Hydrogen atom transfer from metal hydrides

An organometallic radical may undergo several different types of reactions Scheme 3 illustrates some different reactions of CpM(CO)3 radicals (21) including (i) dimerization (ii) halide abstraction from an alkyl hahde or metal halide (iii) hydrogen atom abstraction from metal hydrides (iv) electron-transfer reduction (v) electron-transfer oxidation and ligand addition (vi) electron-transfer induced disproportionation (see Electron Transfer in Coordination Compounds). 

Hydrogen atom transfer from the hydride form of the catalyst to monomer (eq 15) is relatively unexplored in comparison with the initial reaction in the catalytic cycle, hydrogen atom abstraction from the growing radical. This is despite the fact that the two reactions are essentially the microscopic reverse, because the substituents on the organic fragment are relatively removed from the metal center. Early investigations of CCT were frustrated by the fact that concentrations of LCoH were below detection lim-... 

Hydride elimination reactions are characterized by hydrogen atom transfer from a ligand to a metal. The most common type of hydride elimination is /3 elimination, with a proton in a j8 position on an aUcyl ligand transferred to the metal by way of an intermediate in which the metal, the a and /3 carbons, and the hydride are coplanar. An example of j8 elimination—the reverse of 1,2-insertion—is in Figure 14.15. Beta eliminations are important in many catalytic processes. 

Scheme 1, which shows reaction pathways available for hydrogenation of alkenes using dihydride catalysts , has been developed largely from studies on Rh catalysts. The steps define the hydride route, and Kg,k2 the unsaturated route via oxidative addition of H2 to the metal-alkene complex. The common key dihydride-alkene intermediate 1 gives the saturate product wifii regeneration of catalyst M via two successive hydrogen atom transfer steps k. The and equilibria are usually established... 

Insertion of 2,3-DHT into the hydride-bridged cluster [H20s3(C0)9PPh3] yields a metalathiacyclopropane complex and thus models a plausible process by which 2,3-DHT could be hydrogenated at two metal centers on a catalyst surface. In contrast, when 2,5-DHT reacts with [Ru3(CO)i2], C-H bond activation occurs and hydrogen is transferred from the heterocycle to the cluster framework. The product complex, [Ru3(//-H)(CO)9(//3-l-4-v -DHT)], has the cyclometalated orga-nosulfur residue bound via both the sulfur atom and the olefin. 

The mechanism of hydride abstraction from rhenium alkyl complexes of the type [Re(Cp)(NO)(PPh3)(R)] (50) has been examined using electrochemical techniques and provides the first rate data for metal alkyl/Ph3C reactions. Both the a-hydride abstraction in equation (17) and )8-hydride abstraction in equation (18) processes are shown to involve a preequilibrium electron-transfer step shown in equation (19), followed by rate-determining hydrogen-atom transfer between... 

The tetracarbonylcobalt radical has a high reactivity. It dimerizes spontaneously (146), reacts with O2 to form 02Co(CO)4 (236,237) and abstracts hydrogen atom from transition metal hydrides (147). According to NMR line-shape analysis of the degenerate hydrogen-atom transfer reaction between HCo(CO)4 and Co(CO)4 proceeds with activation parameters of AH = (23.5 2.5) kJ/mol and AS = (-68.3 4.3) J/(mol K) (147). 

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