Transition metal hydrides exchange reactions

Titanium tetrachloride is a very effective catalyst for the addition of LiAlH4 or alane to the olefinic double bond. The mechanism of this reaction involves intermediate transition metal hydrides, as in the case of reaction of NaBPU and Co11-salts. The hydrotitanation of the double bonds is probably followed by a rapid metal exchange reaction (Scheme 3)94. 

In summary the calculations presented in Ref. [62] represent the missing theoretical link between the phenomena of incoherent and coherent dihydrogen exchange in transition metal hydrides and the PHIP effect. Moreover, PHIP is identified as a powerful and sensitive tool to study reaction pathway effects via analysis of the polarization patterns of the final hydrogenation products. 

The transition-metal-catalysed exchange of halogen atoms may also be viewed as a heteroatom coupling reaction. It is particularly useful for the conversion of the cheaper and more available chloro compounds to the more reactive iodo compounds. Nickel catalysis may be used (Scheme 2.185). Cyanide can also be coupled (Scheme 2.186). The use of hydride sources, such as formate or tri-n-butyltinhydride is a method of removing functionality (Scheme 2.187). The mechanism of formate reduction is illustrated in Scheme 5.50. 

The exchange reaction between aminoorganosilanes and complexed transition metal hydrides occurs as follows (Eq. 3.252) ... 

How does the anionic alkyl of the original trialkylaluminum or of the dialkylaiuminum chloride, which has sufficient anionic character to undergo anionic hydride exchange or CH3OT reaction, form a catalyst which becomes cationic under certain polymerization conditions No studies of this have been reported. One possibility is an internal oxidation-reduction reaction that converts an anionic alkyltitanium trichloride to a cationic alkyltitanium trichloride (Equation 10). Basic and electrophilic catalyst components would determine the relative contributions of the anionic and cationic forms. This type of equilibrium or resonance structures could also explain the color in transition metal compounds such as methyltitanium trichloride (73). 

Concomitant with continued olefin insertion into the metal-carbon bond of the titanium-aluminum complex, alkyl exchange and hydrogen-transfer reactions are observed. Whereas the normal reduction mechanism for transition-metal-organic complexes is initiated by release of olefins with formation of hydride followed by hydride transfer (184, 185) to an alkyl group, in the case of some titanium and zirconium compounds a reverse reaction takes place. By the release of ethane, a dimetalloalkane is formed. In a second step, ethylene from the dimetalloalkane is evolved, and two reduced metal atoms remain (119). 

The reverse process, decarbonylation, is also fast but can be arrested by maintaining a pressure of carbon monoxide above the reaction mixture. The reverse of hydrometallation involves the elimination of a hydride from the adjacent carbon of a metal alkyl to form an alkene complex. This process is known as [3-hydride elimination or simply [3 elimination. It requires a vacant site on the metal as the number of ligands increases in the process and so is favoured by a shortage of ligands as in 16-electron complexes. The metal and the hydride must be syn to each other on the carbon chain for the elimination to be possible. The product is an alkene complex that can lose the neutral alkene simply by ligand exchange. So (3 elimination is an important final step in a number of transition-metal-catalysed processes but can be a nuisance because, say, Pd-Et complexes cannot be used as p elimination is too fast. 

The reactions of silyl hydrides with low-valent metal complexes is a common method of preparing silyl derivatives of transition metals (14-16). However, relatively little work has been directed to the study of H/H (H/D) exchange. From the studies of Sommer et al. (17), it has been established that chloroplatinic acid, (QH4 P CU, Co2(CO)8, L Ir(CO)Cl,... 

The intensive research activities recently focused on late transition metal complexes as polymerization catalysts are justified by decisive improvements in the development of new polymer materials and indicate even more their high potential for future applications. However, up to now they often only dimerize or oligomerize a-olefms due to competing hydride elimination and associative olefin exchange reactions [19]. A new concept of new diimine ligands and complexes bearing 2,6-diphenyl-modified aniline moieties (Fig. 2.5) is stiU focused on sterically demanding substituents in the 2,6-positions, but it aims further towards the facile modification of the steric and electronic properties of the active species. 

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