Hydrogen atoms, direct transfer

A relatively common interaction in molecular coordination or organometallic compounds that nominally are coordinately unsaturated is the formation of a three-center two-electron bond between a metal center in the compound and a C-H bond of a hydrocarbon, a hydrocarbon fragment, or a hydrocarbon derivative that is a ligand in the complex. This interaction can be the prelude, the intermediate or transition state, to a subsequent reaction in which the CH hydrogen atom is transferred to the metal center and a direct a bond is formed between the carbon atom and the metal atom especially if the C-H bond is an activated bond. Internal oxidative addition of CH is a term often applied to this subsequent reaction step. The overall sequence is schematically outlined in 1. Factors that materially... 

As in hydrogen transfer between alcohols and saturated ketones, the rate-determining step in the reaction with a,p-unsaturated ketones is hydrogen abstraction from the a-carbon atom. It has been suggested that the hydrogen atom is transferred directly to the P-carbon of the enone, yielding an in -oxaallyl complex which following protonation yields the saturated ketone. ... 

Sizer and Gierer 56) give arguments to show that the proton is taken up by the enzyme molecule, rather than the water. Vennesland (57) has established that both yeast and liver enzymes transfer H to the same side of the pyridine ring in DPN. The authors consider this hydrogen atom is transferred directly, both molecules being held rigidly as shown on the enzyme surface. 

The transfer to styrene monomer is imlikely to proceed via a direct hydrogen abstraction, because the chain transfer coefficient is higher than that of ethylbenzene with its weaker bound hydrogen atoms. The transfer step rather involves the abstraction of a weakly bound hydrogen of the Diels-Alder adduct (see eq. 14), formed by two styrene molecules (183). 

Combination vs Disproportionation. There are two modes of termination one is the direct coupling (combination) of two free macroradicals to give a dead polymer chain of chain length i +j, with the rate coefficient At,c- The other mode is the so-called disproportionation, where a hydrogen atom is transferred from one of the radical chain ends to another radical, yielding two stabilized polymer chains, of which one carries a double bond. This reaction is associated with the rate coefficient t,d. The process is illustrated in equation 44 using polyethylene macroradicals as the example. It is important to notice that—in the case of macroradicals derived from other monomers—in principle any -hydrogen may be abstracted. 

The reaction is very fast in both directions, and so is always at equilibrium in water and in aqueous solutions. In every glass of water, protons from the hydrogen atoms are ceaselessly migrating between the molecules. This type of reaction, in which one molecule transfers a proton to another molecule of the same kind, is called autoprotolysis (Fig. 10.9). 

Methylmalonyl-CoA mutase (MCM) catalyzes a radical-based transformation of methylmalonyl-CoA (MCA) to succinyl-CoA. The cofactor adenosylcobalamin (AdoCbl) serves as a radical reservoir that generates the S -deoxyadenosine radical (dAdo ) via homolysis of the Co—C5 bond [67], The mechanisms by which the enzyme stabilizes the homolysis products and achieve an observed 1012-fold rate acceleration are yet not fully understood. Co—C bond homolysis is directly kineti-cally coupled to the proceeding hydrogen atom transfer step and the products of the bond homolysis step have therefore not been experimentally characterized. 

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