Bromination olefins

Many mechanistic results on this electrophilic addition are available but most of them deal with the first steps of the reaction in which the ionic intermediate is formed, rather than with the last steps in which the products are obtained by nucleophilic attack on this intermediate (ref. 2). The present paper reports results on the selectivity of olefin bromination, which have been obtained more or less systematically with a view to improving the existing rules which are too naive to be useful in synthesis (ref. 3). 

The use of secondary deuterium kinetic isotope effects in mechanistic studies of olefin bromination... 

Excluding free radical bromination processes, a schematic picture of the mechanistic pathways involved in olefin bromination is shown in Scheme 1. 

First of all, the reaction pathways shown in Scheme 1 involve the formation of charge transfer complexes (CTC) between olefin and Br2- The formation of molecular complexes during olefin bromination had been hypothesized often (ref. 2), but until 1985, when we published a work on this subject (ref. 3), complexes of this type had been observed only in a very limited number of circumstances, all of which have in common a highly reduced reactivity of the olefm-halogen system, i.e. strongly deactivated olefins (ref. 4), or completely apolar solvents (ref. 5) or very low temperatures (ref 6). 

Of course the observation of olefin-Br2 CTCs in solutions of olefins and Bt2 does not necessarily mean that these are essential intermediates in olefin bromination. However the above thermodynamic and kinetic parameters allowed us to answer the question of the mechanistic role played by the CTCs. In fact, if they were unreactive species whose only effect is to reduce the concentration of the actual reactants, Scheme 2 would be valid. The observed k3 would be given by... 

In these solvents at sufficiently low Br2 concentration (< 10-3 m) the kinetics are first order both in the olefin and in Br2 and the main solvent effect consists of an electrophilic solvation of the departing Br ion. A nucleophilic assistance by hydroxylic solvents has also been recognized recently (ref. 26) (Scheme 10). So far, return during the olefin bromination in methanol had been admitted only for alkylideneadamantanes, and was ascribed to steric inhibition to nucleophilic attack at carbons of the bromonium ion (ref. 26). 

In halogenated solvents, olefin bromination is second-order in bromine... 

After these preliminary observations, the problem was to establish whether these CTCs are essential intermediates in olefin bromination (Scheme 3). Are... 

To summarize, when the kinetic data predict that only bromonium ions or only bromocarbocations are formed, the bromination products are obtained stereospecifically and regiospecifically, respectively, whatever the solvent. Olefin brominations involving open intermediates lead to more solvent-incorporated products in methanol or acetic acid than those involving bridged ions. This chemoselectivity can be interpreted in terms of the hard and soft acid and base theory (Dubois and Chretien, 1978). Methanol assistance to intermediate formation also plays a role in determining product-selectivity (Ruasse et al, 1991). 

The stereospecificity observed in olefin bromination is only possible if the inherent facial relationship of the olefinic bond is maintained throughout the addition process and only one bromine atom adds to each face. In bromination, the electrophilic addition leads to a bridged bromonium ion which not only maintains the initial olefin geometry but also forces the second bromine to add from the opposite direction (anti). 

Using the UV/VIS spectrophotometric method, the equilibria of Scheme 12 have been investigated93 as a model of the bromonium tri- and pentabromides, the intermediates of the reactions of olefin bromination. 

Bromonium ion is generated by reactions of both erythro (38) and threo 2-bromo-1,2-diphenylethane with hydrogen bromide95 (in 1,2-dichloroethane and in chloroform) as illustrated by Scheme 15. Stilbene (39) and the 1,2-dibromo derivative are the main reaction products. Product distribution and kinetic investigations on the bromination of stilbenes clearly confirm the reversibility of the bromonium/bromide formation in bromination of olefins, through an olefin/bromine CT complex. 

The chemoselectivity of olefin bromination is reported84 to occur after the attack of the bromine on the double bond, but the formation of the bromonium ion is the slow step of the reaction. As a consequence, the distribution of products and the selectivity of addition of nucleophiles can hardly be explained by substituent effects (both steric and electronic) bonded to the C=C double bond in a fast step of the reaction. 

M. F. Ruasse, Bromonium Ions or 3-Bromocarbocations in Olefin Bromination. A Kinetic Approach to Product Selectivities, Acc. Chem. Res. 1990, 23, 87-93. 

Once again a good deal of bromine chemistry is covered in the section on oscillatory reactions. The species BrJ may be involved in olefin brominations at high concentrations of bromine. The kinetics and equilibria of reaction (58) have been studied, and rate constants for (59) and (60) evaluated. The mechanism of oxidtion of ferrocyanide by bromate involves [Fe(CN)sH20] " as an intermediate. The uncatalyzed and the vanadium(V) catalyzed reaction of bromate with methylene blue have been reported. ... 

Bellucci G, Chiappe C, Marioni F (1987) Evidence for a reversible electrophilic step in olefin bromination. The case of stilbene. J Am Chem Soc 109 515-522... 

Olefins are essentially soft donors, tt Complexation of alkenes with soft metal ions, such as Ag , Pt , and Pd , is well established. The addition of halogens and pseudohalogens is commonly regarded as proceeding via the n complexes, as detected spectroscopically for the olefin-bromine charge-transfer species (24). 

The ion of this type was spectrally proved in the case of the reaction of adamantylideneadamantane with Bt2. The intermediate formation of the cation in olefin halogenation is also evidenced by the fact that halide ions accelerate this reaction. In acetic acid olefin bromination in the presence of Bf or Cf occurs with the rate... 

At a sufficiently high concentration of Br2, olefin bromination in acetic acid occurs as a reaction of the II order with respect to bromine... 

Aliphatic olefins bromine number, electrometric titration p oleum distillates... 

Cationic alkene-Fp complexes are rather stable and easier to handle than their neutral alkene(tetracarbonyl)iron congeners. Due to their positive charge, they are inert towards electrophiles and, thus, can be employed as protecting groups for olefins. Bromination and hydrogenation of other double bonds in the molecule leaves them unaffected. On the other hand they readily react with various nucleophiles such as enamines, enolates, silyl enol ethers, phosphanes, thiols, and amines to give alkyl-Fp... 

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