Bromine precursors

The evolution of the mass spectra of the ion-molecule reaction products of m/z 77 ions with ammonia as a function of the nature of the precursor halobenzene is also of interest. The m/z 94 m/z 93 branching ratio measured as 22 78 for the chlorinated precursor becomes 30 70 for the brominated precursor and 49 51 for the iodinated precursor. Moreover, this ratio is still significantly modified (60 40) if ionized iodobenzene is prepared by charge exchange with methylene chloride in the Cl source. The degree of fragmentation of the m/z 94 ions thus appears extremely dependent on the distribution energies of the precursor m/z 77 ions, but the occurrence of isomeric species of the phenyl cation cannot also be ruled out completely. 

The main synthetic route to CFC, HCFC and Halons is the Swarts fluorination. Technically this is often achieved by reaction of a chlorinated or brominated precursor with anhydrous hydrofluoric acid in the presence of a solid Lewis acid catalyst, for example chromia. Other important reactions are Lewis acid-catalyzed halogen isomerization and hydrogenolysis of chlorine or bromine. 

Alkyl bromides react at lower temperamres than chlorides in dehydrohalo-genation reactions, but the reactions are complex since brominated precursor molecules appear, indicating that some bromine is formed from the hydrogen bromide [182], It is worthwhile mentioning that carbons also catalyze hydrogenol-ysis of C-Hal bonds in monosubstituted benzenes and substituted chlorobenzenes [183,184],... 

An intramolecular addition of an ester enolate to a benzyne intermediate generated from the brominated precursor 18 has been reported by Castedo et al. as the key step in the total synthesis of ( )-clavizepine (Scheme 12.7) [19]. 

The direct derivatization of the chiral brominated precursor shown below (x = 0.2) gave the corresponding silyl phosphazene copolymers (188) where the phosphine ligands sit inside wide and sterically demanding chiral pockets, and therefore, with potential interest to support catalysts for enantioselective synthesis. ... 

Dianions 126, alkenyllithium compounds of type VIII, have been obtained by halogen-lithium exchange from the corresponding chlorinated or brominated precursors 125 using lithium naphthalenide it was necessary to carry out a previous deprotonation with PhLi. The reaction of 126 with electrophiles gave regio-selective functionalized allyl amines 127 (Scheme 2.17) [106]. 

This method is by far the easiest of the two methods I descnbe, but because it uses bromine liquid as a precursor to the dibromodioxane crystals a fume cupboard (or a fucking good method of fume extraction) is absolutely essential. Surgically removing ones gonads with a blunt knife would be a much less painful way of harming yourself than messing with this stuff in the kitchen. 

Less activated substrates such as uorohaloben2enes also undergo nucleophilic displacement and thereby permit entry to other useful compounds. Bromine is preferentially displaced in -bromofluoroben2ene [460-00-4] by hydroxyl ion under the following conditions calcium hydroxide, water, cuprous oxide catalyst, 250°C, 3.46 MPa (500 psi), to give -fluorophenol [371-41-5] in 79% yield (162,163). This product is a key precursor to sorbinil, an en2yme inhibitor (aldose reductase). 

Additions of halogen fluorides to the more electrophilic perfluonnated olefins generally require different conditions Reactions of iodine fluoride, generated in situ from iodine and iodine pentafluoride [62 102 103, /05] or iodine, hydrogen fluoride, and parapeiiodic aud [104], with fluormated olefins (equations 8-10) are especially well studied because the perfluoroalkyl iodide products are useful precursors of surfactants and other fluorochemicals Somewhat higher temperatures are required compared with reactions with hydrocarbon olefins Additions of bromine fluoride, from bromine and bromine trifluonde, to perfluonnated olefins are also known [lOti]... 

Allylic bromination of pregnenolone acetate with dibromodi-methylhydantoin affords the 7-bromo compound (155) of undefined stereochemistry. Dehydrobromination by means of collidine followed by saponification affords the 5,7 endocyclic cis,cis-diene, 156. This compound contains the same chromophore as ergosterol, a steroid used as a vitamin D precursor. The latter displays a complex series of photochemical reactions among the known products is lumisterol, in which the stereochemistry at both C9 and Cio is inverted. Indeed, irradiation of 156 proceeds to give just such a product (158). This reaction can be rationalized by... 

Next ask yourself, "What is an immediate precursor of p-bromotoluene " Perhaps toluene is an immediate precursor because the methyl group would direct bromination to the ortho and para positions. Alternatively, bromobenzene might be an immediate precursor because we could carry out a Friedel-Cralts methylation and obtain a mixture of ortho and para products. Both answers are satisfactory, although both would also lead unavoidably to a product mixture that would have to be separated. 

What is an immediate precursor of toluene " Benzene, which could be methylated in a Friedel-Crafts reaction. Alternatively, "What is an immediate precursor of bromobenzene " Benzene, which could be brominated. 

Azabicyclo[4.1.0]hept-3-enes, e. g. 13, are available from 1,4-dihydrobenzenes 12 by the route indicated, and are useful precursors for the synthesis of specifically substituted 1//-azepines free from isomeric contamination.61 For example, low temperature addition of bromine to bicycle 13 yields the dibromo derivative 14 which, with powdered sodium methoxide in tet-rahydrofuran, undergoes a double dehydrobromination followed by electrocyclic ring opening of the resulting 7-azabicyclo[4.1.0]hepta-2,4-diene 15 to give methyl l//-azepine-l-carboxylate (16). 

Side-chain bromination occurred when 5,6-dimethyl-2,l,3-benzothiadi-azole was treated with NBS (87CB1593). Both the 4- and the 4,7-dihaloge-nated compounds are readily made from the diazotized amino precursors (63JGU1714). 

The optically active D- and L-em/rn.i-2-morpholmones 1 (only the d-erythro series is depicted) are efficiently brominated with (V-bromosuccinimide to afford the bromo derivatives 2 (X = Br) that serve as versatile precursors for electrophilic glycine templates which permit construction of either d- or L-a-atnino acids in high optical purity71. The corresponding chlorides 2 (X = Cl) are similarly obtained by chlorination of 1 with tert-butyl hypochlorite71. 

Braverman and Reisman111 have found that addition of a carbon tetrachloride solution of bromine to bis-y,y-dimethylallenyl sulfone 20 at room temperature unexpectedly resulted in spontaneous and quantitative fragmentation of the sulfone, with formation of the cyclic a, /3-unsaturated sulfmate (y-sultine) 43a and the tribromo products 44 and 45 (equation 38). Analogously, treatment of the same sulfone with trifluoroacetic acid gives rise to y-sultine 43b. It is interesting to note that from a synthetic point of view it is not even necessary to prepare the diallenyl sulfone 20, since one can use its sulfinate precursor (equation 24) to obtain exactly the same results, under the same conditions. The authors suggested that the fragmentation-cyclization of sulfone 20 may take place by the mechanism depicted in equation 39. 

Various bromophenols are useful precursors for medicinal drugs, agricultural chemicals, dyes, and flame retardants. It is difficult to synthesize directly ortho-bromophenols by use of bromine with which bromination afford para-substitution predominantly (ref. 1). Thus desired ort/io-bromophenols were generally prepared via tedious steps as shown in Scheme 1 (refs. 2,3). 

The production of aromatic bromo-compounds is rarely an end in itself. More commonly the bromo-compounds are precursors for some further synthetic steps. Often, the bromine is replaced either in a direct substitution reaction or via... 

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