Disproportionation of aldehydes

For several decades after this, the metal alkoxides remained objects of interest only for organic chemists, who applied them for reduction of carbonyl compounds and as catalysts for disproportionation of aldehydes, condensation and polymerization. Among these works, of special interest is the work of Meerwein and Bersin [1101], who were the first to prepare the bimetallic alkoxides that were then called Meerwein alkoxosalts in analogy... 

To account for the vanom oxygenated products observed in catalytic CO reductions, a Cannizarro-iype reaction should also be considered. Maiilis reports the disproportionation of aldehydes to acids and alcohols using complexes of rhodium and ruthenium 7l),... 

The corresponding iodo complexes are readily prepared by metathesis reactions with sodium iodide. Other reactions have been reviewed and recently [M(> -CjMe5)Cl2]2 have proved to be valuable precursors to novel bridged carbene complexes of the type [ M(> -C5Me5)Y 2(/i-CH2)2]- In the presence of base, [Rh(> -C5Me5)Cl2]2 is an effective catalyst for the hydrogenation of olefins and for the hydrosilylation of olefins and the disproportionation of aldehydes. ... 

There is also a group of reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic polarity or tendency to break in the way required for hydride transfer. These reactions usually proceed via cyclic transition states in which new C-H bonds are formed simultaneously with the cleavage. Hydride transfer is also facilitated by high charge density on the donor carbon atom. The Cannizzaro reaction, the base-catalyzed disproportionation of aldehydes, is one example of a hydride-transfer reaction. A general mechanism is outlined below. The hydride transfer is believed to occur from a species bearing two... 

The following is called the Cannizarro reaction. This is a base-catalyzed disproportionation of aldehydes that have no a-hydrogens (e.g., R = phenyl). Write a mechanism for this reaction showing all electron pushing. Derive a kinetic expression for your mechanism and predict the kinetic order in aldehyde and hydroxide. What experiments would you perform to test the validity of your mechanism Can you write an alternative mechanism that would give different experimental... 

Alkali metal t-butoxides, hydrides and bis(TMS)amides efficiently catalyse Claisen-Tishchenko disproportionation of aldehydes to the corresponding carboxylic esters. Potassium bases were more effective than sodium, and 18-crown-6 further accelerates the reaction. Kinetic studies suggest that the rate-determining step is a second-order concerted hydride transfer from a potassium hemiacetal to another molecule of aldehyde. 

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. 

Menashe N, Shvo Y. Catalytic disproportionation of aldehydes with ruthenium complexes. Orgcmometallics 1991 10 3885-3891. 

Another method for producing petoxycatboxyhc acids is by autoxidation of aldehydes (168). The reaction is a free-radical chain process, initiated by organic peroxides, uv irradiation, o2one, and various metal salts. It is terrninated by free-radical inhibitors (181,183). In certain cases, the petoxycatboxyhc acid forms an adduct with the aldehyde from which the petoxycatboxyhc acid can be hberated by heating or by acid hydrolysis. If the petoxycatboxyhc acid remains in contact with excess aldehyde, a redox disproportionation reaction occurs that forms a catboxyhc acid ... 

The most efficient intramolecular secondary processes competing with the acyl-alkyl diradical recombination in five-membered and larger cyclic ketones are hydrogen shifts resulting in the disproportionation of the diradical to either ketenes or unsaturated aldehydes [cf. (5) (4) (6)]. 

Cannizzaro Reaction The formation of an acid and an alcohol through the base-catalyzed disproportionation of an aliphatic or aromatic aldehyde with no a-hydrogen atoms. 

The Cannizzaro reaction, that is, the base-catalysed disproportionation of a carbonyl compound to an alcohol and a carboxylic acid, has gained some importance as an economically viable alternative to the reduction with borohydrides. However, the reaction is restricted to carbonyl compounds without any a-hydrogen, which do not undergo competing aldol reactions. Thus, mainly aromatic aldehydes are used for this kind of transformation. The protocols developed for microwave applications typically involve solvent-free conditions using alumina as the solid support. Under these conditions, a significant acceleration of the reaction was achieved. 

This redox disproportionation of non-enolizable aldehydes to carboxylic acids and alcohols is conducted in concentrated base. 

The dianion becomes a much more stable carboxylate monoanion, and a second molecule of aldehyde has been reduced to an alcohol. This is the Cannizzaro reaction in this case it takes the form of a disproportionation of two molecules of aldehyde to one HO as nucleophile of carboxylate and one of alcohol. 

Habermehl and Schunk289 have drawn attention to the analogy between the pseudobase disproportionation and the Cannizzarro reaction of aldehydes, which can be considered to be a disproportionation of the aldehyde hydrate to a carboxylic acid and alcohol. This reaction is usually considered to involve hydride transfer from either the mono- or dianion (i.e., 143 or 144) of the aldehyde hydrate to the carbonyl group of another aldehyde molecule. The anions 143 and 144 are clearly quite similar electronically... 

A convenient synthesis [Eq. (13)] of 2-ary lidenehydrazono-4-thia-zolidinone (43) from ethyl thiocyanoacetate (44), arylideneazines (45), and hydrazine hydrate (46) has been reported.79 Presumably, the reaction proceeds via the intermediary hydrazone, which condenses with 44 to give 43. Subsequently, condensation of 43 with the aromatic aldehyde, generated in situ from disproportionation of the azine with hydrazine, affords the 5-arylidene derivative. The reaction of 44 with ammonium acetate and an aromatic aldehyde gives 5-arylidene-2-imino-4-thiazolidinones.80... 

Intramolecular disproportionation of [1] could conceivably result in the formation of unsaturated aldehyde or ester. Both paths are well known to occur with cyclic alkanones (la). Abstraction of H by the acyl radical portion of [1] would produce alkenal [3] while abstraction of by the alkyl radical portion of [1] would produce ketene [4] which may be efficiently trapped in nucleophilic alcoholic solvents to yield ester [5]. The intramolecular nature of alkenal formation is supported by deuterium labeling experiments (5), while the intramolecular nature of ketene formation is supported by (a) deuterium labeling experiments (6,7), and (b) the observed decrease in ketene formation with decreasing ring size (8). 

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