Alkyl radicals, photolytic generation

Chemical Properties. Acychc di-Z f/-alkyl peroxides efftciendy generate alkoxy free radicals by thermal or photolytic homolysis. 

Alkyl radicals have also been prepared by reaction of alkylbromides with photolytically generated Re(CO)5 (from Re2(CO)io) [17], photolysis of cobalt-alkyl complexes [20], photolysis of AIBN [17, 21, 22] or thermolysis of TEMPO adducts [23]. 

Chemical Properties. Acyclic di-ferf-alkyl peroxides efficiently generate alkoxy free radicals by thermal or photolytic homolysis. Primary and secondary dialkyl peroxides undergo thermal decompositions more rapidly than expected owing to radical-induced decompositions. Such radical-induced peroxide decompositions result in inefficient generation of free radicals. 

Finally, as examples of similar types of reactions, photolytic treatment of O-acyl ester (D) of benzophenone oxime, A-acyloxy-phthalimide (E), and O-acyl ester (F) of A-hydroxy-2-pyridone with a mercury lamp generates the corresponding alkyl radicals via decarboxylation. However, these reactions can be used only for the alkylation of aromatics (solvents such as benzene) and reduction [86-89], so their synthetic utility is extremely limited. 

Kolbe electrolysis also allows some comparisons with analogous homogeneous reactions with regard to dimerization, substitution, or addition reactions of the generated radicals. Photolytic or thermal decarboxylation of diacylperoxides is a source of alkyl radicals similar to those afforded by the Kolbe electrolysis. The anodic oxidation of propionate has been compared with the thermal decomposition of dipropionyl peroxide [28]. Examination of the yields shows that reaction between radicals is favored in the electrochemical process, whereas in peroxide decomposition hydrogen atom abstraction from the solvent or the substrate occurs to a higher extent. This illustrates the effect of the higher radical concentration at the electrode. 

Two articles have reviewed conformations, deduced from e.s.r. hyperfine coupling constants, of radicals generated by abstraction of Br, I or SePh from C-1, -2, -3, or of alkylated and acylated pyranoses with photolytically generated stannyl radicals. The preferred conformations of ir-type C-1 1-deoxypyranosyl radicals were... 

The Hunsdiecker Reaction. The classical Hunsdiecker reaction is somewhat restricted due to the relatively harsh conditions required. In the Barton version, alkyl radicals generated from O-acyl thiohydroxamates, under either thermal or photolytic conditions, are efficiently trapped either by CI3C-X (X=C1 or Br CbC is the chain carrier) or by IodoformThe method is applicable to sensitive substrates for which the classical methods are unsuitable. thus allowing the preparation of a wide range of alkyl chlorides, bromides, and iodides by the one-carbon degradation of a carboxylic acid. Similar reactions of aromatic acid derivatives tend to require an additional radical initiator (e.g. Azo-bisisobutyronitrile), if high yields (55-85%) are to be obtained. ... 

Heating or photolytic treatment of A,A-dialkyl-A-haloamine in sulfuric acid or trifluoroacetic acid, followed by neutralization with a base, generates a pyrrolidine or piperidine skeleton. This is the Hofmann-Loffler-Frey tag reaction, and the reaction comprises of the formation of an electrophilic aminium radical, 1,5-H shift (6-membered transition state) or 1,6-H shift (7-membered transition state), formation of 4-haloalkyl ammonium or 5-haloalkyl ammonium, and its polar cyclization by neutralization with a base. Eq. 6.16 shows the formation of A-alkyl pyrrolidine (31) from A-chloro-A-alkyl-A-butylamine (30) in sulfuric acid [46, 47]. 

Trapping of short-lived radical intermediates as generated by photolytic, thermal or chemical processes with the help of NO-containing spin traps snch as alkyl nitrones or nitroso compounds [31]... 

These radicals may be generated by a variety of methods and we may compare these methods with those that are well established for alkyl (SO) and alkoxyl (18) radicals. Thus, there is an essential similarity between the three sources of azo compounds R—N=N—R, hyponitrites RO—N=N—OR, and tetrazenes R2N—N=N—NR2, all of which would be expected to produce the appropriate radicals (either photolytically or pyrolytically). The literature evidence for the four simplest radicals is summarized here. 

Pasto has reported the generation of alkoxyl radical by the photo-induced ho-molytic dissociation of alkyl 4-nitrobenzenesulfenate (52]. Cekovic has examined the photolytically induced decomposition of benzenesulfenates in the presence of hexabutylditin [53]. This reaction allows selective introduction of a phenylthio group at the -position via a 1,5-hydrogen transfer. The overall reaction is an O to C transfer of a phenylthio group. This reaction has been applied for the synthesis of acetyl scopine starting from (Af-ethoxycarbonyl)nortropine benzenesulfenate (Scheme 8) [54]. The sulfenate esters are stable compounds and are easily prepared by reaction of alcohols with PhSCl/EtyN. 

The photodecomposition of the various oxidation products of the alkanes, alkenes, and the aromatic hydrocarbons play important roles in the chemistry of the urban, mral, and remote atmospheres. These processes provide radical and other reachve products that help drive the chemistry that leads to ozone generation and other important chemistty in the troposphere. In this chapter, we have reviewed the evidence for the nature of the primary processes that occur in the aldehydes, ketones, alkyl nitrites, nittoalkanes, alkyl nitrates, peroxyacyl nitrates, alkyl peroxides, and some representative, ttopospheric, sunlight-absorbing aromatic compounds. Where sufficient data exist, estimates have been made of the rate of the photolytic processes that occur in these molecules by calculation of the photolysis frequencies ory-values. These rate coefficients allow estimation of the photochemical lifetimes of the various compounds in the atmosphere as well as the rates at which various reactive products are formed through photolysis. 

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