Bromine atoms, abstraction reactions

That the mechanism of allylic bromination is of the free-radical type was demonstrated by Dauben and McCoy, who showed that the reaction is veiy sensitive to free-radical initiators and inhibitors and indeed does not proceed at all unless at least a trace of initiator is present. Subsequent work indicated that the species that actually abstracts hydrogen from the substrate is the bromine atom. The reaction is initiated by small amounts of Br. Once it is formed, the main propagation steps are... 

The final product analysis of the bromomethane reactions revealed the appearance of a peak at m/e 36 that is attributed to HC1 molecule. For CH3Br and CH2Br2 reactions there was no evidence for Br atoms (at m/e 79, 80) or BrCl molecules (at m/e 114, 116, 118) in the products, suggesting the absence of bromine substitution or abstraction pathways. However, for CHBr3 reaction there were small peaks at m/e 114, 116, and 118 the intensity of 114 ( Br Cf) peak was ca. 4 % of the intensity loss of Cl atoms. Since the calibration factor for the parent peak of BrCl at m/e 114 was 1.29 0.09 times higher than that of Cl atoms at m/e 35, the yield of the bromine atom abstraction pathway is ca. 3 %, indicating the absence secondary reactions. Moreover, the HC1 yield was always equal to the Cl atom consumption, within 10%. 

The rates of bromine atom abstraction by tris(trimethylsilyl)silyl radicals from a range of /Jara-substitutcd benzyl bromides has indicated that the silyl radical is nucleophilic. In addition both the polar and spin-delocalization effects of the substituents play a role in the abstraction reaction with the latter effect greater than for H-atom abstractions.166 The perfluoroalkylation of aromatics and alkenes has been investigated using C4F9I as the source of C,. Measurement of rate constants indicated that perfluoroalkyl radicals were 2-3 orders of magnitude more reactive than the corresponding alkyl radicals. This was attributed primarily to the reaction enthalpy and far less to the electrophilic nature of the radicals.167... 

In contrast to eq. 2.29, eq. 2.30 shows the oxidative conversion of aldehydes (62) to amides (63) via acyl bromides with NBS/AIBN/R2NH under refluxing conditions in CC14 [74]. The reaction comprises of the abstraction of the formyl hydrogen atom by the succinimidyl radical, bromine atom abstraction from NBS by the acyl radical, and lastly,... 

In the first propagation step of the Wohl-Ziegler bromination, the bromine atom abstracts a hydrogen atom from the allylic position of the alkene and thereby initiates a substitution. This is not the only reaction mode conceivable under these conditions. As an alternative, the bromine atom could react with the C=C double bond and thereby start a radical addition to it (Figure 1.27). Such an addition is indeed observed when cyclohexene is reacted with a Br2/AIBN mixture. 

Reaction of the tris-amido titanium(in) compound Ti(NRAr)3 (R = Buc, Ar = 3,5-C6H3Me2) with 0.5equiv. of 0-bromophenyl allyl ether results in bromine atom abstraction followed by cyclization of the intermediate aryl radical to generate a titanium-bound 3-methylenedihydrobenzofuran product (Scheme 28), which has been spectroscopically characterized in solution.79... 

Acetylated 2-, 3- and 4-deoxypyranosyl radicals can be generated similarly, by photolysis of hexamethylditin (hexamethyl distannane, (043)380-80(043)3) in the presence of the appropriate bromodeoxy or iododeoxy precursor, the reaction being a bromine atom abstraction by (C4l3)38n. Their splitting patterns were entirely as expected for 71-type radicals with the trivalent carbon sp hybridised, the unpaired electron in a p orbital and the sugar in a Ci conformation. ... 

The trend of reactivity tert > sec > pri is consistently observed in various hydrogen atom abstraction reactions, but the range of reactivity is determined by the nature of the reacting radical. The relative reactivity of pri, sec, and tert positions toward hydrogen abstraction by methyl radicals is 1 4.8 61. An allylic or benzylic hydrogen is more reactive toward a methyl radical by a factor of about 9, compared to an unsubstituted C—H. The relative reactivity toward the t-butoxy radical is pri 1, sec 10, tert 50. In the gas phase, the bromine atom is much more selective, with relative reactivities of pri 1, sec 250, tert 6300. Data for other types of radicals have been obtained and tabulated. ... 

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 hydrogen-abstraction process. Another example of transannular cyclization of unsaturated radicals is found in the reaction of 1,4-cyclooctadiene with acetaldehyde in the presence of benzoyl peroxide, which gives a cyclized ketone by a process involving an intramolecular addi-... 

Chain Propagation Chain propagation involves the formation of products. Reaction of a radical and a nonradical gives a new radical. (Both radicals formed in the initiation can abstract hydrogen atoms. We show only the Br reaction.) In the first propagation step, a bromine atom abstracts an allylic hydrogen (the weakest C—H bond in propene) to produce an allyl radical. The allyl radical, in turn, reacts with a bromine molecule to form allyl bromide and a new bromine atom. 

Photobromination more closely approaches the goal of perfect selectivity because the bromine atom abstracts tertiary hydrogens much more readily than secondary or primary hydrogens. But even this reaction is not without its problems. It is a slow process and the side products of di- and polybromination are formed. 

When one compares the selectivity of Br and Cl, the more reactive species (Cl) is clearly the less selective one. For example, bromine atoms abstract tertiary H-atoms 17(X) times faster than the primary H-atom. By comparison, chlorine radical is only 7 times faster. In fact, the electronic factor m for those two radicals has almost identical values (m close to 2) and RSP is obeyed. When changes in reactivity are not due only to changes in reaction energy (A °, A/7° or AG°), the postulates of physical organic chemistry are no longer verified. For example, Br and CHj have almost the same reactivity with respect to CH4. According to RSP one would expect that both radicals should have identical selectivities for H-atom abstractions. Nevertheless, Br is much more selective, because the electrophilicity... 

The relative rates of reaction of ethane toluene and ethylbenzene with bromine atoms have been measured The most reactive hydrocarbon undergoes hydrogen atom abstraction a million times faster than does the least reactive one Arrange these hydrocarbons in order of decreasing reactivity... 

Introduction of a 3-bromosubstituent onto thiophene is accompHshed by initial tribromination, followed by reduction of the a-bromines by treatment with zinc/acetic acid, thereby utilizing only one of three bromines introduced. The so-called halogen dance sequence of reactions, whereby bromothiophenes are treated with base, causing proton abstraction and rearrangement of bromine to the produce the most-stable anion, has also been used to introduce a bromine atom at position 3. The formation of 3-bromotbiopbene [872-31-1] from this sequence of reactions (17) is an efficient use of bromine. Vapor-phase techniques have also been proposed to achieve this halogen migration (18), but with less specificity. Table 3 summarizes properties of some brominated thiophenes. 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

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 ... 

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