Alkyl radicals, secondary, irradiated

In a preliminary study, in situ generated B-alkylcatecholboranes were allowed to react with PTOC-OMe under irradiation with a standard 150 W lamp. The S-pyridyl products coming from primary, secondary and tertiary alkyl radicals were isolated in moderate to good yields [88]. Based on these initial results, a procedure for conjugate addition to various activated alkenes was developed. A one-pot procedure involving hydroboration of an alkene with catecholborane followed by irradiation in the presence of five equivalents of an activated alkene and three equivalents of the chain transfer reagent PTOC-OMe was developed (Scheme 36) [88]. 

Alkyl Free Radicals. The primary alkyl free radical, RCH./, has been postulated to exist in gamma-irradiated polyethylene (23), but its presence has never been unambiguously demonstrated. If all the a and p protons interacted equally with the unpaired electron, the primary alkyl free radical ESR spectrum should consist of five lines, in contrast to those of the secondary and tertiary alkyl radicals whose ESR spectra should consist of six and eight lines, respectively (the latter calculated for the tertiary free radical in polypropylene). The existence of the secondary... 

The electron beam irradiation of poly(e-CL) and cross-linked PDXO in argon yielded a secondary alkyl ether radical and a tertiary alkyl radical, respectively. When the irradiation was carried out in an air atmosphere, peroxy radicals were detected in poly(e-CL) but not in PDXO (150). Oxygen permeabilities of poly(e-CL), and its tri-block copolymer poly(e-CL-b-PEG-b-e-CL) were in the range of 10 10 to 10 9 cm3 (STP) cm/cm2sec cm Hg [184]. 

Trifluoromethylation of pyrrole (and indole and imidazole) occurs on irradiation of a mixture of the compound with difluorodiiodomethane, again via a SET mechanism involving fragmentation of the radical anion (Scheme 31). The presence of the CF3 group in the final products, 45 and 46, is a result either of secondary decomposition of the initially formed—but not isolated—difluoroiodomethyl derivatives, or of formation of the CF3 anion or radical in situ [94], Perfluoroalkyla-tion of pyrroles can also be achieved by an SrnI mechanism—by reaction with perfluoroalkyl iodides in the presence of magnesium or zinc [95]. Indole, on the other hand, gives a mixture of the seven possible alkylated derivatives when irradiated in the presence of ethyl chloroacetate [96],... 

The photochemical and photophysical behavior of the N,N-dimethylaminoalkyDstyrenes 37-42, in which the amino group is attached to the styrene ci- or p-carbon by an ethyl, propyl, or butyl polymethylene linker, has been investigated in the author s laboratory. The preparative results are summarized in Scheme 7 and display remarkable dependence upon the chain length and point of attachment to styrene. Whereas formation of the intramolecular adduct 43 upon irradiation of 37 requires a-C- H transfer from a Al-methyl to the styrene P-carbon, formation of adduct 45 upon irradiation of 40 requires a-C-H transfer from a A -methyl to the styrene a-carbon. The latter process results in formation of a secondary alkyl radical rather than the more stable benzyl radical intermediate. The (aminopropyl)sty-renes 38 and 41 both fail to undergo intramolecular photoaddition. The (aminobutyl)styrenes 39 and 42 form adducts which result from a-C—H transfer from a Al-methylene to the styrene P-carbon, the former reaction leading to a mixture of dias-tereomeric aminocyclopentanes 44 and the later reaction to ami-nocyclohexanes 46. 

This initiation reaction can be induced by different factors, both physical (temperature, shear stress, gamma irradiation, etc) and chemical (pro-oxidants, radical initiators...). In the presence of oxygen, it can react easily with the originated radicals practically without activation energy leading to primary and secondary peroxide radicals (ROO ), which feed the complex oxidation cycle (Reactions 1 and 2, Scheme 2.2) [62, 63]. The rate constant for the most alkyl radicals with oxygen is of the order of 10 -10 moF sT... 

At the initial stage, the Ru catalyst, [Ru(ll)(bpy)3] , is excited by irradiation with a 15 W fluorescent light bulb to form the excited species, [Ru(ll)(bpy)3] , which is reduced by enamine 3 derived from the reaction of the secondary chiral amine catalyst with aldehydes 1 [31, 32]. The resulting strong reductant, [Ru(l) (l py)3], undergoes SET to alkyl halide 2, leading to formation of alkyl radical and... 

The importance of the secondary reactions can be expressed as a radical chain length, which is the total rate of all reactions involving a particular radical divided by the primary rate of formation of that radical. For a particular set of conditions chosen in the modeling studies, chain lengths of about 4 for the hydroperoxy radical and about 8 for the alkyl-peroxy and hydroxyl radicals were calculated early in the irradiation period. After further irradiation, the chains became shorter. 

Dialkyl peroxides (1), R-O-O-R (R and R are = or primary, secondary, tertiary alkyl, cycloalkyl, aralkyl and heterocyclic radicals Homolytic decompn when heated or irradiated with prodn of free radicals for org synthesis difficult to hydrolyze and reduce rearrangement crosslinking and polymerization polymeric peroxides are thick liqs or amorph wh powds used as polymerization catalysts Primary radicals are unstable, lowest members such as dimet peroxide are shock sens and dangerous expls sensitivity lessens with increasing mw polymeric peroxides (copolymers of olefins and Oj) explode on heating... 

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