Metal-oxygen complexes, initiation

In the examples above, one or both of the reaction centers are already attached to the metal center. In many cases, the reactants are free before reaction occurs. If a metal ion or complex is to promote reaction between A and B, it is obvious that at least one species must coordinate to the metal for an effect. It is far from obvious whether both A and B enter the coordination sphere of the metal in a particular instance. A number of metal-oxygen complexes can oxygenate a variety of substrates (SOj, CO, NO, NO2, phosphines) in mild conditions. Probably the substrate and O2 are present in the coordination sphere of the metal during these so-called autoxidations. In the reaction of oxygen with transition metal phosphine complexes, oxidation of metal, of phosphine or of both, may result. The initial rate of reaction of O2 with Co(Et3P)2Cl2 in tertiary butylbenzene. 

Direct initiation by either mechanism is characterized by a lack of induction period (47) and is most efficient by metals that are strongly oxidizing (Co and Fe) or can form metal-oxygen complexes (Co and Cu). 

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

Further evidence against initiation by direct oxygen activation in the oxidation of olefins is provided by the following two observations.185 First, no reaction was observed between olefins (e.g., cyclohexene, 1-octene, and styrene) and metal-dioxygen complexes, such as I, II, and V, when they were heated in an inert atmosphere (nitrogen). Second, no catalysis was observed with these metal complexes in the autoxidation of olefins, such as styrene, that cannot form hydroperoxides. 

Figure 4-27. Changes in the two distances between the olefin carbons and the metal (M — C(ir) and M — C tt)) as well as the metal-oxygen distance M-O from constrained MD and die following relaxation (unconstrained) simulations for the methyl acrylate ir-complex — > a-complex interconversion with the Ni-based diimine catalyst. The initial and finalgeometries are shown in the figure...

An additional method to determine the nature of the metal-nucleotide complex that is preferred by the enzyme is in the study of the effects of cation substitution on the stereoselectivity of chiral-specific thiophosphate nucleotides. Thiophosphate derivatives of cAMP, AMP, ADP, and ATP at the a-, /3-, and/or y-positions were initially introduced by Eckstein and co-workers (66, 67) and expanded to several other nucleotides. Substitution of a phosphate oxygen by sulfur at the a-position of cAMP, ADP, or ATP or at the /3-position of ATP gives rise to a new chiral center, Substitution in the a-position of AMP, j8-position of ADP, and the y-position of ATP changes an achiral center to a prochiral center. This substitution gives rise to a small decrease in p a for the phosphate, and with many enzymes these thiophosphate analogs have a decreased substrate ac-... 

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