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 tl 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 . 2H(2)O is efficient, but in the case of aryl triflates bearing electron donating groups, KF . 2H(2)O as a base makes the unexpected phenylaryl exchange between the phenyl of Pd(PPh3)(4) and the aryl of triflate to occur. The use of K3PO4. 3H(2)O instead of KF . 2H(2)O 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.