Dehydrohalogenation bromide

Dehydrogenation of alkylbenzenes although useful m the industrial preparation of styrene is not a general procedure and is not well suited to the laboratory prepara tion of alkenylbenzenes In such cases an alkylbenzene is subjected to benzylic bromi nation (Section 11 12) and the resulting benzylic bromide is treated with base to effect dehydrohalogenation... 

Cyclohexyl bromide, for exfflnple, is converted to cyclohexene by sodium ethoxide in ethanol over 60 times faster than cyclohexyl chloride. Iodide is the best leaving group in a dehydrohalogenation reaction, fluoride the poorest. Fluoride is such a poor leaving group that alkyl fluorides are rarely used as starting materials in the preparation of alkenes. 

Quaternary ammonium salts of pyrrolines 106) can be prepared only indirectly 197). Addition of bromine to l-dimethylamino-4-pentene followed by removal of hydrogen bromide afforded, depending upon the dehydrohalogenation conditions, quaternary bromides derived from either l,2-dimethyl-/f -pyrroline (107) or l-methyl-2-methylenepyrrolidine (108) (Scheme 7). 

The alkylation reaction is limited to the use of primary alkyl bromides and alkyl iodides because acetylide ions are sufficiently strong bases to cause dehydrohalogenation instead of substitution when they react with secondary and tertiary alkyl halides. For example, reaction of bromocyclohexane with propyne anion yields the elimination product cyclohexene rather than the substitution product 1-propynylcyclohexane. 

The knowledge of the valence tautomerization of benzene oxides to oxepins12 prompted several groups to synthesize oxepins by dehydrohalogenation of 7-oxabicyclo[4.1.0]heptane derivatives. Numerous examples have been described for the base-catalyzed elimination of hydrogen bromide from the 3,4-dibromo-7-oxabicyclo[4.1.0]heptane system. The reaction products are usually obtained as mixtures of oxepin 1 and benzene oxide 2. The 2,7-bis(hydroxy-methyl)oxepin 1 p obtained by this route can be converted to the 2,7-dicarbaldehyde with man-ganese(IV) oxide.23... 

The dehydrohalogenation reaction has been extended to benzannulated oxepins. Elimination of hydrogen bromide from 3-bromo-4-phenyl-2,3-dihydro-l-benzoxepin with 1,5-diazabicyclo-[4.3.0]non-5-ene gives 4-phenyl-1-benzoxepins 15a15 and 15b16 in low yield. 

Hydrogen bromide is eliminated from 10,11-dibromo-l 0,1 l-dihydrodibenz[7>,/]oxepin with potassium tert-butoxide at room temperature to give 10-bromodibenz[i,/]oxepin (17a).160161 When the elimination reaction was performed in boiling toy-butanol the yield increased from 58 to 92%.261 Dehydrohalogenation of 10-chloro-2,3-dimethoxy-10,ll-dihydrodi-benz[/),/]oxepin afforded 2,3-dimethoxydibenz[6,/]oxepin (17b) in 52% yield.162... 

Triphenylmethylpotassium rapidly dehydrohalogenates secondary alkyl bromides and iodides, in over 90% yields, at 0 C Anton, D.R. Crabtree, R.H. Tetrahedron Lett., 1983, 24, 2449. 

The addition of bromine to the ylide (69) gave a bromophosphonium salt which could be isolated. Dehydrohalogenation with dimethyl-formamide and lithium bromide afforded 1-phenylvinyltriphenylphos-phonium bromide (70). 

From the results of the small scale thermal analysis experiments previously reported (23,25), it was concluded that the antimony volatilization and bromide release observed for ternary mixtures containing organobromine compounds, which did not undergo intermolecular dehydrohalogenation, could not be accounted for solely on the basis of HBr formation during degradation. 

Several dehydrohalogenation reactions have been carried out with the help of DMSO. Thus isopropyl bromide has been successfully converted to propene by heating with potassium tertiary butoxide-DMSO mixture at 550°. 

Allenyl ethers are useful key building blocks for the synthesis of a-methylene-y-butyrolactones [129, 130], The synthesis of the antileukemic botryodiplodin was accomplished with the crucial steps briefly presented in Scheme 8.56. Bromoallenyl ethers 225 were easily prepared by base-induced isomerization from the corresponding /3-bromoalkyl alkynyl ether compounds and then subjected to electrophilic bro-mination with NBS. The resulting acetals 226 were converted into 2-alkoxy-3-methy-lenetetrahydrofurans 227 by dehydrohalogenation of the alkenyl bromide unit to an alkyne and subsequent radical cyclization employing tributyltin hydride [130],... 

The dehydrohalogenation of 1- or 2-haloalkanes, in particular of l-bromo-2-phenylethane, has been studied in considerable detail [1-9]. Less active haloalkanes react only in the presence of specific quaternary ammonium salts and frequently require stoichiometric amounts of the catalyst, particularly when Triton B is used [ 1, 2]. Elimination follows zero order kinetics [7] and can take place in the absence of base, for example, styrene, equivalent in concentration to that of the added catalyst, is obtained when 1-bromo-2-phenylethane is heated at 100°C with tetra-n-butyl-ammonium bromide [8], The reaction is reversible and 1-bromo-l-phenylethane is detected at 145°C [8]. From this evidence it is postulated that the elimination follows a reverse transfer mechanism (see Chapter 1) [5]. The liquidrliquid two-phase p-elimination from 1-bromo-2-phenylethanes is low yielding and extremely slow, compared with the PEG-catalysed reaction [4]. In contrast, solid potassium hydroxide and tetra-n-butylammonium bromide in f-butanol effects a 73% conversion in 24 hours or, in the absence of a solvent, over 4 hours [3] extended reaction times lead to polymerization of the resulting styrene. 

Co(CO)4] ) dehydrohalogenation [Eq. (15)] followed by addition of HCo(CO)4, or whether splitting out of HCo(CO)4 occurs from the alkyl-cobalt [Eq. (14)], which is the malonate precursor, followed by HCo(CO)4 addition in the opposite direction. In one case [Eq. (15)], olefin formation proceeds directly from the bromide and no reversibility of any steps is required, while according to Eq. (14) olefin formation proceeds from elimination of HCo(CO)4. 

Lithium bromide is used in absorption, refrigeration and air-conditioning systems. A highly concentrated solution of the salt is an efficient absorbent of water vapor. The vapor pressure of such solution is very low. Other applications include the use of the salt as a swelling agent for wool, hair and other organic fibers as a catalyst in dehydrohalogenation reactions and as a sedative and hypnotic in medicine. 

The intermediacy of aryne intermediates generated by dehydrohalogenation of the corresponding aryl bromide is useful in preparing polycyclic isoquinolines with the phenanthridine skeleton <99T5195, 990L985>. 

Similarly, the 2-cyano-6-oxazolopiperidine 75 (Scheme 16) can be used to provide a variety of substituted piperidines <99TL3731, 99H(51)2065>. Conversion to the enamide 76 provides a means to introduce C-3 alkyl groups by Michael reaction <99TL3699>. Electrochemical bis-bromination and dehydrohalogenation affords the vinyl bromide 77, which can imdergo substitution at the 4-position by the addition of nucleophiles as simple as water <99T8931>. 

As a dehydrohalogenating reagent, phenylmagnesium bromide is not so effective as are lithium dialkylamides, but in hexamethylphosphoramide it reacts with a,/9-dichloroenamines to give a 35% yield of the corresponding ynamine [18] (Eq. 23). 

The reagent is unstable and so is generated in the presence of the carbonyl compound by dehydrohalogenation of the alkyltriphenylphos-phonium bromide with phenyllilhium in dry ether in a nitrogen atmosphere. There are various modifications, such as the phosphonate. in which djethylbenzylphosphonate, cinnamaldehyde, and sodium methoxide yield 1,4-diphenylbutadiene. 

Electrophilic nitration of olefins can also be carried out with nitronium salts in pyridinium poly (hydrogen fluoride) (PPHF) solution491 (which also acts as solvent) to give high yields of nitrofluorinated alkanes. In the presence of added halide ions (iodide, bromide, chloride) the related haloalkanes are formed, and these can be dehydrohalogenated to nitroalkenes492 [Eq. (5.183)]. 

Condensation [274] of the bromide (412) with the unsaturated derivative (378) using silver triflate gave an a p yield ratio of 42 21% and condensation of (412) with (377) gave 26 8%. From these results the authors concluded that the 3-P-hy-droxyl group on the neuraminic acid halide prevents dehydrohalogenation and assists glycosidation. 

The necessary alkene, styrene, is available by dehydrohalogenation of the given starting material, 1-phenylethyl bromide. 

Dehydrohalogenation of this compound can be accomplished under E2 conditions by treatment with base. Sodium methoxide in methanol would be appropriate, for example, although almost any alkoxide could be employed to dehydrohalogenate this tertiary bromide. 

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