. Melting-point, 1340- Yield, about 90% of the theory. 

Heterazolium salts such as thiazolium salts (12.86) and triazohum salts " are able to catalyse the conversion of benzaldehyde (12.87) into benzoin (12.88). The mechanism involves deprotonation of the thiazolium salt to give the true catalytic species (12.89), which acts as a nucleophile towards benzaldehyde, and subsequently promotes the formation of benzoin. 

Milller and co-workers recently developed an enantioselective benzoin dimerization using purified enzymes from Pseudomonas. The thiamine diphosphate (ThDP) dependent enzymes benzaldehyde lyase (BAL) and benzoylformate decarboxylase (BED) were found to catalyze the reversible benzoin condensation of aromatic aldehydes. The reaction is driven in the forward direction by the poor solubility of the benzoin products in aqueous media. A wide variety of aromatic aldehydes are accepted by BAL, and products of the (/ )-configuration are produced in excellent yield and enantiomeric purity. The (S)-enantiomer of benzoin is also available in high enantiomeric purity from a BAL-catalyzed kinetic resolution of rac-benzoin. In the presence of excess acetaldehyde, BAL selectively converts (i )-benzoin into (/ )-2-hydroxy-l-phenylpropanone, while the (iS)-benzoin enantiomer is not a substrate for the enzyme. At 49% conversion, (5)-benzoin is resolved to > 99% ee. BED can produce (i )-benzoin from benzaldehyde in comparable yield and enantiomeric purity with respect to BAL, but the substrate scope appears more limited. ... 

A very suitable oxidant for the conversion of acyloins into a-diketones is ammonium nitrate in the presence of catalytic amounts of cupric acetate. This reagent converts benzoin into benzil in 90% yield [476], The same result is obtained with bismuth sesquioxide 481] and sodium bromate 740 (equation 450). On the other hand, ceric ammonium nitrate does not give benzil but cleaves the bond between the alcoholic and the keto groups and cleaves benzoin into benzaldehyde and benzoic acid [425]. 

A detailed Organic Synthesis procedure is available for the conversion of benzil into benzilic acid. It is advantageous to isolate the acid as its potassium salt, for this enables the removal of the more soluble potassium benzoate, which results from a competitive cleavage reaction. Benzilic acid is then obtained in 77-79% yield by acidifying an aqueous solution of potassium benzilate. Since benzil is usually obtained by the oxidation of benzoin, both this conversion and rearrangement can be performed in tandem by using alkaline sodium bromate. Thus, benzilic acid is obtained in 84-90% yield from benzaldehyde via benzoin. 

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