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Regioselectivity in aromatic Claisen rearrangements
Fábio Cesar Gozzo1, Sergio Antonio Fernandes, Denise Cristina Rodrigues
1Institute of Chemistry, State University of Campinas-UNICAMP, CP 6154, 13083-970 Campinas, SP, Brazil.
The Journal of Organic Chemistry
|July 4, 2003
Summary
A new NMR method accurately predicts aromatic Claisen rearrangement regioselectivity by analyzing reactant conformation. This approach surpasses traditional orbital interaction models for predicting outcomes in meta-substituted allyl aryl ethers.
Area of Science:
- Organic Chemistry
- Reaction Mechanism Studies
- Computational Chemistry
Background:
- The Claisen rearrangement is a fundamental carbon-carbon bond-forming reaction in organic synthesis.
- Predicting regioselectivity in aromatic Claisen rearrangements of substituted allyl aryl ethers is crucial for synthetic planning.
- Traditional methods using frontier orbital interactions have limitations in accurately predicting these rearrangements.
Purpose of the Study:
- To validate a novel nuclear magnetic resonance (NMR) methodology for predicting aromatic Claisen rearrangement regioselectivity.
- To assess the reliability of predicting regioselectivity based on the ground-state conformational preference of the allyloxy group.
- To compare the predictive power of ground-state conformation with transition-state energetics and orbital interactions.
Main Methods:
- Synthesis and characterization of eight meta-substituted allyl aryl ethers.
- Application of a new (1)H NMR methodology to determine reactant ground-state conformations.
- Theoretical calculations using B3LYP/6-31G(d,p) to analyze reactant conformations, transition states, and orbital interactions.
Main Results:
- The (1)H NMR methodology reliably predicts the isomeric product composition of aromatic Claisen rearrangements.
- Ground-state conformational preference of the allyloxy group accurately reflects the observed regioselectivity.
- Frontier molecular orbital (HOMO-LUMO) interactions failed to accurately predict transition-state behavior for these systems.
Conclusions:
- The novel (1)H NMR approach offers a reliable alternative for predicting regioselectivity in aromatic Claisen rearrangements.
- Ground-state conformational analysis is a superior predictor of regioselectivity compared to classical orbital interaction theories for these substrates.
- Computational methods (B3LYP/6-31G(d,p)) confirm predictions derived from ground-state conformations or transition-state energies.