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Chelated aryllithium reagents: ring size and chelating group effects
H J Reich1, W S Goldenberg, A W Sanders
1Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706, USA. Reich@chem.wisc.edu
Organic Letters
|June 30, 2001
Summary
Chelation in phenyllithium reagents was studied using NMR spectroscopy. Five-ring chelates formed strongly with ethers and amines, while larger rings showed limited complexation, impacting reagent aggregation and structure.
Area of Science:
- Organometallic Chemistry
- Supramolecular Chemistry
- Spectroscopy
Background:
- Phenyllithium reagents are crucial in organic synthesis.
- Understanding their aggregation state and coordination behavior is key to controlling reactivity.
- Chelating substituents can significantly influence these properties.
Purpose of the Study:
- To investigate the chelation and aggregation of phenyllithium reagents with varying ortho-substituents.
- To determine the impact of ether and amine chelating groups on reagent structure.
- To explore the influence of ring size (5-, 6-, and 7-ring) on chelation strength and aggregation.
Main Methods:
- Variable-temperature 6Li and 13C Nuclear Magnetic Resonance (NMR) spectroscopy.
- 6Li and 15N isotope labeling studies.
- Investigation of solvent additive effects.
Main Results:
- Strong 5-ring chelation was observed for both ether and amine substituents.
- 6-ring ether chelates effectively competed with Tetrahydrofuran (THF).
- 6-ring and 7-ring amine chelates showed minimal or no complexation, respectively.
- o-Methoxymethylphenyllithium formed an open dimer and a pentacoordinate monomer with PMDTA.
Conclusions:
- The size and type of chelating substituent profoundly affect phenyllithium aggregation and coordination.
- Five-membered chelate rings provide the most effective coordination.
- Amine chelates are generally less effective than ether chelates, especially for larger ring sizes.