Iridacycles as Catalysts for the Autotandem Conversion of Nitriles into Amines by Hydrosilylation: Experimental Investigation and Scope

The set of iridacycles [{C,N}Cp*Ir-III-Cl] ({C,N} = benzo[h]quinoline, dibenzo[f,h]quinoline) containing the (pentamethylcyclopentadienyl)iridium(III) unit were synthesized and derivatized into cations [{C,N}Cp*Ir-NCMe](+) associated with BArF-type anions. The latter salts were benchmarked for their potential catalytic properties toward HSiEt3 in a H-2-releasing test reaction. The best-performing BArF-type salts demonstrated the capability to promote with a low catalytic load of ca. 0.5-1 mol % the autotandem hydrosilylation of acetonitrile, propionitrile, and a series of arylnitrile substrates. Mechanistic investigations confirmed the preliminary formation of a silane-iridacycle adduct by electrophilic and heterolytic activation of the Si-H bond. The molecular structure of a new example of such an adduct was resolved by X-ray diffraction analysis. Theoretical considerations support a donor-acceptor [{C,N}Cp*Ir-III-H]->[SiEt3](+)({C,N} = benzo[h]quinolinyl) formulation where the cationic silyl moiety acting as a Z ligand binds both Ir and H centers. Under the conditions of the catalysis, the latter adduct is assumed to transfer readily the electrophilic [SiEt3](+) moiety to the nucleophilic nitrile substrate to form a N-silylnitrilium cation and the neutral [{C,N}Cp*Ir-H]. The latter reduces the N-silylnitrilium into the corresponding N-sflylimine, which undergoes further N-silylation and reduction to yield the final N,N-disilylamine. Under optimal conditions of low catalyst load (70 degrees C, 0.5 mol %) the autotandem hydrosilylation of arylnitriles produces the silylated amines in yields >80% in 24 h.