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Related Concept Videos

Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation02:24

Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation

Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
Reduction of Alkenes: Catalytic Hydrogenation02:13

Reduction of Alkenes: Catalytic Hydrogenation

Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the surface of...
Reduction of Alkenes: Asymmetric Catalytic Hydrogenation02:17

Reduction of Alkenes: Asymmetric Catalytic Hydrogenation

Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an...
Electrophilic Addition to Alkynes: Hydrohalogenation02:35

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.
ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3

All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation02:47

Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation

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.

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P-H activation using alkynylgold substrates: steric and electronic effects.

Gerald F Manbeck1, Mark C Kohler, Meghan R Porter

  • 1Department of Chemistry, Bucknell University, Lewisburg, Pennsylvania 17837, USA.

Dalton Transactions (Cambridge, England : 2003)
|October 21, 2011
PubMed
Summary

Gold complexes catalyze P-H activation in hydrogen phosphonates. Ligand choice affects reaction dynamics but not P-H activation rates, with electronic effects on the alkyne influencing reaction speed.

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Area of Science:

  • Organometallic Chemistry
  • Catalysis
  • Phosphorus Chemistry

Background:

  • Gold complexes are effective catalysts for various organic transformations.
  • P-H activation is a key step in functionalizing phosphonates.
  • Alkynylgold complexes offer unique reactivity profiles.

Purpose of the Study:

  • To investigate the P-H activation of hydrogen phosphonates using alkynylgold complexes.
  • To understand the influence of ligands and substrate structure on reaction dynamics and rates.
  • To elucidate the reaction mechanism.

Main Methods:

  • Synthesis of alkynylgold complexes with varying ligands (triphenylphosphine, bulky biaryldialkylphosphines, N-heterocyclic carbenes).
  • Reaction of these complexes with hydrogen phosphonates.
  • Analysis of reaction kinetics and product distributions.
  • Computational studies to explore potential mechanisms.

Main Results:

  • Alkynylgold complexes efficiently promote P-H activation of hydrogen phosphonates.
  • Fluxional behavior was observed with triphenylphosphine ligands due to ligand exchange.
  • Bulky phosphine ligands and N-heterocyclic carbenes stabilized the complexes but did not significantly alter P-H activation rates.
  • Electron-donating groups on the alkyne accelerated the reaction, while electron-withdrawing groups decelerated it.
  • Product distributions were generally unaffected by incorporating propargyl alcohol moieties.

Conclusions:

  • The P-H activation of hydrogen phosphonates by alkynylgold complexes is a viable synthetic route.
  • Ligand choice impacts reaction dynamics but not the fundamental P-H activation step.
  • Electronic properties of the alkyne significantly influence reaction rates.
  • Potential for gold-catalyzed reactions involving alkynylgold intermediates requires careful consideration of labile P-H donors to avoid catalyst/intermediate interception.