Radicals generated by hydrogen abstraction

Abstraction of hydrogen by the excited state molecules makes possible the generation of radicals from molecules with low excitation energies, which effectively extends photoionization that can initiate photopolymerization of reactive monomers into the visible part of the spectrum. Examples of H-abstraction reactions 

In the presence of a reactive monomer, the radicals formed by hydrogen abstraction initiate chain polymerization. However, in the absence of any other reactive species, the radicals can combine to form dimers. The species that absorbs the radiation (anthraquinone) and gets promoted to an excited state is termed the initiator, while the hydrogen donor (THF) is termed the co-initiator. The tetrahydrofuryl radical is the principal initiating species.  

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Alcohols are not only source of ketyl radicals generated by hydrogen abstraction from the a-C-H position (Eq. (7), Table 1). Oxidation of alcohols with Pb(OAc)4, PhI(OAc)2, and S2082 with Ag(I) as catalyst produces alkoxy radicals (RO-) which may further undergo /3-scission (Eq. 13), intramolecular hydrogen abstraction, or intra- and intermolecular addition to alkenes, generating a nucleophilic carbon-centered radical useful for heteroaromatic substitution (Scheme 6) [2]. 

In parallel the cocatalysis of cobalt compounds with A7-hydroxyphthalimide 398 was developed extensively for oxidative radical reactions [434]. Ishii and colleagues showed that these conditions can be used in radical additions (Fig. 92). Ketyl radicals generated by hydrogen abstraction from secondary alcohols 396 add to a.p-unsaturated esters 397 affording 2,4-dihydroxy esters, which cyclized to lactones 399 under the reactions conditions [435]. Using 0.1 mol% of Co(OAc)2, 1 mol% of Co(acac)3, and 10 mol% of 398 under 1 atm of oxygen, the cyclic products 399 were isolated in 14—90% yield. As observed for similar reactions, Co(III) alone needs an induction period (see below). 

The ipso cyclization of an ort/io-(phenylsulfonyloxy)benzoyl radical generated by hydrogen abstraction from the corresponding aldehyde <98J(P1)67>, followed by rearomatization of the spirodienyl radical intermediate by elimination of sulfur dioxide constitutes an isolated example of acyl radical cyclization upon a benzene ring, developed by Motherwell for the synthesis of 2-hydroxyaryl aryl ketones <00TL9667>. 

Arylimidoyl radicals, generated by hydrogen abstraction from A-arylideneanilines (22) with diisopropyl peroxydicarbonate, react with alkynes to give quinolines 23 and 24 in ca 80% overall yield (equation 32). A mechanistic scheme where arylimidoyl, vinyl and spirocyclohexadienyl radicals are involved as intermediates has been presented and discussed. 

Radical generation by hydrogen abstraction some photoinitiators tend to abstract a hydrogen atom from a H-donor molecnle via an exciplex, to generate a ketyl radical and the donor radical. The H-donor radical initiates the polymerization, the inactive ketyl radical disappears by a radical coupling process (5). This type of photoinitiators includes benzophenone and thiox-anthone. 

Fig. 17. Experimental (a) and simulated b) ESR spectra of mid-chain radical generated by hydrogen abstraction from polyfBA by peroxide radicals b). The spectrum was reasonably simulated based on the structure shown in the figure.

In the presence of radical initiators such as benzoyl peroxide (BPO), azobisisobutyronitrile (AIBN), persulfates (S208 ), etc., grafting of vinyl monomers onto polymeric backbones involves generation of free radical sites by hydrogen abstraction and chain transfer processes as described below ... 

The plan for a tandem cyclization as the key step in the synthesis of morphine alkaloids has been successfully performed [95]. Three examples are given in Scheme 7.10. Specifically, the initially formed aryl radical, generated by bromine abstraction from compound 74, underwent a tandem cyclization to construct the desired carbocyclic skeleton. Depending on the nature of the Z substituent to the double bond, the product radicals either abstracted hydrogen from silane (Z = C02Me or CN) or eliminated thiyl radical (Z = SPh). 

The arachidonic acid cascade is a biological free radical oxidation of unsaturated fatty acids leading to formation of the prostaglandins (equation 102). Cyclization of a peroxy radical intermediate 66 leading to endoperoxide 67 was proposed as a pathway for this process, and this was demonstrated in chemical model systems, in which the peroxyl radical 66 was generated by hydrogen abstraction from the hydroperoxide corresponding to 66. 

At elevated temperatures, the radical, 5-8, generated by hydrogen abstraction from the aldehyde functional group fragments to give a new carbon radical and carbon monoxide. The fragmentation is temperature-dependent the higher the temperature, the more favorable the loss of CO from the initial radical. 

Cyclopropanoxyl radicals have also been generated by hydrogen abstraction from cyclopropanols by ketone triplets.In the photochemical reaction of cyclopropanols with... 

Cyclopropanaminyl radicals have been far less studied. When generated by hydrogen abstraction from cyclopropylamines, they rearrange too rapidly for observation by EPR spectro-... 

The electron donor radicals in Schemes 26 and 27 are generated by hydrogen abstraction reactions by radicals which themselves are either acceptors (PhCOj, CI3C , I, SO " ) or have little donor ability such as Me or Ph. Another route to a donor radical is the addition of a radical to an unsaturated system such as an enamine or ynamine. Thus, the addition of Cp3, CrjCl, "CFjBr or "CFjCFjBr to an enamine converts an acceptor radical into a donor radical (e.g. R2NCHC(CF3)R2) and the free radical process of Scheme 28 occurs (Cantacuzene and Dorme, 1975 Rico et al., 1981, 1983). 

A number of additional photoinduced radical-promoted systems incorporating N-alkoxypyridinium salts were also described by the Yagci group. These include the use of photo-excited phthalaldehyde and polysilanesto generate reducible carbon- and silicon-centered radicals, respectively, by hydrogen abstraction and cleavage of silicon-silicon bonds. 

Coupling of macroradicals generated by hydrogen abstraction through reactive radicals for example, triplet nitrene or benzoyl radicals... 

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