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

Abstract

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 [H 2 ]. These findings are understood as arising from the formation of the saturated, 18-electron resting state (PNP)IrH 3 Bpin (5a), which is required to dissociate H 2 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 ΔG 298 ⧧ = 17.6 kcal/mol and ΔH ⧧ = 13.9 kcal/mol match the experimentally determined values (ΔG 298 ⧧ = 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.

Examination of a Series of Ir and Rh PXL Pincer Complexes as (Pre)catalysts for Aromatic C–H Borylation

“…In the 31 P{ 1 H} NMR spectrum in C 6 D 6 at room temperature, four 31 P NMR resonances were observed. 31 P NMR resonances at δ 148.7 and 147.9 were assigned to P−O of two isomers, respectively, and 31 P NMR resonances at δ 61.2 and 60.6 were assigned to P−N of two isomers, respectively. The 1 H NMR spectrum also showed two sets of resonances that correspond to two isomers.…”

“…It should also be noted that the SiNN-and PNP-supported Ir pincer catalysts for DHBTA do not catalyze aromatic C−H borylation and furthermore operate by different mechanisms in DHBTA. 27,31 On the whole, there exists a remarkable mechanistic diversity of possible C−H borylation pathways with Ir, depending on the supporting ligands and the substrate.…”

“…The synthesis of derivatives of organoboronic acids via homogeneous catalysis of C–H bond borylation (Scheme A) is an attractive method of preparation of versatile synthetic building blocks. − The most impressive foundational successes in this chemistry have been associated with catalysis of aromatic C–H borylation by Ir complexes supported by neutral bidentate ligands. − Catalytic borylation of aromatic and other sp 2 and sp 3 C–H bonds has also been realized with a variety of transition-metal and main-group complexes. − Our group has reported on the catalysis of dehydrogenative borylation of the sp C–H bonds in terminal alkynes (DHBTA) using Ir complexes supported by diarylamido-centered SiNN , or PNP − pincer − ligands. Catalysis of DHBTA with other metals has also been reported, − but the activity and the chemoselectivity of the Ir catalysts are superior.…”

“…Interestingly, the (POCOP)Ir catalysis proceeded by a different mechanism (Scheme C), which required the use of a sacrificial olefin for kinetic reasons. , This is in contrast to the catalysis by the bidentate-ligand-supported Ir, which does not require and is indeed poisoned by simple olefins. It should also be noted that the SiNN- and PNP-supported Ir pincer catalysts for DHBTA do not catalyze aromatic C–H borylation and furthermore operate by different mechanisms in DHBTA. , On the whole, there exists a remarkable mechanistic diversity of possible C–H borylation pathways with Ir, depending on the supporting ligands and the substrate.…”

Air- and Water-Tolerant (PNP)Ir Precatalyst for the Dehydrogenative Borylation of Terminal Alkynes

“…The mechanism of DHBTA catalysis by the SiNN-and PNPsupported Ir complexes has been thoroughly investigated computationally and experimentally. 12,13 Other groups reported DHBTA catalysts based on Zn, 14,15 Cu, 16 and Fe, 17 and we reported a Pd-based DHBTA catalyst (Scheme 1). 18 Chirik et al also reported a Co catalyst that produces alkynylboronates from terminal alkynes en route to 1,1-diborated alkene products.…”