Reductive elimination binuclear reactions

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). 

Catalytic dehalogenation cycles with a binuclear reductive elimination step have not yet been reported, but many examples are known with a single metal. The RH product eliminates easily from d8 metals (Ni(II), Pd(II)) or d6 metals (Ru(II), Rh(III)) (Eq. (25)) [196]. This reaction is believed to go by a three-center transition state [193, 194, 212] ... 

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. 

Binuclear reductive eliminations followed by oxidative addition may also be involved in reactions such as Equation 8. 

We suggest that binuclear reductive elimination also occurs as the initial step in reactions of (I) with methanethiol or diphenylphosphine (equation 4) (13). 

A final example of the apparent occurrence of binuclear reductive elimination is found in reactions of (I) with alkynes bearing electronegative substituents. These reactions occur as shown in equation (5)... 

The formation of a Th(C5H5)3 complex was suggested because the subsequent thermal reaction of the photogenerated thbrium monohydride with unreacted starting hydrocarbyl complexes, gives the binuclear reductive elimination of alkane ... 

The reaction starts by the substitution of two CO ligands by the diene to give [Fe(CO)3(T] -diene)], then continues by the substitution of a 3" CO ligand and the intramolecular oxidative addition of the endo C-H bond leading to [FeCp(CO)2H], It ends by the dimerization with formation of the Fe-Fe bond and the loss of H2. This last step is not mechanistically clear-cut, but it could be taken into account by partial decoordination of the Cp ring (from t] - to t] -), coordination of the Fe-H bond of another molecule on the vacant site and binuclear reductive elimination of H2. 

Several useful reviews have appeared. Mondal and Blake have collected thermochemical data on oxidative addition, Halpern has investigated the formation of C-H bonds by reductive elimination, while in a thought-provoking article on activation of C[5/ ]-X bonds, Chanon stresses the importance of electron transfer in oxidative addition (among other topics). In a discussion of oxidation addition and reductive elimination involving two metal centers, Halpem classifies and gives examples of three mechanisms whereby binuclear reductive elimination can occur concerted two center (77), concerted one center (78), and free-radical [(79)-(81)] reactions given in Scheme 6. 

Figure 34. Binuclear reactions between [RhMesftacn)] and its reductive elimination product...

Behind the interest manifested in binuclear complexes stands the expectation that they will display fundamentally new modes of reactivity. Naturally, we also expect that they can show the patterns of reactivity known for mononuclear complexes. These include Lewis base associ-ation/dissociation, Lewis acid association/dissociation, ligand migration (insertion/deinsertion), oxidative addition/reductive elimination, and oxidative ligand coupling/reductive ligand uncoupling, as well as electron-transfer. While these reaction patterns do occur with binuclear... 

Step is hydrogen addition to an unsaturated, mononuclear, acyl-cobalt complex have been challenged on the basis of in situ infrared spectroscopic studies/ ° The mechanism has been shown to include a binuclear reaction (Equation 3.7) as the major or only reductive elimination step. ... 

Oxidative addition reactions of Ar-X to Ni (bipy) are often followed by disproportionation of the initially formed Ni(Ar)(X)bipy complex into NiAr2(bipy) and NiX2(bipy). It is believed that the latter process involves the intermediacy of binuclear species with bridging aryl groups, formed by dissociation of X , and hence is favored by polar solvents, such as dmf. " Relatively bulky aryls, such as mesityl or naphthyl, help to prevent disproportionation, while smaller aryl groups tend to favor it. The formation of NiAr2(bipy) can be followed by reductive elimination of biaryl, although the final outcome of the overall process depends markedly on the reaction conditions. 

Nickel dimethyl complexes NiMe2(PRs)2 decompose readily in solution, either thermally or photochemically, providing an efficient method for the generation of Ni(0) complexes. The complex NiMe2(dtbpe) undergoes reductive elimination in benzene at 50-60 °C, giving rise to a binuclear Ni(0) 7r-arene complex. Careful thermolysis in THF-/ 8 over several days at 20 °G affords a 1 2 1 mixture of the ethylene complex 159, the hydride 160, and ethane (Scheme 47). If the reaction is carried out in the presence of ethylene, equimolecular amounts of 159 and ethane are formed. These results can be explained by assuming that the decomposition process involves the formation of a complex. In contrast with this behavior, the bis(carbene) dimethyl complex 45 decomposes... 

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