Palladium-Catalyzed Cross-Coupling Reaction of Cyclopropylboronic Acids with Aryl Triflates

In the presence of appropriate base and NaBr, the Suzuki-type reaction of aryl triflates and trans-cyclopropylboronic acids proceed readily to give pure trans-cyclopropylarenes in good to high yields. For the reaction of general aryl triflates, the base KF · 2 H₂O is efficient, but in the case of aryl triflates bearing electron donating groups, KF · 2 H₂O as a base makes the unexpected phenyl-aryl exchange between the phenyl of Pd(PPh₃)₄ and the aryl of triflate to occur. The use of K₃PO₄ · 3 H₂O instead of KF · 2 H₂O can inhibit the unexpected phenyl-aryl exchange, but the yield of the product is somewhat decreased. The coupling reaction of aryl triflates with the optically active cyclopropylboronic acids, which were easily obtained in good yield and with good to excellent ee (up to 94%) by the asymmetric cyclopropanation of alkenylboronates with optically pure TMTA auxiliary, was also investigated. During the coupling reaction, the absolute configuration of the cyclopropyl group was retained. Thus, a novel method to prepare aryl-substituted cyclopropanes, including highly optically active cyclopropanes, from phenols or arylamines was provided.

Suzuki-coupling reaction - cyclopropylboronic acids - aryl triflates - optically active cyclopropylboronic acids - trans-cyclopropylarenes