Halo-alkenes, radical

The radical cyclizations of diacetoneglucose-derived halo-alkenes for example (66) to give cyclohexane and cycloheptane annulated furanoses has been described (Scheme 16). A paper by two of the same authors describing similar work has also appeared. ... 

The free radical additions of sulfonyl halides to alkenes, catalyzed by light or typical chemical radical initiators (In), were first investigated in the 1950s69. The products which are / -halo sulfones (22) were obtained via a chain reaction in which RSO j acts as the chain carrier, namely61-62,70,71... 

Similar additions have been successfully carried out with carboxylic acids, anhydrides, acyl halides, carboxylic esters, nitriles, and other types of compounds. These reactions are not successful when the alkene contains electron-withdrawing groups such as halo or carbonyl groups. A free-radical initiator is required, usually peroxides or UV light. The mechanism is illustrated for aldehydes but is similar for the other compounds ... 

The groups R2N and Cl can be added directly to alkenes, allenes, conjugated dienes, and alkynes, by treatment with dialkyl-V-chloroamines and acids. " These are free-radical additions, with initial attack by the R2NH- radical ion. " N-Halo amides (RCONHX) add RCONH and X to double bonds under the influence of UV light or chromous chloride. " Amines add to allenes in the presence of a palladium catalyst. ... 

Nitryl chloride (NO2CI) also adds to alkenes, to give p-halo nitro compounds, but this is a free-radical process. The NO2 goes to the less-substituted carbon. Nitryl chloride also adds to triple bonds to give the expected l-nitro-2-chloro alkenes. The compound FNO2 can be added to alkenes by treatment with HF in HNOa or by addition of the alkene to a solution of nitronium tetrafluoroborate (NOJBF4, see 11-2) in 70% polyhydrogen fluoride-pyridine solution (see also 15-37). 

The alkenylation reaction of ct-halo carbonyl compounds with alkenylindium in the presence of triethylborane proceeds via a radical process. Unactivated alkene moieties as well as a styryl group can be introduced by this method. The geometry of the carbon-carbon double bonds of the alkenylindium is retained. Preparation of alkenylindium via hydroindation of 1-alkyne followed by the radical alkenylation establishes an efficient one-pot strategy (Scheme 113).384... 

There are no one-step syntheses of A(-alkylaziridines direct from alkenes of proven generality, the most thoroughly investigated being the reactions of iV-halo- and iV,iV-dihalo-alkylamines with alkenes referred to earlier (Scheme 6). They are probably radical reactions and are not stereospecific the A(-haloamine reagents are difficult to prepare cleanly, and are rather dangerous. Representative examples (6 of 17) prepared from halogenated alkylamines are listed in Scheme 14. 

When metal ion complexed amino radicals are produced by the reaction of A -chloro amines with reducing metal salts in the presence of alkenes, /6-halo amines are produced12-39 41. The reaction of 1-chloropiperidine with cyclohexene, iron(II) sulfate and iron(III) chloride in methanol afforded mainly the d.s-adduct of 2. The diastereoselectivity is attributed to coordination of the unprotonated amino group with the iron(III) salt, which is mainly responsible for the chlorine atom transfer. With A-chlorodimethylarnine and 4-chloromorpholine lower yields are obtained. 

The amide N-H may also be halogenated, oxidized and nitrosated. A -Bromosuccinimide (NBS), like a number of other iV-halo compounds, readily undergoes a radical fission to give a bromine radical. This provides a useful reagent for radical bromination at, for example, allylic or benzylic positions. In the presence of acid, NBS is also a mild source of the halonium ion, which is used for the addition of hypobromous acid (Scheme 3.74) to alkenes or for the bromination of reactive aromatic rings. 

By use of a series of l-aroyl- 0-haloalkanes [95a] and 1-aryl-oj-haloalkanes [95b] intramolecular ET between the n radical anion site and the distal a (C-X) acceptor was evaluated for longer distances. The correlation with the acceptor number of the solvent indicates that the negative charge is more localized in the transition state than in the initial radical anion [95b], Notably, rate constants for the dissociation of Ar-Cl show the same correlation with solvent polarity as aroyl-a -haloalkanes [95b]. Experimental and theoretical investigations on co-halo-l-alkene [96] and phenyl-substituted 4-benzoyloxy-l-methylcyclohexylbromide [97] shed further light on intramolecular dissociative ET processes. 

The addition of polyhaloalkanes and related halo-genated compounds to alkenes can occur via a classical radical chain process (Scheme 13), which is often called the Kharasch reaction.38 In 1961, Minisci et al.39 and Asscher and Vofsi40 discovered that this reaction is catalyzed by transition metal ions in their lower valent state such as Cu+ and Fe2+, and they formulated the mechanism in Scheme 14. The catalysis of the additions by simple metal salts or complexes such as Cu(I)-2,2 -bipyridyl41a and ruthe-... 

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