Radical reactions, atom transfer

Triplet carbenes have a singly occupied p orbital, as is the case for radicals, and hence react like those radicals. Hydrogen atom transfer reactions are fundamental reaction pathways of triplet carbenes. The reaction of a triplet carbene with a hydrocarbon is quite analogous to the free radical hydrogen atom transfer process (Scheme 9.6). 

By contrast, for iodide 18 having the triple bond activated by a phenyl group, conversion to the cyclic organozinc species 25 occurred effectively and the latter could be efficiently functionalized, provided that traces of moisture were excluded by pre-treatment of zinc powder with Mel. The substituted benzylidene cyclopentanes 26 and 27 were respectively obtained after iodinolysis and palladium-catalyzed cross-coupling reaction with benzoyl chloride (equation 10). However, it could not be assessed whether the formation of organozinc 25 was attributable to an anionic or a radical cyclization pathway (or both) as, had iodide 26 been produced by a radical iodine atom-transfer, it would have been converted to 25 by reaction with metallic zinc due to the presence of the activating phenyl group21. 

Free-Radical H-Atom Transfer. In competition with molecular H-atom transfer reactions, radical-induced transfer may occur,... 

Apart from the above described radical-forming reactions, the influence of solvent on the reactions of the radicals themselves has also been thoroughly investigated [159-166]. The most important elementary reactions of radicals are atom transfer, combination, addition, disproportionation, and electron transfer, as listed in Table 5-10 [15, 213]. 

This catalytic sequence is known as Kharasch addition or atom transfer radical addition (ATRA) [4]. Various polyhalogenated compounds such as CCI4 and CCI3CO2R are used as the organic halides, and transition metal salts or complexes are used as the catalyst [3]. Intramolecular version of the Kharasch addition reaction (atom transfer radical cyclization, ATRC) has opened novel synthetic protocols to the synthesis of carbocyde or heterocyles catalyzed by transition metals [5-7], and this has become a very important field in free radical cydization in organic synthesis. Transition metal-catalyzed Kharasch reactions sometimes afford telomers or poly-... 

While this is a typical reaction of metal-metal-bonded complexes and radicals, rapid atom transfer was also observed between the formed I-CrfCOTCsRs and unreacted CrCCCOsCsRs ... 

The organic radical ( )CPhHMe generated by a slow atom transfer step in Equation 10.47 can either combine with a second Cr radical as shown in reaction 10.49 or undergo H-atom transfer as shown in reaction 10.48. Reaction 10.50 involves (3-H-atom elimination and serves to generate the metal hydride. This can be also involved in radical H-atom transfer reactions such as that shown in Equation 10.51. Finally, H-atom transfer to PhCH=CH2 can generate radical pairs as shown in... 

Cyelopropyl is a bent n radical 9I-9,2, not a planar n radical like a typical alkyl radical, and it is about () times as reactive as a typical alkyl radical in atom-transfer reactions [94). This suggests that the solvent-attack rate constant ks will be of the order 10 s in DEE. Indeed, a measurement for the 2-phenyIcyclopropyl radical gave ks = 1.6 x 10 s 95. ... 

This is similar to the dissociation-combination scheme, but the release and return of the controlling species (X) are catalyzed by an activator (A) which is a transition metal complex. The controlling species is a halide radical in the most common form of this reaction, atom transfer radical polymerization (ATRP), and this technique will be described further in Section 13.5. It is also possible to use a quinone instead of a... 

Atom Transfer Radical Polymerization. Atom transfer radical polymerization (ATRP) (80,81) involves reversible homol5d ic cleavage of a carbon-halogen bond by a redox reaction between an organic halide (R-X) and a transition metal, such as copper(I) complexed with 2,2 -bipyridine (bpy), as illustrated in reaction 20 ... 

Fuchs and co-workers used radical-mediated atom-transfer addition of iodomethyl trlflone (14) [158530-86 0] to substituted alkynes to afford functionalized allyl triflones. The reaction was complete within 5 10 h In most cases. For example, heating a... 

Extensive data exists on the physiochemical, biological and pharmacokinetic release properties of chitosan [30,32], making it an ideal system for the assessment and performance of the three fundamental free radical transformations radical addition reactions, atom transfer reactions and radical cascade reactions (Figure 11.5) [34,33]. These reactions are our main focus of investigation since they represent the three major transformations commonly associated with cell death and damage in a disease as a result of free radical generation in-vivo [18]. Our studies illustrate that the chitosan model is a successful experimental setup as a molecular prototype carrier system, suitable for evaluation of the fundamental aspects of a delivery system under laboratory conditions [35,33]. 

Other typical reaction mode for free radicals is atom transfer reactions such as H atom abstraction. In the case of ArS, H atom abstraction reactivity is low, although studies in 1978 reported high reactivity of PhS in H atom abstraction from the benzylic H atom ( = 1 x 10 M s" ) [28]. However, a lower k was estimated by Xe flash photolysis. The actual value of could not evaluated for... 

Durmaz, H., Kaiatas, F., Tunca, U., and Hizal, G. (2(X)6b) Preparation of ABC miktoarm star terpolymer containing polyfethylene glycol), polystyrene, and poly(tert-butylacrylate) arms by combining Diels-Alder reaction, atom transfer radical, and stable free radical polymerization routes. Journal of Polymer Science Part A-Polymer Chemistry, 44,499. 

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

A number of chemiluminescent reactions have been studied by producing key reactants through pulsed electric discharge, by microwave dissociation, or by observing the reactions of atoms and free radicals produced in the inner cone of a laminar flame as they diffuse into the flame s cool outer cone (182,183). These are either combination reactions or atom-transfer reactions involving transfer of chlorine (184) or oxygen atoms (181,185—187), the latter giving excited oxides. 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

Cyclizations involving iodine-atom transfers have been developed. Among the most effective examples are reactions involving the cyclization of 6-iodohexene derivatives. The 6-hexenyl radical generated by iodine-atom abstraction rapidly cyclizes to a cyclo-pentylmethyl radical. The chain is propagated by iodine-atom transfer. 

The bicyclic product is formed by coupling of the two radical sites, while the alkene results from an intramolecular hydrogen-atom transfer. These reactions can be sensitized by aromatic ketones and quenched by typical triplet quenchers and are therefore believed to proceed via triplet excited states. 

FIGURE 24.21 A mechanism for the methylmalonyl-CoA mntase reaction. In the first step, Co is rednced to Co dne to homolytic cleavage of the Co —C bond in cobalamin. Hydrogen atom transfer from methylmalonyl-CoA yields a methylmalonyl-CoA radical that can undergo rearrangement to form a snccinyl-CoA radical. Transfer of an H atom regenerates the coenzyme and yields snccinyl-CoA. 

Vitamin E actually consists of a family of compounds, the most active of which is a-tocopherol. The mechanism of the vitamin s action is not completely certain, but it seems likely that it might undergo hydrogen atom transfer reactions with free radicals to give a stable radical (see also Chapter 17, Problem 7). 

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