Preparation and dehydrohalogenation

Preparation of cathylates, 414 Preparation of cholesta-3,5-diene, 331 Preparation of 5 -cholest-2-ene, 343 Preparation of 5a-cholest-3-ene, 347 Preparation of 5a-cholest-6-ene, 354 Preparation of cyanohydrins, 412 Preparation and dehydrohalogenation of a-halo ketals, 301... 

Improvements in the preparation and dehydrohalogenation of w o-stilbene dibromide [Org. Syntheses, Coll. Vol. 3, 350 (1955)] are as follows. ra 5-Stilbene (20 g.) is heated with 400 ml. of acetic acid on the steam bath until dissolved, 40 g. of pyridinium bromide perbromide is added, and the mixture is heated on the steam bath and swirled for 5 minutes, meso-Stilbene dibromide separates at once in pearly white plates. The mixture is cooled to room temperature, and the product is collected and washed with methanol. The yield of dibromide, m.p. 

Halohydrins are easily prepared and dehydrohalogenation occurs readily at low temperatures. Another way epoxides can be formed is by catalytic vapor-phase oxidation of alkenes to form oxiranes. 

Alkenylbenzenes are prepared by the various methods described m Chapter 5 for the preparation of alkenes dehydrogenation dehydration and dehydrohalogenation... 

Cyclohexenone has been prepared by dehydrohalogenation of 2-bromocyclohexanone, by the hydrolysis and oxidation of 3-chlorocyclohexene, by the dehydration of a-hydroxycyclohexa- ione, by the oxidation of cyclohexene with chromic acid or hydrogen peroxide in the presence of a vanadium catalyst, by I lie addition of acroleiti to ethyl acetoacctate followed by cycliza-lion, hydroly.sis, and decar])oxylation, by the reduction of N,N-dimelliyliiniline with sodium and ethanol itt liquid ammonia... 

A number of halogen acceptors, such as alkali and alkaline hydroxides, aceutes, ethoxides, amines, and ammonia, have been used successfully. The pyrida2ine 9 was prepared via dehydrohalogenation of 8 using sodium acetate as acceptor (37). 

Nitrile oxides are usually prepared via halogenation and dehydrohalogenation of aldoximes [11] or via dehydration of primary nitro alkanes (Scheme 1) [12]. However, it is important to note that nitrile oxides are relatively unstable and are prone to dimerization or polymerization, especially upon heating. 1,3-Dipolar cycioaddition of a nitrile oxide with a suitable olefin generates an isoxazoline ring which is a versatile synthetic intermediate in that it provides easy access to y-amino alcohols, )5-hydroxy ketones, -hydroxy nitriles, unsaturated oximes, and a host of other multifunctional molecules (Scheme 1) [5a]. Particularly for the formation of )5-hydroxy ketones, nitrile oxide-olefin cycioaddition serve as an alternative to the Aldol reaction. 

Sulfonyl imides (78) are, like sulfenes, prepared by dehydrohalogenation of the corresponding sulfonyl chlorides (79) (usually called sulfamoyl chlorides). Like sulfenes, they take part in [2 + 2] and [4 + 2] cycloaddition reactions with electron-rich alkenes or with 1,3-dienes, yielding 1,2-thia-zetidine 1,1-dioxides (80)104 or dihydro-1,2-thiazines (81),105 respectively. 

Halogenation and dehydrohalogenation have been performed as a one-step operation from aldoximes using NCS in the presence of base such as pyridine (67), NBS and triethylanfine (68), or a heterogeneous base such as basic alumina or Florisil (69). Solvent-free reaction conditions also proved useful for the preparation... 

By far the most thoroughly investigated azocines are the 2-methoxy derivatives prepared and studied by Paquette and coworkers (7lAG(E)ll). The synthesis involves addition of chlorosulfonyl isocyanate to a cyclohexadiene, conversion to the imidate and introduction of another double bond by allylic bromination and dehydrohalogenation. Valence isomerization then ensues and 2-methoxyazocine (98) or alkyl homologs are isolated from this sequence in multigram quantities as stable yellow oils. NMR data (see 98) clearly indicate the monocyclic azocine structure the spectra are invariant from -70 to 180 °C and indicate less than 2% of the bicyclic valence isomers. 

The needed alkene is prepared by dehydrohalogenation and then brominated. 

Isoxazoles and their partially or fully saturated analogs have mainly been prepared, both in solution and on insoluble supports, by 1,3-dipolar cycloadditions of nitrile oxides or nitrones to alkenes or alkynes (Figure 15.10). Nitrile oxides can be generated in situ on insoluble supports by dehydration of nitroalkanes with isocyanates, or by conversion of aldehyde-derived oximes into a-chlorooximes and dehydrohalogenation of the latter. Nitrile oxides react smoothly with a wide variety of alkenes and alkynes to yield the corresponding isoxazoles. A less convergent approach to isoxazoles is the cyclocondensation of hydroxylamine with 1,3-dicarbonyl compounds or a,[3-unsatu-rated ketones. 

Dehalogenation and dehydrohalogenation are the main synthetic methods for the preparation of all classes of fluorinated unsaturated aliphatic compounds, which are often used as monomers and/or chemical intermediates. 

Direct introduction of a vinyl substituent onto an aromatic ring is not a feasible reaction. p-Methoxystyrene must be prepared in an indirect way by adding an ethyl side chain and then taking advantage of the reactivity of the benzylic position by bromination (e.g., with N-bromosuccinimide) and dehydrohalogenation. 

Phenylvinylsulfones (II) have been prepared by dehydrohalogenation of S-haloethylphenylsulfones and are discussed (4). 

The procedure for the elimination of HBr from the dibromo ester is a modification of the method of Lawton and co-workers for sui generis generation of the methyl or ethyl ester during a reaction. Methyl a-(bromomethyl)acrylate has also been prepared by bromination of methyl methacrylate in 700°C steam and by dehydrohalogenation with sodium acetate in acetic acid. Ethyl a-(bromomethyl)acrylate has been prepared by dehydrohalogenation with the monosodium salt of ethylene glycoP and ethyl diisopropylamine." The latter reaction was reported by Ohler et al. with no experimental details for the elimination reaction. The use of triethylamine as reported in this procedure appears to be the most efficient and convenient method for dehydrobromination to these acrylate esters. 

The parent stibabenzene (60) and bismabenzene (61) have been prepared by dehydrohalogenation of 59a and 59b, respectively (equation 170 ). While stibabenzene (60) has been isolated, it is a labile compound and rapidly polymerizes at room temperature. Bismabenzene (61) exists mainly as a dimer, but has been detected spectroscopically and via chemical trapping by hexafluorobutyne (equation 171 ). 

Aziridines can be prepared by various methods which invoive dehydration of 3-amino alcohols and dehydrohalogenation of 3-amino halides. Therefore, the reactions of aziridines with oxygen nucleo-... 

Isoprene epoxidized on the vinyl group (l,2-epoxy-3-methyl-3-butene, 14) would also be an attractive synthon for monoterpenoids if it were more readily accessible. Among the possible preparative methods (Scheme 2), acid-catalyzed isomerization of 2,3-epoxy-l-halo-3-methylbutanes (epoxidized prenyl halides) and dehydrohalogenation of the resulting isomers seemed the best route until chlorination of prenyl acetate (15, R = OAc) with hypochlorous acid was published. Dehydration of the mixture of products gave a maximum of the chloroacetate 16, which yielded the epoxide 14 with sodium hydroxide in methanol. ... 

Haloalkenes can be prepared by dehydrohalogenating saturated hydrogen-containing polyhalocarbons using liquid alkali metal acid fluoride and/or alkali metal fluoride compositions [75], HCFC-133a can be converted to CF2=CHC1 using these catalyst systems as shown in eq 21. 

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