Radical transfer, catalytic

Other complexes also react with propagating radicals by catalytic chain transfer.110 These include certain chromium,151 152 molybdenum152 1" and iron154 complexes. To date the complexes described appear substantially less active than the cobaloximes and are more prone to side reactions. 

In the fully reduced model, four electrons are transferred to dioxygen through sequential one-electron oxidations of heme as s iron ion, the Cub ion, the heme a iron ion, and one of the bimetallic center s Cua ions. The sequence of electron transferal differs in the mixed valence model, and a tyrosine radical (tyr) is generated. The proposed formation of a tyrosine radical during catalytic turnover arises from the known post-translational modification in most CcO s in which a covalent bond is formed between the his240 ligand of Cub... 

Caps and coworkers studied the solvent effect in the epoxidation of stilbene by varying solvents and the supports [200], In methylcyclohexane (MCH), the activated radical species proposed were MCH peroxy radicals, which were formed by the radical transfer from TBHP and reaction with molecular oxygen. Except for MCH, the solvent effect is not fully understood however, the choice of solvent and supports that can trap or stabilize the radical species affected the catalytic performance of Au. 

Fig. 17. Electron-transfer catalytic cycle, where the chain-carrying species is an alkyl radical. 

Scheme 13. The catalytic reaction was proposed to go through a sulfuranyl radical by addition of a thiyl radical to the sulfur atom of the thioether MeSPh. Interestingly, these studies are consistent with the proposed mechanism of the enzyme the thiyl radical of cofactor HS-CoB resulting from one-electron reduction of F430 by S-CoB couples with Me-CoM to afford the sulfuranyl radical, CoB-S-S (Me)-CoM. The sulfuranyl radical transfers a methyl radical to Ni -p43o to generate Me-Ni -F43o and the heterodisulfide CoM-S-S-HTP...

It is possibly to carry out chain transfer catalytically. The process is related to atom transfer radical polymerization (ATRP) [67, 68] and related living polymerizations which keep the concentration of chain-carrying radicals low. ATRP employs a halide complex (often Ru X) that is subject to facile one-electron reduction that complex reversibly donates X to the chain-carrying radical (1.23) and thereby decreases the concentration of the latter [69, 70]. 

Fig. 8. Reaction mechanism of (6-4) photolyase. The enzyme binds to DNA containing a (6-4) photoproduct and flips out the dinucleotide adduct into the active site cavity, where the open form of the photoproduct is converted to the oxetane intermediate by a light-independent general acid-base mechanism. Catalysis is initiated by light MTHF absorbs a photon and transfers energy to FADH , which then transfers an electron to the oxetane intermediate bond rearrangement in the oxetane radical regenerates two canonical pyrimidines, and back-electron transfer restores the flavin radical to catalytically competent FADH form. The repaired dipyrimidine flips back into the DNA duplex, and the enzyme is dissociated from the substrate.

The radical process begins with the radical-transfer agents R and ROO" (R = CgHii). Cobalt acts as an electron-transfer catalyst and redox initiator in the process. In a one-electron step, the oxidation state of the metal varies between +2 and +3, and radicals are released from the cyclohexane hydroperoxide. Since the cobalt is also involved in a cyclic process, its function is purely catalytic, and thus only small amounts of catalyst are required. Other metals such as V, Cr, Mo, Mn can also be used. Industrial variants of the process have been developed by companies such as BASF, Bayer, DuPont, ICI, Inventa, Scientific Design, and Vickers-Zimmer [T9]. 

Haddleton, D. M., et al. (1997). Identifying the nature of the active species in the polymerization of methacrylates inhibition of methyl methacrylate homopolymerizations and reactivity ratios for copolymerization of methyl methacrylate/n-butyl methacrylate in classical anionic, alkyUithium/trialkylaluminum-initiated, group transfer polymerization, atom transfer radical polymerization, catalytic chain transfer, and classical free radical polymerization. Macromolecules, 30(14) 3992-3998. 

Alkyl- and aryl-acetylenes, e.g. (70), but not acetylene itself, co-dimerize with allylic halides (71) in the presence of catalytic amounts of palladium complexes to give halogenated 1,4-dienes (72) in excellent yields. " The most active catalyst appears to be the [PdX2(PhCN)2] complex. The procedure involves very careful addition of the acetylenic compound to the allylic halide solution at 20 °C to prevent polymerization in this exothermic reaction. The co-dimerization of isobutene with trichloroethylene can produce useful quantities of l,l-dichloro-4-methylpenta-1,4-diene in the presence of t-butyl peroxide at 500 °C in a gas-phase reactor. The reaction probably occurs by a radical transfer mechanism. 

Use of metal complexes with weaker Mt -H bonds (i.e., leading to a more stabilized metal-based radical) led to chain transfer catalytic activity in MMA polymerization. The penta-phenylcyclopentadienyl compound (C5Ph5)Cr (CO)3, XXXVIIIa, is a stable radical (no tendency to form dimer XLa) and the corresponding hydride complex (C5Ph5)Cr (CO)3H, XXXIXa, is also available (Figure 29). The reaction of XXXIXa with a large excess of MMA produces low-Mn PMMA, rather than hydrogenated MMA. The same process is initiated by thermal AIBN... 

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

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.)...

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