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Acid-Catalyzed α-Halogenation of Aldehydes and Ketones01:21

Acid-Catalyzed α-Halogenation of Aldehydes and Ketones

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By replacing an α-hydrogen with a halogen, acid-catalyzed α-halogenation of aldehydes or ketones yields a monohalogenated product
In the first step of the mechanism, the acid protonates the carbonyl oxygen resulting in a resonance-stabilized cation, which subsequently loses an α-hydrogen to form an enol tautomer. The C=C bond in an enol is highly nucleophilic because of the electron-donating nature of the –OH group. Consequently, the double bond attacks an electrophilic halogen to form a...
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Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule02:17

Regioselectivity of Electrophilic Additions to Alkenes: Markovnikov's Rule

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If a set of reactants can yield multiple constitutional isomers, but one of the isomers is obtained as the major product, the reaction is said to be regioselective. In such reactions, bond formation or breaking is favored at one reaction site over others.
The hydrohalogenation of an unsymmetrical alkene can yield two haloalkane products, depending on which vinylic carbon takes up the halogen. However, one product usually predominates, where hydrogen adds to the vinylic carbon bearing the...
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Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene01:14

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Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide.
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Electrophilic Addition to Alkynes: Hydrohalogenation02:35

Electrophilic Addition to Alkynes: Hydrohalogenation

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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.
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Halogenation of Alkenes02:46

Halogenation of Alkenes

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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.
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Base-Promoted α-Halogenation of Aldehydes and Ketones00:51

Base-Promoted α-Halogenation of Aldehydes and Ketones

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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...
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Engineered Flavin-Dependent Halogenases Catalyze C-C Bond Formation via Enantioselective Semipinacol Rearrangement.

Xuzhong Shen1, Paras Gupta1, Pratibha Gandhi1

  • 1Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States.

Journal of the American Chemical Society
|January 15, 2026
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Summary

Flavin-dependent halogenases catalyze enantioselective semipinacol rearrangements of allylic alcohols, creating chiral ketones. A T52G mutation enabled this novel biocatalytic transformation, expanding access to valuable molecular scaffolds.

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Area of Science:

  • Synthetic organic chemistry
  • Biocatalysis
  • Enzyme engineering

Background:

  • Semipinacol rearrangement is crucial for synthesizing molecular scaffolds.
  • Existing semipinacolases are limited, with only one new-to-nature example using Brønsted acid catalysis.
  • Flavin-dependent halogenases (FDHs) are known for halogenation reactions.

Purpose of the Study:

  • To explore the potential of flavin-dependent halogenases (FDHs) in catalyzing semipinacol rearrangements.
  • To develop a biocatalytic method for enantioselective semipinacol rearrangement.
  • To investigate enzyme engineering strategies for novel catalytic functions.

Main Methods:

  • Utilized flavin-dependent halogenases (FDHs) for catalytic semipinacol rearrangement.
  • Employed prochiral allylic alcohols as substrates.
  • Investigated the role of specific mutations, such as T52G, in enzyme activity.
  • Analyzed substrate scope and enantioselectivity of the reaction.

Main Results:

  • FDHs successfully catalyzed enantioselective halogenative semipinacol rearrangement of allylic alcohols.
  • The biocatalytic platform demonstrated a broad substrate scope, yielding chiral ketones with quaternary stereocenters.
  • Achieved high enantioselectivity and a kcat value of 16.86 ± 0.97 min-1, the highest for FDH catalysis.
  • Identified the T52G mutation as critical for enabling this non-native transformation by reshaping the enzyme's active site.

Conclusions:

  • Flavin-dependent halogenases can be engineered to perform semipinacol rearrangements, a non-native function.
  • This study presents the first example of asymmetric C-C bond construction using an FDH.
  • The developed biocatalytic platform provides access to enantioenriched carbocycles and heterocycles, highlighting enzyme catalytic flexibility.