Alkyl radicals, generation

Reactions of pyridine with a number of alkyl radicals generated by this method have been studied (Section... 

N-Alkoxylamines 88 are a class of initiators in "living" radical polymerization (Scheme 14). A new methodology for their synthesis mediated by (TMSlsSiH has been developed. The method consists of the trapping of alkyl radicals generated in situ by stable nitroxide radicals. To accomplish this simple reaction sequence, an alkyl bromide or iodide 87 was treated with (TMSlsSiH in the presence of thermally generated f-BuO radicals. The reaction is not a radical chain process and stoichiometric quantities of the radical initiator are required. This method allows the generation of a variety of carbon-centered radicals such as primary, secondary, tertiary, benzylic, allylic, and a-carbonyl, which can be trapped with various nitroxides. 

Alkyl radicals generated from azoalkanes as in (7) react with oxygen added to argon matrices giving alkylperoxy radicals. In this manner radicals... 

Alkyl radicals generated by reduction of organomercury compounds can also add to alkenes having EWG groups. Radicals are generated by reduction of the organomercurial by NaBH4 or a similar reductant. These techniques have been... 

Alkyl radicals generated efficiently from allylsulfones in 80% aqueous formic acid induced a cyclization reaction on aromatic and heteroaromatic compounds to provide polycyclic aromatic and heteroaromatic derivatives (Eq. 7.17).37... 

Clerici and Porta reported that phenyl, acetyl and methyl radicals add to the Ca atom of the iminium ion, PhN+Me=CHMe, formed in situ by the titanium-catalyzed condensation of /V-methylanilinc with acetaldehyde to give PhNMeCHMePh, PhNMeCHMeAc, and PhNMeCHMe2 in 80% overall yield.83 Recently, Miyabe and co-workers studied the addition of various alkyl radicals to imine derivatives. Alkyl radicals generated from alkyl iodide and triethylborane were added to imine derivatives such as oxime ethers, hydrazones, and nitrones in an aqueous medium.84 The reaction also proceeds on solid support.85 A-sulfonylimines are also effective under such reaction conditions.86 Indium is also effective as the mediator (Eq. 11.49).87 A tandem radical addition-cyclization reaction of oxime ether and hydrazone was also developed (Eq. 11.50).88 Li and co-workers reported the synthesis of a-amino acid derivatives and amines via the addition of simple alkyl halides to imines and enamides mediated by zinc in water (Eq. 11.51).89 The zinc-mediated radical reaction of the hydrazone bearing a chiral camphorsultam provided the corresponding alkylated products with good diastereoselectivities that can be converted into enantiomerically pure a-amino acids (Eq. 11.52).90... 

Alkyl mercury halides participate in a photo-stimulated radical chain reaction of the anion of nitroalkanes (see Eq. 5.38) in which a 275-W sun lamp is used.56 c Primary, secondary, and tertiary alkyl radicals generated from alkyl mercury halides react with the anion of nitroalkanes to form new C-C bonds. 

Intramolecular addition of trialkylboranes to imines and related compounds have been reported and the main results are part of review articles [94, 95]. Addition of ethyl radicals generated from Et3B to aldimines affords the desired addition product in fair to good yield but low diaster control (Scheme 40, Eq. 40a) [96]. Similar reactions with aldoxime ethers [97], aldehyde hydrazones [97], and N-sulfonylaldimines [98] are reported. Radical addition to ketimines has been recently reported (Eq. 40b) [99]. Addition of triethylborane to 2H-azirine-3-carboxylate derivatives is reported [100]. Very recently, Somfai has extended this reaction to the addition of different alkyl radicals generated from trialkylboranes to a chiral ester of 2ff-azirine-3-carboxylate under Lewis acid activation with CuCl (Eq. 40c) [101]. 

Nozaki reported the reaction of trialkylboranes with styryl sulfoxides and sulfones. Alkyl radicals generated from trialkylboranes add at the -position of /3-styryl sulfoxides and sulfones (a- to the sulfur atom). The resulting radicals fragment and deliver the -styryl adducts [108]. Interestingly, the sulfoxides eliminate very rapidly leading to partially stereospecific substitution (Scheme 44). The radical nature of the process is demonstrated by the presence of a side product derived from the solvent (THF) by hydrogen atom abstraction. 

The a-silyloxy alkyl radical generated by the addition of (TMS)3Si radical to the aldehyde moiety of 45 has been employed in radical cyclization of (3-aminoacrylates (Reaction 7.53) the trans-hydroxy ester and the lactone in a 2.4 1 ratio were the two products [62]. 

In the absence of a three-electron bond, it is possible that some interaction (Van der Waals, electrostatics, etc.) between the product radical and anion exists. This situation has been discussed in some detail for the interaction between a halide ion and an alkyl radical generated in the gas phase by dissociative electron attachment to an alkyl halide. " It is expected that these interactions will be more important in the gas phase, as a solvent tends to screen charge. Wentworth suggested that an appropriate potential... 

In this connection investigations are to be mentioned in which a mass-spectrometric analysis has been made of neutral radicals, e.g., CHjCO, split off from acetone by u.v. photons in the ordinary range.27-28 In the first a flash lamp has been used and the radicals were ionized as usual by electron impact. In the second the same radical ionized at a field emission electrode. Recently, several alkyl radicals generated by pyrolysis have been studied. Their values of lv and of the photoionization cross sections could be obtained in the mass spectrometer under monochromatic vacuum u.v. irradiation.29... 

A potential method for the preparation of novel amino acids via the highly selective addition of radicals to the glyoxylic oxime derivative of Oppolzer s camphor sultam (88) has been reported.181 Both Lewis acid and non-Lewis acid-mediated reaction conditions for the addition of alkyl radicals generated from alkyl iodides and Et3B/Bu3SnH were examined. A new chiral auxiliary based upon (R,R)-2,5-diphenylpyiTolidine has been used in the addition of phenylthiyl radicals to unsaturated methacrylamides. The selectivity was found to be better than that reported for the structurally related 2,5-dimethylpyrrolidine derivative.182... 

The mechanism of aminyl radical generation from PTOC carbamates follows closely the radical chain mechanism of alkyl radical generation from PTOC esters. The chain reaction sequence involves the series of steps shown in Scheme 8. Several methods for inducing N —O bond cleavage are possible. Photochemical decomposition of 29 via visible light irradiation is used to initiate the chain reaction sequence at ambient or subambient temperature reactions have been run as low as -78°C (91TL6493). 

Intramolecular addition of carbon-centered radicals to alkynes under reductive conditions has been examined by Dulcere and Rodriguez. Alkyl radical generation from 39 resulted in the formation of a substituted tetrahydrofuran 40, a nucleoside analog [95SL923]. An improved 5-exo-dig process for the formation of iodomethylene lactone has been reported [95T4665],... 

Perfluorinated alkyl radicals, generated by photoinduction from heptadecafluoro-octyl iodide, were added to SWCNTs and the perfluorooctyl-derivatized CNTs obtained (Scheme 1.14). No difference in the solubility of the fluoroalkyl-substituted nanotubes and the starting materials was observed [148]. A pathway to the radical functionalization of CNTs sidewalls was predicted by classical molecular dy-... 

Hydroaminomethylation reactions can be accomplished by addition of nucleophilic a-amino alkyl radicals (generated from amines) onto electron-deficient alkenes [7, 21], Amines have a low oxidation potential, and are easily oxidized by excited aromatic ketones bearing electron-donating substituents (e.g., Michler s ketone) or... 

The concept of tungstate-catalyzed radical reactions was recently extended by Albini and coworkers to efficient radical addition reactions to electrophilic alkenes 147 (Fig. 40). Dondi et al. described the photocatalyzed addition of alkyl radicals generated by 310 nm irradiation of 2 mol% decatungstate 139 to ot,(3-unsaturated nitriles [241] or ot,(3-unsaturated ketones [242]. The yields of saturated nitriles 148a were 30-66%, while those of saturated ketones 148b amounted to 21-56% after... 

Alkyl radicals generated from the reduction of halides or sulfones with Sml2 have been successfully exploited in intramolecular additions to alkenes that result in the generation of a variety of functionalised small carbocyclic and heterocyclic ring systems. Substrates containing an oxygen atom within the... 

Minisci, F., Gain, R., Cecere, M., Malatesta, V., Caronna, T. Nucleophilic character of alkyl radicals new syntheses by alkyl radicals generated in redox processes. Tetrahedron Lett. 1968, 5609-5612. 

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