Cannizzaro final product

Under similar conditions, reactions between pyrrolidine derivatives 632 and MTAD proceed much more slowly and less cleanly with formation of a polymeric material. When the reaction is stopped before 50% conversion is reached, starting compound 632 is isolated as the main component (c. 40%) and compound 637 as a minor product (10-14%). Mechanistically, the most difficult problem lies in the fact that a reduction step has to be involved and no particular reduction agent is present. A proposed mechanism is shown in Scheme 103. The pathway includes a Cannizzaro-type hydride transfer between dipole 633 and product 634 (keto tautomer), resulting in the formation of the iminium derivative 635, which might be responsible for the polymeric material, and hydroxy derivative 636, the direct precursor of the final products 637. The low experimental yield of 637 could be explained by this mechanism <2003EJ01438>. 

Even this is not all. A fourth molecule of formaldehyde reacts with hydroxide ion and then reduces the third aldol. This reduction is known as the Cannizzaro reaction, and is described in the box. The final product is the highly symmetrical pentaerythritol , CfCTbOH) with four CH2OH groups joined in a tetrahedral array about the same carbon atom. 

Figure P. OPA neutralization by Cannizzaro reactions final product IS independently confirmed by GC/MS.

Sugars (hexoses) yield formic acid as a product of the Cannizzaro reaction of formaldehyde and as one of the final products of the dehydration of sugars in addic media and their degradation in neutral and alkaHne media. At high concentrations (in quantities of up to 2%), formic add therefore occurs in acid protein hydrolysates, where it is formed by 5-hydroxymethyl-2-furancarbaldehyde degradation. In randd fats, formic acid arises by oxidative decomposition of aldehydes. Formic acid is sometimes used as a preservative. 

The proposed mechanism of this transformation (Scheme 39), as supported by GC/MS and isotope labeling studies, involves removal of the hydroxide proton of ethanol by a base, thus giving a molecule of water and ethanolate (Step 1). This anion rapidly reacts with water to give H2, acetaldehyde, and hydroxide ion (Step 2). Reaction of the hydroxide ion with the carbonyl carbon of acetaldehyde (via nucleophilic addition) forms alkoxide which is then deprotonated and gives a dianionic Cannizzaro intermediate, that subsequendy reacts with another molecule of acetaldehyde to give the final product (sodium acetate) and a molecule of ethanol (Step 3). 

Coupled voltammetric and pH measurements, performed as previously described during the prolonged electrolysis, are necessary to obtain a complete balance between the quantity of electricity measured and that of the compounds consumed or produced. Such information, concerning both C- and 0-containing species, are particularly useful when the electrochemical process is accompanied or followed by significant chemical transformations like hydrolysis, aldolisation, Cannizzaro reaction, enolisation, etc., involving reactants and the intermediate or final products. At room temperature, the chemical transformation of blank"... 

The Crossed Cannizzaro Reaction.90- Into a 2-1. three-necked flask fitted with a dropping funnel, mercury-sealed stirrer, and reflux condenser are introduced one mole of the aromatic aldehyde, 700 cc. of methanol, and 100 cc. (1.3 moles) of formalin. The solution is heated to 65°, and the flask is surrounded by cold water while a solution of 120 g. of sodium hydroxide or 168 g. (3 moles) of potassium hydroxide in 120 cc. of water is added rapidly, the temperature being maintained at 65-75°. The mixture is then heated at 75° for forty minutes and finally refluxed for twenty minutes. The solution is cooled, diluted with 300 cc. of water, the oily layer separated, and the aqueous layer extracted with four 150-co. portions of benzene. The combined oil and benzene extracts are dried, the benzene removed, and the product distilled under reduced pressure. The yields are 85-95%. 

As mentioned above, hydrogen atoms, removed from the alcohol substrate, can return to form the product however, if the final hydrogenation step could not occur, a product that is more oxidized than the starting material is obtained. The formation of esters from alcohols and of amides from alcohols and amines concern the most representative and studied reactions of this type. In these cases, aldehydes, formed on the first oxidation stage from alcohols, undergo Tishchenko- and Cannizzaro-type reactions, where esters or carboxylates and alcohols are formed upon fusion or disproportionation of aldehydes, respectively. 

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