Iridium Diphosphine Complexes Featuring Pendant Lewis Acids: Efforts toward Selective Heteroarene Borylation

European Journal of Inorganic Chemistry(2023)

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Abstract
The selective forging of carbon-boron bonds via C-H borylation stands as a central means to access fine chemical precursors. Notwithstanding, achieving selectivity in this reaction is difficult, calling for the design of molecular catalysts that offer a vector for mechanistic control. This report aims to achieve such through the strategic placement of Lewis acids in the ligand periphery, permitting engagement with a substrate through non-covalent Lewis acid/base interactions. Various diphosphine iridium(I/III) complexes having 1,2-bis(di-n-propylphosphino)ethane) (dnppe), tetrakisallylphosphinoethane (tape) and 1,2-bis(di(3-dicyclohexylboranyl)propylphosphino)ethane (P2B4Cy) ligands were prepared. The P2B4Cy ligand scaffold boasts four Lewis acidic boron groups in its secondary coordination sphere, which are shown to engage with N-heterocycles, tape is the precursor to P2B4Cy, and dnppe is a saturated n-propyl analogue devoid of boron functionality. Select combinations of such iridium salts/diphosphine ligands were assayed in the catalytic borylation of 2-methylpyridine using B(2)Pin(2) (Pin=pinacol).
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Key words
Boron,Iridium,Ligand design,Phosphorus,Secondary coordination sphere
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