Intramolecular Hydrogen-abstraction Reactions

Maeda K, Terazima M, Azumi T and Tanimoto Y 1991 CIDNP and CIDEP studies on intramolecular hydrogen abstraction reaction of polymethylene-linked xanthone and xanthene. Determination of the... 

Intramolecular hydrogen abstraction reactions have also been observed in mediumsized rings. The reaction of cyclooctene with carbon tetrachloride is an interesting case. As shown in the equation below, whereas bromotrichloromethane adds to cyclooctene in a completely normal manner, carbon tetrachloride gives some 4-chloro-l-trichloromethyl-cyclooctane as well as the expected product ... 

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... 

The earliest report on the effect of micelles on Type II reactions dealt with the dependence of the ratio of disproportionation to cyclobutanol formation for the intramolecular hydrogen abstraction reaction of octanophenone (2<5 R=C4H9) and valerophenone (25 R = CH3) (Scheme XI)32). The reaction in homogeneous solu-... 

A variety of intramolecular hydrogen-abstraction reactions from amino groups by carbonyl groups have been studied. There is no direct evidence that these reactions involve an intramolecular electron-transfer reaction. However, on the basis of studies on intermolecular systems, this is likely to be the case when the carbonyl group is part of an electron-accepting system, e.g. a quinone or phthalimide group. Quinone [153] was found to give [154], [155] and [156] (Scheme 35) (Maruyama et al., [911 Falci el al., 1977). 

The feasibility of utilising similar intramolecular hydrogen abstraction reactions for photoinitiation was therefore tested (55). Reactions of 3,3, 4,4 -benzophenone tetracarboxylic dianhydride (BTDA) with primary n-alkyl amines readily yield a series of N-substitut i imides. 

E. Suarez and co-workers prepared chiral 7-oxa-2-azabicyclo[3.2.1]octane and 8-oxa-6-azabicyclo[3.2.1]octane ring systems derived from carbohydrates via an intramolecular hydrogen abstraction reaction promoted by A/-centered radicals. The A/-centered radicals were obtained under mild conditions (Suarez modification) from phenyl and benzyl amidophosphates and alkyl and benzyl carbamate derivatives of aminoalditols by treatment with PIDA/I2 or PhlO/l2. The initial A/-radical undergoes a 1,5-hydrogen abstraction to form an alkyl radical, which is oxidized to the corresponding stabilized carbocation (oxocarbenium ion) under the reaction conditions. The overall transformation may be considered as an intramolecular N-glycosidation reaction. 

Francisco, C. G., Herrera, A. J., Suarez, E. Intramolecular Hydrogen Abstraction Reaction Promoted by N-Radicals in Carbohydrates. Synthesis of Chiral 7-Oxa-2-azabicyclo[2.2.1]heptane and 8-Oxa-6-azabicyclo[3.2.1]octane Ring Systems. J. Org. Chem. 2003, 68, 1012-1017. 

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]. 

Abstraction reaction in polymers can occur intramolecularly or intermolecularly. The former possibility is important in polymers which possess lateral groups (PP and PS), whereas it is nonexistent in linear polymers (HOPE). The possibility of intramolecular H abstraction has been advanced as one of the reasons for the high sensitivity of PP (in comparison to HOPE) toward oxidative degradation. Infrared studies have revealed that more than 90% of hydroperoxides in PP are hydrogen-bonded in sequences of two or more. This result supports an intramolecular hydrogen abstraction reaction, facilitated by a six-membered ring stereochemical arrangement [Eq. (40)]. 

The photolysis of nitrite esters for intramolecular hydrogen abstraction reactions, a procedure that has become known as the Barton reaction, is a useful process for functionalization of unactivated sites. The carbon radical thus generated may be quenched by a variety of reagents (i- ii), D. H. R. Barton, J. M. Beaton, L. E. Geller, M. M. Pechet, J. Amer. Chem. Soc., 83, 4076 (1961) M. Akhtar, D. H. R. Barton and P. G. Sammes, J. Amer. Chem. Soc., 87, 4601 (1965). 

The chiral auxiliary method yields exceptionally high de values even during intramolecular hydrogen abstraction reactions of carbonyl systems. A single isomer of highly substituted cyclopentanols was obtained when the chiraUy modified aryl alkyl ketones 27 shown in Scheme 26 were irradiated. ... 

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