Miscellaneous Radical Reactions

Samarium(II) iodide has been utilized quite extensively as a reducing agent to create aryl or alkenyl radicals that are used in a variety of atom transfer reactions [104]. Both intra- and intermolecular atom transfer reactions have been observed. Al- 

Acyl radicals can be generated by the interaction of Sml2 with acyl halides [106], For those substrates wherein decarbonylation is slow and cyclization is fast, cyclopropanols can be created in reasonable yields (Eq. 94). 

In an interesting transformation, ketyls have been demonstrated to undergo an intramolecular Sh2 reaction on benzylseleno ethers, generating seleno hemiacetals [107]. Carbohydrate derived precursors have been utilized to create several seleno-pyranoses (Eq. 95). 

Aromatic nitriles have been coupled with nitro groups in the presence of Sml2, providing a unique synthesis of amidines [108]. The intramolecular version of the reaction provides an excellent synthesis of 2-aminoquinolines (Eq. 96). 

Finally, chiral nitroxyl radicals can be synthesized by the intramolecular coupling of enones with nitro compounds under reductive conditions [109]. Thus reaction of appropriately substituted nitro enones with Sml2, followed by a quench with reactive acyl chloride electrophiles, provides a-asymmetric nitroxide radicals in excellent yields (Eq. 97). 

Problem 5.12. Draw a reasonable mechanism for the following reaction. Why aren t any of the C atoms adjacent to the ring N oxidized  

The synthetic potential of cycloaromatizations has only just begun to be explored. The two aryl radicals that are generated in the cycloaromatization are usually trapped by abstraction of H- from a compound such as 1,4-cyclohexadiene. Alternatively, one of the aryl radicals may add to a nearby ir bond to give a new radical, which may itself be quenched by H- abstraction. 

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MISCELLANEOUS REACTIONS OF DIHYDROPYRIDINES Additional tests for net hydride transfers initiated by single-electron transfer include the use of substrates in which such pathways would necessarily involve readily ring-opened cyclopropylmethyl or readily cyclized 5-hexenyl radicals. Products from these radical reactions are not formed in NAD+/ NADH dependent enzymic reductions or oxidations (Maclnnes et al., 1982, 1983 Laurie et al., 1986 Chung and Park, 1982). Such tests have also been applied in non-enzymic reductions. Thus cyclopropane rings in cyclopropyl 2-pyridyl ketones, or imines of formylcyclopropane (van Niel and Pandit, 1983, 1985 Meijer et al., 1984) survive Mg+2 catalysed reduction by BNAH or Hantzsch esters but are opened by treatment with tributylin hydride. 

Miscellaneous reactions Oxidation at sulfur atoms Radical reactions... 

Metathetical reactions of miscellaneous radicals containing one carbon atom... 

One other miscellaneous cycloaddition reaction has been reported. Arynes generated from anthranilic acids 205 have been found to undergo cycUzation with Barton esters 206 to afford polycycUc heterocydes 207 in moderate yields (Scheme 12.59) [109], with the reaction presumably proceeding through a radical pathway. 

Miscellaneous OSCIR reactions This subsection is a brief synopsis of sulfur-centered radical ion reactions that are congruent with the subjects discussed in previous sections. A list of these miscellaneous reactions is contained in Table 12. The material of this subsection is not necessarily cohesive however, it has been included for completeness. 

Miscellaneous Reactions. Dy(OTf)3 has also be used for the synthesis of (V-substituted pyrroles,the synthesis of 2,3-unsaturated glycopyranosides, for heterocycle synthesis, radical reactions, and polymer synthesis. 

Other miscellaneous compounds that have been used as inhibitors are sulfur and certain sulfur compounds (qv), picryUiydrazyl derivatives, carbon black, and a number of soluble transition-metal salts (151). Both inhibition and acceleration have been reported for styrene polymerized in the presence of oxygen. The complexity of this system has been clearly demonstrated (152). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which at above 100°C decomposes to initiating alkoxy radicals. Therefore, depending on the temperature, oxygen can inhibit or accelerate the rate of polymerization. 

Miscellaneous Reactions. Epoxy compounds yield chlorosubstituted carbonates (45). The reaction of chloroformates with hydrogen peroxide or metal peroxides results in the formation of peroxydicarbonates that are used as free-radical initiators of polymerization of vinyl chloride, ethylene, and other unsaturated monomers (46,47). 

Unimolecular thermal or photochemical reaction Reactivity tcward electrophiles Reactivity toward nucleophiles Reactivity toward radicals and carbenes Cyclic transition state reactions Miscellaneous reactions... 

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