Addition of bromine to a double bond

Selective monoprotection of dienes. Since the rate of addition of bromine to a double bond increases with substitution, monoprotection is possible. The selectivity is higher with pyridinium hydrobromide perbromide than with bromine, especially at low temperatures (- 60°). The dibromide can be deprotected by cathodic reduction at the mercury cathode in DMF. 

Addition of bromine to a double bond has been observed in several cases. The hydrolysis of the resulting dibromide leads to a substance which may be regarded as an oxidation product of the original unsaturated compound. Fischer and Dangschat prepared glycolic aldehyde triacetate by the action of bromine on vinyl acetate and treatment of the resulting dibromide with acetic acid. 

The addition of bromine to a double bond occurs in two steps, the first of which forms a bridged bromonium ion intermediate. This intermediate accounts for the fact that geminal, rather than vicinal, addition occurs, and that the addition is anti. 

The mechanism for the addition of oxygen to a double bond to form an epoxide is analogous to the mechanism described in Section 4.7 for the addition of bromine to a double bond to form a cyclic bromonium ion. In one case the electrophile is oxygen, and in the other it is bromine. So the reaction of an alkene with a peroxyacid, like the reaction of an alkene with Br2, is an electrophilic addition reaction. 

To the best of our knowledge, the hrst paper which mentioned an A-(l-haloalkyl)pyridinium compound appeared 66 years ago in the Chemische Berichte (Krohnke 33CB1386). Tlie author described the reaction of phenacyl pyridinium derivatives 1 with bromine in acetic acid to give the halides 2 (36CB2006 37CB864). Tire addition of bromine to the double bonds of A-vinylpyridinium salts 3 and 4 giving the adducts 5 and 6 has also been reported (51CB399) (Scheme 1). 

Benzotriazol-l-yl)methyl]triphenylphosphonium chloride 848 reacts with BunLi and aldehydes to give l-(alken-l-yl)benzotriazoles 849. Addition of bromine to the double bond of derivatives 849 followed by a reaction with amines furnishes amides 850. A variety of primary or secondary amines can be used. This way aldehydes are conveniently homologated and converted to amides with a one-atom longer chain (Scheme 136) <2004ARK(ix)44. 

In the examples of electrophilic substitution of vinylsilanes described rove, collapse of the carbon-silicon bond to form a carbon-carbon 7t bond faster than nucleophilic attack on the carbocation by the anionic counterion l the electrophile. This is not always the case. For example, on treatment of -l-trimethylsilylpropene with bromine followed by aqueous ethanol, (Z)--bromopropene is formed almost exclusively (Figure Si5.10). This is pnsistent with anti addition of bromine to the double bond followed by anti limination of trimethylsilyl bromide. 

NBS has proved to be especially useful in brominations at allylic positions because competition from the addition of bromine to the double bond is not a problem. Apparently, the fact that only a low concentration of Br2 is ever present in NBS brominations somehow inhibits the addition reaction. Examples are provided in the following equations. Note that the reaction is best if only a single type of allylic hydrogen is available to be abstracted. In addition, the resonance-stabilized allylic radical provides two sites that can abstract a bromine atom. If these two sites are different, a mixture of products is formed, as shown in the second example ... 

The kinetics of halogenation has been the object of special study. Calculations showed that the addition of iodine to a double bond should proceed nearly as rapidly as the addition of bromine. It is known that under ordinary conditions, iodine will not add to unsaturated compounds, but that in many cases bromine will. 

It can be concluded from these calculations that the addition of halogen to a double bond to form the symmetrical dihalide (a) is much easier than the substitution reaction (c). The former requires activation energies of the order of 25,000 calories while the latter requires energies of the order of 50,000 calories. The reaction rate (a) is approximately the same, as already discussed for all the halogens, chlorine, bromine, and iodine. It is evident that the activation energies are approximately the same for the atom-catalyzed chain reaction and for the bimolecular reaction. Ap-... 

The addition of bromine across a double bond proceeds with attachment of each bromine atom to opposite faces of the starting olefin. In the case of the present example, the products are illustrated below. Please note that the two illustrated products are enantiomers of one another. 

Addition of bromine to the double bond of 3-piperideines (preferably in a solution of hydrobromic acid or acetic acid) is often used to... 

Other closed voids (cavities) have been found (in addition to small channels along a, b, and c) in the crystals of ewdo-bicyclo[2.2.2]oct-5-ene-2,3-dicarboxyanhydride (P2j/c), the Diels-Alder adduct of 1,3-cyclohexadiene and maleic anhydride [1]. The voids extend as tall three-axis ellipsoids under the skew faces of about (01 10 ) and (118). These crystals include up to 0.4 equivalents of diluted bromine vapor without melting. No addition of bromine to the double bond occurs, as no large channels and no cleavage planes are available for anisotropic migrations in the 3D-interlocked structure. The bromine can be evaporated in a vacuum at 20-50 °C without having attacked the double bond of the host [1]. It can enter into the crystal and exit via the small channels. Numerous crystallographic closed voids are to be expected in various crystals. 

The first step is the photolysis of the azide giving a nitrene which forms a dihydrobenzazirine 13 by a [1+2] cycloaddition with the arene the latter isomerizes yielding 1//-azepine. An alternative synthesis of l//-azepines 14 is based on bicyclic aziridines 15, which are transformed to 14 by addition of bromine to the double bond, two-fold dehydrobromination and valence tautomerization of the resulting 13. The bicyclic aziridines 15 are available from cyclohexa-1,4-dienes via l-alkoxycarbonylamino-2-iodocyclohexenes 16 and their SNi-cyclization [15]. 

The addition of bromine to the double bond is useful as a simple chemical test for alkenes. Brj is red while the product of its addition to an alkene is colorless. This allows one to easily detect by a color change whether some unknown sample is an alkene. If the red color of bromine persists when bromine is added to an unknown, the unknown does not contain a double bond. If the red color disappears, the unknown contains a double bond. 

In this section, we consider the determinations based on the addition of bromine on double bonds. Most of the time, these addition reactions take place in nonaqueous media. We limit ourselves to recall the notions of iodine value and fat indices. Some drugs are determined by the addition of bromine on a double bond that their structures contain. We ll first consider cyclobarbital and hexobarbital, which are derived from barbituric acid ... 

Since we start with achiral starting materials, all products (if chiral) are racemates (Section 5-7). If the first step of addition of bromine to the double bond had given a carbocation, the bromide ion released in this process could have attacked the positively charged carbon atom from either side, resulting in both syn and anti addition products, which is not observed. 

Halogen-substituted succinimides are a class of products with important appHcations. /V-Bromosuccinimide [128-08-5] mp 176—177°C, is the most important product ia this group, and is prepared by addition of bromine to a cold aqueous solution of succinimide (110,111) or by reaction of succinimide with NaBr02 iu the presence of HBr (112). It is used as a bromination and oxidation agent ia the synthesis of cortisone and other hormones. By its use it is possible to obtain selective bromine substitution at methylene groups adjacent to double bonds without addition reactions to the double bond (113). 

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... 

Bromination and chlorination of alkanes and cycloalkanes can also take place by an electrophilic mechanism if the reaction is catalyzed by AgSbF. "2 Direct chlorination at a vinylic position by an electrophilic mechanism has been achieved with benzenescleninyl chloride PhSe(0)Cl and AIC13 or AIBr3. n However, while some substituted alkenes give high yields of chloro substitution products, others (such as styrene) undergo addition of Cl2 to the double bond (5-26).113 Electrophilic fluorination has already been mentioned (p. 690). 

Bromination of allylic positions cannot be achieved specifically by using elementary bromine, unless electrophilic addition to the rc-bond (Eq. 18) is unfavorable because the substituents have a high negative inductive effect. Efficiency of electrophilic addition of Br2 may also be diminished by steric effects. In addition to these secondary thermal reactions, the specificity of photochemical brominations of allylic positions using Br2 will also suffer from the competing (radical) addition of Br to the double bond (Eq. 19) [31]. 

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