Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 138, Issue 30, Pages 9487-9497Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jacs.6b03656
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Funding
- National Science Foundation [CHE-1300299]
- Welch Foundation [A-1717]
- LSAMP NSF Fellowship [HRD-1406755]
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [1300299] Funding Source: National Science Foundation
- Division Of Human Resource Development
- Direct For Education and Human Resources [1406755] Funding Source: National Science Foundation
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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 H-2 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 Jr complexes supported by neutral bidentate ligands, where the C-H activating step is understood to involve trivalent Ir-boryl intermediates.
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