1.2- Dibromides, dehydrobromination

Vinyl Bromide. Vinyl bromide [593-60-2] is prepared by the base-promoted dehydrobromination of ethylene dibromide [106-93 ]. It is used as a comonomer in the production of acrylic fibers. 

Chemical/Physical. In an aqueous phosphate buffer solution (0.05 M) containing hydrogen sulfide ion, ethylene dibromide was transformed into 1,2-dithioethane and vinyl bromide. The hydrolysis half-lives for solutions with and without sulfides present ranged from 37 to 70 d and 0.8 to 4.6 yr, respectively (Barbash and Reinhard, 1989). Dehydrobromination of ethylene dibromide to vinyl bromide was observed in various aqueous buffer solutions (pH 7 to 11) in the temperature range of 45 to 90 °C. The estimated half-life for this reaction at 25 °C and pH 7 is 2.5... 

An attempted approach to bis(bicyclopropylidenyls) of type 87 utilizing the Kulinkovich reaction turned out to be unfruitful although the twofold cyclo-propanation of the diester 84 prepared from butadiene, worked perfectly well, and the conversion of the biscyclopropanol 85 to the dibromide 86 also gave a good yield (60%), the twofold dehydrobromination of 86 afforded only 1.7% of a mixture of meso- and d,l-S7 (Scheme 15) [56 a]. However, the direct oxidative coupling of two molecules of lithiobicyclopropylidene was accomplished under... 

The observation that the bis-alkyne 5 was more stable that the enyne 13 led us to believe that the non-annelated bis-alkyne 19 might be a stable molecule. Our preliminary experiment to synthesize 19 was similar to the method used by Krebs. Unfortunately, dehydrobromination of the dibromide 2()3 3 with an excess of KO-t-Bu only gave an unseparable mixture of 21.3 jg therefore no direct support for the formation of 19 as an intermediate. 

In practice, bromination of 35 gave the tetrabromide 37, which was dehydro-brominated carefully to the dibromide 38. Further dehydrobromination with an excess of KO-r-Bu furnished only a mixture of 40 and 42. It was likely that the dehydrobromination pathway was of a stepwise manner, going through 39 and 41 as intermediates (Scheme 9), although none of which could be trapped. The presence of 39 and 41 was indirectly proved by the isolation of an adduct 43 through a dehydrobromination of 38 in the presence of 1,3-diphenylisobenzofuran." It was quite clear that the instabilities of 39 and 41 were also due to the severe pen-H-H interaction, as indicated in 44. 

The cyclization of a number of chalcone dibromides to flavones has been achieved by treatment with pyridine (81JOC638). It is thought that the reaction may proceed through debromination and dehydrobromination to the chalcone and bromochalcone, respectively (Scheme 166). The formation of pyridine perbromide would account for the nuclear brominated flavones which sometimes accompany the flavone (63CB913). 

The dibromides 45 on treatment with potassium carbonate or triethylamine are easily and regioselectively dehydrobrominated to the a-bromo-a,/J-enones 46.98... 

The loss of a bromide ion when situated in a -position to a carboxylate group occurs very readily under mildly basic conditions. Thus in an alternative synthesis of phenylacetylene (Expt 5.24), cinnamic acid dibromide is converted into /1-bromostyrene under the influence of hot aqueous sodium carbonate solution. Dehydrobromination to yield the acetylenic compound is then achieved in the usual manner with potassium hydroxide. 

Alkynes.10 Alkynes can be prepared by double dehydrobromination of pic-dibromides in petroleum ether by use of powdered KOH and catalytic amounts of a phase-transfer catalyst. Tetraoctylammonium bromide, 18-crown-6, or even Aliquat 336 are much more effective than more hydrophilic quarternary ammonium salts, such as tetrabutylammonium hydrogen sulfate, previously used (7, 354-355)." Isolated yields are 80-98%. Yields are generally lower when mc-dichlorides are used as the starting material. 

Synthesis of the other classes of polycyclic aziridines is reported in [106]. Derivatives of l,3,4,6a-tetrahydroazireno[l,2-a]pyrazines 88 and 1,1 a-dihydro-l,2-diarylazireno[l,2-a]quinoxaline 90 were obtained in the reaction of dibromides 86 with ethylenediamine 87 or 1,2-phenylenediamine 89 (Scheme 1.25). The authors showed the possibility of synthesizing dihydroazirenoquinoxalines 90 from a-bromochalcones. This can be confirmed by the following reaction of ce,/2-dihalogen ketones via dehydrobromination. 

The dibromide 73 dehydrohalogenates to give a cyclopropa(c)thiophene which is trapped in 57% yield by added isobenzofuran. A stepwise dehydrobromination-trapping has yet to be completely excluded and no products are obtained from attempted gas-phase dehydrobromination90. 

The dehydrobromination of 2,3-dihydrotelluropheno[2,3-6]quinoline 1,1-dibromide, prepared from the dihydrotellurophenoquinoline, /V-bromosuccinimide, and a crystal of benzoyl peroxide in carbon tetrachloride, by DBU gave yields approximately half of those obtained with dehydroiodination . 

The selectivity of DBN is illustrated by the dehydrobromination of the dibromide (7). Use of potassium /-butoxide leads to (8), whereas the product obtained with DBN is (9). 

Dehydrobrommatiom. In a procedure for the preparation of tri-/-butylcyclo-propcnyl fluoroborate (4), Ciabattoni et at. prepared dineopenlylketone (I) by the Grignard synthesis formulated, converted it to the a, -dibromide (2), and effected double dehydrobromination by treatment of this intermediate with potassium t-buloxide. The resulting cyclopropcnonc (3) was then brought into reaction with a solution of commercial t-butyllithium in pentane. The mixture was quenched with water, and the pentane layer was washed, dried, and evaporated on a rotary evaporator. The resulting pale-yellow oil is taken up in ether and treated at 0° under rapid magnetic... 

Bromination of an olefin and double dehydrobromination of the resulting 1,2-dibromide is a classical method for the generation of 1,3-dienes (Table 1). Bromination of a double bond can be done with molecular bromine or, more conveniently, with pyridinium bromide perbromide . A variety of bases has been employed for dehydrobromination. While potassium hydroxide and sodium methoxide have been used for a long time, lithium carbonate-lithium chloride in DMF or hexamethylphosphoric triamide (HMPA) works well in many cases . Double dehydrobromination with hindered bases such as potassium r-butoxide or diazabicyclononene (DBN) and diazabicycloundecene (DBU) give good results. 

Microwave facilitates the KF/Al Oj-promoted condensation of oxindole with aldehydes and dehydrobromination of vtc-dibromides to bromoalkenes. ... 

For the conversion of oleic acid into stearolic acid via the dibromide, Khan and co-workers found that dehydrobromination with sodium amide doubles the yield obtainable by use of ethanolic potassium hydroxide. A suspension of sodium amide in liquid ammonia prepared from 1.87 g. atoms of sodium was stirred under Br2, SNaNHj HCl ... 

Double dehydrobromination was also used in the preparation of 2-chloro-3a,7a-methanoindane (6). The dibromide, formed by brominating the corresponding monoalkene 5 at — 78 C, was treated with potassium ferr-butoxide in /ert-butyl alcohol, which gave 6 in 56% overall yield. Notably, this treatment left the chlorine atom intact. The chloride 6 served in turn as the starting material for the synthesis of 3a,7a-methano-l//-indene (see Section 5.2.2.1.1.3.)." ... 

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