Combined Experimental and Computational Studies of the Mechanism of Dehydrogenative Borylation of Terminal Alkynes Catalyzed by PNP Complexes of Iridium

Iridium complexes supported by four different diarylamido/bis(phosphine) PNP ligands have been studied as catalysts for the dehydrogenative borylation of terminal alkynes (DHBTA). Experimental mechanistic investigations on a representative ligand system established that the reaction rate is zeroth order in the concentration of the borylating agent HBpin, first order in [alkyne] and in [Ir], and inverse first order in [H2]. These findings are understood as arising from the formation of the saturated, 18-electron resting state (PNP)IrH3Bpin (5a), which is required to dissociate H2 in order to enter the C–H oxidative addition reaction with the alkyne. However, the overall highest transition state corresponds to the B–H rotation following C–H oxidative addition. This realization follows from the density functional theory (DFT) calculations and is consistent with the modest experimentally determined kinetic isotope effect of 1.5(1) for C–H/C–D. DFT-calculated ΔG298⧧ = 17.6 kcal/mol and ΔH⧧ = 13.9 kcal/mol match the experimentally determined values (ΔG298⧧ = 20(2) kcal/mol and ΔH⧧ = 16(2) kcal/mol) reasonably well. DFT calculations were used to demonstrate that the boryl migration from the amido N to Ir is too high in energy to play a part in the DHBTA mechanism in the (PNP)Ir system. This is in surprising contrast to the seemingly closely related (SiNN)Ir system studied previously. DFT calculations further provide insights into the greater kinetic barrier for activating a C–H bond of a terminal alkyne trans to a boryl ligand versus trans to a hydride, explaining why diboryl Ir compounds are not catalytically competent in this system.