Synthesis and Structures of Polyphenylphenanthrenes
Yonglong Xiao1, Joel T Mague1, James P Donahue1
1Department of Chemistry, Tulane University, New Orleans, LA, 70118, USA.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|May 8, 2020
Summary
Synthesizing octaphenylphenanthrene and decaphenylphenanthrene yielded highly twisted structures. Decaphenylphenanthrene is configurationally unstable at room temperature due to its low racemization barrier.
Area of Science:
- Organic Chemistry
- Supramolecular Chemistry
- Crystallography
Background:
- Polycyclic aromatic hydrocarbons (PAHs) with extensive phenyl substitutions exhibit unique structural and electronic properties.
- Steric hindrance in highly substituted PAHs can lead to significant deviations from planarity.
- Chirality and racemization dynamics are crucial aspects of molecular stability and function.
Purpose of the Study:
- To synthesize and characterize novel poly-phenylated phenanthrenes, specifically 1,2,3,4,5,6,7,8-octaphenylphenanthrene and decaphenylphenanthrene.
- To investigate the impact of extensive phenyl substitution on the molecular geometry and conformational flexibility of the phenanthrene core.
- To determine the configurational stability and racemization barriers of these sterically hindered molecules.
Main Methods:
- Short synthetic routes involving tetraphenylfuran and polybrominated benzene derivatives.
- X-ray crystallography for detailed structural elucidation of the synthesized compounds.
- Chiral chromatography for the resolution of enantiomers.
- Determination of activation energy for racemization via experimental measurements and computational studies.
Main Results:
- Successful synthesis of 1,2,3,4,5,6,7,8-octaphenylphenanthrene (4) and decaphenylphenanthrene (5) in two to three steps.
- X-ray structures revealed significant twisting of the phenanthrene cores (approx. 40°) due to steric clashes between phenyl groups.
- Compound 4 was resolved, with a determined free energy of activation for racemization of 24.6 kcal/mol at 40°C.
- Computational studies predicted a racemization barrier for compound 5 that is ~6 kcal/mol lower than that of compound 4.
Conclusions:
- Extensive phenyl substitution in phenanthrene derivatives leads to highly non-planar, sterically congested structures.
- The degree of steric hindrance directly influences the racemization barrier and configurational stability.
- Decaphenylphenanthrene is predicted to be configurationally unstable at room temperature, highlighting the limits of molecular stability under extreme steric pressure.
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