A Crystalline Dimeric Steroidal Diboronate with Electronically Impeded Rotation
Josué Vazquez-Chavez1, Fátima C Martínez-Torres1, Armando Navarro-Huerta2
1Facultad de Química, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 Ciudad de México, México.
The Journal of Organic Chemistry
|December 8, 2022
Summary
The steroid SMR-3, designed for molecular motion, was found to be unexpectedly rigid. Electronic factors, not steric hindrance, were identified as the cause of its high rotational barrier.
Area of Science:
- Organic Chemistry
- Solid-State Chemistry
- Computational Chemistry
Background:
- Steroids are versatile molecules with potential applications in molecular machinery.
- Understanding intramolecular dynamics is key to designing functional molecular rotors.
Purpose of the Study:
- To synthesize and investigate the intramolecular dynamics of the dimeric steroid SMR-3.
- To elucidate the factors contributing to the rotational barrier of its central phenyldiboronic ester component.
Main Methods:
- Single-crystal X-ray diffraction and Hirshfeld analyses for structural insights.
- Solid-state NMR (VT 13C{1H} CPMAS and 2H spin-echo) on SMR-3D to assess molecular motion.
- Classical molecular dynamics, molecular mechanics, and ab initio calculations for mechanistic understanding.
Main Results:
- Structural analyses indicated minimal steric hindrance around the aromatic core, suggesting favorable motion.
- Solid-state NMR revealed that SMR-3 is rigid, even at elevated temperatures (85 °C).
- Computational methods distinguished between steric and electronic contributions to the rotational barrier.
Conclusions:
- Electronic factors, rather than steric effects, significantly restrict the rotation of the central component in SMR-3.
- The observed rigidity of SMR-3 challenges expectations for steroidal molecular rotors.
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