Disproportionation Cannizzaro

The Cannizzaro disproportionation, 2RCHO + HO- — -RCIUO- + RCO2H, has been studied in the gas phase by a variety of MS techniques, for R = Bu and Ph,173 and results have been compared with calculations on the prototypical system, i.e. R = H. 

Although known as an explosive since 1894, PETN was used very little until after World War I when the ingredients to make the starting material became commercially available. The symmetrical, solid alcohol starting material, pentaerythritol, is made from acetaldehyde and formaldehyde, which react by aldol condensation under basic catalysis followed by a crossed Cannizzaro disproportionation to produce the alcohol and formate salt. Although the reaction takes place in a single mixture, it is shown below in two steps for clarity. 

Reactions of pyrazinecarbaldehydes already discussed include reduction to hydroxyalkylpyrazines (Section 5.2.1) and oxidation to pyrazinecarboxylic acids (Section 8.1.1) no Cannizzaro disproportionations appear to have been reported recently. 

A wide variety of reaction conditions have been proposed for the reaction of starch with aldehydes. Early studies entailed use of alkaline solutions, perhaps in order to gelatinize starch.1276 Because of the known behavior of aldehydes in alkaline media (Cannizzaro disproportionation and aldol condensations of aldehydes with active active a-methylene groups), this reaction has no simple outcome. However, a process of acetalation in alkaline solutions was subsequendy patented.1293 An exothermic reaction was observed within 3 4 min using an excess of formaldehyde. It was also reported that a,(3-unsaturated aldehydes reacted readily in slightly basic media.1294... 

Aldehydes and alcohols can be oxidized to their corresponding carboxylic acids on certain oxide surfaces. Carboxylate formation can also take place by the breakdown of certain adsorbates, such as an ester to carboxylate and alkoxy, or by hydride transfer from one adsorbate to another, as takes place in the Cannizzaro disproportionation reaction of aldehydes to alkoxy and carboxylate. The oxidation reactions are less likely on hydroxylated surfaces, because the surface is less nucleophilic. Hydrogenation of an acid to an aldehyde can take place on oxides of intermediate M-O bond strength at temperatures of 623-723 K, and at lower temperatures on the Group VIII metals with low M-O bond strength. Obviously, once the carboxylates are formed, condensation to ketones could occur according to one of the mechanisms given above. 

Full details have been published on the oxidative rearrangement of pyruvates to malonates. Whereas the reduction of dimethyl bismethylmalonate with sodium in xylene is reported to yield the keten acetal (9) in the presence of trimethylchlorosilane, when the solvent is ammonia a mixture of products is formed, including the cyclopropanediol di(trimethylsilyl)ether (10). This diol, on treatment with sodium methoxide, gives the products shown in Scheme 39, possibly by Cannizzaro disproportionation of the aldehyde (11). The mechanism of decarboxylation of monoethyl oxaloacetate has been studied as part of a general investigation of ) -keto-acid decarboxylation. 

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