A Stereospecific Pd-Catalyzed Suzuki Cross-Coupling Reaction and a Statistical Study of the Ligand\u27s Role in Stereoselection

The development of palladium-catalyzed carbon-carbon cross-coupling reactions has profoundly influenced the manner through which we approach the synthesis of complex organic molecules. Among established cross-coupling reactions, which include Kumada, Negishi, and Stille reactions, the Suzuki cross-coupling reaction exhibits particularly wide functional group compatibility, while employing non-toxic, air-stable organoboron nucleophiles. Conventional studies of Pd-catalyzed Suzuki cross-coupling reactions have focused on C(sp2)-C(sp2) cross-coupling reactions, which result in the formation of planar products. Conceptually, the use of C(sp3) (secondary) nucleophiles would enable the reliable manipulation of organic molecules in three dimensions. However, this process has not been achievable due to the slow rate of transmetallation of secondary alkylboron nucleophiles, and the subsequent isomerization of alkyl units following transmetallation to palladium. Herein, we describe the development of a general process for Pd-catalyzed cross-coupling reactions involving alkylboron nucleophiles using racemic and enantioenriched alkylboron nucleophiles. The origin of stereoselectivity was investigated through correlation of parameterized phosphine properties to stereochemical outcomes. A stereodivergent cross-coupling reaction was developed in which stereoretention and stereoinversion could be selectively achieved through the proper choice of ligand. A broad range of aryl chloride or bromide and inactivated secondary alkylboron nucleophiles are applicable in these reactions