name reaction c

name reaction c

Cadiot-Chodkiewicz Coupling

The copper(I)-catalyzed coupling of a terminal alkyne and an alkynyl halide offers access to unsymmetrical bisacetylenes.

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MECHANISM OF THE CADIOT-CHODKIEWICZ COUPLING

Cannizzaro Reaction

This redox disproportionation of non-enolizable aldehydes to carboxylic acids and alcohols is conducted in concentrated base.
α-Keto aldehydes give the product of an intramolecular disproportionation in excellent yields.

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MECHANISM OF THE CANNIZZARO REACTION

An interesting variant, the Crossed Cannizzaro Reaction, uses formaldehyde as reducing agent:

At the present time, various oxidizing and reducing agents can be used to carry out such conversions (with higher yields), so that today the Cannizzaro Reaction has limited synthetic utility except for the abovementioned conversion of α-keto aldehydes.
The Cannizzaro Reaction should be kept in mind as a source of potential side products when aldehydes are treated under basic conditions.

Corey-Bakshi-Shibata Reduction
Itsuno-Corey Reduction

The enantioselective reduction of ketones using borane and a chiral oxazaborolidine as catalyst (CBS catalyst). Usually, MeCBS is used (R'' = Me, but selectivity may be increased by varying this substituent).

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MECHANISM OF THE COREY-BAKSHI-SHIBATA REDUCTION

The mechanism depicted portrays the rationale for the enantioselectivity and high reaction rates, which are influenced only by the CBS catalyst. This catalyst is a combination of both a Lewis acid and a chiral auxiliary!

Chan-Lam Coupling

This reaction allows aryl carbon-heteroatom bond formation via an oxidative coupling of arylboronic acids, stannanes or siloxanes with N-H or O-H containing compounds in air. Substrates include phenols, amines, anilines, amides, imides, ureas, carbamates, and sulfonamides. The reaction is induced by a stoichiometric amount of copper(II) or a catalytic amount of copper catalyst which is reoxidized by atmospheric oxygen.
The Chan-Lam Coupling may be conducted at room temperature in air, which gives it a certain advantage over the  Buchwald-Hartwig Cross Coupling.

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MECHANISM OF THE CHAN-LAM COUPLING

The reaction with a stoichiometric amount of copper(II) is also facilitated by oxygen, because reductive elimination from a copper(III) species is faster.

Acetoacetic-Ester Condensation
Claisen Condensation

The Claisen Condensation between esters containing α-hydrogens, promoted by a base such as sodium ethoxide, affords β-ketoesters. The driving force is the formation of the stabilized anion of the β-keto ester. If two different esters are used, an essentially statistical mixture of all four products is generally obtained, and the preparation does not have high synthetic utility.
However, if one of the ester partners has enolizable α-hydrogens and the other does not (e.g., aromatic esters or carbonates), the mixed reaction (or crossed Claisen) can be synthetically useful. If ketones or nitriles are used as the donor in this condensation reaction, a β-diketone or a β-ketonitrile is obtained, respectively.
The use of stronger bases, e.g. sodium amide or sodium hydride instead of sodium ethoxide, often increases the yield.
The intramolecular version is known as Dieckmann Condensation.

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MECHANISM OF THE CLAISEN CONDENSATION

Claisen Rearrangement

The aliphatic Claisen Rearrangement is a [3,3]-sigmatropic rearrangement in which an allyl vinyl ether is converted thermally to an unsaturated carbonyl compound.
The aromatic Claisen Rearrangement is accompanied by a rearomatization:


The etherification of alcohols or phenols and their subsequent Claisen Rearrangement under thermal conditions makes possible an extension of the carbon chain of the molecule.

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MECHANISM OF THE CLAISEN REARRANGEMENT

The Claisen Rearrangement may be viewed as the oxa-variant of the Cope Rearrangement:
 Mechanism of the Cope Rearrangement
Mechanism of the Claisen Rearrangement
The reaction proceeds preferably via a chair transition state. Chiral, enantiomerically enriched starting materials give products of high optical purity.

A boat transition state is also possible, and can lead to side products:

The aromatic Claisen Rearrangement is followed by a rearomatization:

When the ortho-position is substituted, rearomatization cannot take place. The allyl group must first undergo a Cope Rearrangement to the para-position before tautomerization is possible.

All Claisen Rearrangement reactions described to date require temperatures of > 100 °C if uncatalyzed. The observation that electron withdrawing groups at C-1 of the vinyl moiety exert a positive influence on the reaction rate and the yield has led to the development of the following variations:
 Ireland-Claisen Rearrangement
 Eschenmoser-Claisen Rearrangement
 Johnson-Claisen Rearrangement

Azide-Alkyne Cycloaddition

"Click Chemistry" is a term that was introduced by K. B. Sharpless in 2001 to describe reactions that are high yielding, wide in scope, create only byproducts that can be removed without chromatography, are stereospecific, simple to perform, and can be conducted in easily removable or benign solvents. This concept was developed in parallel with the interest within the pharmaceutical, materials, and other industries in capabilities for generating large libraries of compounds for screening in discovery research. Several types of reaction have been identified that fulfill these criteria, thermodynamically-favored reactions that lead specifically to one product, such as nucleophilic ring opening reactions of epoxides and aziridines, non-aldol type carbonyl reactions, such as formation of hydrazones and heterocycles, additions to carbon-carbon multiple bonds, such oxidative formation of epoxides and Michael Additions, and cycloaddition reactions.
For example, an examination of the azide-alkyne cycloaddition shows that it fulfills many of the prerequisites. Many of the starting monosubstituted alkynes and organic azides are available commercially, many others can easily be synthesized with a wide range of functional groups, and their cycloaddition reaction selectively gives 1,2,3-triazoles.

Unfortunately, the thermal Huisgen 1,3-Dipolar Cycloaddition of alkynes to azides requires elevated temperatures and often produces mixtures of the two regioisomers when using asymmetric alkynes. In this respect, the classic 1,3-dipolar cycloaddition fails as a true click reaction. A copper-catalyzed variant that follows a different mechanism can be conducted under aqueous conditions, even at room temperature. Additionally, whereas the classic Huisgen 1,3-dipolar cycloaddition often gives mixtures of regioisomers, the copper-catalyzed reaction allows the synthesis of the 1,4-disubstituted regioisomers specifically. By contrast, a later developed ruthenium-catalyzed reaction gives the opposite regioselectivity with the formation of 1,5-disubstituted triazoles. Thus, these catalyzed reactions comply fully with the definition of click chemistry and have put a focus on azide-alkyne cycloaddition as a prototype click reaction.

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MECHANISM OF THE HUISGEN AZIDE-ALKYNE 1,3-DIPOLAR CYCLOADDITION

For the mechanism, please refer to the text on 1,3-dipolar cycloaddition. This reaction is highly exothermic, but the high activation barrier is responsible for a very low reaction rate, even at elevated temperature. Another drawback is the formation of regioisomers, as the two possible HOMO-LUMO interactions of the substrates are closely related in terms of energy. The thermal reaction therefore often gives approximately 1:1 mixtures of both the 1,4-substituted and the 1,5-substituted regioisomers.