Allyl radicals reactions

With R = benzyl and in the absence of 02, the major product (73%) is the de-carbonylation product [reaction (209) possible formed to a large extent within the solvent cage], and the dimer of the allylic radical [reaction (207)] is formed only in small amounts. Addition of a thiol increases the yield of Thd [reaction (208)]. If an evaluation of the data reported for the reduction of the allylic OH-adduct to 1,3-cylohexadiene by a thiol (Pan et al. 1988), estimated at 104 dm3 mor1 s"1, is a good guide the rate constant for reaction (208) should be similar. This would revise an assumed rate constant of 106 dm3 mol-1 s-1 and the conclusions as to the repairability of allylic Thy in DNA radicals by cellular thiols (Anderson et al. 2000). 

Although propene oxide is not a major product, very importantly from measurements of its rate of formation, [HO2] may be calculated reliably as kn is known accurately [37]. As [CH2CHCH2] can similarly be calculated from the rate of formation of HDE, Walker and co-workers have made a comprehensive analysis of allyl radical reactions in both propene oxidation and the decomposition of 4,4-dimethylpentene-l (DMP) in the presence of O2. Table 1.29 summarizes the Arrhenius parameters for various reactions of allyl radicals with O2. Kinetic data for a variety of other allyl reactions has also been critically examined recently [9]. 

Allylic Systems. Allylic Halides. Allylic halides also undergo homolytic carbon-halogen cleavage by pentacyanocobaltate(II) to form equimolar quantities of halo- and allylcobalt complexes (21, 22, 23). It is assumed that this reaction involves generation of the allylic radical (Reaction 19), which then reacts with pentacyanocobaltate(II) (Reaction 20). 

The 1,3-cyclohexadiene could not be prepared with higher purity than 98% and hence the analysis based on the final products is less meaningful. The yield of 3- and 4-hydroxycyclohexenes show that only 31% (0.18 pmolJ /0.58 pmolJ ) of the OH radicals add to the double bonds. There is no information about the missing 44% (100% — 25%—31%). Von Sonntag and coworkers suggested that the yield of hydroxy cyclohexenes is not indicative of the OH addition to the double bonds due to non-quantitative reaction of the ally lie radical 1 (equation 10) with RSH. Since, in the case of 1,4-cyclohexadiene, they found complete material balance, they concluded that the alky lie radical formed in reaction reacts quantitatively with the thiolic compound. Thus, radical 2 formed in reaction (11) will react quantitatively with RSH. The inefficiency of the reduction of the allylic radical by the thiol is probably due to the weak ally lie C—H bond which leads to a six orders of magnitude lower rate constant for the RSH-I- allylic radical reaction compared with the RSH-I- alkyl radical reaction. If all the material imbalance is due to incomplete reduction of the allylic radical, its formation is the main path of reaction of OH with 1,3-cyclohexadiene. 

Chemical activation and fall-off. Gas-phase reactions that form an energized product cause particular difficulty in kinetic model-construction. For example, in Reaction (1) the unstable product will be formed with excess energy (due to the exothermicity of the bond-forming addition reaction), that it will rapidly dissociate to ethyl acetate + allyl radical, Reaction (3). 

The degree to which allylic radicals are stabilized by delocalization of the unpaired electron causes reactions that generate them to proceed more readily than those that give simple alkyl radicals Compare for example the bond dissociation energies of the pri mary C—H bonds of propane and propene... 

Allylic carbocations and allylic radicals are conjugated systems involved as reactive intermediates m chemical reactions The third type of conjugated system that we will examine conjugated dienes, consists of stable molecules... 

Alkenes react with N bromosuccimmide (NBS) to give allylic bromides NBS serves as a source of Br2 and substitution occurs by a free radical mechanism The reaction is used for synthetic purposes only when the two resonance forms of the allylic radical are equivalent Otherwise a mixture of isomeric allylic bromides is produced... 

Reaction Mechanism. High temperature vapor-phase chlorination of propylene [115-07-17 is a free-radical mechanism in which substitution of an allyhc hydrogen is favored over addition of chlorine to the double bond. Abstraction of allyhc hydrogen is especially favored since the allyl radical intermediate is stabilized by resonance between two symmetrical stmctures, both of which lead to allyl chloride. 

The transition state involves six partially delocalized electrons being transformed from one 1,5-diene system to another. The transition state could range in character from a 1,4-diradical to two nearly independent allyl radicals, depending on whether bond making or bond breaking is more advanced. The general framework for understanding the substituent effects is that the reactions are concerted with a relatively late transition state with well-developed C(l)—C(6) bonds. 

An example of this reaction is the reaction of cyclohexene with t-butyl perbenzoate, which is mediated by Cu(I). " The initial step is the reductive cleavage of the perester. The t-butoxy radical then abstracts hydrogen from cyclohexene to give an allylic radical. The radical is oxidized by Cu(II) to the carbocation, which captures benzoate ion. The net effect is an allylic oxidation. 

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 ... 

The chain propagation step consists of a reaction of allylic radical 3 with a bromine molecule to give the allylic bromide 2 and a bromine radical. The intermediate allylic radical 3 is stabilized by delocalization of the unpaired electron due to resonance (see below). A similar stabilizing effect due to resonance is also possible for benzylic radicals a benzylic bromination of appropriately substituted aromatic substrates is therefore possible, and proceeds in good yields. 

As mentioned in an earlier section (cf. Chapter 1, Section III), allylic positions are subject to attack by free radicals resulting in the formation of stable allyl radicals. A-Bromosuccinimide (NBS) in the presence of free-radical initiators liberates bromine radicals and initiates a chain reaction bromination sequence by the abstraction of allylic or benzylic hydrogens. Since NBS is also conveniently handled, and since it is unreactive toward a variety of other functional groups, it is usually the reagent of choice for allylic or benzylic brominations (7). 

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 addition to its effect on stability, delocalization of the unpaired electron in the allyl radical has other chemical consequences. Because the unpaired electron is delocalized over both ends of the nr orbital system, reaction with Br2 can occur at either end. As a result, allylic bromination of an unsymmetrical alkene often leads to a mixture of products. For example, bromination of 1-octene gives a mixture of 3-bromo-l-octene and l-bromo-2-octene. The two products are not formed in equal amounts, however, because the intermediate allylic radical is... 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

When the allylic cation reacts with Br to complete the electrophilic addition, reaction can occur either at Cl or at C3 because both carbons share the positive charge (Figure 14.4). Thus, a mixture of 1,2- and 1,4-addition products results. (Recall that a similar product mixture was seen for NBS bromination of alkenes in Section 10.4, a reaction that proceeds through an allylic radical.)... 

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

This allyl transfer reaction, which is a valuable synthetic method, has been shown to be a free-radical chain substitution (SH2 ), namely... 

A small library of highly functionalized pyrrolines 95 was synthesized by reaction of allylic and propargylic isocyanides 94 with thiols followed by radical cyclization (Scheme 33). The radical reaction was carried out using a radical initiator (AIBN) under flash heating microwave irradiation [67]. 

The reaction is usually quite specific at the allylic position and good yields are obtained. However, when the allylic radical intermediate is unsymmetrical, allylic rearrangements can take place, so that mixtures of both possible products are obtained, for example. 

Several studies characterizing the reactions of alkenyl radicals with quinone dumines and quino-neimines were published in the late 1970s. Quinone dumines react with allylic radicals yielding both the reduced PPD and the alkylated product. In these experiments 2-methyl-2-pentene served as a model olefin (model for NR). Samples of the olefin and quinoneimines or quinone diimine were heated to 140°C. Isolation and analysis of products demonstrated that 40%-70% of the imine or diimine was reduced to the corresponding PPD, while 20%-50% was isolated as the alkylated product. This alkylation reaction (via an allylic radical) represents the pathway to the formation of rubber-bound antidegradant. ... 

Evidence indicates [28,29] that in most cases, for organic materials, the predominant intermediate in radiation chemistry is the free radical. It is only the highly localized concentrations of radicals formed by radiation, compared to those formed by other means, that can make recombination more favored compared with other possible radical reactions involving other species present in the polymer [30]. Also, the mobility of the radicals in solid polymers is much less than that of radicals in the liquid or gas phase with the result that the radical lifetimes in polymers can be very long (i.e., minutes, days, weeks, or longer at room temperature). The fate of long-lived radicals in irradiated polymers has been extensively studied by electron-spin resonance and UV spectroscopy, especially in the case of allyl or polyene radicals [30-32]. 

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