Hydrogen atom transfer compounds

Vitamin E actually consists of a family of compounds, the most active of which is a-tocopherol. The mechanism of the vitamin s action is not completely certain, but it seems likely that it might undergo hydrogen atom transfer reactions with free radicals to give a stable radical (see also Chapter 17, Problem 7). 

In triethylamine instead of benzene the reaction products are completely different, and are indicative of a homolytic process involving an initial electron transfer from triethylamine followed by a hydrogen atom transfer. Scheme 10-68 gives the major products, namely 1,3,5-tri-tert-butylbenzene (10.36, 20%), the oxime 10.39 (18%), formed from the nitroso compound 10.38, and the acetanilide 10.37 (40%). ESR and CIDNP data are consistent with Scheme 10-68. In their paper the authors discuss further products which were found in smaller yields. 

Enantioselective synthesis of /1-amino acids is important as they are present in various natural products and in many biologically active compounds [26,27]. Several methods exist for the enantioselective synthesis of -substituted /1-amino acids (/l3-amino acids) however, synthesis of a-substituted /1-amino acids (/l2-amino acids) is very limited [28,29]. A report on highly enantioselective hydrogen atom transfer reactions to synthesize /l2-amino acids (Scheme 9) has recently been described [30]. 

Figure 2,7. Absorption and fluorescence spectra of DCM-crown and its complexes with perchlorate salts in acetonitrile. DCM-H is the compound in which the crown is replaced by a hydrogen atom this compound cannot undergo intramolecular charge transfer and its fluorescence quantum yield is very low (3 x 1(C) (adapted from Ref, 40.)...

Shorthand notations such as ET (electron transfer), HAT (hydrogen atom transfer), BDE (bond dissociation energy), NHE (normal hydrogen electrode), CV (cyclic voltammetry), LFP (laser flash photolysis), EPR (electron paramagnetic resonance) and KIE (kinetic isotope effect) will be used throughout the chapter. In addition, recurring chemical compounds such as TEMPO (2,2,6,6-tetramethylpiperidine-Ai-oxyl), HBT (1-hydroxyben-zotriazole), BTNO (benzotriazole-A-oxyl), HPI (iV-hydroxyphthalimide), PINO (phthal-imide-iV-oxyl), NHA (A-hydroxyacetanilide) and a few others will be referred to by means of the capital-letter acronym. 

Synergistic behavior by two antioxidants is not confined to compounds which inhibit by entirely different mechanisms—for example, two chain-breaking phenolic antioxidants may synergize one another. This homosynergism is caused by the suppression of the unfavorable chain propagation reactions of one phenoxy radical by a hydrogen atom transfer from the second phenol. 

The formation of benzocyclobutene and styrene in different ratios from the three isomeric tolylcarbenes is not easily explained by the Baron mechanism, and led to more than one clever (but ultimately wrong) alternative proposal. The question is best resolved by assuming that in the ortho system hydrogen atom transfer in the diazo compound leads to extra benzocyclobutene... 

The commonest compounds containing O—H bonds are water, alcohols, phenols and carboxylic acids (if inorganic acids and bases are neglected). Hydrogen atom transfer from water to a radical seems to be unknown (Kondratiev, 1970 Bennett et al., 1970 for deuterium atom transfer from D20 to N see Felder et al., 1970). 

The specially synthesized l-arylcyanomethyl-9,10-anthraquinones with a long lifetime for the photoinduced form showed photodegradation in alcohols as well.36 It was found that despite the variable lifetime of the photoinduced form of the different compounds of this type, the dark reaction of its interaction with alcohol dominated over the hydrogen atom transfer, which was due to the photochromic process. The efficiency of the photochromic transformations depended on the nature of the... 

Radical stabilization energies for a wide variety of carbon-centered radicals have been calculated at G3(MP2)-RAD or better level. While the interpretation of these values as the result of substituent effects on radical stability is not without problems, the use of these values in rationalizing radical reactions is straight forward. This is not only true for reactions involving hydrogen atom transfer steps but also for other reactions involving typical elementary reactions such as the addition to alkene double bonds and thiocarbonyl compounds. 

The quantum-yields for reaction of compounds 120a, 120b, and 120 c in benzene solution were 0.054, 0.50, and 0.57 (0.62 in t-butyl alcohol) respectively 100). Rates of hydrogen atom transfer were obtained from Stem-Vollmer treatment of quenching... 

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