Homolytic catalysis

Homolytic catalysis is observed with both organometallic and coordination complexes. It is involved in a wide variety of metal-mediated transformations, often in competition with electrophilic or nucleophilic catalysis [11], For example, many metal-catalyzed oxidations involve substrate activation by homolytic catalysis (Eq. 5) [12], Similarly, oxidative additions (Eq. 6) and dioxygen activation (Eq. 7) can proceed via two-step homolytic mechanisms. 

Reactions with H2O2 may be divided into two classes arising from the homolytic vy. heterolytic cleavage of the 0-0 bond (173). In homolytic catalysis, the oxygen-centered radicals are intermediates the participation of concerted processes in heterolytic catalysis precludes paramagnetic intermediates. Product selectivity is usually higher in the latter class. Transition metal cations in low oxidation states, such as Cu, Ti, Cr, and Fe " ", catalyze the homolytic... 

A similar case is the catalysis of Gomberg-Bachmann arylations by A,A-diphenyl-hydroxylamine, which was discovered by Cooper and Perkins (1969). As Scheme 8-46 shows, the covalent adduct cation 8.62 first loses a proton. This facilitates the homolytic dissociation, as a stable radical, A/,A-diphenylnitroxide (8.63), is formed. This... 

It is likely that Cohen s method is a homolytic reaction (Sec. 8.6). The catalysis can be represented by Schemes 10-7 to 10-9. The last step of this mechanism is based on competitive hydroxy-/halo-de-diazoniations investigated by Hanson et al. (1991) and discussed in Section 10.5. 

Holmium, tris(2,2,6,6-tetramethyl-3,5-heptanedione)-photosubstitution, 1,408 Holmium complexes 1,3-diketones, 2,387 phenanthroline, 3,1069 Homoazaporphyrins synthesis, 2, 817 Homolytic cleavage catalysis... 

The phenomenon of acid catalysis of peroxide homolytic splitting will be discussed in Chapter 10. 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

The homolytic decomposition of hydroperoxides was proved to be catalyzed by Bronsted as well as Lewis acids (for example, BF3, A1C13, SbCls) [230]. Experimental data on acid catalysis of the homolytic decomposition of hydroperoxides are collected in Table 10.9. 

Acid Catalysis of the Homolytic Decomposition of Hydroperoxides (Experimental Data)... 

In the field of enzyme catalysis, heme-proteins such as cytochrome P450, for example, exhibit both types of 0-0 bond cleavages in organic hydroperoxides and peroxy acids (178). Heterolytic cleavage of HOOH/ROOH yields H20 or the corresponding alcohol, ROH and a ferryl-oxo intermediate (Scheme 4). Homolytic 0-0 bond cleavage results in the formation of a hydroxyl (HO ) or an alkoxyl (RO ) radical and an iron-bound hydroxyl radical. 

Catalysis by transition metals in liquid-phase oxidation has been thor- oughly investigated. The roles of other ions have not been sufficiently studied. This paper is concerned with catalysis by hydrogen ions and some anions, in the chain oxidation of secondary alcohols such as cyclohexanol and 2-propanol. Secondary alcohols, because of their polarity, are convenient for studying ionic homolytic reactions and their role in chain oxidation. 

Similar results were obtained for tert-butyl hydroperoxide and perchloric acid in 2-propanol. Thus, it is evident from the decomposition of hydrogen peroxide into free radicals that both heterolytic and homolytic reactions may be catalyzed by hydrogen ions. Further research is needed to investigate proton catalysis in certain homolytic reactions. 

Competition between Homolytic and Heterolytic Catalytic Decompositions of Hydroperoxides Reactions of Transition Metals with Free Radicals Reactions of Transition Metal Ions with Dioxygen Catalytic Oxidation of Ketones Cobalt Bromide Catalysis Oscillating Oxidation Reactions... 

Autoxidation of alkanes may be carried out by metal catalysis.2,14 17 Although metal ions participate in all oxidation steps, their main role in autoxidation is not in their ability to generate free radicals directly by one-electron oxidation [Eq. (9.14)] but rather their activity to catalyze the homolytic decomposition of the intermediate hydroperoxide according to Eqs. (9.15) and (9.16). As a result of this decomposition, metal ions generate chain-initiating radicals. The overall reaction is given in Eq. (9.17) ... 

The homolytic and heterolytic nature of the oxidation is principally governed by the nature of the metal, but also by the nature of the ligands, the availability of coordination sites, the nature of the substrate and the temperature conditions. Consequently, the chemist has to overcome several difficulties, some inherent to homogeneous catalysis, some inherent to oxidation chemistry ... 

Perbenzoic acid fcrf-butjl ester (51) is the source — under copper I) bromide catalysis —of a benzoate anion (52) and radical 53. Radical 53 subsequently abstracts a hydrogen atom selective ) from the 11 -position of 55 in a homolytic bond cleavage to give a buta diene system with opening of the cyclopropane ring.Jn... 

Roelfes et al. prepared a non-heme iron(II) complex 26 from pentadentate ligand N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine 25 (Scheme 3.32) [126]. In the presence of H202, complex 26 reacted to a low-spin Fe(III)OOH intermediate, which was cleaved homolytically to an oxo Fe(IV) species and a hydroxy radical. Both species are capable of oxidizing various organic substrates via a radical pathway (Scheme 3.32). Under the catalysis of complex 26, cyclohexene la was oxidized with excess H202 to a mixture of products 2a, 3a and 4a. The TON was found to be solvent dependent, with higher TON in acetonitrile than in acetone (Scheme 3.32). In no case were isolated yields given and, furthermore, the allylic oxidation is limited to cyclohexene la. 

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