Dioxygen reactive intermediates

The model proposed by Brandt et al. is consistent with the experimental observations, reproduces the peculiar shape of the kinetic curves in the absence and presence of dioxygen reasonably well, and predicts the same trends in the concentration dependencies of t, p that were observed experimentally (80). It was concluded that there is no need to assume the participation of oxo-complexes in the mechanism as it has been proposed in the literature (88-90). However, the model provides only a semi-quantitative description of the reaction because it was developed at constant pH by neglecting the acid-base equilibria of the sulfite ion and the reactive intermediates, as well as the possible complex-formation equilibria between various iron(III) species. In spite of the obvious constraints introduced by the simplifications, the results shed light on the general mechanistic features of the reaction and could be used to identify the main tasks for further model development. 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

In the presence of trace amounts of water, the tetrameric p,2-oxo complex (182) in 1,2-dimethoxyethane is transformed into a p, -oxo tetrameric complex (183 equation 254), characterized by an X-ray structure.574 In contrast, (182) 572,575 is inactive towards the oxidation of phenols. The reaction of N,N,N, AT -tetramethyl-l,3-propanediamine (TMP) with CuCl, C02 and dioxygen results in the quantitative formation of the /z-carbonato complex (184 equation 255).s76 This compound acts as an initiator for the oxidative coupling of phenols by 02. 6 Such jz-carbonato complexes, also prepared from the reaction of Cu(BPI)CO with 02 [BPI = 1,3 bis(2-(4-methyl-pyridyl)imino)isoindoline],577 are presumably involved as reactive intermediates in the oxidative carbonylation of methanol to dimethyl carbonate (see below).578 Upon reaction with methanol, the tetrameric complex (182 L = Py X = Cl) produces the bis(/z-methoxo) complex (185 equation 256), which has been characterized by an X-ray structure,579 and is reactive for the oxidatiye cleavage of pyrocatechol to muconic acid derivatives.580,581... 

In the past 25 years, Fe =0 and Mn =0 have also emerged as reactive intermediates in the oxidation of hydrocarbons they are not formed by interaction with dioxygen but rather by monooxygen donors (see below). 

Figure 8 Formation of an initial superoxo species has been long suspected in the formation of Cuj -peroxo species. For example the formation of (3) from [Cu (Nn)] proceeds with a very low (and sometimes negative) activation enthalpy, suggestive of the formation of an initial species Cu -Op - Cu (a). Direct observation of such an intermediate came from dioxygen reactivity studies with [Cu (TMPA)] complexes (b)...

Hence, the consequence of the addition of an electron (or hydrogen atom) to dioxygen is the production of finite fluxes of an entire group of reactive intermediates [including the strongest oxidant of the oxygen family, hydroxyl... 

In summary, the Coll(bpy)2 /HOOH/(4 1 MeCN/py) system forms a reactive intermediate (20) that selectively ketonizes methylenic carbon, and as such is closely similar to the intermediate of the Fe KPA)2/HOOH/(2 1 py/HOAc) system.36 The ability of FeP(DPAH)2 to activate O2 to an intermediate that has the same unique selectivity for hydrocarbon ketonization is further support for a common stabilized-dioxygen reactive complex (see Chapter 6), Several cobalt-dioxygen complexes exhibit oxygenase reactivity with organic substrates, 0,4l which is consistent with the dioxygen formulation for species 20. 

The dioxygenation of unsaturated a-diols (catechol and benzoin. Table 6-2) by the O2/Fe l(DPAH)2 system parallels that of the catechol dioxygenase enzymes, which are nonheme iron proteins. -l Hence, the reactive intermediate (1, Scheme 6-1) of the Feh(DPAH)2/O2 reaction may be a useful model and mimic for the activated complex of dioxygenase enzymes. ... 

A specific example from Chapter 4 will illustrate the point. In pyridine/acetic acid the combination of Fe(II)(PA)2 and HOOH yields a reactive intermediate that dioxygenates c-PhCH =CHPh... 

Borovik, A. S. Zinn, P. J. Zart, M. K. Dioxygen binding and activation reactive intermediates. In Activation of Small Molecules, Tolman, W. B. Wiley-VCH Weinheim, 2006 pp 187-234. 

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