Hydroperoxide complexes

Thermal insertion occurs at room temperature when R is XCH2CHAr-, at 40° C when R is benzyl, allyl, or crotyl (in this case two isomeric peroxides are formed), but not even at 80° C when R is a simple primary alkyl group. The insertion of O2 clearly involves prior dissociation of the Co—C bond to give more reactive species. The a-arylethyl complexes are known to decompose spontaneously into CoH and styrene derivatives (see Section B,l,f). Oxygen will presumably react with the hydride or Co(I) to give the hydroperoxide complex, which then adds to the styrene. The benzyl and allyl complexes appear to undergo homolytic fission to give Co(II) and free radicals (see Section B,l,a) in this case O2 would react first with the radicals. 

The complex formation between hydroperoxides and HALS derivatives proposed for the preceding reaction was recently postulated by two different groups of investigators. First, Carlsson determined a complex formation constant for +00H and a nitroxide on the basis of ESR measurements—. Secondly, Sedlar and his coworkers were able to isolate solid HALS-hydroperoxide complexes and characterize them by IR measurements. The accelerated thermal decomposition of hydroperoxides observed by us likewise points to complex formation. It is moreover known that amines accelerate the thermal decomposition of hydroperoxides-. Thus Denisov for example made use of this effect to calculate complex formation constants for tert.-butyl hydroperoxide and pyridineitZ.. 

In real systems (hydrocarbon-02-catalyst), various oxidation products, such as alcohols, aldehydes, ketones, bifunctional compounds, are formed in the course of oxidation. Many of them readily react with ion-oxidants in oxidative reactions. Therefore, radicals are generated via several routes in the developed oxidative process, and the ratio of rates of these processes changes with the development of the process [5], The products of hydrocarbon oxidation interact with the catalyst and change the ligand sphere around the transition metal ion. This phenomenon was studied for the decomposition of sec-decyl hydroperoxide to free radicals catalyzed by cupric stearate in the presence of alcohol, ketone, and carbon acid [70-74], The addition of all these compounds was found to lower the effective rate constant of catalytic hydroperoxide decomposition. The experimental data are in agreement with the following scheme of the parallel equilibrium reactions with the formation of Cu-hydroperoxide complexes with a lower activity. 

Such a species cannot be ruled out in reactions of iron-EDTA complexes with hydroperoxides recently described by Bruice and coworkers (27). On the other hand, a hydroperoxide complex that reacts with the substrate such that bond formation fiom O to substrate is concerted with 0-0 bond breaking, as proposed by Klinman for dopamine P-monooxygenase (18), could provide compensation for the cost of 0-0 bond cleavage in the transition state. In fact, it is interesting to speculate that for each of these enzymes, the mechanism by which the substrate is oxidized may be dependent on the reactivity of the substrate. One could envision certain substrates that would react with the metal-bound hydroperoxide ligand prior to or concerted with 0-0 bond cleavage. This possibility is difficult to assess because of our lack of information concerning the reactivity of HQ2" when complexed to different metal ions. 

Electrons from ferrous ions in deoxyhemerythrin are transferred to O2 during formation of oxyhemerythrin, so that the latter is a diferric hydroperoxide complex. The electron transfer is reversed upon oxygen release. Oxidation of the met centers by other processes yields inactive methemerythrin or one of its complexes with small anions. Our discussion of the various states of hemerythrin... 

Epoxidation of olefins over Mo containing Y zeolites was studied by Lunsford et al. [86-90]. Molybdenum introduced in ultrastable Y zeolite through reaction with Mo(C0)g or M0CI5, shows a high initial activity for epoxidation of propylene with t-butyl hydroperoxide as oxidant and 1,2-dichloroethane as solvent [88]. The reaction is proposed to proceed via the formation of a Mo +-t-butyl hydroperoxide complex and subsequent oxygen transfer from the complex to propylene. The catalyst suffers however from fast deactivation caused by intrazeolitic polymerization of propylene oxide and resulting blocking of the active sites. 

Square-planar Pd (23) and octahedral Rh (24) hydroperoxide complexes containing hydrotris(3,5-diisopropylpyrazolyl)borate ligand have been prepared as well and characterized in the solid state. Strong hydrogen-bond interaction between —OOH moiety and ligands has been detected in these complexes. 

However, our kinetic data with molybdenum hexacarbonyl and other observations appear more consistent with a mechanism which proceeds through a polarized hydroperoxide complex. Reaction 2 appears to be faster than Reaction 3. 

The molybdenum-hydroperoxide complex (Step 3) reacts with the olefin in the rate-determining step to give the epoxide, alcohol, and molybdenum catalyst. This mechanism explains the first-order kinetic dependence on olefin, hydroperoxide, and catalyst, the enhanced reaction rate with increasing substitution of electron-donating groups around the double bond, and the stereochemistry of the reaction. 

Platinum-alkylperoxo and -hydroperoxo complexes are much less effective ketonization reagents than their palladium analogs. The platinum-hydroperoxide complex generated by protonation of Pt(PPh3)202 as in equation (90) was found to be inactive,133 as well as Pt(CF3)(OOH)(depe) obtained from the reaction of H202 with the corresponding hydroxo complex.265... 

Computational studies showed that the nature of the reactive species in the oxidation of trimethylamine, iodide ion, and dimethyl sulfide with lumiflavin is a C4 a-hydroperoxide complexed with water. The other two species, C4 a-hydroperoxide and C4 a-peroxide, yielded higher activation energies.237 Kinetic and spectroscopic studies on the effect of basic solvents, ethers, esters, and amides, on the oxidation of thianthrene-5-oxide with substituted peroxybenzoic acids indicated the involvement of the basic solvent in the transition state of the reactions. A solvent parameter, Xtc, based on the ratio of the trans to the cis form of thianthrene-5,10-dioxide, has been introduced.238... 

Fig 3.36. Mechanistic details of Sharpless epoxidations, part I the actual oxidant is a stereouniform tert-butyl hydroperoxide complex of a titanium tartrate "dimer" with the least hindrance possible. 

The selectivity to epoxide is determined by the realtive rates of reaction of the catalyst-hydroperoxide complex with the olefin [Eq. (311)] in competition with its homolytic decomposition [Eq. (312)]. 

A wide variety of solvents has been used for epoxidations, but hydrocarbons are generally the solvent of choice 428 Recently, it has been shown434 that the highest rates and selectivities obtain in polar, noncoordinating solvents, such as polychlorinated hydrocarbons. Rates and selectivities were slightly lower in hydrocarbons and very poor in coordinating solvents, such as alcohols and ethers. The latter readily form complexes with the catalyst and hinder both the formation of the catalyst-hydroperoxide complex and its subsequent reaction with the olefin. 

All of the reactions just described closely parallel the reactions of the same substrates with organic peracids. They probably involve rate-determining oxygen transfer from a metal-hydroperoxide complex to the substrate via a cyclic transition state, described earlier for the epoxidation of olefins with these reagents433,435... 

Rouchaud and co-workers492 494 studied the liquid phase oxidation of propylene in the presence of insoluble silver, molybdenum, tungsten, and vanadium catalysts. Moderate yields of propylene oxide were obtained in the presence of molybdenum catalysts. These reactions almost certainly proceed via the initial formation of alkyl hydroperoxides, followed by epoxidation of the propylene by a Mo(VI)-hydroperoxide complex (see preceding section). 

It would appear, therefore, that the sulfoxide-hydroperoxide complex retards decomposition of the hydroperoxide to free radicals. However, active sulfoxides decompose readily according to the scheme ... 

Efforts aimed at fully understanding the mechanism of these oxidation reactions are still needed but new insights regularly appear in the literature [203-205]. The isolation and characterization of a dioxygen-derived palladium(II)-hydroperoxide complex—species generally postulated as intermediates in this reaction—has been achieved for the first time by Stahl et al. [206] (Scheme 27). The capability of IMes ligands to undergo cis-trans isomerization has been pointed out as essential for the formation of this complex. 

Figure 19-6. Hypothetical active site complex of phenylalanine hydroxylase. The nonheme iron atom of phenylalanine hydroxylase is coordinated with protein amino acid residues (top of the figure) and molecular oxygen. The BH4 cofactor is also coordinated with molecular oxygen (bottom of the figure). It has been suggested that this complex decomposes to either an iron-oxygen complex or a biopterin-4 hydroperoxide complex prior to interaction with the amino acid substrate.

The reactions of Al, Ga, and In trialkyls with hydroperoxides or 02 give isolable hydroperoxide complexes (6-XX) which act as oxidizing agents.38... 

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