Bromine atoms, abstraction

This result shows than the initially added trichloromethyl group has little influence on the stereochemistry of the subsequent bromine atom-abstraction. The intermediate 2-(trichlor-omethyl)cyclohexyl radical presumably relaxes to the equatorial conformation faster than bromine-atom abstraction occurs. In contrast with addition to A -octahydronaphthalene, the addition is exclusively /ran -diaxial ... 

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 intramolecular hydrogen abstraction. 

The lack of steric effects in oxidations of hydrocarbons by Cr(VI) renders D and E unacceptable. The activated complex of scheme C is non-linear and hence does not comply with the magnitude of the observed isotope effect. Two pieces of evidence are quoted which indicate A to be the more probable of the remaining two. Firstly, the p constant of —1.17 is more in agreement with that obtained for bromine atom abstraction from toluenes (—1.369 to —1.806) than those found for solvolyses involving electron-deficient carbon ( — 2.57 to —4.67) . Secondly, the correlation between the relative rates of oxidation of the series... 

For example, radical allylic bromination of pent-2-ene must produce a mixture of three products. There are two allylic positions in the substrate, and either can suffer hydrogen abstraction. If hydrogen is abstracted from the methylene, then the two contributing resonance structures for the allylic radical are equivalent, and one product results when this captures a bromine atom. Abstraction... 

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. ... 

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-... 

The trans-fused decalin system is conformationally rigid, and the stereochemistry of the product indicates that the initial addition of the trichloromethyl radical is from an axial direction. This would be expected on stereoelectronic grounds, because the radical should initially interact with the tt orbital. The axial trichloromethyl group then shields the adjacent radical position enough to direct the bromine abstraction in the trans sense. The results from the addition to cyclohexene indicate that the flexible 2-trichloromethylcyclohexyl radical must undergo conformational relaxation faster than bromine atom abstraction. 

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... 

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