Radicals intramolecular hydrogen abstraction

In the case of carbon tetrachloride, the radical intermediate undergoes two competing reactions intramolecular hydrogen abstraction is competitive with abstraction of a chlorine atom from carbon tetrachloride ... 

It has been proposed that oxygen adds to the excited keto group [- (112)]. The rearrangement of the resulting hydroxyhydroperoxy diradical (112) could then proceed by intramolecular hydrogen abstraction involving a six-membered cyclic transition state, followed by fission of the former C —CO bond to form the unsaturated peracid (113) as the precursor of the final product. Such a reaction sequence demands a hydrogen atom in the J -position sterically accessible to the intermediate hydroperoxy radical. 

Interniolecular hydrogen abstraction, 239 Intramolecular free radical reactions, 238 Intramolecular hydrogen abstraction, 239 Iodine azide, 24... 

Upon the irradiation the nitrous acid ester 1 decomposes to give nitrous oxide (NO) and an alkoxy radical species 3. The latter further reacts by an intramolecular hydrogen abstraction via a cyclic, six-membered transition state 4 to give an intermediate carbon radical species 5, which then reacts with the nitrous oxide to yield the 3-nitroso alcohol 2 ... 

With a radical-scavenging compound present in the reaction mixture, an alkyl radical species like 5 can be trapped, thus suggesting a fast conversion of the alkoxy radical 3 by intramolecular hydrogen abstraction, followed by a slow intermolecular reaction with nitrous oxide. 

PE produced by a high-pressure polymerization process (pressure 1000-3000 atm) using a free radical initiator is a highly branched material that contains both LCBs and SCBs. The polymer so produced is a low-density material (density up to about 0.925 g/cc) and is known as high-pressure low-density PE (HP LDPE). The LCBs are formed via intermolecular hydrogen transfer [19], whereas SCBs are formed by intramolecular hydrogen abstraction [16]. 

Among the seven reaction pathways, the intramolecular hydrogen abstraction from the methyl group in the MPP radical by the radical peroxy oxygen atom leading to 2-hydroperoxybenzyl radical (HPB, pathways I, Scheme 2.15) is one of the kinetically most favored since its activation enthalpy is only 26.5 kcal/mol. This benzylic radical is fairly unstable and rapidly generate the o-QM by OH radical loss through a very low activation barrier (7.6 kcal/mol). 

Intramolecular hydrogen abstraction by primary alkyl radicals from the Si-H moiety has been reported as a key step in several unimolecular chain transfer reactions.59,60 In particular, the 1,5-hydrogen transfer of radicals 14-17 [Eq. (5)], generated from the corresponding iodides, was studied in... 

For example, 2-hexanone in MeCN gave 5-acetamino-2-hexanone (44) in 40% yield (Scheme 17) [82], and 2-pentanone was oxidized in trifluoroacetic acid to give a mixture of 4-trifluoroacetoxy-2-pentanone (24%) and 3- and 4-penten-2-one (6%) [83]. A mechanism involving intramolecular hydrogen abstraction by a ketone cation radical that forms a car-benium ion via a [l,5]-hydrogen shift was proposed. 

The autoxidation mechanism by which 9,10-dihydroanthra-cene is converted to anthraquinone and anthracene in a basic medium was studied. Pyridine was the solvent, and benzyl-trimethylammonium hydroxide was the catalyst. The effects of temperature, base concentration, solvent system, and oxygen concentration were determined. A carbanion-initi-ated free-radical chain mechanism that involves a singleelectron transfer from the carbanion to oxygen is outlined. An intramolecular hydrogen abstraction step is proposed that appears to be more consistent with experimental observations than previously reported mechanisms that had postulated anthrone as an intermediate in the oxidation. Oxidations of several other compounds that are structurally related to 9,10-dihydroanthracene are also reported. 

A 1,4 shift of this type was postulated by Duncan and Cvetanovid27 to explain some of their results on the reaction of CH2 with isobutene and butene-2. Previously, Gordon and McNesby56 demonstrated that intramolecular hydrogen abstraction occurs in the case of n-pentyl radicals ... 

The involvement of an intramolecular hydrogen abstraction in the Barton reaction is not necessarily limited to those molecules with rigid stereochemistry. In fact, simple aliphatic nitrites undergo the Barton reaction with equal ease. Thus, the principal product obtained from the photolysis of ra-octyl nitrite20 in benzene solution is the dimer of 4-nitroso-l-octanol however, photolysis of n-octyl nitrite in n-heptane20 produced a mixture of 7/-nitroso heptanes in addition to 4-nitroso-l-octanol in the ratio l 4.5,f respectively. The formation of y-nitroso heptane obviously results from an attack of the intermediate alkoxy radical on the solvent molecule. The intermediate alkyl radical then collapses, according to eq. 2. For the sake of convenience we have indi-... 

Second, alkylperoxy radicals must react predominantly via Reaction 4 and not via Reaction 3. The ratio of the rates of intermolecular and intramolecular hydrogen abstraction by CmH2w + iOO is k3[C H2n+2]/k4. When [CttH2w + 2] = 200 mm. of Hg = 10l8r> molecules per cc., k3[CwH2n + 2] has values between 102 8 and 10°1 sec.1 (see Table II). For intramolecular abstraction, the corresponding range of values of k4... 

Viewed in this way, reactions yielding cyclic ethers should be thought of as two-step processes, epoxides probably arising from peroxy radical additions to olefins followed by 1,3-displacements, and larger rings arising via intramolecular hydrogen abstraction by peroxy radicals followed by 1,4- or 1,5-displacements. In all such reactions it is probably the first step which is slow and which determines the yield of product observed. 

That both singlets and triplets should be able to take part in the intramolecular hydrogen abstraction reaction is not surprising,305 although some authors have been inclined toward the conclusion that only the triplet should show reactivity reminiscent of that of an alkoxy radical. Actually both species have the requisite electron deficiency at oxygen and do not differ much in excitation energy. It is probable that... 

In the last example, a 8-hydrogen must be abstracted by the excited carbonyl oxygen, a situation perfectly analogous to that encountered with alkoxy radicals, where the preference for intramolecular hydrogen abstraction from adjoining carbon atoms is y > 8 /3.3S4... 

Epimines. Irradiation of nitroamines and cyanoamines in the presence of phenyliodine(III) diacetate and iodine generates an aminyl radical which undergoes intramolecular hydrogen abstraction to produce epimines. 

Intramolecular hydrogen abstraction is an important synthetic reaction because it provides a method of introducing a functional group at a saturated carbon bonded to only hydrogen and other carbons, a difficult task to accomplish by other means.196 The abstracting radical need not be carbon the Barton reaction (Scheme 15) is a standard method in which the abstracting radical is oxygen.197 This technique has been used extensively for selective introduction of functionality in steroid synthesis.198... 

In contrast to aminyl radicals, alkoxyl (and acyloxy) radicals are highly reactive. As illustrated in equation (7), their cyclization reactions are extremely rapid and irreversible. However, the rapidity of such cyclizations does not guarantee success because alkoxyl radicals are also reactive in inter- and intramolecular hydrogen abstractions, and -fragmentations (see Section 4.2.S.2). This lack of selectivity may limit the use of alkoxyl radicals in cyclizations, but S-exo cyclizations are so rapid that they should succeed in many cases, and other types of cyclizations may also be possible. 

Nitro compounds, in particular aromatic and heterocyclic derivatives, absorb strongly in the near UV. They have properties similar to ketones in their excited state. These compounds are characterized by an unpaired electron in the n0 orbital and thus by a radical character. A typical example of this radical character is the easy intramolecular hydrogen abstraction in nifedipine and related vasodilators (Sch. 7) (18). Another manifestation of the radical character of the nitro group is the rearrangement often observed with nitrated five-membered heterocycles, as in the case of metronidazole (Sch. 8) (19). 

A synthetic variation of the methodology reported in Eq. (6) is shown in Eq. (8), in which the selective intramolecular hydrogen abstraction by aminium radical... 

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]. 

Heating the diselenoallene 140 leads to an intramolecular cycloaddition affording the fused selenin 141 (Scheme 15) <2001JOG1787>. The reaction appears to take place by an initial intramolecular hydrogen abstraction by the sp-hybridized carbon of the allene affording a diradical intermediate. Radical coupling then leads to the selenin. The cyclic allene precursor could be prepared by dilithiation of the benzylacetylene followed by treatment of the bis-selenocyanate. 

The latter example (reaction 36) already indicates that the Yang cydization can also be used to synthesize four-membered heterocycles. After light absorption, the a,(3-unsaturated carbonyl compound 84 undergoes intramolecular hydrogen abstraction at the a-position of the carbonyl moiety (reaction 37), leading to the 1,4-biradical intermediate XXX [87]. A radical combination then efficiently yields the spirocyclic P-lactam derivative 85, and only one stereoisomer is formed in this case. In this transformation, the a,P-unsaturated carbonyl function can be considered as being vinylogous to a simple ketone. 

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