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Related Concept Videos

Carbocations02:10

Carbocations

11.9K
Carbocations are one of the reaction intermediates formed during several nucleophilic substitutions or elimination reactions. A carbocation is an electron-deficient species with the central carbon atom having six electrons and three bonded atoms. The central carbon in a carbocation is sp2 hybridized with trigonal planar geometry. It has an empty p orbital perpendicular to the plane of the structure that can accept electrons. Thus, carbocations act as strong electrophiles and may react with any...
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Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

8.8K
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.
8.8K
Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation02:47

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

19.1K
Introduction
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
19.1K
Acidity of 1-Alkynes02:42

Acidity of 1-Alkynes

10.2K

The acidic strength of hydrocarbons follows the order: Alkynes > Alkenes > Alkanes. The strength of an acid is commonly expressed in units of pKa — the lower the pKa, the stronger the acid. Among the hydrocarbons, terminal alkynes have lower pKa values and are, therefore, more acidic. For example, the pKa values for ethane, ethene, and acetylene are 51, 44, and 25, respectively, as shown here.
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Aldehydes and Ketones to Alkenes: Wittig Reaction Mechanism01:14

Aldehydes and Ketones to Alkenes: Wittig Reaction Mechanism

4.0K
The Wittig reaction, which converts aldehydes or ketones to alkenes using phosphorus ylides, proceeds through a nucleophilic addition‒elimination process.
The reaction begins with the nucleophilic addition between a phosphorus ylide and the carbonyl compound. Due to its carbanionic character,  phosphorus ylide acts as a strong nucleophile and attacks the electrophilic carbonyl group. This generates a charge-separated dipolar intermediate called betaine. The negatively charged oxygen atom and...
4.0K
α-Alkylation of Ketones via Enolate Ions01:10

α-Alkylation of Ketones via Enolate Ions

3.3K
Ketones with α protons are deprotonated by strong bases like lithium diisopropylamide (LDA) to form enolate ions. The anion is stabilized by resonance, and its hybrid structure exhibits negative charges on the carbonyl oxygen and the α carbon. This ambident nucleophile can attack an electrophile via two possible sites: the carbonyl oxygen, known as O-attack, or the α carbon, known as C-attack. The nucleophilic attack via the carbanionic site is preferred. This is due to the...
3.3K

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Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of PhosphorusI
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Preparation and Reactivity of a Triphosphenium Bromide Salt: A Convenient and Stable Source of PhosphorusI

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Alkylphosphinites as Synthons for Stabilized Carbocations.

Lukas Ochmann1, Mika L Kessler1, Peter R Schreiner1

  • 1Institute of Organic Chemistry, Justus Liebig University, 35392 Giessen, Germany.

Organic Letters
|February 11, 2022
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Summary

A novel acid-free method generates carbocations via redox condensation, enabling SN1 reactions. This approach utilizes phosphinites and diisopropyl azodicarboxylate for alkylating bioactive molecules.

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Area of Science:

  • Organic Chemistry
  • Synthetic Chemistry

Background:

  • Carbocation generation often requires harsh acidic conditions.
  • Developing milder methods for carbocation synthesis is crucial for complex molecule synthesis.

Purpose of the Study:

  • To introduce a new acid-free carbocation generation method.
  • To demonstrate its utility in SN1 reactions and alkylation of bioactive molecules.

Main Methods:

  • Utilizing readily synthesized phosphinites activated by diisopropyl azodicarboxylate.
  • Formation of betaine intermediates that collapse to yield carbocations.
  • Reaction of carbocations with various nucleophiles and bioactive molecules.

Main Results:

  • Successful generation of carbocations under acid-free conditions.
  • Demonstration of SN1 reactions with diverse nucleophiles.
  • Application of the method for alkylating bioactive compounds.

Conclusions:

  • The developed redox condensation method offers a mild and effective route to carbocations.
  • This approach expands synthetic possibilities for SN1 reactions and functionalization of bioactive molecules.