Allylic hydrogens abstraction

Alkenes. When the substrate molecule contains a double bond, treatment with chlorine or bromine usually leads to addition rather than substitution. However, for other radicals (and even for chlorine or bromine atoms when they do abstract a hydrogen) the position of attack is perfectly clear. Vinylic hydrogens are practically never abstracted, and allylic hydrogens are greatly preferred to other positions of the moleeule. Allylic hydrogen abstraction from a cyclic alkenes is usually faster than abstraction from an acyclic alkene. ... 

The formation of ROO and RO radicals, and M" =0 species is expected when phthalocyanines and porphyrins are used as catalysts (21, 22). The formation of the epoxide, Fignre 49.1, may be associated with the attack of metal oxo species (Fe = O) to the double bond (23). For a-pinene this attack is possibly favored by its rigid structure that causes an orbital overlapping, making the allylic hydrogen abstraction difficult (24). 

Additionally, it should be observed that the thermal oxidability and oxidative polymerization of the unsaturated fatty acids follows the trend linolenic > linoleic > oleic > > palmitoleic (Martinenghi, 1963). The oxidation involves, as first step, the abstraction of a hydrogen atom in allylic position to the double bonds. Certainly, this process is favoured in the case of fatty acids with two or more unconjugated double bonds where the formation of a free radical by allylic hydrogen abstraction leads quite necessarily to double bonds slippage with formation of conjugated double bonds ... 

For example, in substrates 94 and 96 the major ene adducts (74% and 90%, respectively) are formed by vinylic hydrogen atom abstraction, whereas the minor ene adducts (26% and 10%, respectively) are produced by allylic hydrogen abstraction. The percentage of... 

In summary, oxygen associated with Bi is responsible for allylic hydrogen abstraction, whereas oxygen polyhedra around Mo are inserted into the allylic intermediate.962-964 Nitrogen insertion in ammoxidation takes place in a similar way965 on an active site containing Mo=NH instead of Mo=0. 

Direct irradiation of diazoacetophenone in cyclohexene yields much the same product distribution as that obtained through the intermediacy of triplet sensitizers, indicating that the reactant benzoyl-methylene most likely is a triplet.60 A small amount of nonstereospecific addition to the olefin occurs, but the major reaction is allylic hydrogen abstraction to yield acetophenone and dicyclohexenyl. In alcohol... 

When the reaction is applied to an unsymmetrically substituted alkene, the major photoproduct or products are those arising from the most stable diradical intermediate. This is illustrated in the irradiation of benzaldehyde in 2-methylbut-2-ene in which the principal products are the stereoisomeric oxetanes arising from the same diradical intermediate [Eq. (70)].278 The oxetanes resulting from addition in the alternative sense [Eq. (71)] are minor products, as are other products arising by allylic hydrogen abstraction. 

In the presence of bromide, the oxidation of alkenes by Mn(OAc)3 in acetic acid readily occurs it 70-80 °C and produces allylic acetates in good yields. Thus cyclohexene is oxidized to cyclo-lexenyl acetate in 83% yield,508 and a-methylstyrene to j3-phenylallyl acetate in 70% yield,509 vith a mechanism involving allylic hydrogen abstraction by bromine atoms coming from the ixidation of bromide by MniU (equation 206). 

Keywords cyclohexenone, photorearrangement, intramolecular allylic hydrogen abstraction... 

C—H bond and the reacting C=C double bond (see Scheme 25). For example, in substrates 93 and 95 the major ene adducts (74% and 90%, respectively) are formed by vinylic hydrogen atom abstraction, whereas the minor ene adducts (26% and 10%, respectively) are produced by allylic hydrogen abstraction. The percentage of allene hydroperoxide is lower (43%) in the photo-oxidation of 94. In this substrate, the steric hindrance at the stereogenic center is lower than in 93 or 95, and the diene is therefore less twisted and the a --rr interaction less profound, or by analogy, the vinylic hydrogen atom is less perpendicular to the double bond. 

Figure 28 Cation-alkene interaction within a supercage of cation-exchanged Y-zeolite may control the conformations of allylic hydrogens. For allylic hydrogen abstraction, Ha and Hb should be parallel to the tt orbital (see top).

Unfortunately, substitution in the y-position, such as in crotyltin, led to poorly reactive allyltins, due to the decrease in the addition rate of the radicals to the double bond. It has been established that, generally, the competitive allylic hydrogen abstraction became predominant, destroying the crotyltin reagent548. The use of y-substituted allyltins for the photo-induced radical allylation of carbonyl compounds represents an interesting... 

Tandem reactions may also occur by initial allylic hydrogen abstraction (with 1,5-hydrogen transfer) by a vinylic radical, followed by a cyclization process. This methodology has been used to synthesize bicyclic and tricyclic systems (equations 135 and 136)778,793,807-809. Electron-withdrawing groups conjugated to the vinylic halide increase the yields and decrease reaction times considerably. 

Co(II) carboxylates were well documented, and for cyclohexene as substrate the cyclohexenyl hydroperoxide is formed in situ by attack of 02 on the allylic radical produced by allylic hydrogen abstraction, Reaction 12 (4, 48). The products, usually those shown in Reaction 13, are formed via the metal-catalyzed decomposition of the hydroperoxide, and any 02 coordination at the metal is incidental there is... 

In contrast to intramolecular cyclizations, the intermolecular addition of O-centered radicals to ti systems as initiating step in complex radical cyclization cascades has only recently attracted considerable attention. The reason for the low number of papers published on O radical addition to alkenes and alkynes could originate from the perception that O-centered radicals, such as alkoxyl radicals, RO, or acyloxyl radicals, RC(0)0", may not react with 7t systems through addition at rates that are competitive to other pathways, for example, allylic hydrogen abstraction and p-fragmentation in the case of RO" or decarboxylation in the case of RC(0)0" (Scheme 2.9). 

The photochemical reactions of arenecarboxylic acid esters with alkenes has received recent attention by Cantrell. - For example, irradiation of 2,3-dimethyl-2-butene and methyl benzoate gave a mixture of alkoxyoxetane (56), carbonyl-alkene metathesis product (57) and ketone (58), resulting from alkoxy radical allylic hydrogen abstraction and radical recombination. Such alkoxyoxetane photoproducts are... 

Ter-butoxy radical is known to be very reactive towards allylic hydrogen abstraction Eq. (2), but a fraction undergoes the 0 scission reaction (1) ... 

Rothenberg, G., Sasson, Y. Cyclic vs. acyclic allylic hydrogen abstraction an entropy motivated process Tetrahedron 1998, 54, 5417-5422. 

H2C CMeC(CMe3)(OH)CH CHCMc20H, which would have arisen from allylic hydrogen abstraction. This implies that the higher reactivity of the vinyl hydrogen can be rationalised in terms of the large orbital interaction... 

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