Bromine addition to olefins

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. 

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-... 

Instead of a bromonium ion, in certain cases an isomeric acyclic and sufficiently stable cation can occur as an intermediate of the bromine addition to olefins. This holds true for bromine-containing benzyl cations. Therefore, the bromine addition to (3-methyl styrene shown in Figures 3.5 and 3.6 takes place without stereocontrol. 

The bromonium ion concept does rationalize the stereochemistry of bromine additions to double bonds very satisfactorily if it is recognized that olefins that can form highly stabilized carbocations need not form such a structure. As al-ready noted, nonconjugated olefins give predominantly anti addition. Cornu-gated olefins, in which the intermediate carbocation would be stabilized by Resonance- however, give a mixture of svn and anti adducts. 

A comparable study of bromine addition to steroidal A -derivatives with varied substitution at C(3> and/or C(x ) also revealed rate retardation by electronegative substituents [103], and an analysis of data by the Taft method [104] yielded a reaction constant in the range —2.0 to — 2.7f. Although the Hammett and Taft reaction constants ( and q ) are not strictly comparable, these values indicate a similarity of reaction mechanism between the two types of olefin. Since chlorination in saturated aliphatic systems is believed to involve chloronium ions rather than classical carbonium ions, we would expect chlorine addition to A -steroids to produce a -value not greatly different from bromination. This does not appear to have been studied. 

Addition to olefins. Depending upon the polarity of the solvent, bromine azide adds to olefins by ionic or free radical processes. Ionic additions are achieved in nitromethane-methylene chloride free radical addition predominates in pentane ... 

Hassner1 has reviewed the addition of bromine azide to olefins. 

Electrophilic additions also occur to cumulated dienes (allenes). Caserio and co-workers established that the mechanisms of both the oxy-mercuration reaction and the electrophilic addition of bromine to allenes are similar to the corresponding additions to olefins. The oxymercuration of ( )-(—)-2,3-pentadiene (82) in methanol produced 83% of (S)-traMS-3-acetoxy-mercuri-4-methoxy-2-pentene (83), confirming the anti pathway for the addition. Also formed was 17% of cis-3-acetoxymercuri-4-methoxy-2-pentene (84), which was presumed to have the R configuration. As shown in Figure 9.48, the product ratios can be explained on the basis of preequilibrium formation of mercurinium ions resulting from attachment of mercury to either the top or the bottom of one of the double bonds. Subsequent rate-limiting attack of methanol on the mercurinium ions is easier for the pathway... 

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. ... 

Additions of halogen fluorides to the more electrophilic perfluonnated olefins generally require different conditions Reactions of iodine fluoride, generated in situ from iodine and iodine pentafluoride [62 102 103, /05] or iodine, hydrogen fluoride, and parapeiiodic aud [104], with fluormated olefins (equations 8-10) are especially well studied because the perfluoroalkyl iodide products are useful precursors of surfactants and other fluorochemicals Somewhat higher temperatures are required compared with reactions with hydrocarbon olefins Additions of bromine fluoride, from bromine and bromine trifluonde, to perfluonnated olefins are also known [lOti]... 

Olefins react with bromine by addition of the latter to the carbon-carbon double bond. In contrast the Wohl-Ziegler bromination reaction using N-bromosuccinimide (NBS) permits the selective substitution of an allylic hydrogen of an olefinic substrate 1 by a bromine atom to yield an allylic bromide 2. 

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... 

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