Canizzaro reactions

CaniZZaro Reaction. Both 2- and 4-hydroxybenzaldehydes undergo this self-oxidation—reduction reaction, but much less readily than benzaldehyde the reaction requires metal catalysts such as nickel, cobalt, or silver to yield the corresponding hydroxybenzoic acids and hydroxybenzyl alcohols (47—48). 

The raw materials are usually mutually contaminated to about 5 %. Thus, DSC can be ruled out. The compounds are made by aldol condensations and cross Canizzaro reactions between acetaldehyde and formaldehyde. An elegant method of distinguishing between the two raw materials takes advantage of the 10 fold solubility difference between the two in methanol and water. 

Carbonyl double bonds can also be photoelectrochemically reduced ZnS sols prepared from cold oxygen-free aqueous solutions ofZnSO4 and NajS induce efficient photodisproportionation of aldehydes, i.e., a photo-Canizzaro reaction Thus, acetaldehyde produces ethanol and acetic acid, together with smaller quantities of biacetyl and acetoin, Eq. (37). 

Photoelectrochemical reduction of double bonds also takes place on ZnS sols. For example, efficient photodisproportionation of aldehydes, that is, a photo-Canizzaro reaction, can be observed on suspensions prepared from cold oxygen-free aqueous solutions of ZnS and Na2S (Eq. 29) [169]. 

Jackson, B. A. Acceleration of a Canizzaro reaction by a polymimic quaternary hydroxide. Chem. Ind. (London) 1964, 2022. 

The methyl ester-forming attack Qq could be due to the Canizzaro-type condensation of two aldehydes, to the reaction of surface carboxylates with formyl or formaldehyde, or to the reaction of surface methoxide with an aldehyde with a hydride elimination. To be consistent with the experimental observations, we shall give an example for the last mechanism because it requires the fewest number of steps, utilizes methoxide which may be favored in hydrogen rich synthesis gas and does not require a strong base catalyst. 

Prior to the advent of organocatalysis, the asymmetric direct a-allqtlation reaction was relatively unknown. Classical methods to access a-allq lated carbonyl products required stoichiometric amounts of preformed aldehyde metal enolates. Additionally, side reactions such as aldol, Canizzaro- or Tischenko-type processes diminished the efficiency of these reactions. In this sense the asymmetric intermolecular Sjj2 a-alkylation of aldehydes with simple allq l halides has been a difficult feat to achieve. 

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