Homolytic additions

Homolytic addition of dithiols to alkynes as a new approach to construction of dithiacyclanes and thiacrown ethers 97IZV1256. 

Den Hertog and Overhoff - observed that when pyridine in sulfuric acid is added to molten potassium sodium nitrate the 3-nitro derivative is formed at 300°C, whereas at 450°C 2-nitropyridme is the main product. The latter is probably a free-radical process. Schorigin and Toptschiew obtained 7-nitroquinoline by the action of nitrogen peroxide on quinoline at 100°C, possibly through the homolytic addition of NOa. Laville and Waters reported that during the decomposition of pernitrous acid in aqueous acetic acid, quinoline is nitrated in the 6- and 7-positions. They considered that the reaction proceeds as shown in Scheme 3. 

ORGANOBROMINE COMPOUNDS IN REACTIONS OF HOMOLYTIC ADDITION AND TELOMERIZATION... 

The use of metal-complex initiating systems proved to be especially promising in carrying out the reactions with acrylic monomers which can be easily polymerized, when the common initiators of radical reactions are excepted. The use of Fe(CO)s -I- DMFA system allows us to perform homolytical addition of bromoform to acrylic monomers selectively at C-Br bond with no essential polymerization (ref. 10). 

Whereas additions of carbon radicals to alkene moieties are the best characterized homolytic additions, carbon radicals are known to add to a wide range of unsaturated systems. These include polyenes, alkynes, arenes, heteroarenes, carbon monoxide,isonitriles, °° ° nitriles, ° imines and derivatives, ° ° aldehydes,nitrones, and thiones. ° Many of these reactions, such as addition of an alkyl radical to a carbonyl group, ° are thermodynamically unfavorable and readily reversible, and they form the basis of composite group-transfer reactions discussed below. 

A second process that has been invoked is heterolytic fission of H2.49 Examples are shown in equations (15) and (16). Although the overall process certainly involves heterolysis, some or all of these reactions may well go via initial oxidative addition followed by fast deprotonation or reductive elimination (equation 17). A third variant closely related to heterolytic fission is the addition of hydrogen across a metal-ligand or metal-metal bond. Examples are shown in equations (18) and (19). Here too, homolytic addition via equation (1) followed by fast reductive elimination may be taking place in some cases. In d° systems, the homolytic fission reaction of equation (1) is unlikely as the metal is already at its maximum valency, so examples of heterolytic fission and addition to... 

The stoichiometric oxidation of alkenes by Mn(OAc)3 in acetic acid at 120 °C affords 7-lactones n good yield, via the homolytic addition of carboxymethyl radical to the double bond (equations 104 and 205).504-506... 

Nishinaga and co-workers isolated a series of stable cobalt(III)-alkyl peroxide complexes such as (170) and (171) in high yields from the reaction of the pentacoordinated Co"-Schiff base complex with the corresponding phenol and 02 in CH2C12. Complex (170 R=Bu ) has been characterized by an X-ray structure. These alkyl peroxide complexes presumably result from the homolytic addition of the superoxo complex Co111—02 to the phenoxide radical obtained by hydrogen abstraction from the phenolic substrate by the CoUI-superoxo complex. The quinone product results from / -hydride elimination from the alkyl peroxide complex (172)561,56,565,566 The quinol (169) produced by equation (245) has been shown to result from the reduction of the CoIU-alkyl peroxide complex (170) by the solvent alcohol which is transformed into the corresponding carbonyl compound (equation 248).561... 

Sternberg and Wender (140a) reviewed several reactions such as these in 1959. At that time they proposed the homolytic addition of dicobalt octa-carbonyl as the initiating step in the reaction, thus,... 

Photoinduced reactions of homolytic addition of bromotrichloromethane C.CI3B1 to allylic derivatives of germanium and tin could be an illustrative example of the potentialities of CIDNP application to study processes where the polarization effects have not been generated at the initiation stage. An earlier proposed46 overall scheme of CChBr addition to the allylic double bond in the R3MCH2CH=CH2 molecule was based on the analysis of the reaction products (Scheme 5). 

Radical reaction mechanisms of and at organic germanium, tin and lead 2. Homolytic addition of bromotrichloromethane to allyltriorganostannanes... 

C. Other Examples of Reactions of Homolytic Addition and Substitution... 

Under appropriate circumstances, free-radical methods can also be used to prepare macrocyclic systems. For example, Maillard and coworkers prepared the 12-membered macrocyclic ether (102) by Bu3SnH-mediated intramolecular homolytic addition to a remote, but activated (a, / unsaturated) olefin as depicted in equation 77460. Tozer and coworkers employed 7-, 8- and 9-endo cyclization in their preparations of conformationally constrained amino acids (103), an example of which is provided in equation 78448. 

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