How Solvents Control the Stereospecificity of Ni-Catalyzed Miyaura Borylation of Allylic Pivalates

Solvents act as a powerful control of stereospecificity in Ni-catalyzed Miyaura borylation of allylic pivalates. Low polar solvents lead to stereoretentive borylation, whereas highly polar solvents deliver low stereoselectivity for retention and acetonitrile produces stereoinversion products. We explored the reaction mechanism and origins of stereospecificity with density functional theory calculations. The Ni-catalyzed allylic C–O bond activation determines the stereospecificity of borylation. A zwitterionic SN2-type transition state inverts the allylic stereogenic center, whereas a neutral cyclic transition state cleaves the C–O bond with stereoretention. This difference in charge separation results in the control of stereospecificity by solvent polarity. Low polar solvents demonstrate the intrinsic selectivity for retention, whereas highly polar solvents lower such selectivity by stabilizing the zwitterionic stereoinvertive transition state. Distinctive from other solvents, acetonitrile further promotes stereoinversion by replacing the coordination of the phosphine ligand and reverses the intrinsic stereoselectivity. We proved this solvent-promoted Ni-catalyzed C–O bond activation with a designed experimental verification of ligandless Ni-catalyzed borylation of allylic pivalates in acetonitrile.