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The Mechanism of Double Olefination Using Titanium-Substituted Ylides
Kelly A. Reynolds1, M. G. Finn
1Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901.
The Journal of Organic Chemistry
|April 18, 1997
Summary
This study details a new method for synthesizing allenes using titanium reagents and phosphorus ylides. Optimizing the titanium precursor, TiCl2(OiPr)2, allows for stepwise transformations, avoiding excess reagents and improving control in allene synthesis.
Area of Science:
- Organometallic Chemistry
- Synthetic Organic Chemistry
- Asymmetric Synthesis
Background:
- The synthesis of allenes often requires harsh conditions and excess reagents, limiting efficiency and control.
- Titanium-based reagents have shown promise in organic synthesis, but their application in allene formation needs further investigation.
- Understanding the reaction mechanisms and intermediate stabilization is crucial for developing selective synthetic routes.
Purpose of the Study:
- To investigate the role of titanium precursors in the synthesis of allenes.
- To develop a more controlled and efficient stepwise method for allene synthesis.
- To explore the stereochemical outcomes and mechanistic pathways in titanium-mediated Wittig-type reactions.
Main Methods:
- Utilized titanium precursors TiCl(3)(OiPr) and TiCl(2)(OiPr)(2) in reactions with phosphorus ylides and aldehydes.
- Employed sodium bis(trimethylsilyl)amide (NaN(SiMe3)2) for ligand substitution and deprotonation steps.
- Characterized reaction intermediates, including vinylphosphonium salts and allenic phosphoranes, using low-temperature multinuclear NMR spectroscopy.
- Investigated stereochemical induction by incorporating chiral elements into the reactants.
Main Results:
- The use of TiCl(2)(OiPr)(2) enabled stepwise transformations, circumventing the need for excess reagents observed with TiCl(3)(OiPr).
- Selective formation of vinylphosphonium salts was observed, attributed to the chloride leaving group on titanium.
- Thermally sensitive allenic phosphoranes were generated and characterized, reacting with aldehydes to form oxaphosphetane and betaine intermediates.
- Stereochemical drift was observed, where the diastereomeric ratio of intermediates differed from the final allene products, indicating an irreversible isomerization step.
Conclusions:
- The choice of titanium precursor significantly impacts the reaction pathway and efficiency in allene synthesis.
- Dimethylamino-substituted phosphorus ylides are crucial for high yields, enhancing reactivity and stabilizing key intermediates.
- The study provides insights into stereochemical control and mechanistic details, including stereochemical drift, in titanium-mediated allene formation.