Binuclear elimination reactions

The potential participation of an alternative route, involving a binuclear elimination reaction between a metal-acyl and a metal-hydride has also been probed [73]. In Rh-catalysed cydohexene hydroformylation, both [Rh4(CO)i2] and [Rh(C(0)R)(C0)4] are observed by HP IR at steady state, the duster species being a potential source of [HRh(CO)4] by reaction with syn-gas. The kinetic data for aldehyde formation indicated no statistically significant contribution from binudear elimination, with hydrogenolysis of the acyl complex dominant. For a mixed Rh-Mn system. 

C. Li, Bimetallic Catalytic Binuclear Elimination Reaction. Experimental, Spectroscopic and Kinetic Elucidation, PhD Thesis, National University of Singapore, 2003. 

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

Complex XV undergoes an interesting binuclear reductive elimination reaction losing H2 upon reacting with ligands such as tertiary phosphines see Equation 6) (15), but pyridine and simple alkenes such as ethylene will not induce this reaction. 

The complex (I) undergoes binuclear reductive elimination reactions induced by tertiary phosphine and other ligands (equation 2) (11). 

N-bridging cyanate in low yields (17-23% after heating for 24 h Scheme 11). Conversion was found to proceed at comparable rates in ethanol or acetonitrile, and it was thus concluded that hydrolytic processes by traces of water do not play a role. Cyanate formation also was observed with Af-methylurea or 7/,7/-dimethylurea, but not with Af,A -dimethylurea or tetramethylurea, which shows that at least one NH2 group is essential for the elimination reaction to occur. A possible interpretation is that bridging of urea over the binuclear core through its O atom and one amino N atom is a key step for the conversion (58). This finding is in line with the discovery of urea-to-cyanate transformation for several pyrazolate-based dinickel complexes with urea bound in the N,0-bridging mode (see below). 

The synergistic effect often observed in bimetallic systems was further explored by Garland and coworkers. The hydroformylation of 3,3-dimethylbut-l-ene to form 4,4-dimethylpentanal in >95% selectivity at room temperature with [Rli4(CO)i2]-[Mn2(CO)io/HMn(CO)5] as catalyst coprecursors was investigated using in situ PT-IR spectroscopic techniques and kinetic studies revealing evidence of a bimetallic catalytic binuclear elimination reaction (CBER). 

Although hydrogen elimination is binuclear, the reaction is not kinetically bimolecular, the rate being first order in H2Os(CO)4. The derived rate law... 

The diversity of results of studies of hydrogenation and hydroformylation catalysis have prompted a report on the factors governing the different mechanisms of binuclear elimination reactions of complexes leading to carbon-hydrogen bond formation. 

The Catalytic Binuclear Elimination Reaction Importance of Non-linear Kinetic Effects and Increased Synthetic Efficiency... 

Keywords Catalytic binuclear elimination reaction (CBER), Hydroformylation, In-situ spectroscopy, Rhodium... 

The expression unusual has been deliberately been left open ended or ill defined since the effect will almost certainly differ between systems having different phenomenological bases for cooperativity or synergism. Nevertheless, a rather useful working definition might be a rate or selectivity dependence which cannot be explained as a strictly additive effect of the metal(s) used. Having said that, the homometallic and heterobimetallic catalytic binuclear elimination reactions (CBERs) which are the focus of this chapter have very well-defined rate dependences which can be traced back to the topology of the reaction mechanisms. 

Circa 35 rather well-defined stoichiometric homometallic and heterobimetallic binuclear elimination reactions have been identified to date. The synthetic products range from molecular hydrogen to hydrocarbons and even more functionalized organics. 

In the decades following the identification of the stoichiometric cobalt binuclear elimination reaction by Heck and Breslow, a number of researchers set out to verify the catalytic analogue. In particular, Whyman [35, 36], Alemdaroglu et al. [37] and Mirbach [38] conducted in situ spectroscopic analyses using high-pressure infrared spectroscopy. All groups observed the simultaneous presence of the mononuclear species HCo(CO)4 and RCOCo(CO)4 with simple alkenes (i.e. R = octyl, cyclohexyl) and the dinuclear complex Co2(CO)g under catalytic alkene hydrofor-mylation conditions. The general conclusion from Whyman and Mirbach was that... 

The Catalytic Binuclear Elimination Reaction Importance of Non-linear. 

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