Titanocene catalyzed 4-exo cyclizations: mechanism, experiment, catalyst design
Joachim Friedrich1, Katarzyna Walczak, Michael Dolg
1Institut für Theoretische Chemie der Universität zu Köln, Greinstr. 4, 50939 Köln, Germany. joachim_friedrich@gmx.de
Journal of the American Chemical Society
|January 15, 2008
Summary
This study introduces a novel radical-based method for synthesizing cyclobutanes. Titanocene(III) chlorides facilitate epoxide cyclization, offering a general approach to these valuable compounds.
Area of Science:
- Organic Chemistry
- Radical Chemistry
- Synthetic Methodology
Background:
- Cyclobutane rings are prevalent in pharmaceuticals and natural products.
- Efficient and general synthetic routes to cyclobutanes remain a challenge in organic chemistry.
Purpose of the Study:
- To develop a general method for cyclobutane synthesis using radical chemistry.
- To investigate the mechanism and diastereoselectivity of the proposed cyclization.
Main Methods:
- Radical generation via electron transfer from titanocene(III) chlorides to epoxides.
- Use of alpha,beta-unsaturated carbonyl compounds as radical traps.
- Combined theoretical and experimental mechanistic studies.
- Development of modified titanocene catalysts with substituted cyclopentadienyl ligands.
Main Results:
- A 4-exo cyclization of radicals provides a general route to cyclobutanes.
- Titanocene-catalyzed radical cyclization is effective for epoxide functionalization.
- Computational studies elucidated key intermediates, transition structures, and origins of diastereoselectivity.
- Modified catalysts enhance the longevity of beta-titanoxy radicals, enabling diastereoselective cyclobutane formation.
Conclusions:
- The presented method is the first general radical-based approach to cyclobutanes.
- The study provides mechanistic insights into titanocene-mediated radical cyclizations.
- Catalyst optimization leads to improved diastereoselectivity in cyclobutane synthesis.
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