Mechanistic Insight into Hydrosilylation Reactions Catalyzed by High Valent Re⋮X (X = O, NAr, or N) Complexes: The Silane (SiH) Does Not Add across the Metal−Ligand Multiple Bond

Treatment of oxo and imido-rhenium(V) complexes Re(X)Cl3(PR3)2 (X = O, NAr, and R = Ph or
Cy) (1−2) with Et3SiH affords Re(X)Cl2(H)(PR3)2 in high yields. Cycloaddition of silane across the Re⋮X
multiple bonds is not observed. Two rhenium(V) hydrides (X = O and R = Ph, 4a; X = NMes and R = Ph,
5a) have been structurally characterized by X-ray diffraction. The kinetics of the reaction of Re(O)Cl3(PPh3)2 (1a) with Et3SiH is characterized by phosphine inhibition and saturation in [Et3SiH]. Hence, formation
of Re(O)Cl2(H)(PPh3)2 (4a) proceeds via a σ-adduct followed by heterolytic cleavage of the SiH bond
and transfer of silylium (Et3Si+) to chloride. Oxo and imido complexes of rhenium(V) (1−2) as well as their
nitrido analogues, Re(N)Cl2(PR3)2 (3), catalyze the hydrosilylation of PhCHO under ambient conditions,
with the reactivity order imido > oxo > nitrido. The isolable oxorhenium(V) hydride 4a reacts with PhCHO
to afford the alkoxide Re(O)Cl2(OCH2Ph)(PPh3)2 (6a) with kinetic dependencies that are consistent with
aldehyde coordination followed by aldehyde insertion into the ReH bond. The latter (6a) regenerates the
rhenium hydride upon reaction with Et3SiH. These stoichiometric reactions furnish a possible catalytic cycle.
However, quantitative kinetic analysis of the individual stoichiometric steps and their comparison to steady-state kinetics of the catalytic reaction reveal that the observed intermediates do not account for the
predominant catalytic pathway. Furthermore, for Re(O)Cl2(H)(PCy3)2 and Re(NMes)Cl2(H)(PPh3)2 aldehyde
insertion into the ReH bond is not observed. Therefore, based on the kinetic dependencies under catalytic
conditions, a consensus catalytic pathway is put forth in which silane is activated via σ-adduct formation
cis to the Re⋮X bond followed by heterolytic cleavage at the electrophilic rhenium center. The findings
presented here demonstrate the so-called Halpern axiom, the observation of “likely” intermediates in a
catalytic cycle, generally, signals a nonproductive pathway.