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Du G, Fanwick PE, Abu-Omar MM: Mechanistic insight into hydrosilylation reactions catalyzed by high valent ReX (X = O, NAr, or N) complexes: the silane (Si-H) does not add across the metal-ligand multiple bond. J Am Chem Soc. 2007 Apr 25;129(16):5180-7. Epub 2007 Mar 28.
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 ReX 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 sigma-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 sigma-adduct formation cis to the ReX 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. |
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