Oxygen atom transfer catalysis

Oxygen Atom Transfer Catalysis and Dioxygen Activation... 

Scheme 4.3 Substituted enzyme mechanism transferred to oxygen atom transfer catalysis (M = Mo, W). In principle, this represents two coupled half-reactions with a pre-equilibrium at each half-reaction. This is the favoured mechanism due to the negative activation entropies observed for this type of reaction.

In order to avoid the unwanted dimerization, steric constraints were incorporated into the used ligands. This hinders the metal centres to approach each other, so either the equilibrium is shifted to the side of the monomeric molybdenum species or the dimerization completely prevented. One of the first bulky sulfur-containing ligands that was able to successfully promote oxygen atom transfer catalysis of its molybdenum complex was developed by Berg and Holm (Scheme 4.7 BuLNS = bis(4-tert-butylphenyl)-2-pyridylmethanthiolate). ... 

Intermolecular oxygen atom transfer from a metal complex to an organic substrate is an archetypical reaction step in oxidation catalysis. As the transformation of O2 into metal 0x0 groups by oxidative addition is a well-precedented process (Sect. 2.2), its combination with transfer of the oxygen atom to an oxidizable substrate ( S ) constitutes a catalytic cycle for aerobic oxidations (Eq. 21). Examples of such cycles exist in organometallic chemistry, by virtue of 0x0 complexes with carbon-based ancillary hgands. 

Friedman, S. H., Massefski, W., and Hollocher, T. C. (1986). Catalysis of intermolecular oxygen atom transfer by nitrite dehydrogenase of Nitrobacter agilis. J. Biol. Chem. 261, 10538-10543. 

Dialkyl peroxides, like diacyl peroxides and peroxyesters, are characterized by homolysis of the 0-0 bond, which is promoted thermally, photochemically or by transition metal catalysis. The combination of steric factors, and the poor leaving group ability makes simple dialkyl peroxides (ROOR) almost unreactive in heterolytic oxygen atom transfer.128 Consequently, no further mention of these species will be made. 

Caradonna, J. P., Reddy, P. R., and Holm, R. H., 1988, Kinetics, mechanisms, and catalysis of oxygen atom transfer reactions of S Oxide and pyridine N Oxide substrates with molybdenum (IV,VI) complexes relevance to molybdoenzymes, J. Am. Chem. Soc. 110 2139n2144. 

A very broad class of primary oxidants act as oxygen atom transfer agents, the most widely nsed oxidants in oxidation catalysis. These inclnde peracids or their anion forms, snch as MCPBA or oxone (O-OSOs ) as well as A-oxides snch as A-methyl morpholine A-oxide or hypochlorite ion. They all have general structure XO, where X is a good leaving gronp. 

The possible role of oxygen atom transfer in molybdenum enzyme catalysis was recognized in the early 1970s (190-194). In the ensuing years, a wealth of chemistry has established molybdenum as the premier exponent of such reactions (7, 195). Importantly, related dioxo-Mo(VI) and oxo-Mo(IV) complexes are interconverted by oxygen atom transfer reactions (Eq. (13)). These reactions are effected by reductants (X) such as tertiary alkyl and aryl compounds of the group 15 elements (especially phosphines) and oxidants (XO) such as S- and N-oxides. In many cases, however, the Mo(VI) and Mo(IV) compounds participate in a comproportionation reaction yielding dinuclear Mo(V) complexes (Eq. (15)). 

Ozone is also a better-oxygen-atom-transfer reagent in the atmosphere because UV light results in the formation of 02 and O atoms, which react with halides. The role of metals in the catalysis of iodide oxidation is also more likely in the atmosphere because of the enrichment of iodide relative to the other halides in rainwater and aerosols (13). 

Khenkin and Neumann discovered that SET from polycyclic arenes to a mixed-addenda heteropoly acid (HPA), Hj[PV2M0jj 0 (j], can be followed by oxygen atom transfer from the HPA to the radical cation (Scheme 14.7) [38], This type of reactivity is well known in the area of heterogeneous gas-phase oxidation as a Mars-van Krevelen mechanism, whereby a metal oxide at high temperature transfers oxygen atom from the lattice to an activated hydrocarbon substrate, but in homogeneous catalysis, it is veiy rare. 

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.)...

The ionization of (E)-diazo methyl ethers is catalyzed by the general acid mechanism, as shown by Broxton and Stray (1980, 1982) using acetic acid and six other aliphatic and aromatic carboxylic acids. The observation of general acid catalysis is evidence that proton transfer occurs in the rate-determining part of the reaction (Scheme 6-5). The Bronsted a value is 0.32, which indicates that in the transition state the proton is still closer to the carboxylic acid than to the oxygen atom of the methanol to be formed. If the benzene ring of the diazo ether (Ar in Scheme 6-5) contains a carboxy group in the 2-position, intramolecular acid catalysis is observed (Broxton and McLeish, 1983). 

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