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

A B(C₆ F₅ )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(C₆ F₅ )₃ with good cis diastereoselectivity. Furthermore, this transformation was applied in the late-stage modification of pharmaceutical compounds.