Cyclization of free radicals

The strong preference for cyclization in the indicated manner in both sensitized reactions lends support to the view that cycloaddition reactions of triplets involve two steps with the intermediate formation of biradicals. It has recently been shown290 that cyclization of free radicals forms five-membered rings in preference to six-membered rings. 

CycUzation with group transfer. Unsaturated organoiodides and tellurides undergo cyclization of free radicals. With a hydrosilane to provide a hydrogen atom the configuration of the alkylidenecycloalkane product is predetermined. There is a distinct preference for cyclization leading to a tetrahydrofuran rather than to a cyclopentane. ... 

Sources of Radical Intermediates Introduction of Functionality by Radical Reactions Addition Reactions of Radicals with Substituted Alkenes Cyclization of Free-Radical Intermediates Fragmentation and Rearrangement Reactions... 

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... 

KAGAN-MOLANDER Samanum reagent Lanthanide reagents, speoflcally samanum, lor generation of free radicals useful in cyclizations, reductions... 

Intramolecular cyclization of sulfonyl radicals is almost absent from literature. The fact that free radical cyclization has been the subject of a large number of studies and applications in the last decade in organic chemistry48 and that sulfonyl radicals add quickly to multiple bonds (vide infra) makes cyclization of sulfonyl radicals a rather attractive area. Recently, Johnson and Derenne49 studied the reaction of 6-methylhept-5-en-2-ylcobaloxime(III) with sulfur dioxide and, based on the product analysis, they suggested reaction 15 to be an intermediate step. 

In addition to the two major processes, cross-linking and chain modification (or cyclization), chain scission doubtless occurs also to varying degrees during conventional vulcanizations. Processes of this nature are not difficult to envisage in the presence of free radicals. The radical intermediate (II) may, for example, undergo /3-fission as follows ... 

Free-radical chain reactions have been reviewed60. The cyclization of dienes by the action of free radicals is illustrated for the case of the 1,6-heptadiene derivative 90 (E = CC Me) in equation 56. Treatment with tosyl radicals, produced from tosyl chloride and a catalytic amount of dibenzoyl peroxide, generates the radicals 91, which cyclize to 92. The latter reacts with tosyl chloride to form 93 and tosyl radicals are regenerated. The product is obtained in 85% yield as a 6 1 mixture of cis- and fraws-isomers61. 

Whereas the design and application of free radical cyclization reactions have been extensively covered in excellent reviews [3-5], there is no comprehensive report on the synthetic application of their charged counterparts radical cations and radical anions. 

Cyclization reactions have boosted the development of free-radical strategies in organic synthesis. The construction of five- and six-membered ring systems has... 

Conversion of 2-chloroacetamides 17 into iodoacetamides by iodide exchange followed by reaction with BusSnH in the presence of AIBN affords 7,12-dihydro-indolo[3,2-d][l]benzazepin-6(5H)-ones 18 as products of free radical cyclization (Equation (4) (2005T5489)). Low (8-25%) yields of azepinones 18 are observed in toluene medium, and they are usually accompanied with the product of spiro cyclization 19 and isomeric compound 20. Yields of the paullone 18 can be increased to 25-52% at higher temperatures (boiling mesitylene). 

The aliphatic alicyclic hydrocarbon precursor is not well recognized as the major aliphatic component in dissolved humic substances, although it was previously postulated to occur (II). This precursor might arise from terpenoid hydrocarbon lipids, but the data presented in this chapter favor polyunsaturated lipid precursors that are oxidatively coupled and cyclized by free-radical mechanisms (20). Degradative studies have not identified this aliphatic component in recognizable fragments. The quantitative, structural-model approach presented here combines the results of 13C NMR, NMR,... 

If, oil the other hand, R- is unsaturated and can undergo cyclization rapidly, it will do so. This competition between reduction of the first formed radical R and its cyclization to a new cyclic radical R - is the same as discussed for the formation of free radicals from alkyl halides and tributyltin radicals. The only difference is in the way in which the carbon-centered radical is produced. 

Since the order of free-radical stabilities falls in the order 3° > 2° > 1°, product stability would dictate that cyclization should preferentially occur to give die more stable secondary radical—a six-membered ring in reaction (9.1) (path a) and a seven-membered ring in reaction (9.2)(path a). 

Chapter 4 by J.J. Li reviews radical cyclization reactions in the total synthesis of indole alkaloids. The use of free radical chemistry in the synthesis of alkaloids has grown markedly because of the mild reaction conditions, tolerance of a wide variety of functional groups, and the good stereoselectivities. 

In germanium chemistry the importance of free radical pathways in substitution reactions of secondary bromides with R3GeLi (R = CH3, CgHs) reagents is strongly indicated by product stereochemistry in cyclohexyl systems and by cyclization of the cA-heptene-2-yl moiety to yield [(2-methylcyclopentenyl)methyl]germanes, with the appropriate cis/trans ratio, as shown in equation 180, Table 9 and equations 181 and 182189. 

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