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Thermal Electrocyclic Reactions: Stereochemistry01:17

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Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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Ring-opening metathesis polymerization or ROMP involves strained cycloalkenes as starting materials. The mechanism of ROMP proceeds by reacting cycloalkene with Grubbs catalyst to give metallacyclobutane intermediate which undergoes a ring-opening reaction to form new carbene. The new carbene reacts with another molecule of cycloalkene. Repetition of these steps leads to the formation of an unsaturated open-chain polymer product. All these steps are reversible, however, relieving the ring...
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In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as...
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In this lesson, we delve into the role of ring conformation and its stability, which determines the spatial arrangement and, consequently, the molecular symmetry and stereoisomerism of cyclic compounds. 1,2-Dimethylcyclohexane is used as a case study to evaluate the possible number of stereoisomers. Here, given the multiple (n = 2) chiral centers, there are 2n = 4 possible configurations that lack a plane of symmetry, as the ring skeleton exists in a non-planar chair conformation. In addition,...
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Molecular ring rotation in poly(vinylferrocene).

Markus Appel1, Bernhard Frick2, Johannes Elbert3

  • 1Institute for Condensed Matter Physics, Technische Universität Darmstadt, Hochschulstraße 8, 64289 Darmstadt, Germany. appel@ill.eu and Institut Laue-Langevin, 71 Avenue des Martyrs, 38000 Grenoble, France.

Physical Chemistry Chemical Physics : PCCP
|October 12, 2016
PubMed
Summary
This summary is machine-generated.

We studied ring rotation in poly(vinylferrocene) using neutron spectroscopy. The rotational barriers are distributed, but dynamics are explained by a jump rate model with Arrhenius behavior.

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Area of Science:

  • Polymer Science
  • Materials Science
  • Spectroscopy

Background:

  • Poly(vinylferrocene) (PVFc) features ferrocene units attached to a polymer backbone.
  • The cyclopentadienyl ring within the ferrocene unit exhibits rotational jump diffusion.

Purpose of the Study:

  • To investigate the ring rotation dynamics in PVFc.
  • To characterize the rotational barrier distribution and temperature dependence.

Main Methods:

  • Incoherent neutron spectroscopy (time-of-flight and backscattering) was employed.
  • Polarized neutron diffraction was used to measure the static structure factor.
  • Data were analyzed using a rotation rate distribution model.

Main Results:

  • The rotational barrier is broadly distributed, not a single value.
  • Arrhenius behavior was observed for the jump rate distribution from 80 K to 350 K.
  • The mean activation energy was determined as 9.61(2) kJ/mol with a width of 3.12(1) kJ/mol.

Conclusions:

  • The rotational dynamics of PVFc can be modeled by a distribution of barriers.
  • Both intramolecular and intermolecular interactions contribute to the heterogeneous rotational barriers.
  • Neutron spectroscopy provides valuable insights into the dynamics of organometallic polymers.