Reversible Borylene Formation from Ring Opening of Pinacolborane and Other Intermediates Generated from Five-Coordinate Tris-Boryl Complexes: Implications for Catalytic C–H Borylation

Abstract

Catalytic C–H borylation using the five-coordinate tris-boryl complex (dippe)Ir(Bpin)3 (5a, dippe = 1,2-bis(diisopropylphosphino)ethane) has been examined using 31P{1H} and 1H NMR spectroscopy. Compound 5a was shown to react rapidly and reversibly with HBpin to generate a six-coordinate borylene complex, (dippe)Ir(H)-(Bpin)2(BOCMe2CMe2OBpin) (6), whose structure was confirmed by X-ray crystallography. Under catalytic conditions, the H2 generated from C–H borylation converted compound 6 to a series of intermediates. The first is tentatively assigned from 31P{1H} and 1H NMR spectra as (dippe)Ir(H2B3pin3) (7), which is the product of formal H2 addition to compound 5a. As catalysis progressed, compound 7 was converted to a new species with the formula (dippe)Ir(H3B2pin2) (8), which arose from H2 addition to compound 7 with loss of HBpin. Compound 8 was characterized by 31P{1H} and 1H NMR spectroscopy, and its structure was confirmed by X-ray crystallography, where two molecules with different ligand orientations were found in the unit cell. DFT calculations support the formulation of compound 8 as an IrIII agostic borane complex, (dippe)IrH2(Bpin)(η2-HBpin). Compound 8 was gradually converted to (dippe)Ir(H4Bpin) (9), which was characterized by 31P{1H} and 1H NMR spectroscopy and X-ray crystallography. DFT calculations favor its formulation as an agostic borane complex of IrIII with the formula (dippe)IrH3(η2-HBpin). Compound 9 reacted further with H2 to afford the dimeric structure [(dippe)IrH2(μ2-H)]2 (10), which was characterized by 1H NMR and X-ray crystallography. Compounds 7–10 are in equilibrium when H2 and HBpin are present.

Silylene–Nickel Promoted Cleavage of B−O Bonds: From Catechol Borane to the Hydroborylene Ligand

“…This unprecedented reactivity for a silylene transition‐metal complex towards catechol borane highlights the remarkable potential of low‐oxidation state heavier Group 14 transition‐metal complexes in small‐molecule activation. It also represents a novel synthetic route to a borylene metal complex, which are otherwise difficult to access …”

Mechanistic Studies of Cobalt-Catalyzed C(sp2)–H Borylation of Five-Membered Heteroarenes with Pinacolborane

“…Obtaining insights into the turnover-limiting step and nature of catalyst deactivation pathways in this class of C−H functionalization reaction is crucial for the rational design of next generation catalysts. Information on the speciation of iridium, 17,18 ruthenium, 19 iron, 6a,d heterobimetallic copper, 6c,20 and frustrated Lewis pair (FLP) 21 catalysts in the presence of HBPin have been reported; however, the turnover-limiting step and possible catalyst deactivation pathways have not been identified, and principles for future catalyst development are not evident.…”

Catalytic borylation of methane

“…Because hydrolysis of 1 and 2 is very slow on the basis of control experiments, we propose O[Bpin] 2 to derive from a combination of hydrolysis of 3 during aerobic workup and analysis by GC-MS, as well as decomposition of B 2 pin 2 or 3 . The decomposition of B 2 pin 2 may be metal-catalyzed, as ring-opening of pinacolborane with Ir catalysts has been documented recently (20). We did not observe any tri- or tetraborylated methyl products, H 4- x C[Bpin] x ( x = 3 or 4), whereas borylation of the solvent is barely detected under our conditions.…”