Intramolecular hydrogen abstraction transition states

Intramolecular hydrogen abstraction. As stated above, photolysis of n-octylnitrite results in a predominance of the product corresponding to intramolecular hydrogen abstraction. Photolysis of 3-heptyl, 2-hexyl, and 2-pentylnitrites yields intramolecular hydrogen abstraction products in 28%, 31%, and 6% yields, respectively/5 In each case the product corresponded to hydrogen abstraction via a six-membered transition state ... 

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. 

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 ... 

Photolysis of 2-propylnitrite, in which intramolecular hydrogen abstraction cannot occur via a six-membered transition state, results in a 30% yield of 2-propanone. As we shall see, this intramolecular hydrogen abstraction has been extensively applied to steroid syntheses by D. H. R. Barton and hence is commonly referred to as the Barton reaction. 

The availability of a six-membered transition state, although necessary, is not always sufficient for an intramolecular hydrogen abstraction to take place. The distance X-Y (structure 95a) also must fall between certain critical limits, usually attainable in nonrigid aliphatic systems but often unsatisfied in rigid situations, i.e., steroids. 

Although an entropy of activation of about —15 cal. mole 1 °K."1 is involved in forming the transition state ring in reactions such as 4, the over-all entropy change accompanying intramolecular hydrogen abstraction is small, and the pre-exponential factor for a unimolecular reaction involving a cyclic transition state has been estimated (10) as 1011... 

Intramolecular hydrogen abstractions occur most readily through six-membered cyclic transition states and thus involve 1,5-shifts 1,6-shifts occur about 1/10 as readily, and 1,4-shifts less than 1/100, since they are rarely detected. 

When an excited aldehyde or ketone has a y hydrogen, intramolecular hydrogen abstraction via a six-membered ring transition state usually occurs. The resulting 1,4-biradical may either cleave or cyclize to give the Norrish Type II products of Scheme 4. 

Remote photocyclization of (dibenzylamino)ethyl benzoylacetate. Intramolecular hydrogen abstraction through a ten-membered cyclic transition state. Journal of the Chemical Society, Chemical Communications, 363-364. 

Hydrogen abstraction can occur intramolecularly and is useful for functionalizing rather remote unactivated sites. With freely rotating alkyl systems, there is a great preference for a five- or six-membered cyclic transition state. With rigid or conformationally restricted systems, molecular models are necessary to predict the site of intramolecular hydrogen abstraction. 

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. 

Photoinduced remote cyclization of amino ketones provides modest yields of mixtures of the cis and trans isomers of oxazocinones (79 R = H, = H, Ph, R = H, Ph R = Me, R = R = Ph) (Scheme 21), resulting from intramolecular hydrogen abstraction by the photoexcited ketone in a ten-membered transition state <85CC363,90JCS(Pl)90l). 

Also, intramolecular hydrogen abstraction is assumed to occur through a seven-membered ring transition state. This can provide a route to chain scission ... 

Both substitution and addition reactions can occur intramolecularly. Intramolecular substitution reactions that involve hydrogen abstraction have some important synthetic utility, since they permit functionalization of carbon atoms relatively remote from the initial reaction site. The preference for a six-membered cyclic transition state in the hydrogen-abstraction step imparts considerable selectivity to these intramolecular hydrogen abstractions. An important example of this... 

In their opinion, the primary radical is formed by the sequence of reactions (7) and (10) and the secondary radical by the intramolecular hydrogen abstraction, proceeding via a seven-membered ring transition state [reaction (13)] 17). 

In practice, the hnear polymer we might expect for alkenes is not the major product of the free radical process. (Cationic polymerization is generally used to prepare linear addition polymers of alkenes.) The product chains have many alkyl branches, which most often are the four-carbon-atom butyl groups produced by short chain branching. These products are the result of intramolecular hydrogen abstraction by way of a six-membered transition state that generates a secondary radical from a primary radical. 

Hasegawa, X, Miyata, K., Ogawa, T., Yoshihara, N., and Yoshioka, M., Remote photocychsation of (dibenzylamino) ethyl benzoylacetate. Intramolecular hydrogen abstraction through a ten-mem-bered cyclic transition state, /. Chem. Soc., Chem. Commun., 363, 1985. 

Scheme 7). The results outlined in Scheme 8 also clearly indicate that intramolecular hydrogen abstraction by the alkoxyl radical via a six-membered transition state predominates over fragmentation and intermolecular allylic hydrogen abstraction. For more details concerning this subject, readers should refer to the original literature and also to a review article."... 

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