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Abstract Because internal alkenes are more challenging synthetic targets than terminal alkenes, metal‐catalyzed olefin mono‐transposition (i.e., positional isomerization) approaches have emerged to afford valuableE‐ orZ‐internal alkenes from their complementary terminal alkene feedstocks. However, the applicability of these methods has been hampered by lack of generality, commercial availability of precatalysts, and scalability. Here, we report a nickel‐catalyzed platform for the stereodivergentE/Z‐selective synthesis of internal alkenes at room temperature. Commercial reagents enable this one‐carbon transposition of terminal alkenes to valuableE‐ orZ‐internal alkenes via a Ni−H‐mediated insertion/elimination mechanism. Though the mechanistic regime is the same in both systems, the underlying pathways that lead to each of the active catalysts are distinct, with theZ‐selective catalyst forming from comproportionation of an oxidative addition complex followed by oxidative addition with substrate and theE‐selective catalyst forming from protonation of the metal by the trialkylphosphonium salt additive. In each case, ligand sterics and denticity control stereochemistry and prevent over‐isomerization.
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A Hydride Migration Mechanism for the Mo‐Catalyzed Z‐2‐Selective Isomerization of Terminal Alkenes
The catalytic one-bond isomerization (transposition) of 1- alkenes is an emerging approach to Z-2-alkenes. Design of more selective catalysts would benefit from a mechanistic understanding of factors controlling Z selectivity. We propose here a reaction pathway for cis-Mo(CO)4(PCy3)(piperidine) (3), a precatalyst that shows high Z selectivity for transposition of alpha olefins (e.g., 1-octene to 2-octene, 18:1 Z:E at 74% conversion). Computational modeling of reaction pathways and isotopic labeling suggests the isomerization takes place via an allyl (1,3-hydride shift) pathway, where oxidative addition of fac- (CO)3Mo(PCy3)(η2-alkene) is followed by hydride migration from one position (cis to allyl C3 carbon) to another (cis to allyl C1 carbon) via hydride/CO exchanges. Calculated barriers for the hydride migration pathway are lower than explored alternative mechanisms (e.g., change of allyl hapticity, allyl rotation). To our knowledge, this is the first study to propose such a hydride migration in alkene isomerization.
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- Award ID(s):
- 2154438
- PAR ID:
- 10484345
- Publisher / Repository:
- Wiley-VCH
- Date Published:
- Journal Name:
- ChemCatChem
- Volume:
- 15
- Issue:
- 23
- ISSN:
- 1867-3880
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1002/cctc.202301052
