Binuclear elimination

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. 

The spectroscopic and kinetic data from this reaction indicated the existence of a long sought catalytic reaction topology, bimetallic catalytic binuclear elimination. The kinetic data provided a linear-bilinear form in organometallics [95]. One term represented the classic unicyclic rhodium catalyzed hydroformylation and the other represented the attack of manganese hydride carbonyl on an acyl rhodium tetracarbonyl species. A representation of the interconnected topology is shown in Figure 4.12. 

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

The first term represents the classic unicyclic rhodium catalysis, while the second indicates a hydride attack on an acyl species. These spectroscopic and kinetic results strongly suggested the presence of bimetallic catalytic binuclear elimination as the origin of synergism of both metals rather than cluster catalysis. This detailed evidence for such a catalytic mechanism, and its implications for selectivity and nonlinear catalytic activity illustrate the important mechanistic knowledge that can be revealed by this powerful in situ spectroscopic technique. 

Donor molecules can displace H2 leading to substitution. Loss of one hydride ligand and one other anionic ligand (e.g. Cl, OAc, Me) is also possible (equation 57). Norton125 has discussed binuclear elimination as an important route for reductive elimination in metal hydrides. An interesting development has been the use of a base such as NEt3 to dehydrochlorinate a metal complex. Richards et al.ut have shown the versatility of this technique (equation 58). 

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). 

Elimination of methane from c/s-HOsMe(CO)4 proceeds by a binuclear mechanism as evidenced by the observation of CD4 evolution from the thermolysis of DOsMe(CO)4 in the presence of DOsCD3(CO)4 (63). Norton (59) proposed that such binuclear elimination should occur whenever co-ordinatively unsaturated alkyls such as AuMe(PPh3) are reacted with metal hydrides as in the reaction of HMn(CO)s with AuMe(PPh3) (76). Methane is also formed from the reaction of H2Os(CO)4 with RhI3(COMe)(CO) or IrCl(COMe)(CO)(AsPh3). Compound 26 is proposed as a general intermediate for such reactions. 

The stability of cluster hydrides seems to follow a similar pattern. Bridging seems to be the predominant structural form, but fluxional behavior, presumably involving intermediate terminal hydride spedes, is common. Norton has discussed binuclear elimination as an important cluster-forming decomposition pathway for metal hydrides (see Section 19.5.3). Structural rules due to Wade, Mingoes, Hoffmann, Lauher and Teo have been proposed to account for the structures of the higher nuclearity clusters. It is quite possible that clusters of several different electron counts will be accessible for any given polyhedron of metals, especially for the larger clusters. The relative usefulness of the different systems is still under discussion. 

It has been suggested that the acyl complex is [RCO-Co(CO)4], or [RCO Co(CO)3PBu3l for modified catalysts, but a site of unsaturation cis to the acyl ligand may be required (c.f.. Reference 60). A more probable formulation is therefore [RCO Co(CO)3], or [RCO Co(CO)2PBu3]. A binuclear, free-radical mechanism for the cobalt-catalyzed hydroformyla-tion of styrene or other conjugated substrates has also been proposed. These studies are far-reaching, especially because similar binuclear elimination steps have not received much consideration in studies of rhodium hydroformylation catalysts. ... 

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. 

Fig. 2 The general structure of a single-product mechanism for a heterobimetallic catalytic binuclear elimination where M represents the set of mononuclear intermediates possessing metal M, M represents the set of mononuclear intermediates possessing metal M and M-M presents the set of heterobimetallic intermediates. The all-important steps a and p which transform mononuclear species to dinuclear species and dinuclear species to mononuclear species are highlighted for emphasis. The symbol fs allows each sequence to possess an arbitrary non-zero number of intermediates...

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... 

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