Journal
ACS CATALYSIS
Volume 8, Issue 9, Pages 7907-7914Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acscatal.8b02206
Keywords
azetidine; methylene aziridines; [2,3]-Stevens rearrangement; aziridinium ylides
Categories
Funding
- NIH [R0I GM11412]
- ACS-PRF [53146-ND1]
- National Science Foundation (NSF) [CHE-9208463, CHE-9629688]
- National Institutes of Health (NIH) [RR08389-01]
- National Institutes of Health [NIH-S10, NIH-1S100D020022-1]
- Spanish MINECO-FEDER [CTQ2016-78205-P, CTQ2016-81797-REDC]
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The synthesis of densely functionalized azetidinesin a highly stereocontrolled manner is challenging, but interest in the bioactivities of these small heterocycles has stimulated methods for their preparation. We recently reported a one-carbon ring expansion of bicyclic methylene aziridines under dirhodium catalysis capable of delivering enantioenriched azetidines. This work explores this ring expansion using computational and experimental studies. DFT computations indicate that the reaction proceeds through formation of an aziridinium ylide, which is precisely poised for concerted, asynchronous ring-opening/closing to deliver the azetidines in a [2,3]-Stevens-type rearrangement. The concerted nature of this rearrangement is responsible for the stereospecificity of the reaction, where axial chirality from the initial allene substrate is transferred to the azetidine product with complete fidelity. The computed mechanistic pathway highlights the key roles of the olefin and the rigid structure of the methylene aziridine in differentiating our observed ring expansion from competing cheletropic elimination pathways noted with ylides derived from typical aziridines.
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