ACYLOIN CONDENSATION Nitesh Sharma, group 10. Science adviser is Svetlana Kozub

ACYLOIN CONDENSATION

Nitesh Sharma, group 10. Science adviser is Svetlana Kozub.

The bimolecular reductive coupling of carboxylic esters by reaction with metallic sodium in an inert solvent under reflux gives an α-hydroxyketone, which is known as an acyloin. This reaction is favoured when R is an alkyl. With longer alkyl chains, higher boiling solvents can be used. The intramolecular version of this reaction has been used extensively to close rings of different sizes, e.g. paracyclophanes or catenanes.

If the reaction is carried out in the presence of a proton donor, such as alcohol, simple reduction of the ester to the alcohol takes place (Bouveault-Blanc Reduction).

The Benzoin Condensation produces similar products, although with aromatic substituents and under different conditions.

When the acyloin condensation is carried out in the presence of chlorotrimethylsilane, the enediolate intermediate is trapped as the bis-silyl derivative. This can be isolated and subsequently is hydrolysed under acidic condition to the acyloin, which gives a better overall yield.

Mechanism of Acyloin Condensation

KUMADA COUPLING REACTION

Namrata Pal, group 10. Science adviser is Svetlana Kozub.

The procedure uses transition metal catalysts, typically nickel or palladium, to couple combination of two alkyl, aryl or vinyl groups.

The groups of Robert Corriu and Makoto Kumada reported the reaction independently in 1972.

The reaction :

Applications :

1. Synthesis of Aliskiren -The Kumada coupling suitable for large-scale, industrial processes, such as drug synthesis. The reaction is used to construct the carbon skeleton of aliskiren (trade name Tekturna), a treatment for hypertension.

2. Synthesis of polythiophenes -The Kumada coupling also shows promise in the synthesis of conjugated polymers, polymers such as polyalkylthiophenes (PAT), which have a variety of potential applications in organic solar cells and light-emitting diodes.

KOLBE–SCHMITT REACTION

Priyanka Shankar, group 10. Science adviser is Svetlana Kozub.

The Kolbe–Schmitt reaction or Kolbe process (named after Adolph Wilhelm Hermann Kolbe and Rudolf Schmitt) is a carboxylation chemical reaction that proceeds by heating sodium phenolate (the sodium salt of phenol) with carbon dioxide under pressure (100 atm, 125 °C), then treating the product with sulfuric acid. The final product is an aromatic hydroxy acid which is also known as salicylic acid (the precursor to aspirin).

By using the potassium salt 4-hydroxybenzoic acid is accessible, an important precursor for the versatile paraben class of biocides used e.g. in personal care products.

Reaction mechanism

The Kolbe–Schmitt reaction proceeds via the nucleophile addition of a phenoxide, classically sodium phenoxide (NaOC6H5), to carbon dioxide to give the salicylate. The final step is reaction of the salicylate with acid to form the desired salicylic acid.

BLAISE REACTION

Spoorthi Spoo, group 10. Science adviser is Svetlana Kozub.

The Blaise reaction is an organic reaction that forms a β-ketoester from the reaction of zinc metal with a α-bromoester and a nitrile. The final intermediate is a metaloimine, which is hydrolyzed to give the desired β-ketoester.

Description: The Blaise reaction

Bulky aliphatic esters tend to give higher yields. Steven Hannick and YoshitoKishi have developed an improved procedure.

It has been noted that free hydroxyl groups can be tolerated in the course of this reaction, which is surprising for reactions of organometal halides.

Mechanism

The mechanism of the Blaise reaction involves the formation of an organozinc complex with the bromine alpha to the ester carbonyl. This makes the alpha carbon nucleophilic, allowing it to attack the electrophilic carbon of the nitrile. The negative nitrile nitrogen resulting from this attack complexes with the zinc monobromidecation. The β-enamino ester (tautomer of the imine intermediate pictured above) product is revealed by work-up with 50% K2CO3 aq. If the β-ketoester is the desired product, addition of 1 M hydrochloric acid hydrolyzes the β-enamino ester to turn the enamino into a ketone, forming the β-ketoester.

APPPEL REACTION

PapnejaAashish, group 10. Science adviser is Svetlana Kozub.

The Appel reaction is an organic reaction that converts an alcohol into an alkyl using triphenylphosphine and carbon tetrachloride. The use of carbon tetrabromide or bromine as a halide source will yield alkyl bromides, whereas using methyl iodide or iodine gives. The reaction is credited to and named after Rofl Appel it had however been described earlier.

Drawbacks to the reaction are the use of toxic halogenating agents and the coproduction of organophosphorus product that must be separated from the organic product. The phosphorus reagent can be used in catalytic quantities. The corresponding alkyl bromide can also be synthesised by addition of lithium bromide as a source of bromide ions.

Mechanism

The Appel reaction begins with the formation of the phosphonium salt 2.Deprotonation of the alcohol, forming chloroform 3, yields an alkoxide ion pair 4. The nucleophilic displacement of the chloride by the alkoxide yields intermediate 5. With primary and secondary alcohols, the halide reacts in a SN2 process forming the alkyl halide 6 and triphenylphosphine oxide 7. Tertiary alcohols form the products 6 and 7 via a SN1 mechanism.

The driving force behind this and similar reactions is the formation of the strong PO double bond The reaction is somewhat similar to the Mitsunobu Reaction, where the combination of an organophosphine as an oxide acceptor, a diazo compound as a hydrogen acceptor reagent, and a nucleophile are used to convert alcohols to esters and other applications like this.

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