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The common conditions for electrophilic fluorination of phenol to introduce fluorine are as follows:
1. **Selectfluor reagent** (1-chloromethyl-4-fluoro-1,4-diazabicyclo[2.2.2]octane difluoride salt): It can efficiently fluorinate under mild conditions (at room temperature, in acetonitrile or water solvents), with good selectivity.
2. **NFSI** (N-fluorobisbenzenesulfonimide): It requires a weak base (such as NaHCO₃) to facilitate the reaction. It is suitable for electron-rich aromatic rings, but the reaction is slow and has fewer by-products.
3. **Electrofluorination**: Using HF/piperidine as the fluorine source, strict control of conditions is necessary to avoid excessive fluorination.
4. **Transition metal catalysis** (such as Pd/Cu): It selectively activates the C-H bond, but the cost is high.
Notes: The activity of the phenol hydroxyl group needs to be regulated (such as esterification protection), to avoid excessive substitution at the ortho/para positions; Strong fluorinating agents (such as F₂) are prone to cause side reactions, and inert gas protection is required.
