Enantioselective Synthesis of cis-1,2-Disubstituted Cyclopentanes and Cyclohexanes by Suzuki-Miyaura Cross-Coupling and Iridium-Catalyzed Asymmetric Hydrogenation

A series of 1,2-disubstituted cyclohexene derivatives was prepared through Suzuki-Miyaura cross-coupling of 2-bromo-1-cyclohexenecarbaldehyde or 2-carbomethoxy-1-cyclohexen-1-yl triflate with arylboronates. These tetra-substituted cyclic alkenes were subjected to Ir-catalyzed asymmetric hydrogenation. In this way cis-1-methoxymethyl-2-arylcyclohexanes were obtained in high yield with excellent enantio- and diastereoselectivities (up to >99 % ee, >99 % cis) by using phosphinomethyloxazolines as ligands. Asymmetric hydrogenation of analogous cyclopentene derivatives, prepared by Suzuki-Miyaura cross-coupling, proved to be more difficult and proceeded with lower enantioselectivities of up to 88 % ee. The synthetic potential of this cross-coupling/asymmetric-hydrogenation strategy was demonstrated by an enantioselective route to chiral hexahydrofluorenones.