CH bond activation in aromatic ketones mediated by iridium-tris(pyrazolyl)borate complexes

Abstract

Reaction of complex [Tp^Me2Ir(η⁴-CH₂C(Me)C(Me)C₂)] (1) with a series of aromatic ketones at 130 °C renders, by means of a selective ortho-CH activation, Ir(III)-metallacycles 2–5, which display an Ir–H bond. When [Tp^Me2Ir(C₆H₅)₂N₂] (6) is treated with 2-(trifluoromethyl)acetophenone and 2-fluoroacetophenone at 80 °C, the formation of dimeric (7) and trimeric architectures (8) is achieved through the meta- and para-CH activation of the aromatic ketone, respectively. The generation of complexes 2–5 is proposed to occur by the initial formation of Ir(III) η¹-ketone adducts as key intermediates, followed by aromatic CH activations and the release of a butadiene ligand. The formation of complexes 7 and 8 involves an assisted process in which a metal center activation of the less sterically hindered C–H bond of the aromatic ketone takes place (releasing a benzene molecule), followed by the coordination of the carbonyl group, which generates the respective dimeric and trimeric structures. Complexes 7 and 8 are efficient catalysts for the transfer hydrogenation of ketones and aldehydes using isopropanol as the hydrogen source. All complexes have been fully characterized by NMR spectroscopy, FT-IR, elemental analysis and, in the cases of 7 and 8, X-ray crystallography. Details of the reaction conditions, isolation of the products, and proposals for the pathways of formation of complexes 2–5 and 7–8 are discussed.