Amino radicals, abstraction reactions

These reactions are believed to have a free radical mechanism where the neutral amino radical abstracts a hydrogen atom from the hydrocarbon. 

Nitrogen oxides. Nitric oxide (NO) itself, has been shown to be a poor nitrosating agent (28), probably because it is unable to abstract an amino-H atom to generate the dialkyl-amino radical, which might then combine with further NO. However, the presence of even a small amount of air results in complete conversion, presumably via oxidation of NO to NO2. Nitrosation by NO is catalyzed by metal salts, such as Znl2, CuCl, and CUSO4. The metal catalyzed reaction is inhibited in acid or aqueous media (29). 

The primary reaction of Type 2 photoinitiators is a hydrogen abstraction from the tertiary amine by a triplet excited ketone. The amino radical thus formed is sufficiently active to initiate the polymerization of vinyl monomers Scheme 2. 

In the case of amines, protonation that withdraws electron density from the center of reaction lowers the rate of reaction by a factor of 30 (Das and von Sonntag 1986). Besides H-abstraction from carbon [reactions (18) and (21)], the formation of N-centered radical cations is observed [reactions (19)/(22) and (20) for amino acids see, e.g Bonifacic et al. 1998 Hobel and von Sonntag 1998]. Reaction (20) is also an H-abstraction reaction. The ET reaction (19)/(22) may proceed via a (bona-fide, very short-lived) adduct (Chap. 7). 

For the first two peptides, CysS radicals abstract hydrogen atoms from the or-carbon of glycine with 7 = (1.0 to 1.1) x 10 s , while the reverse reaction proceeds with = (8.0 to 8.9) x 10 s . For the latter peptide, CysS radicals abstract hydrogen atoms from the ce-carbon of alanine with = (0.9 to 1.0) x 10 s while the reverse reaction proceeds with k -j = 1.0 x 10 s" The order of reactivity, Gly > Ala, is in accordance with previous studies on intermolecular reactions of thiyl radicals with these amino acids. The fact that < k y suggests that some secondary structure prevents the adoption of extended conformations for which calculations of homolytic bond dissociation energies would have predicted k j > k y. 

In this section, reactions are featured that involve the ortho trapping of an iminium ion generated oxidatively without the abstraction of H by the adjacent group. (In principle, this process could also involve an a-amino radical). Some years ago, we demonstrated that Mn02 oxidation of o-substituted /-anilines can lead to intermediates that can be trapped by appropriate ortho nucleophiles [68JCS(C)1722 69CI(L)443]. Thus o-amino- and o-carboxyphenyldialkylamines were cyclized to give... 

Silverman and Zieske have rationalized how a protein nucleophile other than flavin is involved in MAO inactivation reactions, and why different inactivator compounds specifically react with flavin, protein amino acids, or both (100). Hydrogen atom donation from a cysteine residue to the flavin semiquinone radical would produce a thiyl radical, which could then capture the primary or secondary alkyl radical generated on cyclopropyl ring opening from the amine radical cation of the inactivator. The hydrogen atom abstraction reaction between the flavin and active site amino acid may be an equilibrium process such that either species could be present at any turnover. Hence, a combination of steric constraints and proximity to either the flavin semiquinone radical or the thiol radical will determine the site of adduct formation for a particular inactivator structure. A two-dimensional representation is shown in Scheme 23 (compounds 40-42), which illustrates the proposed equilibrium between the flavin semiquinone radical and amino acid as well as the proposed intermediates for the inactivation of MAO by A-(l-methylcyclopropyl)benzylamine 40 (104), rrradical center relative to the particular protein radical is consistent with proposed site of attachment of inactivator to protein 40 is near the flavin radical, such that exclusive flavin attachment occurs, 41 is positioned closer to the amino... 

It is particularly typical of grafting amine-containing monomers peroxides cause oxidation of amino groups, which leads to a loss of commercial value of a grafted product. It should be remembered that most of the azo compounds are unsuitable as initiators not only because of half-life (tq.s) but also because cyanoalkyl radicals formed are inactive in abstraction reactions of hydrogen from chains. An exception is phenylazo-compounds (33) being a source of phenyl radicals that are most active in hydrogen abstraction reactions (Table 10.3). 

A CIDNP study of the irradiation of (7) has shown that a-fission occurs on irradiation. The resultant radical (8) eliminates diethyl phosphoric acid and yields the radical cation (9). When irradiation is carried out in methanol this species is trapped as the ether (10). Other products are methyl benzoate and the acetal (II). A laser irradiation study of the reaction of acetophenone with amines constrained in NaY zeolites has been carried out and evidence has been collected that shows that ketyl and amino radicals are formed by hydrogen abstraction pathways. ... 

The radical processes that appear to be important with amino acids are H-abstraction, radical combination (Reaction K), p-scission of amino acid oxy radicals (Reaction G) and possibly radical addition (Reaction L). 

As shown in Scheme 7.9, triplet nitrene can abstract a hydrogen atom from neighboring macromolecules, thus forming an amino radical and a carbon macroradical (reaction (a)). The two radicals have correlated spins and can. 

In synthetic contexts, thiyl radicals are known to engage in a number of useful reactions [191-198], including C-H bond abstraction [197, 199-203]. Recently, MacMillan has demonstrated the capacity of catalytically generated thiyl radicals to cleave the C-H bond of benzylic ethers for radical coupling reactions (Fig. 27) to form diaryl methanols (a) [204] or p-amino ethers (b) [205]. The light source is either a Blue LED or a Compact fluorescent lamp (CFL), as designated above. In both reactions, a wide variety of arene components and ether substitutions are tolerated to furnish a diverse set of products and only a catalytic amount of methyl thioglycolate is necessary to affect the desired C-H HAT event. In the formation of diaryl ethers (Fig. 27a), phosphate serves as the base, whereas the P-amino ether... 

Shinohara H, Imamura A, Masuda T et al (1979) Molecular-orbital study on the partial reactivity of hydrogen of various amino-acids in the abstraction reaction by hydroxyl radical. Bull Chem Soc Japan 52 1-7... 

A representative mechanism for the oxidative nitro-Mannich reaction of THIQs is shown in Scheme 10.5. Oxidation of the THIQ with excited state Ir gives the amine radical cation (la) and Ir. Catalyst turnover is presumed to occur by Ir -mediated reduction of either adventitious 0 q gen or the nitroalkane to the corresponding radical anion species. This radical anion can abstract the a-H atom of the amine radical cation, resulting in the formation of the iminium ion (Ila), which is then trapped by the nitroalkane. Alternatively, deprotonation of la can occur to form a-amino radical nia, which is then oxidized to form Ila. Stephenson and co-workers were able to rule out the role of singlet ojqrgen in the predominate reaction pathway by demonstrating that photosensitizers were ineffective in promoting this transformation. 

---