There are several transformations that are conceptually related to carbene reactions but do not involve carbene, or even varbenoid intermediates. Usually these are reactions in which the generation of a carbene is circumcented by a concerted rearrangement process. An important example of this type of reaction is the thermal and photochemical reactions of ?-diazoketons. When ?-diazoketons are decomposed thermally or phtochemically, the usually undergo rearrangement to ketenes. This reaction is known as Wolff Rearrangement.
If this reaction procedes in a concerted fashion, a carbene intermediate is avoided. Mechanistic studies have been aimed at determining if migration is concerted with loss of nitrogen. The conclusion that has emerged is that a carbene is generated in photochemical reactions but the reaction can be concerted under the thermal conditions. A realted issue is weather the carbene, when involved, is in equillibrium with a ring closed isomer, and oxirene. this aspect o the reaction has been probed by isotopic labelling. If a symmetrical oxirene is formed, teh label should be distributed to both the carbony carbon and the ?-carbon. A concerted reaction or a carbene intermediate that did not equillibriate with the oxirene should ahve been only in the carbonyl carbon. The extendt to which teh exorene is formed depends upon the structre of the diazo compound. For diazoacealdehyde, photolysis leads to only 8% migration of label which would correspond to the formation of 16% product through oxirene.
The diphenyl analog shows about 20-30 % rearrangement. ?-Diazocyclohexanone gives no evidence of an oxirene intermediate, since all the label remains at the carbonyl carbon.
The main synthetic application of Wolff rearrangement is for the one-carbon homologation of carboxylic acids. In this procedure, a diazomethyl ketone is syntehsized from an acyl chloride. The rearrangement is then carried out in a nucleophilic solved which traps the ketene to form a carboxylic acid or an ester. Silver oxide is often used as a catalyst because it seems to promote the rearrangement over carbene formation. The photolysis of ?-diazoketons results in ring contraction to a ketene which is usually isolated as the corresponding ester.
Today, I am going to write on a very basic type of reaction. The reaction is famously called aldol condensation. Most of the chemists of the world would be aware of this reaction and its mechanism. Anyhow, I am getting used to this writing, and would appreciate you people to bear with my new found interest.
Aldol condensation is a very useful reaction in the field of chemistry. It is a “bond forming” reaction and thus has wide applications in the field of synthetic chemistry. Independently discovered by Wurtz and Borodin, it involves a neucleophilic addition of a ketone moiety to an aldehyde molecule. This finally results into the synthesis of a ?-hydroxy ketone. Sometimes the ?-hydroxy ketone loses a molecule of water to give ?,?-unsaturated ketone. This reaction is called aldol condensation.
Aldehydes posessing atleast 1 Hydrogen atom is partially converted to its enolate anion by bases such as hydroxide and alkoxide ions.
Because the pKa’s of the aldehyde and water are similar, the solution contains signifi-
cant quantities of both the aldehyde and its enolate. Moreover, their reactivities are com-
plementary. The aldehyde is capable of undergoing nucleophilic addition to its carbonyl
group, and the enolate is a nucleophile capable of adding to a carbonyl group. And as
shown in Figure 18.4, this is exactly what happens. The product of this step is an alkox-
ide, which abstracts a proton from the solvent (usually water or ethanol) to yield a
[3-hydroxy aidehyde. A compound of this type is known as an aldol because it contains
both an aldehyde function and a hydroxyl group (ald + ol = aldol). The reaction is
called aldol addition.
