Alkoxy radical hydrogen abstraction, intramolecular

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. 

The small and mobile "OH can readily abstract hydrogen from a nearby polymer chain, creating a secondary or tertiary macroalkyl radical. The tertiary alkoxy radical can abstract hydrogen intramolecularly, forming a primary alkyl radical as in Eq. (41). 

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 ... 

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-... 

Intramolecular abstraction of hydrogen by the intermediate alkoxy radical B to furnish the alkyl radical C always takes place through a six-membered transition state. In the photolysis of organic nitrites, no exception to this rule of six has been recorded to date. Thus, photolysis of 3-phenyl-l-propyl nitrite does not yield any product corresponding to attack on the a-carbon through a five-membered transition state (90), although abstraction of such a hydrogen should be exothermic to the extent of ca. 25 kcal./mole. 3 Also, irradiation of 5-phenyl-l-pentyl... 

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... 

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]. 

Jenner, E. L., Intramolecular Hydrogen Abstraction in a Primary Alkoxy Radical, J. Org. Chem 1962, 1031, 27. 

The generation of alkoxy radicals that can undergo intramolecular hydrogen abstraction can also be achieved by photolysis of hypohahtes. Photolysis of a hypochlorite (RO—Cl) gives a 1,4-chloro-alcohol, formed as expected by abstraction of a hydrogen atom attached to the 8-carbon atom. The 1,4-chloro-alcohol can be converted readily to a tetrahydrofuran product (4.27). The hydrogen abstraction reaction proceeds through a six-membered cyclic transition state as in the photolysis of nitrites. 

The intramolecular hydrogen abstraction reactions promoted by alkoxy radicals in carbohydrates are particularly useful for the stereoselective synthesis of various polycyclic oxygen-containing ring systems [643-647], This reaction can be illustrated by the intramolecular 1,8-hydrogen abstraction between glucopy-ranose units in disaccharide 613 promoted by alkoxy radicals and leading to the 1,3,5-trioxocane derivative 614 (Scheme 3.240) [644]. 

Intramolecular hydrogen abstraction by alkoxy radicals generated from hypochlorites has also been observed ... 

The lowest-lying excited state of ketones most often corresponds to a o 7t c=o transition. The maximum of this band is around 280 nm with simple aldehydes or ketones and is shifted to the red for conjugated or aryl derivatives. As hinted above, the unpaired electron on the hq orbital gives to these states electrophilic properties similar to those of alkoxy radicals, and indeed the observed chemistry is similar in the two cases. Typical reactions are a-fragmentation, inter- or intramolecular (from the easily accessible y position) hydrogen abstraction and attack of alkenes (finally resulting in a formal 2h-2 cycloaddition to give an oxetane, the Paterno-Btichi reaction). 

The crucial step for functionalization of the saturated chain involves an intramolecular hydrogen atom abstraction by an alkoxy radical ... 

For a recent application of this reaction, see (a) Boto, A., Hernandez R., and Suarez, E., Oxidative decarboxylation of alpha-amino acids a mild and efficient method for the generation of N-acyliminium ions, Tetrahedron Lett., 40,5945,1999 (b) Francisco, C. G., Herrera, A. J., and Sudrez, E., Intramolecular hydrogen abstraction reaction promoted by alkoxy radicals in carbohydrates, synthesis of chiral 2,7-dioxabicyclo[2.2.1]heptane and 6,8-dioxabicyclo[3.2.1]octane ring systems, /. Org. Chem., 67, 7439, 2002. 

De Armas, R, Concepcion, J.I., Francisco, C.G., Hernadez, R., Salazar, J.A., and Suarez, E., Intramolecular hydrogen abstraction. Hypervalent organoiodine compounds, convenient reagents for alkoxy radical generation, J. Chem. Soc., Perkin Trans. 1,405, 1989. 

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