92284-89-4Relevant articles and documents
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
Du, Guodong,Fanwick, Phillip E.,Abu-Omar, Mandi M.
, p. 5180 - 5187 (2008/02/05)
Treatment of oxo and imido-rhenium(V) complexes Re(X)Cl3(PR 3)2 (X = O, NAr, and R = Ph or Cy) (1-2) with Et 3SiH 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)(PPh 3)2 (4a) proceeds via a σ-adduct followed by heterolytic cleavage of the Si-H bond and transfer of silylium (Et 3Si+) to chloride. Oxo and imido complexes of rhenium(V) (1-2) as well as their nitrido analogues, Re(N)Cl2(PR 3)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)Cl 2(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.
Mechanistic Features of C-H Activation by ReH72
Zeiher, E. H. Kelle,DeWit, David G.,Caulton, Kenneth G.
, p. 7006 - 7011 (2007/10/02)
The synthesis and characterization of ReH7(PCy3)2 (Cy=cyclohexyl) is reported.This compound is thermally resistant to the dimerization reaction exhibited by other ReH7(PR3)2 compounds, which allows mechanistic features of C-H activation to be studied.Thus, in the temperature range 60-80 deg C, ReH7(PCy3)2 exchanges hydrogen with deuteriobenzene.Deuterium is incorporated into the complex not only as hydride ligands but also selectively at the C2 and C3 carbons of all cyclohexyl rings.Only one of the two methylene hydrogens at each of these carbons is exchangeable.It is also shown that exchange at the two adjacent carbons does not proceed by concerted dehydrogenation via a cyclohexenyl intermediate; the rate of deuterium incorporation at C3 differs from that at C2.On the basis of observation of exchange with D2, and also kinetic studies of the reaction of ReH7(PCy3)2 with phosphine and olefinic nucleophiles, all of the thermal reactions observed here are concluded to arise from the reductive elimination transient ReH5(PCy3)2.
