Chemoselective Borane‐Catalyzed Hydroarylation of 1,3‐Dienes with Phenols

Abstract

A B(C6F5)3‐catalyzed hydroarylation of a series of 1,3‐dienes with various phenols has been established through a combination of theoretical and experimental investigations, affording structurally diverse ortho‐allyl phenols. DFT calculations show that the reaction proceeds through a borane‐promoted protonation/Friedel–Crafts pathway involving a π‐complex of a carbocation–anion contact ion pair. This protocol features simple and mild reaction conditions, broad functional‐group tolerance, and low catalyst loading. The obtained ortho‐allyl phenols could be further converted into flavan derivatives using B(C6F5)3 with good cis diastereoselectivity. Furthermore, this transformation was applied in the late‐stage modification of pharmaceutical compounds.

Beyond Friedel and Crafts: Innate Alkylation of C−H Bonds in Arenes

“…Mechanistic experiments and computational studies support a radical pathway, whereas an alternative mechanism based on the generation of a silylated intermediate could not be totally ruled out. Finally, some other more specific transformations have also been reported: they comprise the regioselective palladium‐catalyzed alkylation of various nucleophilic arenes, including phenols, with alkenes possessing an aminoquinoline‐based directing group, the bismuth‐ or iridium‐catalyzed benzylation of toluene and anisole derivatives with styrenes, the palladium‐catalyzed atroposelective C−H allylation of biaryls, the chemoselective borane‐catalyzed allylation of phenols with 1,3‐dienes, as well as the polymerizing C−H alkylation of dimethoxyarenes with unconjugated dienes by using rare‐earth catalysts …”

Triflic Acid‐Catalyzed Chemo‐ and Site‐Selective C−H Bond Functionalization of Phenols With 1,3‐Dienes

“…However, the C−H bond allylation of free phenols with 1,3‐dienes has not been well explored. In 2019, the group of Li realized an elegant B(C 6 F 5 ) 3 ‐catalyzed ortho ‐selective allylic alkylation of phenols with1,3‐dienes, in which the coordination between hydroxyl and B(C 6 F 5 ) 3 was the key issue for the ortho ‐selectivity (Scheme 1c) [13] . In line with our continuous interest in the construction of phenol derivatives, [14] we envisioned that the para ‐selective allylic alkylation would become dominant if the interaction between phenolic hydroxyl and the intermediate of 1,3‐dienes with the catalyst could be prevented.…”

“…If there is no interaction between intermediate IA and phenol, the phenolic hydroxyl will push allylic cation far away from ortho ‐position ( IB ) owing to its steric hindrance, and eventually lead to the para ‐selective product 3 via Friedel‐Crafts type alkylation. On the contrary, if the interaction of − OTf with phenol forms IC , [18a] or the interaction of allylic cation with phenolic anion forms ID , [13] the intramolecular type Friedel‐Crafts reaction on the ortho ‐position of phenol will occur to give the ortho ‐product [18] as the major one. The low nucleophilicity of triflate anion inhibits the formation of the hydrogen bond between O and H in IC [15a–c,19] .…”

Jenseits von Friedel und Crafts: immanente Alkylierung von C‐H‐Bindungen in Arenen

“…Mechanistische Experimente und Rechnungen deuten auf einen radikalischen Reaktionsweg hin, während ein alternativer Mechanismus basierend auf der Erzeugung eines silylierten Intermediats nicht vollständig ausgeschlossen werden konnte. Schließlich wurde auch über einige speziellere Transformationen berichtet: Diese umfassen die regioselektive palladiumkatalysierte Alkylierung diverser nucleophiler Arene einschließlich Phenolen mit Alkenen, die eine auf Aminochinolin zurückzuführende dirigierende Gruppe enthalten, die bismut‐ oder iridiumkatalysierte Benzylierung von Toluol‐ und Anisolderivaten mit Styrolen, die palladiumkatalysierte atroposelektive C‐H‐Allylierung von Biarylen, die chemoselektive borankatalysierte Allylierung von Phenolen mit 1,3‐Dienen sowie die polymerisierende C‐H‐Alkylierung von Dimethoxyarenen mit nichtkonjugierten Dienen mithilfe von Seltenerdmetallkatalysatoren …”