Alkene hydrogenated bromo

Bromo-l-alkenes. Hydrogen bromide adds to terminal acetylenes to form 1-bromo-l-alkenes. However, it adds to 1-trimethylsilyl-l-alkynes to form 2-bromo-l-alkenes. Although the reaction is a free radical reaction, a peroxide initiator is not required and may be deleterious. Trimethylsilyl bromide is the... 

Another method for preparing alkyl halides from alkenes is by reaction with jV-brotnosuccinimide (abbreviated NBS) in the presence of light to give products resulting from substitution of hydrogen by bromine at the allylic position—the position next to the double bond. Cyclohexene, for example, gives 3-bromo-cyclohexene. 

DL-Valiolamine (205) was synthesized from the exo-alkene (247) derived from 51 with silver fluoride in pyridine. Compound 247 was treated with a peroxy acid, to give a single spiro epoxide (248, 89%) which was cleaved by way of anchimeric reaction in the presence of acetate ion to give, after acetylation, the tetraacetate 249. The bromo group was directly displaced with azide ion, the product was hydrogenated, and the amine acety-lated, to give the penta-A, 0-acetyl derivative (250,50%). On the other hand. 

Disubstituted alkynes and terminal alkynes form E-dibromoalkenes [4] when the tribromide is formed in situ in an essentially basic medium, an addition reaction followed by elimination of hydrogen bromide results in the conversion of terminal alkynes into the 1-bromoalkynes [5]. When the addition reaction is conducted in methanol, l,l-dibromo-2,2-dimethoxyalkanes are produced, in addition to the 1,2-dibromoalkenes [6], The dimethoxy compounds probably result from the initial intermediate formation of bromomethoxyalkenes. Under similar conditions, alkenes yield methoxy bromo compounds [7]. 

It is possible to obtain anti-Markovnikov products when HBr is added to alkenes in the presence of free radical initiators, e.g. hydrogen peroxide (HOOH) or alkyl peroxide (ROOR). The free radical initiators change the mechanism of addition from an electrophilic addition to a free radical addition. This change of mechanism gives rise to the anh-Markovnikov regiochemistry. For example, 2-methyl propene reacts with HBr in the presence of peroxide (ROOR) to form 1-bromo-2-methyl propane, which is an anh-Markovnikov product. Radical additions do not proceed with HCl or HI. 

As a rule, alkynes react in bromo- and iodofluorinations by monoaddition because vinylic a./f-halofluorides as electron-deficient alkenes are generally less reactive. However, some aryl-conjugated alkenes, such as cinnamate or stilbene derivatives 3 or 5, respectively,184 may also be halofluorinated by hydrogen fluoride/pyridine/A-halosuccinimide reagent.185... 

Selected examples are given in Table 9 for bromofluorinations of alkenes using A-bromosuccin-imide in combination with 70% hydrogen fluoride/pyridine (Method A) and hydrogen fluo-ride/polyvinylpyridine (Method B), respectively. Table 10 shows examples of the selective monoaddition of in situ produced halofluorides (70 % hydrogen fluoride/pyridine and A-iodo-. A-bromo- or /V-ehloro-succinimide) to symmetrically substituted alkynes. 

The addition of bromine monofluoride to alkenes using equimolar quantities of /V-bro-moamides in excess anhydrous hydrogen fluoride gives the expected l-bromo-2-fluoroalkanes and small amounts (3-8 %) of isomeric 2-bromo-l-fluoroalkanes.29 1-Bromo-2-fluoroheptane can be prepared in 60-77% yield from hept-l-cnc, /V-bromoacetamide and hydrogen fluoride in diethyl ether at — 78"C.30 The bromofluorination of methyl methacrylate (1) with 1,3-di-bromo-5.5-dimethylhydantoin (DBH) and liquid hydrogen fluoride exemplifies this procedure.31... 

Potassium fluoride-poly(hydrogen fluoride) salts are useful fluorine sources for bromo- and iodofluorinations of various alkenes (Method F).49 The best yields arc obtained with KH3F4 and KH4F5. The salts are prepared by mixing the calculated amounts of potassium hydrogen difluoride and anhydrous hydrogen fluoride in a reaction vessel made of polyethylene or Hastel-loy B. The mixture is stirred and heated at 60-70 C to form a homogeneous solution of the salt which solidifies at room temperature. 

Like alkenes, the double bonds of ,/3-unsaturated acids can be brominated, hydroxylated, hydrated, and hydrobrominated, although the reactions often are relatively slow. In the addition of unsymmetrical reagents the direction of addition is opposite to that observed for alkenes (anti-Markownikoff). Thus propenoic (acrylic) acid adds hydrogen bromide and water to form 3-bromo-and 3-hydroxypropanoic acids ... 

The situation is more complicated in the case of 5-bromohexopyranose derivatives, because products of both endo- and exo-elimination may be formed, and available evidence indicates that, with 5-bromo-/ -D-glucopyr-anose esters, base-catalyzed elimination favors the production of endo-alkenes following loss of axial hydrogen and bromine atoms. Alternatively, treatment with zinc-acetic acid gives, mainly, the products of exo-elimination.90 From the acetate 164 (R = Ac) and the benzoate 164 (R = Bz), the 4-enes (165, R = Ac, Bz) were both obtained in 65% yield following treatment with 1,5-diazabicyclo[5.4.0]undec-5-ene, whereas zinc - acetic acid afforded the 5-enes (166, R = Ac, Bz) in 59 and 67% yield. The isomeric endo products 167 (R = Ac, Bz) were isolated in 15 and 11% yield, and, from the... 

Before discussing examples of the reactions, it should be pointed out that most types of vinylic halides used in this reaction are easily available. The UV-catalyzed addition of HBr to terminal acetylenes forms the 1-bromo-l-alkenes. The cis isomer is formed almost exclusively if the addition is carried out at dry ice temperature. The 2-bromo or 2-iodo-l-alkenes are obtained from reacting aqueous hydrogen halides with alkynes. The 2-substituted-1-bromo-l-alkenes are available by the bromination-base dehydrobromination reactions. 

An entirely different approach to pyranoid 4-enes depends on eliminations from 5-bromo compounds obtained from pyranoid hexuronic acid derivatives by photobromination. Treated with zinc-acetic acid, the bromide 249 gives the glycal -like 250 (62%), while the 4-acetoxy compound 251 is formed when DBU is used to promote elimination. Similarly, base treatment of penta-0-acetyl-5-bromo-/J-D-glucose with DBU causes the analogous loss of hydrogen bromide and formation of the 4-acetoxy-4-ene, but use of zinc-acetic acid affords mainly the 5-exo-methylene alkene by the alternative available elimination process.237... 

The Kopecky method for the formation of 1,2-dioxetanes involves the dehydrohalogenation of /3-halo hydroperoxides and is promoted by the action of a base or a silver salt and was first reported in 1975 <1975CJC1103>. Scheme 18 depicts the general process in which /3-bromo hydroperoxides are prepared by bromination of alkenes with 1,3-dibromo-5,5-dimethylhydantoin (DDH) in the presence of concentrated hydrogen peroxide. 

In the polar reaction, a proton in HBr first adds to the terminal sp2 carbon in isobutene to produce a stable tert-butyl cation (8), and then it reacts with the counter bromide anion to form tert-butyl bromide. Thus, the proton in HBr adds to the less substituted sp2 carbon in alkene to produce a more stable carbocation. This is based on the Markovnikov rule. In radical reactions, the hydrogen atom of HBr is abstracted first by the initiator, PhCO (or Ph ) derived from (PhC02)2, and the formed bromine atom then adds to the terminal sp2 carbon in isobutene to form the stable (3-bromo tert-butyl radical (9), and then it reacts with HBr to produce /so-butyl bromide and a bromine atom. This bromine atom again... 

By addition of bromine at low temperature methyl 2-siloxycyclopropanecarboxylates are regioselectively cleaved to give a-bromo-y-oxoesters 126 quantitatively. These can be isolated in singular cases, but usually they are directly transformed to oe,P-unsaturated y-oxoesters 127 by subsequent elimination of hydrogen bromide with triethylamine 80). Thereby -alkenes are obtained in good overall yields, which are interesting acceptor building blocks for further synthetic operations. 

A chemist allows some pure (2S,3R)-3-bromo-2,3-diphenylpentane to react with a solution of sodium ethoxide (NaOCH2CH3) in ethanol. The products are two alkenes A (cis-trans mixture) and B, a single pure isomer. Under the same conditions, the reaction of (2S,3S)-3-bromo-2,3-diphenylpentane gives two alkenes, A (cis-trans mixture) and C. Upon catalytic hydrogenation, all three of these alkenes (A, B, and C) give 2,3-diphenylpentane. Determine the structures of A, B, and C, give equations for their formation, and explain the stereospecificity of these reactions. 

Fluorine can also be added to alkenes by reaction with a solution of anhydrous hydrogen fluoride in pyridine (Olah s reagent) containing either A-bromo- or Y-iodosuccinimide, or iodine or bromine and silver nitrate. The final addition of silver fluoride to the reaction mixture leads to the formation of vicinal difluorides 19 23133... 

Decarboxylation precedes dehydrohalogenation, however, as noted by analysis of the gas formed in the preparation of 2-butene. Decarboxylation occurs at 20-30° in sodium carbonate solution. Butene is then evolved at higher temperatures. The reaction is important in the preparation of cis and tra s-2-alkenes from cis- and trans-alkylacrylic acids, RCH= =CR COjH, respectively. ° Either the )3-iodo or /3-bromo acids prepared by the addition of hydrogen halide are suitable sources. 

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