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Base-Promoted α-Halogenation of Aldehydes and Ketones

Base-Promoted α-Halogenation of Aldehydes and Ketones

α-Halogenation of aldehydes and ketones is a reaction involving the substitution of α hydrogens with halogens in the presence of a base. The reaction begins with the abstraction of α hydrogen by the base to produce a nucleophilic enolate ion. This intermediate undergoes a subsequent nucleophilic substitution with the halogen to produce a monohalogenated carbonyl compound. If the starting substrate has more than one α hydrogen, it is difficult to stop the reaction...

Halogenation of Alkenes

Halogenation of Alkenes

Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.

Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.

Alkyl Halides

Alkyl Halides

Structural Properties
Alkyl halides are halogen-substituted alkanes wherein one or more hydrogen atoms of an alkane is replaced by a halogen atom such as fluorine, chlorine, bromine, or iodine. The carbon atom in an alkyl halide is bonded to the halogen atom, which is sp3-hybridized and exhibits a tetrahedral shape.
Unlike alkyl halides, compounds in which a halogen atom is bonded to an sp2 -hybridized carbon atom of a carbon-carbon double bond (C=C) are called vinyl halides. Whereas aryl...

Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene

Electrophilic 1,2- and 1,4-Addition of X₂ to 1,3-Butadiene

Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide.

Electrophilic Addition to Alkynes: Halogenation

Electrophilic Addition to Alkynes: Halogenation

Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.

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Efficient Synthesis of All-Carbon Quaternary Centers via the Conjugate Addition of Functionalized Monoorganozinc Bromides

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Programmed Sequential Additions to Halogenated Mucononitriles.

Adam J Zahara¹, Elsa M Hinds¹, Andrew L Nguyen¹

¹Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States.

Organic Letters

|October 6, 2020

Summary

This summary is machine-generated.

Dihalomucononitriles act as versatile linchpin reagents, enabling controlled synthesis of complex molecules. Their reactivity with nucleophiles allows for sequential additions, forming natural product-like structures.

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Area of Science:

Organic Chemistry
Synthetic Chemistry

Background:

Linchpin reagents are crucial for constructing complex molecular architectures.
Dihalomucononitriles offer potential as novel linchpin reagents due to their unique structure.

Purpose of the Study:

To synthesize dihalomucononitriles and evaluate their utility as linchpin reagents.
To explore the reactivity of these compounds with various nucleophiles.

Main Methods:

Synthesis of bis(2-chloroacrylonitrile) and bis(2-bromoacrylonitrile) from 2,1,3-benzothiadiazole.
Conjugate addition/elimination reactions with nitrogen and carbon nucleophiles.

Main Results:

Achieved 40-95% yield with nitrogen nucleophiles and 72-93% yield with carbon nucleophiles.
Demonstrated controlled monosubstitution with secondary amines and disubstitution with carbon nucleophiles.
Successfully synthesized mixed addition products by controlling nucleophile addition sequence.

Conclusions:

Dihalomucononitriles are effective linchpin reagents for organic synthesis.
The controlled reactivity allows for the construction of diverse molecular scaffolds.
Intramolecular cyclization of products yields natural product-like motifs.