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

The
set of iridacycles [{C,N}­Cp*IrIII-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 H2-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*IrIII-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-silylimine, which undergoes
further N-silylation and reduction to yield the final N,N-disilylamine. Under optimal conditions
of low catalyst load (70 °C, 0.5 mol %) the autotandem hydrosilylation
of arylnitriles produces the silylated amines in yields >80% in
24
h.