Benzilic acid mechanism

Although several mechanisms have been proposed for this reaction, only cyclopropanone mechanism (Scheme 1) and benzilic acid mechanism (Scheme 2) are displayed here. 

SCHEME 2. Favorskii rearrangement occurring a benzilic acid mechanism. 

The following reaction, called the benzilic acid rearrangement, tajkes place by typical carbonyl-group reactions. Propose a mechanism fPh = phenyl). 

Hammond postulate has been used to explain the effect of substituents on the rate of benzilic acid rearrangements, mechanism of electrophillic aromatic substitution reactions and reactions involving highly reactive intermediates such as carbonium ions and carbon ions. 

Aldoses generally undergo benzilic acid-type rearrangements to produce saccharinic acids, as well as reverse aldol (retro-aldol) reactions with j3-elimination, to afford a-dicarbonyl compounds. The products of these reactions are in considerable evidence at elevated temperatures. The conversions of ketoses and alduronic acids, however, are also of definite interest and will be emphasized as well. Furthermore, aldoses undergo anomerization and aldose-ketose isomerization (the Lobry de Bruyn-Alberda van Ekenstein transformation ) in aqueous base. However, both of these isomerizations are more appropriately studied at room temperature, and will be considered only in the context of other mechanisms. 

Komoto detected lactic acid in the mixture from reaction of D-glucose with ammonia,4 and presumed that it was produced from pyruvaldehyde formed by decomposition of D-glucose. Lactic acid has, indeed, been found as a product of the action of alkali (lime-water) on substituted D-glucose and substituted D-fructose,81,83,96 and the mechanism of its formation involves the reversible aldol reaction, followed by formation of pyruvaldehyde, and the benzilic acid rearrangement already described for saccharinic acid this is illustrated83,96 in Scheme 11. 

Exercise 17-43 Write a mechanism analogous to that for the Cannizzaro reaction for the benzilic acid transformation. What product would you expect to be formed from diphenylethanedione with potassium terf-butoxide in ferf-butyl alcohol Would you expect a benzilic acid-type rearrangement to occur with 2,3-butanedione Give your reasoning. 

Liebig observed the first intramolecular rearrangement in 1838 when he found that benzil in basic solution forms a new compound.136 In 1870 Jena correctly established the product of the reaction as benzilic acid, but proposed an incorrect structure for the starting material to avoid postulating a skeletal rearrangement.137 In 1928 Ingold proposed the mechanism shown in Equation 6.50, which today is solidly supported by experimental evidence.138... 

Organic Reaction Mechanism BENZILIC ACID REARRANGEMENT... 

The mechanism of this benzilic acid rearrangement starts with attack of hydroxide on one of the carbonyl groups. The tetrahedral intermediate can collapse in a reaction reminiscent of a semipinacol rearrangement, carbonyl group is formed here... 

With alkoxides, the benzilic acid rearrangement can lead directly to esters by the same sort Ph of mechanism. 

Nef s theory of the mechanism of formation of the saccharinic acids is outlined, in its original form, in a paper published in 1907 and, in its final form, in his comprehensive article of 1910. The theory proposes that the reaction takes place in two major steps (a) the isomerization of the sugar, with loss of water, to an a-dicarbonyl compound, and (b) a benzilic acid type of rearrangement of the latter, with hydration, to the saccharinic acid. The second step involves chain rearrangement in the production of the saccharinic, isosaccharinic, and Kiliani s parasaccharinic acids, but not in the production of the metasaccharinic acids. 

The final phase of the Nef mechanism, which involves a benzilic acid type of rearrangement of a-dicarbonyl intermediates to the saccharinic acids, is at present accepted as a feature of saccharinic acid formation. Nef s concept of the conversion of reducing sugars to the a-dicarbonyl structures required revision, however, when it became evident that the formation, in this step, of the proposed methylenic intermediates is highly improbable. A departure from the methylenic intermediates was suggested in 1926 by Evans and Benoy, who proposed that the a-dicarbonyl intermediates of the Nef mechanism might arise by successive dehydration and rehydration from the enediols. It is now recognized, however, that forma-... 

The Isbell ionic mechanism for the formation of the various types of saccharinic acid, as well as for Nicolet s conversion of 2-hydroxy-3-methoxy-3-phenylpropiophenone to 2,3-diphenyllactic acid, involves the following successive steps (1) the formation and ionization of an enediol (2) the /3-elimination of a hydroxyl or an alkoxyl group (3) rearrangement to an a-dicarbonyl intermediate and (4) a benzilic acid type of rearrangement to the saccharinic acid. 

Considering the formation of C02" on CdS-DMF, we successfully applied the CdS-DMF photocatalysis to the fixation of CO2 into benzophenone (BP), acetophenone (AP), and benzyl halides (BnCl and BtiBr) (Table 1). Four substrates gave benzilic acid, atrolactic acid, and phenylacetic acid as respective fixation products, with dimerized and hydrogenated products. Considering the mechanism proposed above, C02 formed through the same route as described therein should participate in the fixation reaction. 

Rajyaguru, I., Rzepa, H. S. A MNDO SCF-MO study of the mechanism of the benzilic acid and related rearrangements. J. Chem. Soc., Perkin Trans. 21987, 1819-1827. 

O Meara, D., Richards, G. N. Mechanism of saccharinic acid formation. IV. Influence of cations in the benzilic acid rearrangement of glyoxal. J. Chem. Soc. 1960, 1944-1945. 

Dienone-phenol rearrangements are mechanistically diverse. They may involve 1,2-shifts of the Wag-ner-Meerwein type, or of the benzil-benzilic acid kind 1,3-shifts by a Claisen-Cope mechanism 1,5-sigmatropic shifts Favorskii-like reactions and other types. They may also be induced photochemically. A number of reviews are available, which discuss mechanistic aspects in detail. In this chapter emphasis is put on preparative aspects of these reactions and the examples are organized on a structural basis, stressing the new bond(s) formed. 

The prototype reaction is the conversion of glyoxal into glycolic acid (equation 2), and here the benzilic acid rearrangement mechanism coincides with that for an intramolecular Cannizzaro reaction. The reaction is observed with other purely aliphatic a-diketones such as f-butyl 2,3-dioxobutyrate and cyclohexane-1,2-dione (equations 3 and 4), but the scope is limited in the aliphatic series by competing (c.g. aldol) reactions. Suitably constructed heterocyclic systems also rearrange, and the conversion of alloxan (3) into alloxanic acid (4) was among the first of the benzilic acid rearrangements to be discovered (equation 5). ... 

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