Hydrogen abstraction, intramolecular

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... 

Butane. The VPO of butane (148—152) is, in most respects, quite similar to the VPO of propane. However, at this carbon chain length an important reaction known as back-biting first becomes significant. There is evidence that a P-dicarbonyl intermediate is generated, probably by intramolecular hydrogen abstraction (eq. 32). A postulated subsequent difunctional peroxide may very well be the precursor of the acetone formed. 

There are several reaction sequences which involve such intramolecular hydrogen abstraction steps. One example is the photolytically intitiated decomposition of N-haloamines in acidic solution, which is known as the Hofinann-Loffier reactionThe reaction leads initially to y-haloamines, but these are usually converted to pyrrolidines by intramolecular nucleophilic substitution ... 

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 ... 

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 ... 

No product derived from the transannular hydrogen abstraction is observed in the addition of bromotrichloromethane because bromine-atom abstraction is sufficiently rapid to prevent effective competition by the intramolecular hydrogen abstraction. 

In ketones having propyl or longer alkyl groups as a carbonyl substituent, intramolecular hydrogen abstraction can be followed by either cleavage of the bond between the a and P carbon atoms or by formation of a cyclobutanol ... 

If a substituent chain is present on the cyclopentenone ring, an intramolecular hydrogen abstraction can take place ... 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

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]. 

The cycloaddition of photoenol of o-methylbenzaldehyde 66 with 5-alkyli-dene-l,3-dioxane-4,6-dione derivatives 67 is an example of a photo-induced Diels Alder reaction in which one component, the diene in this case, is generated by irradiation [48]. The yields of some cycloadducts 68, generated by photo-irradiation of a benzene solution of 66 and 67 at room temperature, are reported in Table 4.14. The first step of the reaction is the formation of (E)-enol 69 and (Z)-enol 70 (Equation 4.7) by an intramolecular hydrogen abstraction of 66 followed by a stereo- and regioselective cycloaddition with... 

In subsequent experiments, it was found that rearrangement of 2 persisted in solid glasses at temperatures as low as 28 K, where the reaction rates became nearly temperature independent, as expected. The fact that similar kinetics were measured in solution and in low temperature solids indicated that matrix effects were minimal, at least in these intramolecular hydrogen abstractions. 

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 KETONES (TYPE H CLEAVAGE)... 

With compounds containing C—H bonds y to the carbonyl moiety, one often observes intramolecular hydrogen abstraction, in contrast to the in-termolecular hydrogen abstraction by aryl ketones discussed earlier in this... 

Intramolecular hydrogen abstraction has been successfully utilized in a number of interesting synthetic preparations. In general the recombination of the biradical resulting from intramolecular hydrogen abstraction to form a cyclobutanol has received more attention from the point of view of photo-... 

Ring opening due to intramolecular hydrogen abstraction has been demonstrated for cyclopropyl ketone derivatives 1035 ... 

Intramolecular hydrogen abstraction has been utilized to remove a methoxy group from a pyranosidulose, 1055... 

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 ... 

Photolysis of -octylnitrite in heptane solution results in formation of the dimers of 2-, 3-, and 4-nitrosoheptane as well as the dimer of 4-nitroso-l-octanol (intramolecular hydrogen abstraction)[Pg.559]

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. 

Norrish Type II photocyclizations have been employed in many instances in the synthesis of nitrogen heterocycles. Type II cyclizations are the result of an intramolecular hydrogen abstraction by an excited carbonyl group followed by cyclization of the resulting biradical. Hydrogen abstraction from the y-carbon atom is normally preferred. The introduction of a nitrogen atom... 

Intramolecular hydrogen abstraction leading to the aziridine 344 has been proposed to account for the unexpected conversion of the /f-amino vinyl phenyl ketones 345 into the pyrroles 346.286... 

Nitro compounds have also been reported to undergo photocyclizations. The intermediacy of an isoxazoline in the photorearrangement of o-nitro-benzaldehyde to o-nitrosobenzoic acid is now in doubt,318 but intramolecular hydrogen abstraction by an excited nitro group in nitrobenzene derivatives can result in the formation of heterocycles. 4-tm-Butyl-3-methoxy-2,6-dinitrotoluene (384) on irradiation in methanolic sodium hydroxide solution... 

In this chapter are summarized the photochemical reactions wherein the primary chemical event is inter- or intramolecular hydrogen transfer to the excited chromophor. In intermolecular reactions hydrogen abstraction usually implies reduction or hydrodimerization of the excited molecule intramolecular hydrogen abstraction is frequently followed by either ring closure of the diradical or fragmentation to afford unsaturated molecules. 

---