Bromine cations

These solutions are brown in color and have a strong absorption at 300 nm with a shoulder at 375 nm. Solutions of Br3+ can also be obtained in a similar way in 

Glemser and Smalc798 have prepared the compound Br 1 AsFfi by the displacement of oxygen in dioxygenyl hexafluoroarsenate by bromine [Eq. (4.196)] and by the reaction of bromine pentafluoride, bromine, and arsenic pentafluoride [Eq. (4.197)]. The compound is chocolate-brown and in solution has absorption bands at 310 nm and 375 nm it has fair thermal stability and can be sublimed at 30-50°C under nitrogen atmosphere. 

Christe et al.799 have obtained the crystal structure of the Br3 hAsF6 salt and showed that it contains discrete ions with some cation-anion interactions. The Br3+ cation is symmetric with a bond distance of 2.270 A and a bond angle of 102.5°. 

Br21 Ion. The Br2+ cation 329 can be prepared797 by oxidation of bromine by S206F2 in the superacid HSO3F—SbF5—3S03 however, even in this very weakly basic medium, the Br2+ ion is not completely stable because it undergoes appreciable disproportionation [Eq. (4.198)]. 

Moreover, the BrOS02F that is formed also undergoes some disproportionation by itself to Br2+, Br3+, and Br(0S02F3)3, [Eqs. (4.199) and (4.200)], and the equilibria in these solutions are quite complex. 

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The isoxazole nucleus is also halogenated in the 4-position by N-bromosuccinimide provided there is no substituent in this position. This reaction does not proceed homolytically, as might have been expected, and appears to represent a simple electrophilic substitution by the bromine cation. Similar cases have been previously described for the bromination of certain aromatic compounds with A -bromo-succinimide. ... 

In contrast, 13C nmr spectra of ring-substituted /J-bromocumyl cations [2] can be unambiguously interpreted in terms of open /1-bromocarbocations, since the ring substituent effects on the chemical shifts are similar to those on the corresponding non-brominated cations [1], even for the electron-attracting p-trifluoromethyl group (Olah et al, 1972). 

To generate the title bromo allene 1 the -enyne was treated with a bromine cation. According to a procedure described by Murai the reagent 2,4,4,6-tetrabromocyclohexadienone (TBCO) 67 is used as the bromine source.20... 

Dichloroselenuranes or dibromoselenuranes such as 194 can be used for the conversion of alcohols into the corresponding halides.336 Diphenyldibromoselenurane 195 has been found to be an efficient source for bromine cations (Scheme 57).337... 

Reaction of pentafluorophenol with rerf-butyl hypobromite starts as an electrophilic substitution in the benzene ring. The electrophile is formed by dissociation of tert-butyl hypobromite to tert-butoxy anion and bromine cation. The bromine cation attacks the para position and forms a positively charged Wheland complex, a nonaromatic species that is converted by ejection of proton from the phenolic hydroxyl to a quinon-oid compound, 4-bromopentafluorocyclohexa-2,5-dienone [39]. 

Parallel i/wo-ni (ration at position 4 and nitration at position 5 of the pyrazole ring were also observed during the action of the sulfuric-nitric acid mixture on 1,3-dim-ethyl-4-bromopyrazole [369], The released bromine cation partly brominates the initial reagent with the formation of l,3-dimethyl-4,5-dibromopyrazole. The latter is subsequently transformed into l,3-dimethyl-5-bromo-4-nitropyrazole (see the Scheme later). The nitration of l,3-dimethyl-4,5-dibromopyrazole leads to the formation of l,3-dimethyl-5-bromo-4-nitropyrazole and l,3-dimethyl-4-nitropyrazole [369] (Scheme 48). 

The reaction of bromine with toluene in the presence of iron(III) bromide (equation 2) proceeds along an entirely different reaction pathway. Here, the actual reagent turns out to be neither molecular bromine nor atomic bromine. In this case, under the action of iron(III) bromide, molecular bromine is converted into a bromine cation and an anionic complex, [FeBrJ , as is shown in Scheme 2.5. This is a reversible process and the stationary concentration of the bromine cation can be rather low (as in the former case of atomic bromine). It is, nevertheless, sufficient to make Br a reactive partner. 

The methyl hydrogens of toluene are relatively inert to the attack of these charged particles. In contrast, the polarizable 7t-electron system of the aromatic nucleus can be easily perturbed by the approach of the bromine cation. In a simplified form, their interaction can be described in the sequence of the following steps. The attacking cation pulls an electron pair of the aromatic system towards itself to form a C-Br bond. A concerted shift of the next electron pair leads to the development of a positive charge on the methylbearing para-carbon atom. Loss of a proton from this c-complex 13 leads to the formation of bromotoluene with restoration of the aromaticity in the system. Besides the para isomer of bromotoluene 8, the corresponding ortho isomer is also formed, but in lesser amounts. 

The initial attack always occurs on the carbon at one end of the conjugated system, and so in this case it occurs at a terminal carbon. Consider the structure of the carbonium ion that would result if the bromine cation had attacked one of the non-terminal carbons, and so suggest a reason why the terminally substituted carbonium ion is favoured. 

Bromine acts as an electrophile. The partial charge becomes a full charge and a bromine cation attaches to the double bond, leaving a Br ion behind. The cyclic intermediate is called a bromonium ion. 

The electrophilic bromine cation can also be generated from A -bromosuccinimide (NBS). When 2-methylene-6,6-dimethylbicyclo[3.1.0]hexane with an ester function at the bridgehead position was treated with NBS in diethyl ether the electrophilic attack took place at the terminal position of the C—C double bond and the cyclopropane ring was opened to give a cyclopentene derivative. Due to the lack of a reactive nucleophile, a proton was eliminated. The reduction of the ester function did not change the course of this reaction. With tert-hvAy hypochlorite, the corresponding chloro product was obtained. ... 

Bromination of alkyl-substituted methyl 2-(trimethylsiloxy)cyclopropanecarboxylates 1 at low temperature (—78 C) provided methyl 2-bromo-4-oxoalkanoates 2 in high yields.The trimethylsilyl group was eliminated as bromotrimethylsilane in the course of the cleavage reaction. The products 2 had a strong tendency to eliminate hydrogen bromide to form conjugated double-bond systems.It is assumed that an electrophilic bromine cation attacks at the acceptor-substituted cyclopropyl carbon atom. 

HS03F-S03-SbFs Used to enhance the stability of bromine cations. See HSO3F. 

Bromates, dinitrato-, 319 Bromine anions BrFj-.SlO Bromine cations Br3+,314 BrF4+,315 BrF6-",316 C1F2+,314... 

The bromine cations are even more electrophilic than the iodine ones. Therefore, still stronger acidic solutions are required for their stabilization. The Br2 cation, simi-... 

If we take the addition of hydrogen bromide to an alkene as a model for the alkene halogenation reaction, we can build a mechanism quite quickly. Just as the alkene reacts with hydrogen bromide to produce a carbocation and bromide (Br ), so reaction with bromine-bromine might give a brominated cation and bromide (Fig. 10.9).The n bond of the alkene acts as a nucleophile, displacing Br from bromine. Addition of bromide to the carbocation would give the dibromide product. 

Figure 12-3 Electron-pushing picture of cyclic bromonium ion fonnation. The alkene (red) acts as a nucleophile to displace bromide ion (green) from bromine. The molecular bromine behaves as if it were strongly polarized, one atom as a bromine cation, the other as an anion. (B) Orbital picture of bromonium ion formation.

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