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Stereoisomerism

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Isomerism in Complexes
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH3)4Cl2]+ ion has two isomers (Figure 1) In the cis...
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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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The rate of acid-catalyzed hydration of alkenes depends on the alkene's structure, as the presence of alkyl substituents at the double bond can significantly influence the rate.
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Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
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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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Anion and solvent induced chirality inversion in macrocyclic lanthanide complexes.

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Chiral hexaaza macrocycles form helical lanthanide complexes. These complexes can undergo diastereomeric conversion, influenced by solvent and the presence of nitrate anions, revealing insights into ligand and metal ion interactions.

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Area of Science:

  • Coordination Chemistry
  • Supramolecular Chemistry
  • Chiral Recognition

Background:

  • Lanthanide(III) and yttrium(III) complexes with chiral hexaaza macrocycles were synthesized.
  • The free macrocycle exhibits a twisted conformation, preorganized for helical complex formation.
  • Two diastereomeric forms of the macrocycle, L(RI) and L(RII), were observed in lanthanide complexes.

Purpose of the Study:

  • To synthesize and structurally characterize lanthanide(III) and yttrium(III) complexes with chiral hexaaza macrocycles.
  • To investigate the diastereomeric behavior and interconversion of these complexes.
  • To explore the influence of counterions and solvents on the stereochemistry of the complexes.

Main Methods:

  • Synthesis of lanthanide(III) and yttrium(III) complexes with chiral hexaaza macrocycles.
  • X-ray crystallography for structural characterization of the free macrocycle and its complexes.
  • Spectroscopic techniques including circular dichroism (CD) and proton nuclear magnetic resonance ((1)H NMR) to study diastereomeric conversion.

Main Results:

  • Crystal structures revealed two diastereomeric forms (L(RI) and L(RII)) of the macrocycle in lanthanide complexes.
  • Nitrate complexes showed slow conversion from L(RI) to L(RII) in solution, unlike chloride complexes.
  • Chloride complexes underwent L(RI) → L(RII) conversion upon addition of nitrate anions, involving ligand exchange and chirality inversion.

Conclusions:

  • The chiral macrocycle can adopt different diastereomeric configurations upon complexation with lanthanides.
  • The interconversion between diastereomers is sensitive to the nature of the counterion (nitrate vs. chloride) and solvent.
  • External nitrate anions can trigger a stereochemical transformation in the lanthanide complexes, highlighting anion-templated chiral control.