Acylperoxyl radicals

El-Agamey, A. and McGarvey, D.J. 2003. Evidence for a lack of reactivity of carotenoid radicals towards oxygen A laser flash photolysis study of the reactions of carotenoids with acylperoxyl radicals in polar and non-polar solvents. J. Am. Chem. Soc. 125 3330-3340. 

Aldehyde produces the acylperoxyl radical as given by the following reactions ... 

The acylperoxyl radical is extremely active due to the high dissociation energy of O—H bond (D0—h = 418 k J mol 1 in benzaldehyde [73]) and accelerates the chain propagation. 

Chain propagation in an oxidized aldehyde is limited by the reaction of the acylperoxyl radical with the aldehyde. The dissociation energy of the O—H bond of the formed peracid is sufficiently higher than that of the alkyl hydroperoxide. For example, in hydroperoxide PhMeCHOOH, Z)0 H = 365.5 kJ mol-1 and in benzoic peracid... 

Carbonyl group of the aldehyde decreases the BDE of the adjacent C—H bond. This is due to the stabilization of the formed acyl radical, resulting from the interaction of the formed free valence with Tr-electrons of the carbonyl group. For example, DC—H = 422kJmol 1 in ethane and D( n 373.8 kJ mol 1 in acetaldehyde. The values of Dc H in aldehydes of different structures are presented in Table 8.1. In addition, the values of the enthalpies of acylperoxyl radical reactions with aldehydes were calculated (D0 H= 387.1 kJ mol-1 in RC(0)00 H). 

The Values of the C—H Bond Dissociation Energies in Aldehydes DC—h and Enthalpies AH of the Reaction of Acylperoxyl Radical (RC(O)OO ) with Aldehydes [2]... 

The chain unit in the thermal and photochemical oxidation of aldehydes by molecular dioxygen consists of two consecutive reactions addition of dioxygen to the acyl radical and abstraction reaction of the acylperoxyl radical with aldehyde. Experiments confirmed that the primary product of the oxidation of aldehyde is the corresponding peroxyacid. Thus, in the oxidation of n-heptaldehyde [10,16,17], acetaldehyde [4,18], benzaldehyde [13,14,18], p-tolualdehyde [19], and other aldehydes, up to 90-95% of the corresponding peroxyacid were detected in the initial stages. In the oxidation of acetaldehyde in acetic acid [20], chain propagation includes not only the reactions of RC (0) with 02 and RC(0)00 with RC(0)H, but also the exchange of radicals with solvent molecules (R = CH3). 

The rate constants of chain termination by disproportionation of two acylperoxyl radicals are collected in Table 8.4. 

There are two channels of the reaction of acylperoxyl radicals with olefins hydrogen atom abstraction and addition to the double bond with epoxide formation [5,35] ... 

Both these reactions occur rapidly. The kinetics and the products of co-oxidation of aldehydes and olefins were studied by Emanuel and coworkers [47-49], The values of the rate constants of the addition of acylperoxyl radical to olefins are presented in Table 8.8. The experimental data on aldehyde co-oxidation are discussed in monographs [4-6]. 

The important role of reaction enthalpy in the free radical abstraction reactions is well known and was discussed in Chapters 6 and 7. The BDE of the O—H bonds of alkyl hydroperoxides depends slightly on the structure of the alkyl radical D0 H = 365.5 kJ mol 1 for all primary and secondary hydroperoxides and P0—h = 358.6 kJ mol 1 for tertiary hydroperoxides (see Chapter 2). Therefore, the enthalpy of the reaction RjOO + RjH depends on the BDE of the attacked C—H bond of the hydrocarbon. But a different situation is encountered during oxidation and co-oxidation of aldehydes. As proved earlier, the BDE of peracids formed from acylperoxyl radicals is much higher than the BDE of the O—H bond of alkyl hydroperoxides and depends on the structure of the acyl substituent. Therefore, the BDEs of both the attacked C—H and O—H of the formed peracid are important factors that influence the chain propagation reaction. This is demonstrated in Table 8.9 where the calculated values of the enthalpy of the reaction RjCV + RjH for different RjHs including aldehydes and different peroxyl radicals are presented. One can see that the value A//( R02 + RH) is much lower in the reactions of the same compound with acylperoxyl radicals. 

Another factor that influences the reactivity of two polar reactants, acylperoxyl radical with aldehyde, is the polar interaction of carbonyl group with reaction center in the transition state. Aldehydes are polar compounds, their dipole moments are higher than 2.5 Debye (see Section 8.1.1). The dipole moment of the acylperoxyl radical is about 4 Debye (/jl = 3.87 Debye for PhC(0)00 according to the quantum-chemical calculation [54]). Due to this, one can expect a strong polar effect in the reaction of peroxyl radicals with aldehydes. The IPM helps to evaluate the increment Ain the activation energy Ee of the chosen reaction using experimental data [1], The results of Acalculation are presented in Table 8.10. 

Rate Constants of Addition of Acylperoxyl Radical to Olefins with Epoxide Formation... 

As alkylaromatic hydrocarbon (toluene, p-xylene, etc.) is oxidized, aldehydes appear radicals and peracids formed from them play an important role. First, aldehydes react rapidly with the Co3+ and Mn3+ ions, which intensifies oxidation. Second, acylperoxyl radicals formed from aldehydes are very reactive and rapidly react with the initial hydrocarbon. Third, aldehydes form an adduct with primary hydroperoxide, which decomposes to form aldehyde and acid. 

The radical reactivity of Craq002+ has been especially well documented. This complex is ideal for mechanistic studies, not only because of the convenient combination of lifetime and reactivity, but also because it is nearly inert to visible-light photolysis. Photochemical generation of reactive partners in the presence of Craq002+ is thus possible, and rapid radical reactions can be observed and studied directly, as shown on the examples of acylperoxyl radicals, NO, and NO2. 

The cross-coupling with acylperoxyl radicals was shown to lead to high-valent metal species and reactive organic intermediates (144). The Craq002+/CMe3C(0)00 reaction appears to be the sole example of such chemistry reported so far. Extension to other metals and types of radicals is essential before one can even begin to understand whether such reactions take place in catalytic oxidation systems and/or in aerobic organisms, and whether or how to exploit or suppress them. 

Addition of water readily reverses this reaction. However, for esters, the reaction proceeds as in reaction [1.3], and it has been suggested that this could be an important reaction for 02 generated in lipid membranes. Both the intermediate and the acylperoxyl radical should be more reactive as radicals than 02 -. 

The acyltrialkylstannanes are readily hydrolysed to the parent aldehydes, and are oxidised in the air to give the corresponding carboxylates, R3SnOCOR 63 in a radical chain reaction which presumably is similar to that of the autoxidation of an aldehyde, and involves as a key step the Sh2 reaction of an acylperoxyl radical at tin rather that at hydrogen (equation 6-26). The trialkylstannyl peroxyester which is formed then reacts with the parent stannyl ketone to give the trialkyltin carboxylate. 

Our interpretation of the results is that the acyl radical reacts with O2 [Eq. (6.13)] to form the acylperoxyl radical which then adds to the olefin and forms a complex which decomposes to produce the epoxide [Eq. (6.14)]. 

Since the decarboxylation of peroxy acids is diagnostic of the formation of acylperoxyl radical intermediates, these results indicate a fundamentally different manner of processing of peroxides by P-450 than by the peroxidases. A... 

Eq. 3.1) and intramolecular mode. Selective oxidation of ethers to esters by irradiation in the presence of benzil and oxygen was reported by Seto [29]. By this procedure, a tricyclic y-lactone is prepared from the corresponding fused tetrahy-drofuran as shown in Scheme 3, Eq. (3.2). The mechanism presumably involves a hydrogen abstraction by an acylperoxyl radical. 

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