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Driving tert-butyl axial: the surprising cyclopropyl effect
Anthony R Izzotti1, James L Gleason1
1Department of Chemistry, McGill University 801 SherbrookeW. H3A 0B8 Montreal QC Canada jim.gleason@mcgill.ca.
A spirocyclic ring adjacent to a six-membered ring forces larger alkyl groups into axial positions. This conformational change, driven by torsional strain, impacts various ring systems and acyclic models.
Area of Science:
- Organic Chemistry
- Computational Chemistry
- Stereochemistry
Background:
- Conformational analysis of six-membered rings is crucial in organic chemistry.
- The influence of adjacent substituents on ring conformation is a key area of study.
Purpose of the Study:
- To investigate how a small spirocyclic ring affects the conformational preferences of six-membered rings.
- To determine the impact of geminal substituents on these conformational preferences.
Main Methods:
- Density Functional Theory (DFT) calculations were employed.
- Low-temperature 1H Nuclear Magnetic Resonance (NMR) experiments were conducted.
Main Results:
- Alkyl groups larger than methyl exhibit negative A-values when geminal to a spirocyclopropane.
- Isopropyl and tert-butyl groups were found to be exclusively axial at -78 °C.
- Similar conformational effects were observed for heteroatoms, electron-withdrawing groups, and other strained ring systems.
Conclusions:
- Adjacent spirocyclic rings significantly alter conformational preferences in six-membered rings.
- Torsional strain and hyperconjugative effects explain the observed axial preference for certain substituents.
- The findings extend to various cyclic and acyclic systems, including heterocycles.
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