Abstract
This account describes the development of novel dearomatization reactions of phenols using transition-metal-catalyzed ipso-Friedel–Crafts-type processes. In general, phenols function as O-nucleophiles in transition-metal-catalyzed allylic substitution reactions, providing the corresponding aryl ethers. We found that, however, an intramolecular ipso-Friedel–Crafts allylic alkylation of phenols proceeded smoothly in the presence of a palladium catalyst, producing various spiro[4.5]cyclohexadienone derivatives. This finding led us to launch detailed investigations into this type of reaction. Pd-catalyzed intramolecular Friedel–Crafts allylic alkylation of phenols was next examined to synthesize 10-vinyl 9,10-dihydrophenanthrene derivatives. This reaction was successfully extended to a catalytic asymmetric process. We also developed a novel synthetic method for spiro[5.5]cyclohexadienones based on a Pd-catalyzed intramolecular ipso-Friedel–Crafts-type addition of phenols to η3-propargylpalladium(II) intermediates. Mechanistic studies revealed that the present reaction proceeds through a rearomatization-assisted oxidative addition. Moreover, a Au-catalyzed intramolecular ipso-Friedel–Crafts alkenylation of phenols with a terminal alkyne is discussed.
1 Introduction
2 Pd-Catalyzed Intramolecular Friedel–Crafts Allylic Alkylation of Phenols
2.1 Synthesis of Spirocyclohexadienone Derivatives by Pd-Catalyzed Intramolecular ipso-Friedel–Crafts Allylic Alkylation of Phenols
2.2 Mechanistic Considerations
2.3 Application to Cascade Reaction Processes
2.4 Asymmetric Synthesis of Dihydrophenanthrene Derivatives by Pd-Catalyzed Asymmetric Intramolecular Friedel–Crafts Allylic Alkylation of Phenols
3 Dearomatization of Phenols by Activation of Propargyl Carbonates with a Pd Catalyst
4 Au-Catalyzed Intramolecular ipso-Friedel–Crafts Alkenylation of Phenols
5 Summary and Outlook
Key words
asymmetric catalysis - dearomatization - Friedel–Crafts reaction - fused ring system - spiro compounds - transition-metal catalysis
