Alkyl radicals, photolytic

In one study, Ingold and coworkers166 measured the rate constants for the reactions of several alkyl radicals with tributyltin hydride using a laser flash photolytic technique and direct observation of the tributyltin radical. They also used this technique with tributyltin deuteride to determine the primary hydrogen-deuterium kinetic isotope effects for three of these reactions. The isotope effects were 1.9 for reaction of the ethyl radical, and 2.3 for reaction of the methyl and n -butyl radicals with tributyltin hydride at 300 K. 

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]. 

Refluxing or photolytic treatment of O-acyl esters (2) in the presence of a hydrogen donor such as Bu3SnH or ter -BuSH, provides the corresponding reduction products via alkyl radicals. This reaction can be applied to primary-, secondary-, and tertiary-alkyl chained carboxylic acids, and can also be used for steroids, sugars, and peptides as shown in eq. 8.4 [9-14]. Racemization does not occur at other chiral centers. 

The same photolytic treatment of O-acyl esters (2) in the presence of disulfides, diselenides, and ditellurides effectively produces the corresponding alkyl sulfides, alkyl selenides, and alkyl tellurides respectively, through SHi reaction on the chalcogen atoms by alkyl radicals, as shown in eq. 8.9. The reactivities somewhat depend on the kind of chalcogenides. Thus, the effective formation of alkyl sulfides requires 30 eq. of disulfides, that of alkyl selenides requires 10 eq. of diselenides, and that of alkyl tellurides requires 2 eq. of ditellurides [27, 28]. 

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. 

The subsequent cycMsation of the alkoxy-radical depends upon its ability to attack a suitably placed C-H bond on the -carbon atom. The alternative to cyclisation under homolytic conditions is fragmentation of the alkoxy radical into a carbonyl compound (ii) and an alkyl radical (10), which affords a mixture of stable products by further transformations. Heusler [44] reached similar conclusions from a study of steroid reactions, and has demonstrated a close similarity between thermally and photolytically-induced homolytic reactions with lead tetraacetate in hydrocarbon solvents. 

The photolytic cleavage of alkyl aryl sulfoxides has been shown to occur via initial C—S bond homolysis, in accordance with the common mechanistic assumption. Secondary and tertiary alkyl groups show high chemoselectivity for alkyl C—S cleavage. Uniquely, alkene products have been isolated, formed by disproportionation of the initial alkyl radical, with the formation of benzaldehyde and racemization of primary alkyl compounds. An investigation into the photochemical conversion of N-propylsulfobenzoic imides into amides in various solvents revealed a solvent dependence of the observed mechanism. In ethanol, sulfur dioxide extension forms a biradical which abstracts a hydrogen atom from the solvent, whereas in aromatic solvents biradical formation by a single electron transfer is implicated. The photolysis and thermolysis of l,9-bis(alkylthio)dibenzothiophenes and /7-aminophenyl disulfide have been studied. 

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. 

Because oxidative decarboxylation of carboxylic acids by lead tetraacetate depends on the reaction conditions, the co-reagents, and the structures of the acids, a variety of products such as acetate esters, alkanes, alkenes, and alkyl hahdes can be obtained. Mixed lead(IV) carboxylates are involved as intermediates as a result of their thermal or photolytic decomposition decarboxylation occurs and alkyl radicals are formed. Oxidation of alkyl radicals by lead(IV) species gives carbocations a variety of products is then obtained from the intermediate alkyl radicals and the carbocations. Decarboxylation of primary and secondary acids usually affords acetate esters as the main products (Scheme 13.41) [63]. 

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. ... 

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

Chemical Properties. Diacyl peroxides (20) decompose when heated or photoly2ed (<300 mm). Although photolytic decompositions generally produce free radicals (198), thermal decompositions can produce nonradical and radical iatermediates, depending on diacyl peroxide stmcture. Symmetrical aUphatic diacyl peroxides of certain stmctures, ie, diacyl peroxides (20, = alkyl) without a-branches or with a mono-cx-methyl... 

Photolytic. Fukuda et al. (1988) studied the photodegradation of acenaphthene and alkylated naphthalenes in distilled water and artificial seawater using a high-pressure mercury lamp. Based upon a rate constant of 0.23/h, the photolytic half-life of acenaphthene in water is 3 h. Behymer and Hites (1985) determined the effect of different substrates on the rate of photooxidation of acenaphthene using a rotary photoreactor equipped with a 450-W medium pressure mercury lamp (X = 300-410 nm). The photolytic half-lives of acenaphthene absorbed onto silica gel, alumina, and fly ash were 2.0, 2.2, and 44 h, respectively. The estimated photooxidation half-life of acenaphthene in the atmosphere via OH radicals is 0.879 to 8.79 h (Atkinson, 1987). 

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