Bromonium complex

At higher concentrations of halide (>0.02 M) in acetic acid it was found by Walker and Robertson that the rate appears to be third-order over-all and second-order in Br2. This can be interpreted, in this poorly ionizing solvent, as a Brircatalyzed ionization of the olefin-Br2 complex [step 3, Eq. (XVI.6.3)] to produce the more stable Br and the cyclic bromonium complex. In similar fashion it is likely that LiBr and LiCl may have catalyzed this step in the work reported by Ogg and Nozaki. In view of the difficulty with salt effects in these low-dielectric solvents, great caution should be exercised in interpreting data obtained with them. 

The rate data of Ogg and Nozaki are extremely complex even under their own analysis, and their final rate constants cannot be taken literally. ITiey reported considerable formation of Br in the salt-free systems, as well as marked H2O catalysis. The Br ion formation can be interpreted in terms of an acetic acid attack on the bromonium complex, competing with the Br (or HBr) present in the solution. 

As with non-aromatic sp or sp nitrogens, there are not many examples (Fig. 6) of N... X. ., N complexes where an sp nitrogen acts as the halogen bond acceptor for the halonium ions. Representative examples are the iodonium and bromonium complexes of quinuclidine (GEXGOF and BUWMOV, respectively) and the iodonium complex of hexamethylenetetramine (HTMA, refcode HMTITI). 

In summary, it appears friat bromination usually involves a charge-transfer complex which collapses to an ion-pair intermediate. The cation can be a carbocation, as in the case of styrenes, or a bromonium ioiL The complex can evidently also be captured by bromide ion when it is present in sufficiently high concentration. 

This scheme represents an alkyne-bromine complex as an intermediate in all alkyne brominations. This is analogous to the case of alkenes. The complex may dissociate to a inyl cation when the cation is sufficiently stable, as is the case when there is an aryl substituent. It may collapse to a bridged bromonium ion or undergo reaction with a nucleophile. The latta is the dominant reaction for alkyl-substituted alkynes and leads to stereospecific anti addition. Reactions proceeding through vinyl cations are expected to be nonstereospecific. 

In reaction with an alkene, initially a three-membered ring Lewis acid/Lewis base-complex 5 is formed, where the carbon-carbon double bond donates r-electron density into the empty p-orbital of the boron center. This step resembles the formation of a bromonium ion in the electrophilic addition of bromine to an alkene ... 

Quantitative information about the equilibrium between olefin and Br2 on the one hand and CTC s and bromonium ion species on the other (Scheme 4) has been obtained by the already mentioned UV-Vis spectrophotometric study of the adamantylideneadamantane Br2 system (ref. 10). The spectrophotometric UV-Vis data of a large set of solutions of different reagents concentrations have been used to dissect, using a program based on NLLSQ fitting procedures, the complex spectra in those of the single species present at the equilibrium, as shown in Figure 5. 

These observations are explainable by a pathway in which one end of a bromine molecule becomes positively polarised through electron repulsion by the n electrons of the alkene, thereby forming a n complex with it (8 cf. Br2 + benzene, p. 131). This then breaks down to form a cyclic bromonium ion (9)—an alternative canonical form of the carbocation (10). Addition is completed through nucleophilic attack by the residual Br (or added Ye) on either of the original double bond carbon atoms, from the side opposite to the large bromonium ion Br , to yield the meso dibromide (6) ... 

A bridged intermediate exactly analogous to a bromonium ion cannot be formed as H has no electron pair available, but it may be that in some cases a n complex (21) is the intermediate. We shall, however, normally write the intermediate as a carbocation, and it is the relative stability of possible, alternative, carbocations (e.g. 23 and 24) that determines the overall orientation of addition, e.g. in the addition of HBr to propene (22) under polar conditions ... 

Early interest of theoretical chemists in bromonium ions was focused on two aspects are they bridged or open and do they resemble n- or c-complexes ... 

The question of bridged and/or open intermediates has been considered in Section 4, where the data on kinetic substituent effects were discussed with the help of the multipathway scheme (Scheme 7) to determine the relative importance of bromonium and carbocation paths. It is not straightforward to obtain significant p-values for each of them from the complex pa relationship (34) and (35) corresponding to this scheme for an a,/f-Ri,R2... 

Indeed, more intermediates are involved in alkenes bromination than were previously considered. The first intermediates formed in the early steps are the alkene-halogen molecular complexes, whose ionization gives the corresponding bromonium or p-bromocarbenium bromide (tribromide) ion pairs. (2) The reversibility of the ionization step has been widely discussed in the last years... 

In contrast, in protic solvents and at low bromine concentration, the addition process is characterized by a second order rate law (first order in bromine), Scheme 2, path b. In this case, due to the ability of the solvent to provide a specific electrophilic solvation to the leaving bromide ion, the reaction occurs via an SN1 -like unimolecular ionization of the 1 1 it complex to form a bromonium or P-bromocarbenium bromide ion pair. It is worth noting that protic solvents can also give nucleophilic assistance, depending on their specific solvent properties. 

One of the possible ways to stabilize the amine-halonium complexes is to increase the basicity of the amine, bearing in mind that an appropriate one must also not have easily removable P-hydrogens which will lead to oxidation of the amine and formation of an imine. Quinuclidine (pKa of quinuclidinium ion is 11.3 (55)) is 105-106-fold more basic than the pyridines and both the bromonium (10 (36)) and iodonium (11 (57)) BF4 salts have been made and characterized by X-ray crystallography. Interestingly, although the reaction must generally occur as outlined in Figure 7, neither of these ions shows any observable reaction... 

The kinetics of the reaction of bis(iym-collidine)bromonium triflate (17) with adamantylideneadamantane (12), pent-4-en-l-ol (20), and cyclohexene (22) have been investigated in 1,2-dichlorethane at 25 °C under a variety of conditions (Scheme 2). The rates of all the reactions proved to be depressed by added collidine, indicating that the first step for all is a reversible dissociation of (17) into free collidine and a reactive intermediate (18), which is then captured by the alkene. The product of the reaction of (12) with (18) is complex (19), while that of reaction of (20) is... 

The formation of the ionic intermediates, long considered to be irreversible, has been shown to be reversible.229-231 The 1 1 complex (23) dominating in protic sol-vents may be converted to 24 in a solvent-assisted process. The ionic intermediate was first postulated as a cyclic bromonium cation by Roberts and Kimball.234 This was later supported by stereochemical results indicating nearexclusive anti addition stereochemistry,111 by kinetic studies,235 and more recently... 

---