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Benzoin Condensation and Stobbe Condensation KNOW MORE Weblinks https://en.wikipedia.org/wiki/Benzoin_condensation http://chemistry.tutorvista.com/organic-chemistry/benzoin- condensation.html http://www.organic-chemistry.org/namedreactions/benzoin- condensation.shtm http://www.chm.uri.edu/mlevine/chm521/Lecture%205%20Notes.p df https://en.wikipedia.org/wiki/Claisen_condensation http://www.aua.gr/~coulad/PDF_files/50_pdf.pdf CHEMISTRY Paper 9: Organic Chemistry-III (Reaction Mechanism-2) Module NO. 21: Benzoin Condensation and Stobbe Condensation Suggested Readings Name reactions: a collection od detailed reaction mechanism By Jie Jack Li Organic Reactions ad their mechanism By P. S Kalsi CHEMISTRY Paper 9: Organic Chemistry-III (Reaction Mechanism-2) Module NO. 21: Benzoin Condensation and Stobbe Condensation Advanced Organic Chemistry: Reations, Mechaninsm and Structure By Jerry March Glossary B Benzoin- It is a hydroxyl ketone attached to two phenyl groups. It appears as off-white crystals, with a light camphor-like odor. Benzoin addition- The benzoin condensation is in effect a dimerization and not a condensation because a small molecule like water is not released in this reaction. Therefore, it is often called as benzoin addition. Benzoin Condensation: The benzoin condensation is a reaction between two aromatic aldehydes, particularly benzaldehyde. The reaction is catalyzed by a nucleophile such as the cyanide anion or an N-heterocyclic carbene. The reaction product is an aromatic acyloin with benzoin as the parent compound. CHEMISTRY Paper 9: Organic Chemistry-III (Reaction Mechanism-2) Module NO. 21: Benzoin Condensation and Stobbe Condensation Clemmensen reduction- It is a chemical reaction described as a reduction of ketones (or aldehydes) to alkanes using zinc amalgam and hydrochloric acid. Clemmensen reduction of benzoin results in stillbene. Condensation reaction- It is a chemical reaction in which two molecules or moieties (functional groups) combine to form a larger molecule, together with the loss of a small molecule S Stobbe’s Condensation: The condensation reaction of aldehyde or ketone with succinic ester in the presence of basic catalyst like sodium hydroxide or potassium tertiary butoxide to form alkylidene succinic acid is known as Stobbe condensation. Do you know? Benzoin Condensation Benzoin condensation: This reaction is catalyzed by a thiazolium‐derived NHC. It was discovered because researcher knew that thiamine (vitamin B12) catalyzed the benzoin condensation, and they realized that the active part of it was the thiazolium. The structure of thiamine is shown below: The overall benzoin condensation is shown below, and involves two molecules of benzaldehyde reacting to form benzoin: And the mechanism of thiazolium catalysis is shown below: CHEMISTRY Paper 9: Organic Chemistry-III (Reaction Mechanism-2) Module NO. 21: Benzoin Condensation and Stobbe Condensation Basically, the thiazolium acts as a nucleophile and attack the carbonyl first. The resulting intermediate acts as a nucleophile to attack a second carbonyl and form the key new carbon‐carbon bond. There are two points I want to emphasize here. First is that when you see this substitution pattern of an alcohol on a carbon right next to a carbonyl, you should think retrosynthetically that this could have come from a benzoin‐type condensation. This is different from recognizing an aldol product, which has a 1,3 relationship between the carbonyl and the alcohol. Both of these structural motifs are shown below: Second point that you should keep in mind is that the role of the NHC is to take something that is a strong electrophile, an aldehyde, and convert it into a nucleophile that is then able to attack another molecule of the aldehyde. It does that through the so‐called “Breslow intermediate,” which has the carbene in a stable neutral enol form. This reversal of polarity – from electrophile to nucleophile – is called “umpolung” – which is a German word that basically means “switching.” There has been some research on asymmetric benzoin condensations – i.e. you are generating a new chiral center in the molecule, so chiral catalysis could give you some enantiomeric excess in the product. In general, these asymmetric reactions work better with the triazole‐based NHCs compared to either the thiazole or imidazole‐based ones. You can also have a “cross‐benzoin reaction,” where a ketone and aldehyde react. Usually this works only in an intramolecular setting. One example of a cross benzoin is shown here: CHEMISTRY Paper 9: Organic Chemistry-III (Reaction Mechanism-2) Module NO. 21: Benzoin Condensation and Stobbe Condensation and the newly formed bond is highlighted in red. CHEMISTRY Paper 9: Organic Chemistry-III (Reaction Mechanism-2) Module NO. 21: Benzoin Condensation and Stobbe Condensation .
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