Alkyl radical intramolecular hydrogen abstraction

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. 

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... 

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-... 

Preliminary work dealing with UV irradiation of long-chain n-alkyl esters of benzophenone-4-carboxylic acid [9,10] allowed cyclic products to be obtained, thus suggesting the formation of intermediate di-radicals by intramolecular hydrogen abstraction (Scheme 3). 

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. 

From B. Pandey s research group comes a straightforward photochemical method for the synthesis of spirocyclic compounds. An intramolecular hydrogen abstraction by an excited enone group is followed by radical combination. It is remarkable that no product arising from photoenolization and subsequent cyclization was observed. This is usually observed in ortho-alkyl substituted benzophenone and acetophenone derivatives. 

Type B reactivity was observed for systems in which substitution was found P to the sulfoxide [37]. Compound 57 is shown as a representative case. It was shown that 64 and 65 are probably derived from secondary photolysis of 63. Two mechanisms were proposed for the Type B transformations, each involving a-cleavage. First, alkyl-S cleavage can lead to the sultene 59. Further photolysis leads to S-0 homolysis. The subsequent loss of atomic sulfur is the difficulty with this mechanism but may result from attack by other radicals in solution. Intramolecular hydrogen abstraction gives the major isolated product 65. The other proposed mechanism has aryl-S cleavage to give the sulfme 62, presumably followed by photochemical desulfurization [25,35]. 

The term "Norrish-Type-2 cleavage" is understood as the photolysis of aldehydes or ketones at 230-330 nm, generating alkyl and acyl radicals, which stabilise themselves, for example, by intramolecular hydrogen abstraction or fragmentation. [215]... 

Both these reactions are observed with n n excited states. The cleavage and intramolecular hydrogen abstraction were first studied in detail by Norrish and are accordingly called Norrish type I and Norrish type II processes. In the type I process, the excited ketone (41 ) undergoes cleavage into acyl and alkyl radicals (42). In the type II process, the n radical center is used to abstract a y hydrogen atom to form a biradical (45), which may either cleave into the enol (46) of a lower ketone (47) or collapse by radical combination into a cyclobutanol (48). 

In the hypochlorite reactions, intramolecular hydrogen abstraction competes with p-cleavage and other reactions. For example, tertiary hypochlorite 15 on photolysis gives ketones and alkyl chlorides from P-cleavage and radical coupling reactions [10]. 

Intramolecular Hydrogen abstraction The terminal alkyl radicals undergo intramolecular Hydrogen abstraction to produce a ketone. 

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. 

In contrast to the intramolecular hydrogen abstraction mentioned above, fragmentation, another principal reaction of alkoxyl radicals, was seldom used in organic synthesis until the 1970s. An analysis of the numerous examples reported in the past and our own results concerning the photochemistry of alkyl nitrites and hypohalites indicated that the direction of 3-fragmentation in unsymmetrical substrates is the outcome of an interplay of multiple factors, such as the relative thermodynamic stability of the resultant radicals, ring strain, and stereoelectronic factors. 

A molecule of linear alkyl ether possesses a very convenient geometry for intramolecular hydrogen atom abstraction by the peroxyl radical. Therefore, chain propagation is performed by two ways in oxidized ethers intermolecular and intramolecular. As a result, two peroxides as primary intermediates are formed from ether due to oxidation, namely, hydroperoxide and dihydroperoxide [62],... 

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... 

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