Ligands alkylperoxo

A mononuclear diastereopure high-spin Fe alkylperoxo complex with a pen-tadentate N,N,N,0,0-ligand 33 (Scheme 17) was reported by Klein Gebbink and coworkers [109, 110]. The complex is characterized by unusual seven-coordinate geometry. However, in the oxidation of ethylbenzene the iron complex with 33 and TBHP yielded with large excess of substrate only low TON s (4) and low ee (6.5%) of 1-phenylethanol. 

Cobalt(III)-alkylperoxo complexes find use in the oxidation of hydrocarbons.1342,1343 Since they release ROO and RO radicals upon mild heating in solution, they are effective oxidants under mild conditions, and produce catalytic systems in the presence of excess ROOH. Aliphatic C—11 bond oxidation by ConOOR (R = Con, alkyl, H) complexes including a hydrotris(pyrazolyl) borate ligand have also been reported, with homolysis of the peroxo O—O bond believed to be important in oxygenation of the C—H bond.1344... 

There would appear to be two distinct modes of reactivity of early transition metal alkyls with O2. When the metal is not in its highest oxidation state, an O2 complex of variable stability may form, and its subsequent reactivity may or may not involve the metal-carbon bond. The formation of remarkable stable 0x0 alkyls is an example of this pathway. In contrast, d°-alkyls react with O2 by a radical chain mechanism that invariable leads to formation of alkoxide complexes labile alkylperoxo ligands are clearly imphcated as intermediates in these reactions. 

Mimoun has given an alternative possibility, suggesting the formation of peroxo metallacyclic adducts following complexafion of the olefin to the metal [171,475]. The reaction sequence (Fig. 1.13) involves a reactive species denoted as Ln-Mo, a Mo(VI) ion with a set of alkoxy ligands. The reaction starts by formation of an alkylperoxo Mo complex by ligand exchange of an alkylperoxide with an alcohol. This is followed by complexafion of the olefin by a coordination bond. A subsequent peroxy metallation of the olefin produces a five-membered... 

The Tpx ligands can mimic the coordination environment created by three imidazolyl groups from histidine residues, which is frequently found in the active sites of metalloenzymes. Higher valent bis(ix-oxo) species, [(Tpx)M( i-0)2M(Tpx)] via 0-0 cleavage of [(Tpx)M( x-r 2 r 2-02)M(Tpx)] intermediates, but also peroxo, hydroperoxo, and alkylperoxo species, active species undergoing oxidative C-C cleavage reaction, stable hydrocarbyl complexes, and dinuclear xenophilic complexes, [(Tpx)M-M L71], are all relevant to chemical and biological processes, most of which are associated with transition metal catalytic species. 

A very similar acylperoxo (or alkylperoxo) intermediate has also been postulated in the peroxidase reactions (48). For example, horseradish peroxidase (HRP) is oxidized by hydrogen peroxide to give Compound I, which has been characterized as an oxo porphyrin ir-cation radical, Fe (=0)Pf [Eq. (7)J (49). The major difference between HRP and P-450 is the fifth ligand, the former having imidazole and the latter a thiol from cysteine. 

The chemistry of cobalt-dioxygen complexes of hydrotiis(p5 azolyl)-borate ligands (Tp(R)) has been reviewed with special reference to the formation of low-valent metal-peroxo and high-valent metal-oxo spedes, such as Co(ll)-superoxo, alkylperoxo and dinuclear Co(in)-bis(proxo) complexes. In the hydrotris(3,5-dnsopropyl-l-pyrazolyl)borate ligand system (TpCPr )), oxygenation of the proximal isopropyl substituents on Tp Pr3) is mediated by the p,-peroxodicobaIt(III) and the alkylperoxocobalt species. 

The vanadium complex 2.73 has a peroxo ligand. An analogue with an alkylperoxo (RO ") ligand that has a similar strucmre is also known. The oxidation state of vanadium in 2.73 is 5+. As we will see later, the alkyl peroxo analogue of 2.73 is a model (see Section 3.4) that mimics the mode of activation of organic hydroperoxides by high-valent metal ions. 

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