Silyl Phosphorus and Nitrogen Donor Chelates for Homogeneous Ortho Borylation Catalysis

Ghaffari, Behnaz; Preshlock, Sean; Plattner, Donald L.; Staples, Richard J.; Maligres, Peter E.; Krska, Shane W.; Maleczka, Robert E.; Smith, Milton R.

doi:10.1021/ja506229s

Cited by 154 publications

(114 citation statements)

References 28 publications

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“…Although catalysts with bidentate nitrogen donors have been used more extensively, the recent chemistry of phosphine-ligated catalysts suggests that they have as yet unrealized potential. 30,31 The work described herein provides a foundation for understanding catalysis in this family of compounds, which may aid in designing new borylation catalysts.…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2015

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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.

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Section: Discussionmentioning

confidence: 99%

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

et al. 2015

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“…This has been accomplished using surface supported phosphines, 28 certain monodentate ligands, 29 hemilabile bidentate ligands, 30 and P,Si-, N,Si-, and N,B-bidentate anionic ligands. 31,32 Alternative approaches to ortho-borylation shown in Scheme 1 include relay direction with silanes, 33 which is also useful in directed C–H silylations. 34,35 …”

Section: Introductionmentioning

confidence: 99%

Ir-Catalyzed ortho-Borylation of Phenols Directed by Substrate–Ligand Electrostatic Interactions: A Combined Experimental/in Silico Strategy for Optimizing Weak Interactions

et al. 2017

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A strategy for affecting ortho versus meta/para selectivity in Ir-catalyzed C–H borylations (CHBs) of phenols is described. From selectivity observations with ArylOBpin (pin = pinacolate), it is hypothesized that an electrostatic interaction between the partial negatively charged OBpin group and the partial positively charged bipyridine ligand of the catalyst favors ortho selectivity. Experimental and computational studies designed to test this hypothesis support it. From further computational work a second generation, in silico designed catalyst emerged, where replacing Bpin with Beg (eg = ethylene glycolate) was predicted to significantly improve ortho selectivity. Experimentally, reactions employing B2eg2 gave ortho selectivities > 99%. Adding triethylamine significantly improved conversions. This ligand– substrate electrostatic interaction provides a unique control element for selective C–H functionalization.

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“…In these systems, the mechanism of Ir-catalyzed C-H borylation is believed to proceed via a 16-electron triboryl intermediate L 2 Ir(Bpin) 3 (I, Scheme 1), which activates the arene C-H bond via oxidative addition (as shown in Scheme 1) or the closely related oxidative, hydrogen migration pathway. 19 Li et al demonstrated that a combination of two pyridine/boryl bidentate ligands on Ir gave rise to a potent catalyst, again with an implied Ir(III) triboryl intermediate (VI). Hartwig and co-workers used pendant Si-H groups in substrates and propose the intermediacy of a silyl/diboryl Ir(III) complex (IV, Scheme 1).…”

Section: Introductionmentioning

confidence: 99%

High-Turnover Aromatic C–H Borylation Catalyzed by POCOP-Type Pincer Complexes of Iridium

et al. 2016

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The catalytic C-H borylation of arenes with HBpin (pin = pinacolate) using POCOP-type pincer complexes of Ir has been demonstrated, with turnover numbers exceeding 10 000 in some cases. The selectivity of C-H activation was based on steric preferences and largely mirrored that found in other Ir borylation catalysts. Catalysis in the (POCOP)Ir system depends on the presence of stoichiometric quantities of sacrificial olefin, which is hydrogenated to consume the H2 equivalents generated in the borylation of C-H bonds with HBpin. Smaller olefins such as ethylene or 1-hexene were more advantageous to catalysis than sterically encumbered tert-butylethylene (TBE). Olefin hydroboration is a competing side reaction. The synthesis and isolation of multiple complexes potentially relevant to catalysis permitted examination of several key elementary reactions. These experiments indicate that the C-H activation step in catalysis ostensibly involves oxidative addition of an aromatic C-H bond to the three-coordinate (POCOP)Ir species. The olefin is mechanistically critical to gain access to this 14-electron, monovalent Ir intermediate. C-H activation at Ir(I) here is in contrast to the olefin-free catalysis with state-of-the-art Ir complexes supported by neutral bidentate ligands, where the C-H activating step is understood to involve trivalent Ir-boryl intermediates.

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Silyl Phosphorus and Nitrogen Donor Chelates for Homogeneous Ortho Borylation Catalysis

Abstract: Ir catalysts supported by bidentate silyl ligands that contain P- or N-donors are shown to effect ortho borylations for a range of substituted aromatics. The substrate scope is broad, and the modular ligand synthesis allows for flexible catalyst design.

Cited by 154 publications

References 28 publications

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

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

Ir-Catalyzed ortho-Borylation of Phenols Directed by Substrate–Ligand Electrostatic Interactions: A Combined Experimental/in Silico Strategy for Optimizing Weak Interactions

High-Turnover Aromatic C–H Borylation Catalyzed by POCOP-Type Pincer Complexes of Iridium

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