Mechanistic insight into hydrosilylation reactions catalyzed by high valent Re equivalent to X (X = O, NAr, or N) complexes: The silane (SiH) does not add across the metal-ligand multiple bond

作者：Du Guodong; Fanwick Phillip E; Abu Omar Mahdi M^*

来源：Journal of the American Chemical Society, 2007, 129(16): 5180-5187.

DOI：10.1021/ja068872+

Treatment of oxo and imido-rhenium(V) complexes Re(X)Cl-3(PR3)(2) (X = O, NAr, and R = Ph or Cy) (1-2) with Et3SiH affords Re(X)Cl-2(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)Cl-3(PPh3)(2) (1a) with Et3SiH is characterized by phosphine inhibition and saturation in [Et3SiH]. Hence, formation of Re(O)Cl-2(H)(PPh3)(2) (4a) proceeds via a sigma-adduct followed by heterolytic cleavage of the SiH 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)Cl-2(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)Cl-2(OCH2Ph)(PPh3)(2) (6a) with kinetic dependencies that are consistent with aldehyde coordination followed by aldehyde insertion into the ReH 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)Cl-2(H)(PCy3)(2) and Re(NMes)Cl-2(H)(PPh3)(2) aldehyde insertion into the ReH 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.

• 出版日期2007-4-25