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Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

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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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Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
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Prochirality

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The concept of prochirality leads to the nomenclature of the individual faces of a molecule and plays a crucial role in the enantioselective reaction. It is a concept where two or more achiral molecules react to produce chiral products. A typical process is the reaction of an achiral ketone to generate a chiral alcohol. Here, the achiral reactant reacts with an achiral reducing agent, sodium borohydride, to generate an equimolar mixture of the chiral enantiomers of the product. For example, an...
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Cyanohydrins are compounds that contain –CN and –OH groups on the same carbon atom. They are formed by the nucleophilic addition of the cyanide ions to the carbonyl group. Cyanide ions are highly basic and nucleophilic and can be generated from HCN under aqueous conditions. However, since HCN is a weak acid, the number of cyanide ions generated is very small. Hence, a small amount of base or KCN/NaCN is added to HCN to increase the concentration of the cyanide ions in the reaction...
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Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
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Nomenclature of Alkynes02:39

Nomenclature of Alkynes

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Alkynes are unsaturated hydrocarbons characterized by the presence of carbon-carbon triple bonds and have a general formula CnH2n-2. The nomenclature of alkynes follows a set of rules similar to alkanes and alkenes; however, alkynes bear the suffix "-yne" instead of "-ane" or "-ene." There are two approaches to naming alkynes:
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Expanded Helicenes as Synthons for Chiral Macrocyclic Nanocarbons.

Gavin R Kiel1, Katherine L Bay2, Adrian E Samkian1

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Journal of the American Chemical Society
|May 27, 2020
PubMed
Summary

Researchers synthesized gram-scale quantities of an alkyne-functionalized expanded [11]helicene. This precursor was transformed into novel macrocyclic derivatives, including a figure-eight dimer and arylene-bridged helicenes, offering pathways to stable chiral nanocarbons.

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Area of Science:

  • Organic Chemistry
  • Materials Science
  • Nanotechnology

Background:

  • Expanded helicenes are versatile π-frameworks for chiral nanocarbon construction.
  • Developing efficient synthetic routes to complex helicene derivatives is crucial.

Purpose of the Study:

  • To achieve gram-scale synthesis of an alkyne-functionalized expanded [11]helicene.
  • To explore its transformation into distinct macrocyclic derivatives.
  • To investigate the configurational stability and enantiomerization mechanisms of the new compounds.

Main Methods:

  • Gram-scale synthesis of alkyne-functionalized expanded [11]helicene.
  • Alkyne metathesis for figure-eight dimer formation.
  • Zirconium-mediated cycloaddition for arylene-bridged helicene synthesis.
  • Density functional theory calculations to probe enantiomerization mechanisms.

Main Results:

  • Successful gram-scale synthesis of the helicene precursor and a figure-eight dimer.
  • Formation of two types of macrocyclic derivatives: figure-eight dimer and arylene-bridged helicenes.
  • The phenylene-bridged helicene exhibited a significantly higher enantiomerization barrier (22.1 kcal/mol) compared to the precursor (<11.9 kcal/mol).
  • The figure-eight dimer, while configurationally labile in solution, formed homochiral single crystals stabilized by helical superstructures.

Conclusions:

  • Alkyne-functionalized expanded helicenes are valuable precursors for diverse macrocyclic structures.
  • Arylene-bridged derivatives offer a promising strategy for accessing configurationally stable expanded helicenes.
  • The figure-eight dimer's unique crystal packing demonstrates supramolecular stabilization of chirality.
  • Computational insights guide future design of non-racemic macrocyclic systems.