Bromine, 1,3-shifts

This mechanism is challenged by Fischer and henderson1039, who suggest an ipso mechanism followed by 1,2 bromine shift in the bromination of methylphenols in trifluoromethanesulfonic acid (equation 156). 

Dyes and Indicators. The effects of bromine ia dye or iadicator molecules, ia place of hydrogen, iaclude a shift of light absorption to longer wavelengths, iacreased dissociation of phenoHc hydroxyl groups, and lower solubiHty (see Dyes and dye intermediates). The first two effects probably result from iacreased polarizatioa caused by bromine s electroaegativity compared to that of hydrogea. 

Halogenated Butyl Rubber. Halogenation at the isoprene site ia butyl mbber proceeds by a halonium ion mechanism leading to a double-bond shift and formation of an exomethylene alkyl haUde. Both chlorinated and brominated mbber show the predominate stmcture (1) (>80%), by nmr, as described eadier (33,34). Halogenation of the unsaturation has no apparent effect on the isobutylene backbone chains. Cross-linked samples do not crystallize on extension due to the chain irregularities introduced by the halogenated isoprene units. 

The product distribution can be shifted to favor the 1 -product by use of such milder brominating agents as the pyridine-bromine complex or the tribromide ion, Br3. It is believed that molecular bromine reacts through a cationic intermediate, whereas the less reactive brominating agents involve a process more like the AdgS and-addition mechanism. 

The stereochemistry of addition is usually anti for alkyl-substituted alkynes, whereas die addition to aryl-substituted compounds is not stereospecific. This suggests a termo-iecular mechanism in the alkyl case, as opposed to an aryl-stabilized vinyl cation mtermediate in the aryl case. Aryl-substituted alkynes can be shifted toward anti addition by including bromide salts in the reaction medium. Under these conditions, a species preceding the vinyl cation must be intercepted by bromide ion. This species can be presented as a complex of molecular bromine with the alkyne. An overall mechanistic summary is shown in the following scheme. 

Molecular bromine is believed to be the reactive brominating agent in uncatalyzed brominations. The brominations of benzene and toluene are first-order in both bromine and the aromatic substrate in trifluoroacetic acid solution, but the rate expressions become more complicated when these reactions take place in the presence of water. " The bromination of benzene in aqueous acetic acid exhibits a first-order dependence on bromine concentration when bromide ion is present. The observed rate is dependent on bromide ion concentration, decreasing with increasing bromide ion concentration. The detailed kinetics are consistent with a rate-determining formation of the n-complex when bromide ion concentration is low, but with a shift to reversible formation of the n-complex... 

In radical reactions not involving bromine or chlorine on the substrate, rearrangements are much rarer One example is the fluorination of di-tert butyl ketone which produces perfluormated / rt-buty isobutyl ketone [J5] Although isolated yields are poor only the rearranged ketone could be isolated This is perhaps only the second example of a 1,2-acyl shift Low fluorine substrate ratios show that this rearrangement occurs after monofluorination... 

Among the halogens, fluorine does not undergo rearrangement, and what evidence there is suggests that the rearrangement of chlorine is an intramolecular process (1,2 shift) whereas that of bromine appears to take place by both inter-and intramolecular routes. Less is known about iodine migration. 

The formation of alkyl shifted products H and 14 can be explained in terms of the formation of endo-intermediate 21 formed by endo attack of bromine to 2 (Scheme 4). The determined endo-configuration of the bromine atom at the bridge carbon is also in agreement with endo-attack. Endo-Intermediate 21 is probably also responsible for the formation of cyclopropane products 12 and 15. The existence of cyclopropane ring in 12 and 15 has been determined by and 13c NMR chemical shifts and especially by analysis of cyclopropane J cH coupling constants (168 and 181 Hz). On the basis of the symmetry in the molecule 12 we have distinguished easily between isomers 12 and 15. Aryl and alkyl shift products IQ, H, and 14 contain benzylic and allylic bromine atoms which can be hydrolized easily on column material. 

Some results are already described in the literature with nickel- or copper catalysts (refs. 3,71-78). It was us possible to develop this exchange and to show that it is under thermodynamic control the equilibrium lies about 60 to 40 % for bromine and iodine, and is much more shifted to the left (95 to 5 %) for chlorine and iodine. 

However, most of the debate over the possibility of 1,3 migrations has concerned not methyl or bromine but 1,3 hydride shifts.There is no doubt that apparent 1,3 hydride shifts take place (many instances have been found), but the question is whether they are truly direct hydride shifts or whether they occur by another... 

Hydride ion can migrate. In example c, it was hydride that shifted, not bromine ... 

Apart from silyl shifts, other reactions that are also characteristic of this class of compounds or their derivatives are due to the easy formation of halogen-silicon bonds. Phosphonium salt 34, resulting from the addition of bromine to 33, undergoes spontaneous desilylation by the action of the bromide anion to give the P-bromophosphazene 35 [138,139] (Scheme 33). 

Products Driving forces Threats Brominated and phosphorous-based products, specialty inorganics Demands for safer end-use Government regulations shift to alternative chemistries or resins... 

The X-ray structure of the dibromine complex with toluene (measured at 123 K) is more complicated, and shows multiple crystallographically independent donor/acceptor moieties [68]. Most important, however, is the fact that in all cases the acceptor shows an over-the-rim location that is similar to that in the benzene complex. In both systems, the acceptor is shifted by 1.4 A from the main symmetry axis, the shortest Br C distances of 3.1 A being significantly less than the sum of the van der Waals radii of 3.55 A [20]. Furthermore, the calculated hapticity in the benzene/Br2 complex (x] = 1.52) is midway between the over-atom (rj = 1.0) and over-bond (rj = 2.0) coordination. In the toluene complex, the latter varies from rj = 1.70 to 1.86 (in four non-equivalent coordination modes) and thus lies closer to the over-bond coordination model. Moreover, the over-bond bromine is remarkably shifted toward the ortho- and para-carbons that correspond to the positions of highest electron density (and lead to the transition states for electrophilic aromatic bromination [12]). Such an experimental location of bromine is in good agreement with the results of high level theoretical... 

Our third approach to 27 addressed the unavailability of 3-methoxy-2-cyclopentenone (31) in bulk quantities which necessitated the discovery of an alternative route (Scheme 7.7). Fortunately, the precursor to 31,1,3-cyclopentandione (35), was available in the required quantities and our efforts shifted to the use of this reagent Bromination of 35 with NBS, employing either KHC03 or KOH as the base, gave brominated dione 36 in 85% isolated yield. Unfortunately, direct cross-coupling of alkyl bromide 36 with boronic acid 12 under a variety of Suzuki-... 

Methylation of 203 with methyl sulfate proceeds on nitrogen with the concomitant shift of the double bond to give 204. Bromination of the active methylene... 

The fact that we have three olefinic hydrogens means that our compound is a primary olefin, the fact that the other two carbons are both methylene carbons means that our substituent, bromine, is terminal. Thus the only possibility we have is that we are dealing with 4-bromo-1-butene (try to find another isomer that fits ). But this simple molecules has a highly complex proton spectrum, which can only be interpreted completely (exact chemical shift, coupling constants) by spectrum simulation. 

Geminal chlorine or bromine substituents deshield vinylic fluorine significantly, whereas a vicinal chlorine substituent shields the fluorine, much as was the case for the saturated systems. Again similarly, a second vicinal chlorine substituent reverses the trend and shifts the fluorine signal downfield (Scheme 3.45). 

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