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Simple aryl halides do not react with nucleophiles. However, nucleophilic aromatic substitutions can be forced under certain conditions, such as high temperatures or strong bases. The mechanism of substitution under such conditions involves the highly unstable and reactive benzyne intermediate. Benzyne contains equivalent carbon centers at both ends of the triple bond, each of which is equally susceptible to nucleophilic attack. This 50–50 distribution of products is...
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All meta-directing substituents are deactivating groups. These substituents withdraw electrons from the aromatic ring, making the ring less reactive toward electrophilic substitution. For example, the nitration of nitrobenzene is 100,000 times slower than that of benzene because of the deactivating effect of the nitro group. The first step in an electrophilic aromatic substitution is the addition of an electrophile to form a resonance-stabilized carbocation. The energy diagrams for...
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Nucleophilic substitution in aromatic compounds is feasible in substrates bearing strong electron-withdrawing substituents positioned ortho or para to the leaving group. The reaction proceeds via two steps: the addition of the nucleophile and the elimination of the leaving group.
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Introduction
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At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
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Radical Formation: Abstraction00:47

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The electron of an atom can be abstracted from a compound by a relatively unstable radical to generate a new radical of relatively greater stability. For example, an initiator which forms radicals by homolysis can abstract a suitable species like a hydrogen atom or a halogen atom from a compound to generate a new radical. This ability of radicals to propagate by abstraction is a crucial feature of radical chain reactions.
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Ring Contraction by NHC-Induced Pnictogen Abstraction.

Martin Piesch1, Stephan Reichl1, Michael Seidl1

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|October 2, 2019
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Summary

New reactions show N-heterocyclic carbenes (NHCs) can induce ring contraction in metal-phosphorus clusters. This creates novel anionic cobalt-phosphorus and arsenic-phosphorus complexes, expanding inorganic chemistry.

Keywords:
arseniccarbenesphosphorusring contractionvanadium

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Area of Science:

  • Organometallic Chemistry
  • Inorganic Chemistry
  • Materials Science

Background:

  • Metal-phosphorus clusters are versatile building blocks in inorganic chemistry.
  • N-heterocyclic carbenes (NHCs) are increasingly used as ligands and reagents in organometallic chemistry.
  • Ring contraction reactions in inorganic clusters are less explored but offer pathways to novel structures.

Purpose of the Study:

  • To investigate the reactivity of metal-phosphorus clusters with NHCs.
  • To explore NHC-induced ring contraction reactions in cobalt, molybdenum, and vanadium complexes.
  • To synthesize and characterize novel anionic metal-phosphorus complexes and NHC-substituted cations.

Main Methods:

  • Reaction of pre-formed metal-phosphorus clusters with 1,3,4,5-tetramethylimidazol-2-ylidene (MeNHC).
  • Spectroscopic characterization (NMR, Mass Spectrometry) of reaction products.
  • X-ray crystallography for structural elucidation of key compounds.

Main Results:

  • The reaction of [Cp'''Co(η⁴-P₄)] with MeNHC yielded the anionic complex [(MeNHC)₂P][Cp'''Co(η³-P₃)], the first anionic CoP₃ complex.
  • NHC-induced ring contraction was also observed for triple-decker sandwich complexes of molybdenum and vanadium.
  • The synthesis of the first structurally characterized NHC-substituted As¹ cation was achieved.
  • Novel vanadium-phosphorus complexes with varying degrees of P-ring contraction and NHC incorporation were formed.

Conclusions:

  • NHC-induced ring contraction is a viable strategy for synthesizing novel metal-phosphorus cluster architectures.
  • This methodology provides access to unprecedented anionic metal-phosphorus complexes and NHC-substituted cations.
  • The study expands the scope of NHC reactivity in inorganic cluster chemistry and opens new avenues for designing functional materials.