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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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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.
16.1K
[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction01:16

[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

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The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.
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Cycloaddition Reactions: Overview01:16

Cycloaddition Reactions: Overview

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Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.
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Cycloaddition Reactions: MO Requirements for Thermal Activation01:16

Cycloaddition Reactions: MO Requirements for Thermal Activation

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Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.
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Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry01:29

Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry

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Diels–Alder reactions between cyclic dienes locked in an s-cis configuration and dienophiles yield bridged bicyclic products.
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Strain-Sensitive On-Surface Ladderization by Non-Dehydrogenative Heterocyclization.

Yujing Ma1, Kazuma Sugawara2, Yusuke Ishigaki2

  • 1Research Center for Advanced Measurement and Characterization, National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki, 305-0047, Japan.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|December 20, 2022
PubMed
Summary

Researchers developed a new low-barrier cyclization method for synthesizing pyrrolopyrrole-bridged ladder oligomers. This approach avoids high temperatures and random fusion, enabling precise molecular construction on surfaces.

Keywords:
hetero-atomsisomerizationpyrazinopyrrolopyrrolopyrazinescanning tunnelling microscopyπ-conjugation

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Area of Science:

  • Surface Chemistry
  • Organic Synthesis
  • Materials Science

Background:

  • On-surface cyclodehydrogenation is crucial for planarizing π-conjugated molecules.
  • High-temperature annealing required for C-H bond cleavage limits precursor structures and can cause unwanted side reactions.
  • Existing methods face challenges in controlling molecular fusion and precursor design.

Purpose of the Study:

  • To develop a novel, low-activation barrier method for synthesizing pyrrolopyrrole-bridged ladder oligomers.
  • To investigate a non-dehydrogenative cyclization pathway for molecular assembly on surfaces.
  • To gain new insights into strain-sensitive ladder-oligomer formation.

Main Methods:

  • Synthesis of pyrrolopyrrole-bridged ladder oligomers from 11,11,12,12-tetrabromo-1,4,5,8-tetraaza-9,10-anthraquinodimethane.
  • Utilized Ag(111) as a surface substrate for molecular reactions.
  • Employed bond-resolved scanning tunneling microscopy (STM) for in-situ characterization.

Main Results:

  • A non-dehydrogenative cyclization pathway was successfully demonstrated.
  • Low-activation barrier formation of an intermediary dimeric oligomer containing dipyrazinopyrrolopyrrolopyrazine units.
  • The reaction proceeds via cyclization between pyrazine and ethynylene/cumulene groups.

Conclusions:

  • The new method offers a milder alternative to high-temperature cyclodehydrogenation.
  • This pathway enables the controlled synthesis of complex ladder oligomers with specific structures.
  • Provides fundamental understanding of strain-sensitive reactions in surface-assisted molecular assembly.