Catalytic centers oxygen atom transfer

The evidence for the proposed mechanism comes from kinetic, spectroscopic (multinuclear NMR), X-ray structure, and theoretical calculations. The kinetic rate law under optimum catalytic conditions is very complex. Under pseudo-first-order conditions, where the concentrations of both 9.35 and the hydroperoxide are much greater than that of allyl alcohol, the rate expression 9.5 is obeyed. In this expression the inhibitor alcohol is an inert alcohol such as isopropanol or f-butanol that is deliberately added to slow down the reaction for convenient rate measurements. The inert alcohol acts as an inhibitor, since it competes with both hydroperoxide and allyl alcohol for coordination to the Ti center. Note that expression 9.5 is consistent with the formation of an intermediate like 9.36, before the rate-determining oxygen atom transfer step. 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

There are several demonstrations that cytochrome cdi catalyzes the reduction of molecular oxygen to water. Exactly how the enzyme catalyzes this reaction is of some interest, because the crystal structure shows that the catalytic center is mononuclear and expected to handle one electron at a time. If we assume that electron transfer between subimits cannot occur, then only two of the four electrons required for reduction of one oxygen atom can obviously be stored on one subimit of the enz5une before reduction of oxygen commences. Thus, it might be anticipated that some intermediates of oxygen reduction are relatively long-lived. 

The Morita-Baylis-Hillman (MBH) reaction is the formation of a-methylene-/ -hydroxycarbonyl compounds X by addition of aldehydes IX to a,/ -unsaturated carbonyl compounds VIII, for example vinyl ketones, acrylonitriles or acrylic esters (Scheme 6.58) [143-148]. For the reaction to occur the presence of catalytically active nucleophiles ( Nu , Scheme 6.58) is required. It is now commonly accepted that the MBH reaction is initiated by addition of the catalytically active nucleophile to the enone/enoate VIII. The resulting enolate adds to the aldehyde IX, establishing the new stereogenic center at the aldehydic carbonyl carbon atom. Formation of the product X is completed by proton transfer from the a-position of the carbonyl moiety to the alcoholate oxygen atom with concomitant elimination of the nucleophile. Thus Nu is available for the next catalytic cycle. 

---