Hydrogen Abstraction from Substrate

It is not known whether H atom transfer from substrate to Tyr 272 is preceded or followed by single electron transfer from substrate to Cu . Whittaker et al. (1998) favour electron transfer being first since the 4-coordinate geometry of Cu following loss of the Tyr 495 ligand would permit fast inner sphere electron transfer. Formation of would 

It is proposed (Whittaker et al., 1998) that dioxygen binds to the Cu in E reduced although there is no direct evidence. Electron and proton transfer steps to the bound oxygen would generate the second product, hydrogen peroxide. 

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Glickman, M.H. and Klinman, J.P. 1996. Lipoxygenase reaction mechanism Demonstration that hydrogen abstraction from substrate precedes dioxygen binding during catalytic turnover. Biochemistry 35 12882-12892. 

Cysl51 is found to be substituted for the radical site of Tyrl22 in the protein R2. Participation of the sulfhydryl group of the Cysl51 in the hydrogen abstraction from substrate hydrocarbon is proposed, vide infra. 

The hydrogen abstraction from —SH groups is faster than from —OH groups. Hebeish et al. [9] and Misra et al. [10,11] reported the chain-transfer method of initiation of graft copolymerization onto cellulosic substrates with azobisisobutyronitrile (AIBN) and benzoyl peroxide (BPO) as initiators. 

Alkenes. When the substrate molecule contains a double bond, treatment with chlorine or bromine usually leads to addition rather than substitution. However, for other radicals (and even for chlorine or bromine atoms when they do abstract a hydrogen) the position of attack is perfectly clear. Vinylic hydrogens are practically never abstracted, and allylic hydrogens are greatly preferred to other positions of the moleeule. Allylic hydrogen abstraction from a cyclic alkenes is usually faster than abstraction from an acyclic alkene. ... 

In accord with this mechanism, free peroxyl radical of the reaction product hydroperoxide activates the inactive ferrous form of enzyme (Reaction (1)). Then, active ferric enzyme oxidizes substrate to form a bound substrate radical, which reacts with dioxygen (Reaction (4)). The bound peroxyl radical may again oxidize ferrous enzyme, completing redox cycling, or dissociate and abstract a hydrogen atom from substrate (Reaction (6)). 

Hydrogen abstraction from a position a to the oxygen of alcohols and ethers provides a simple route to a-oxyalkyl radicals. Resonance stabilization and polar factors have been used to explain the ease of radical attack on these substrates. Recent studies appear to exclude the possibility that the oxygen atom in position a to the free C-radical may cause stabilization by resonance. The ease of hydrogen abstraction Avould be determined only by polar factors, arising with electrophilic radicals (X ) in contributions from the polar forms 13-15 to the transition state. 

The radical initiator is usually a diacyl peroxide (see Section 9.1) that dissociates to radicals that in turn add on to the alkene. This starts the chain reaction, which is terminated by hydrogen abstraction from some suitable substrate, e.g. another... 

The stereospecific labeling of the anti methyl by deuterium in compound 20 to produce substrate 21 (Table 3) was required in order to study the syn/anti regioselectivity of the ene allylic hydroperoxides. The ene products in different solvents showed a preference for hydrogen abstraction from the methyl syn to the phenyl group. The magnitude of this selectivity depends on solvent polarity. On increasing the solvent polarity, a substantial increase in the amount of syn product occurs (Table 3). 

A number of substrates having a benzylic ether moiety were reacted with 51 to afford the corresponding benzylic esters in good yields (equation 84). For evaluating the effects of p-substiments on the oxidation of a series of benzylic ethers, a competitive oxidation of p-substimted benzylic propyl ethers with 51 was carried out. The Hammett correlation plot for the oxidation reaction gave a better correlation of the relative ratio factors with the a rather than with the a+ substituent constants and afforded a reaction constant p+ = —0.57 (r = 0.99). This p+ value shows that 51 is an electrophilic species and appears to be comparable to the p+ value of —0.65 for benzylic hydrogen abstraction from dibenzyl ethers by the benzoyloxy radicaP . 

If hydrogen abstraction from the zinc salt is fast enough to compete with that from the hydrocarbon substrate, then Scheme 2 amounts to a chain-transfer reaction similar to that illustrated in Reaction 9 for phenolic inhibition. Also, an alternative termination via the peroxyalkyl-ester radical (X) is conceivable since this radical might be expected to cyclize... 

Protonated pyridines and derivatives readily undergo acylation at C-2 or C-4 (Table 28) (76MI20503). Acyl radicals are usually generated either by hydrogen abstraction from aldehydes (Scheme 210), or by oxidative decarboxylation of a-keto acids (Scheme 211). In the former case (Scheme 210) with acridine as the substrate, reduction can take place to give a dihydroacridine. 

The bound peroxyl radical may again oxidize ferrous enzyme, completing redox cycling, or dissociate and abstract a hydrogen atom from substrate (Reaction (6)). 

For the reactions described so far in this section, the ketone substrates have lowest excited states that are (n.ii ) in character aliphatic ketones may react by way of the singlet or the triplet state, and aryl ketones normally through the triplet because intersystem crossing is very efficient. The efficiency of photochemical hydrogen abstraction from compounds such as alcohols or ethers is very much lower if the ketone has a lowest (Ji,n triplet state, as does I - or 2-acetylnaphthalene (CmH-COMe). However, all aryl ketones, regardless of whether their lowest triplet state is fn,Jt l or (Jt.Ji ), react photochemically with amines to give photoreduction or photoaddition products. A different mechanism operates (4.38), that begins... 

The most common bimolecular reaction of triplet ketones in solution is hydrogen abstraction from a suitable substrate to yield alkyl radicals and semipinacol radicals. The radicals so formed perform their own characteristic reactions depending upon the actual conditions. Yang has recently proposed that in high-intensity irradiations, these... 

Photoinduced polymerization can also be obtained through the dissociation of organic salts such as sulfonium, diazonium and similar salts. The photodissociation leads to several species, a radical cation, a neutral free radical and a closed-shell anion for example. The radical cation can then react further, e.g. through hydrogen abstraction from a substrate, ZH, to form another free radical Z. ... 

Nishinaga and co-workers isolated a series of stable cobalt(III)-alkyl peroxide complexes such as (170) and (171) in high yields from the reaction of the pentacoordinated Co"-Schiff base complex with the corresponding phenol and 02 in CH2C12. Complex (170 R=Bu ) has been characterized by an X-ray structure. These alkyl peroxide complexes presumably result from the homolytic addition of the superoxo complex Co111—02 to the phenoxide radical obtained by hydrogen abstraction from the phenolic substrate by the CoUI-superoxo complex. The quinone product results from / -hydride elimination from the alkyl peroxide complex (172)561,56,565,566 The quinol (169) produced by equation (245) has been shown to result from the reduction of the CoIU-alkyl peroxide complex (170) by the solvent alcohol which is transformed into the corresponding carbonyl compound (equation 248).561... 

Even when the reductive processes, so evident in metal-stimulated processes, are avoided, several side reactions can still cause reductions in the yield of the desired a-arylated ketones. The first, abstraction of 3-hydrogen atoms from the enolate ion by the aryl radical, has already been mentioned (Section 2.2.2.1) and is sometimes a serious, chain-terminating process.43 5 This abstraction reaction, however, appears to be quite unpredictable. 3-Hydrogen abstraction from the enolate of 2,4-dimethyl-3-pentanone (PriCOPr Table 1) which severely disrupts the reaction with iodobenzene, does not prevent high-yielding reactions of the same enolate (and those from other ketones with a-branching) with many other substrates. In in-... 

If the clock reaction has been calibrated, i.e. if kp is known under the precise reaction conditions employed, then the absolute values of kAH (rate constant for hydrogen abstraction from the substrate AH) can be obtained using Equation 10.12. The relative rates of hydrogen abstraction for two different substrates AH and BH in separate reactions can also be determined using this simple reaction as long as the temperature, solvent system and concentrations remain the same for both reactions. It is then possible to compare kAH and ksH since kp remains constant throughout and can be eliminated from the expression. 

Studies by electron-spin resonance spectroscopy showed that the reaction of hydroxy radicals with carbohydrates produces new radicals via hydrogen abstraction from a C-H group. When such carbohydrates as glucose are substrates, the H abstraction from a C-H bond is relatively nonselective, and all six possible radicals can be formed. The fate of these radicals is strongly influenced by the type of starting sugar and by the species present in the reaction medium. All of these aspects have been discussed in the review article by Sheldon et al.69... 

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