A Heterogeneous Iridium Catalyst for the Hydroboration of Pyridines

Sulfated zirconium oxide (SZO) capped with silylium-like ions reacts with (cod)­Ir­(py)Cl (cod = 1,5-cyclooctadiene; py = pyridine) to form [Ir­(cod)­py]­[SZO] (1) and Me3SiCl. 1 can also be formed in reactions of phosphonium functionalized SZO and [Ir­(cod)­(OSi­(OtBu)3]2, which forms [Ir­(cod)­P­(tBu)2Ph]­[SZO] (2), followed by reaction with pyridine to form 1. FTIR and 15N­{1H} MAS NMR spectroscopy are consistent with coordination of pyridine in 1 to an electrophilic iridium. 1 is moderately active in the dearomative hydroboration of pyridine. The primary product of this reaction is 1,2-dihydropyridine, which converts to the 1,4-dihydropyridine product at long reaction times. 1 catalyzes the dearomative hydroboration of a variety of substituted pyridines and is also reactive toward pyrazines and N-methylimidazole.