Bromine to olefins

A cyclic complex derivable by addition of positive bromine to a double bond has already been discussed as an example of the neighboring group effect. The same intermediate, except perhaps for the nature of the bonds to bromine, is formed in the addition of bromine to olefins and is responsible for the stereochemistry of the addition reaction and the nature of the by-products.232... 

One example of selective photochemistry with lasers is the gasphase photochemical addition of bromine to olefin molecules induced by the monochromatic light near 6940 A from a pulsed, tunable ruby laser, as studied by Tiffany... 

Several acetylenic derivatives are prepared by dehydrohalogenation of vicinal dibromo compounds, which are obtained by adding bromine to olefinic compounds. 

A typical example of a stereospecific olefin addition reaction is the addition of bromine to olefins. If d.v-2-pentene is used as the substrate, only the 2R,3R and 2S,3S pair will be produced (they are enantiomers). 

This process is quite common for carbon-centered free radicals because die carbon-carbon a bond which is formed is stronger by about 30 kcal dian die n bond which is broken. Other radical species, however, are well known to undergo olefin additions as well. The addition of bromine to olefins is die key step in the anti-Markovnikov addition of HBr to olefins. 

It is clear that the mechanism in Scheme 25 parallels (at least from the qualitative point of view) the mechanism of the addition of bromine to olefins shown in Scheme 11. Kinetic investigations indicate that the oxymercuration reaction involves a series of fast equilibria until the mercuronium ion (53) is formed. The subsequent nucleophilic attack of the solvent is probably the rate-limiting step, as indicated by steric requirements in bulky alkenes111. In the bromine addition, the formation of the bromonium ion is the rate-limiting step (or the rate-limiting equilibrium). However, the olefin reactivities in both reactions (bromination and oxymercuration) are identical when steric effects in the TS of the two addition reactions are taken into account110. 

We referred above to the large substituent effect on the heterolytic addition of bromine to olefins, which is a polar process actually involving intermediate ionic species. By contrast, the addition of alkyl radicals to olefins involves... 

Here the reaction is assisted by conjugative and hyperconjugative effects, and is retarded by the inductive effect which withdraws electrons from the reaction site. A similar effect is found in the addition of bromine to olefins (RCH=CH2) in acetic acid at 25°, viz-... 

The addition of chlorine and bromine to olefins is a very general reaction. 

Addition of bromine to olefins was effected using tetrabutylammonium tribromide in chloroform solution (Eq. 21).126 The expected z ic-dibromides are formed quantitatively in all the cases tested and the reactions are extremely rapid. The short reaction times can explain the absence of decomposition of the dibromides. The conventional addition normally proceeds via an electrophilic attack of the double bond, but the sonochemical mechanism was not studied. 

A -Bromosuccinimide (prepared by the action of bromine on succinimide at o in the presence of sodium hydroxide) is a valuable specific reagent for brominating olefines in the a-methylene position to the double bond without simultaneously adding bromine to this bond. For example, if A -bromosuccini-mide is represented by (C4H40t)NBr —... 

Loaded Adsorbents. Where highly efficient removal of a trace impurity is required it is sometimes effective to use an adsorbent preloaded with a reactant rather than rely on the forces of adsorption. Examples include the use of 2eohtes preloaded with bromine to trap traces of olefins as their more easily condensible bromides 2eohtes preloaded with iodine to trap mercury vapor, and activated carbon loaded with cupric chloride for removal of mercaptans. 

Alkyl fluorides have been prepared by reaction between elementary fluorine and the paraffins, by the addition of hydrogen fluoride to olefins, by the reaction of alkyl halides with mercurous fluoride, with mercuric fluoride, with silver fluoride, or with potassium fluoride under pressure. The procedure used is based on that of Hoffmann involving interaction at atmospheric pressure of anhydrous potassium fluoride with an alkyl halide in the presence of ethylene glycol as a solvent for the inorganic fluoride a small amount of olefin accompanies the alkyl fluoride produced and is readily removed by treatment with bromine-potassium bromide solution. Methods for the preparation of alkyl monofluorides have been reviewed. ... 

In related work fluorine reacts with iodine in fluorotnchloromethane, and the iodine lluonde thus fonned adds the elements of iodine and fluorine to olefins at -78 °C with full regio and stereoselectivity [67] Bromine-fluonnc on the other hand. 

Azaloxan (12) is an antidepressant agent. Its synthesis can be accomplished starting with the reaction of catechol (7) and 3,4-dibromobutyronitrile (obtained by addition of bromine to the olefin) to give l,4-benzodioxan-2-ylacetonitrile (8). A series of functional group transformations ensues [hydrolysis to the acid (9), reduction to the alcohol (10) and conversion to a tosylate (11)] culminating in an SN-2 displacement reaction on tosylate 11 with l-(4-piperidinyl)-2-imidazolidi-none to give azaloxan (12) [3]. 

Dipolar cycloadditkm reactions of nitrones to olefins, 46, 1,3-Dipolar cycloadditions with 3-phenylsydnone, 45, 98 Dispiro[5 1 5 l]tetradecane-7,14-dione, photolysis to cyclohexylidene-cyclohexane, 47, 34 preparation from cyclohexanecarbonyl chlonde and triethylamine, 47,34 Displacement of bromine from 1-bromo-2-fluoroheptane to give 2-fluoro-heptyl acetate, 46, 37... 

This symposium addressed several important issues in bromine chemistry. A major part has been devoted to stereochemistry and mechanism of electrophilic bromination of olefins. Other topics included new selective methods of bromination and oxybromination, brominations in presence of solid supports and catalysts, organobromine compounds as synthons, recent developments in brominated fire retardants and toxicological and environmental aspects of brominated compounds. 

The generally accepted mechanism for electrophilic bromination of olefins in hydroxylic solutions of medium to high polarity, and at low [Br2], is given in equation 1 (ref. 1). 

Apart from a few studies (ref. 7), the use of deuterium kinetic isotope effects (kie s) appears to have had limited use in mechanistic studies of electrophilic bromination of olefins. Secondary alpha D-kie s have been reported for two cases, trans-stilbene fi and p-substituted a-d-styrenes 2, these giving relatively small inverse kie s of... 

If return occurs during the bromination of cw-stilbenes and rotation around the C-C bond is faster than collapse of the intermediates to dibromides, this process will lead to fra j-stilbene (Scheme 9). We used this test to check the possibility of return in the bromination of unsubstimted, 4-methyl, 4-trifluoromethyl-, and 4,4 -bis(trifluoromethyl)-stilbenes in DCE (ref. 24). All these olefins gave clean third-order rate constants spanning 7 powers of 10. For each cis-trans couple the cis olefin was brominated 3.5 to 5.5 times faster than the trans isomer. Reactions for products analysis were performed at initial molar ratios of Br2 to olefin of 1 to 2, so that products arose only from the cis olefin, the trans isomer being accumulated in the reaction medium. 

Tetrazolium ylides are quite reactive and are easily alkylated.168 The mesoionic tetrazolium thiolate 117 readily adds bromine to yield 174 which can then react with a number of active methylene compounds to give mesoionic compounds, e.g., 175.293,294 They also undergo 1,3-dipolar cycloaddition with olefins and acetylenes to yield bicyclic tetrazolo-thiazolines... 

Syntheses of the 1,2- and the 3,4-dihydrodiols of MBA via Methods II and IV have been described (74). The 1,2- and 3,4-diol dibenzoates of 1,2,3,4-tetrahydro-MBA prepared from MBA via the Li/-NH reduction route were readily separable by crystallization. Introduction of the olefinic bond into the 1,2-position of the 3,4-diol dibenzoate by the usual bromination-dehydrobromination procedure was complicated by the greater facility of bromination by NBS on the methyl group than the 1-position. This problem was solved (Figure 14) by allowing bromination to proceed to the dibromo stage, followed by selective reduction of the bromomethyl group with NaBH in diglyme. The monobromo derivative underwent dehydrobromination... 

Since bromine addition to olefins leads to brominated compounds of synthetic interest, there are many studies of bromination products in the literature. Typical examples have been reviewed by Schmid and Garratt (1977). However, there are few systematic product analysis studies related to the role of the structure and the solvent in determining bromination selectivities. 

The cyclic bromonium ion was first used to explain the stereochemistry and course of the bromination of olefins and later proposed as an intermediate in the displacement reaction 232 288 Y... 

But catalytic reduction of the same phenyl propionic acid gives cis cinnamic acid. Therefore by adding hydrogen under various conditions, one can obtain a desired isomer. The conversion of acetylene into olefinic compounds has been carried out not only for the sake of obtaining the adduct, but Michael studied the various addition reactions for the sake of obtaining a desired product cis or trans. For example, he found that the addition of bromine to acetylene-dicarboxylic acid leads predominantly to the formation of trans isomer. 

It is noteworthy that the nature of the ionic intermediate formed in bromine addition to olefins and the solvent properties also govern the competition between nucleophilic trapping and elimination. Thus 1,1-diphenylethylene, 11, gives the corresponding dibromide 13 (or solvent incorporated products, 14) and vinyl bromide, 12, in a ratio changing from 99 1 to 5 95 depending on solvent and on bromine concentration.(20) (see Table III results)... 

Lifetimes of the ionic intermediates of nucleophilic substitution are generally correlated to the pathways followed under given reaction conditions. Information on the lifetimes of ionic intermediates formed by bromine addition to olefins in methanol, as determined by the azide clock method, do not allow the different reaction pathways to be distin-... 

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