Allylic hydrogen atom

The allylic bromination of an olefin with NBS proceeds by a free-radical chain mechanism. The chain reaction initiated by thermal decomposition of a free-radical initiator substance that is added to the reaction mixture in small amounts. The decomposing free-radical initiator generates reactive bromine radicals by reaction with the N-bromosuccinimide. A bromine radical abstracts an allylic hydrogen atom from the olefinic subsfrate to give hydrogen bromide and an allylic radical 3 ... 

This allylic bromination with NBS is analogous to the alkane halogenation reaction discussed in the previous section and occurs by a radical chain reaction pathway. As in alkane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br- radical, which cycles back... 

In the photooxygenation of electron-rich olefins with allylic hydrogen atoms, ene reactivity usually dominates [96]. Nevertheless, other reactions become the preferred reaction mode. Inagaki et al. [92] attributed the exclusive [2h-2] cycloaddition... 

Allylic substitution any reaction in which an allylic hydrogen atom is... 

As the allylic hydrogen atom is abstracted from propene, the. s/r3-hybridized carbon atom of the methyl group changes its hybridization state to sp. ... 

Olefins containing at least one allylic hydrogen atom react with 02 to form an allylic hydroperoxide. The analogous formal reaction of such alkenes with ethylene is known as ene-reaction. The tricyclic lactone peroxypartheno-lide has been prepared by such a reaction (6.13)619). 

The ene reaction (equation 42) is the indirect substituting addition of an unsaturated compound X = Y to an olefin possessing an allylic hydrogen atom, which is transferred in the process44,45. The intramolecular version of the reaction (equation 43) has been applied to the formation of five-, six- and seven-membered rings46. 

The mechanism of the Kharasch-Sosnovsky reaction remains unclear. The generally accepted version, as proposed by Kochi and co-workers (94-96) and later improved by Beckwith and Zavitsos (97), is illustrated in Scheme 8. Cuprous ion reduces the perbenzoate to Cu(II)OBz (Bz = benzoyl) and free /-BuO radical. The radical abstracts an allylic hydrogen atom generating an allyl radical that combines with the cupric salt to form an allylcopper(III) species. Reductive elimination with... 

C=C-CH Allyl compounds (olefins with allylic hydrogen atoms), including most alkenes... 

Clustering of crosslinks can be explained by a kinetic chain reaction occurring through the C=C double bonds. Crosslinking by the conventional vulcanization process with sulfur has been shown by NMR to proceed through the allylic hydrogen atoms. 

Not only can acids catalyze olefin isomerization, but strong bases can also effect isomerization. These base-catalyzed isomerizations proceed through proton abstraction of an allylic hydrogen atom followed by protonation of the allylic anion to regenerate either the original or the isomeric olefin ... 

The factors that influence the diastereoselectivity for the class of monocyclic hydrocarbons, from which monoterpenes are the most studied group, are mainly steric and conformational. Monoterpenes, in general, show relatively low overall syn/anti selectivity, usually attributed to the availability of allylic hydrogen atoms in the right alignment for abstraction. 

Ene reaction (Hydro-allyl addition)-A reaction of an allylic compound with an olefin which resembles both a cycloaddition and a [1, 5]-sigmatropic shift of hydrogen. In this reaction an alkene having an allylic hydrogen atom reacts thermally with a dienophile (enophile) C = C, C = 0, N = N, N = 0, C = S... 

A small portion of the reaction proceeds by what appears to be abstraction of the allylic hydrogen atom from the weaker C-H bond (e.g., Lee and Rowland, 1977). In the case of the isoprene reaction, for example, about 15% of the overall reaction at 1 atm pressure and room temperature proceeds by abstraction (Ragains and Finlayson-Pitts, 1997). It should be noted,... 

The bromine radical abstracts an allylic hydrogen atom of the cyclohexene, and forms a resonance stabilized allylic radical and hydrogen bromide. 

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