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The hydrofluorination of alkenes represents an attractive strategy for the synthesis of aliphatic fluorides. This approach provides a direct means to form C(sp3)–F bonds selectively from readily available alkenes. Nonetheless, conducting hydrofluorination using nucleophilic fluorine sources poses significant challenges due to the low acidity and high toxicity associated with HF and poor nucleophilicity of fluoride. In this study, we present a new Co(salen)-catalyzed hydrofluorination of simple alkenes, utilizing Et3N·3HF as the sole source of both hydrogen and fluorine. This process operates via a polar- radical-polar crossover mechanism. We also demonstrated the versatility of this method by effectively converting a diverse array of simple and activated alkenes with varying degrees of substitution into hydrofluorinated products. Furthermore, we successfully applied this methodology to 18F-hydrofluorination reactions, enabling the introduction of 18F into potential radi- opharmaceuticals. Our mechanistic investigations, conducted using rotating disk electrode voltammetry and DFT calcula- tions, unveiled the involvement of both carbocation and CoIV–alkyl species as viable intermediates during the fluorination step, and the contribution of each pathway depends on the structure of the starting alkene.
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Stereoretention in styrene heterodimerisation promoted by one-electron oxidants
Radical cations generated from the oxidation of CC π-bonds are synthetically useful reactive intermediates for C–C and C–X bond formation. Radical cation formation, induced by sub-stoichiometric amounts of external oxidant, are important intermediates in the Woodward–Hoffmann thermally disallowed [2 + 2] cycloaddition of electron-rich alkenes. Using density functional theory (DFT), we report the detailed mechanisms underlying the intermolecular heterodimerisation of anethole and β-methylstyrene to give unsymmetrical, tetra-substituted cyclobutanes. Reactions between trans -alkenes favour the all-trans adduct, resulting from a kinetic preference for anti -addition reinforced by reversibility at ambient temperatures since this is also the thermodynamic product; on the other hand, reactions between a trans -alkene and a cis -alkene favour syn -addition, while exocyclic rotation in the acyclic radical cation intermediate is also possible since C–C forming barriers are higher. Computations are consistent with the experimental observation that hexafluoroisopropanol ( HFIP ) is a better solvent than acetonitrile, in part due to its ability to stabilise the reduced form of the hypervalent iodine initiator by hydrogen bonding, but also through the stabilisation of radical cationic intermediates along the reaction coordinate.
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- Award ID(s):
- 1955876
- PAR ID:
- 10316174
- Date Published:
- Journal Name:
- Chemical Science
- Volume:
- 11
- Issue:
- 34
- ISSN:
- 2041-6520
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/d0sc03059g
