Metal/bromine displacement

Nitrogen Displaces the Metal Phosphorus Displaces the Metal Oxygen Displaces the Metal Sulfur Displaces the Metal Fluorine Displaces the Metal Chlorine Displaces the Metal Bromine Displaces the Metal Iodine Displaces the Metal... 

The iodides of the alkaU metals and those of the heavier alkaline earths are resistant to oxygen on heating, but most others can be roasted to oxide in air and oxygen. The vapors of the most volatile iodides, such as those of aluminum and titanium(II) actually bum in air. The iodides resemble the sulfides in this respect, with the important difference that the iodine is volatilized, not as an oxide, but as the free element, which can be recovered as such. Chlorine and bromine readily displace iodine from the iodides, converting them to the corresponding chlorides and bromides. 

Nucleophilic substitution reactions, to which the aromatic rings are activated by the presence of the carbonyl groups, are commonly used in the elaboration of the anthraquinone nucleus, particularly for the introduction of hydroxy and amino groups. Commonly these substitution reactions are catalysed by either boric acid or by transition metal ions. As an example, amino and hydroxy groups may be introduced into the anthraquinone system by nucleophilic displacement of sulfonic acid groups. Another example of an industrially useful nucleophilic substitution is the reaction of l-amino-4-bromoanthraquinone-2-sulfonic acid (bromamine acid) (76) with aromatic amines, as shown in Scheme 4.5, to give a series of useful water-soluble blue dyes. The displacement of bromine in these reactions is catalysed markedly by the presence of copper(n) ions. 

The brominated chelates are chemically much less reactive than the organic ligands themselves. Most nucleophilic substitution reactions are ineffective, but bromine can be displaced by thiols (equation 57).280 The halo complexes do not react with lithium or magnesium metals, but can be hydrogenolyzed to the unsubstituted complexes (equation 58).281... 

Chlorine, added to a solution of bromide or iodide of a metal, displaces the bromine or iodine here the non-ionised chlorine becomes ionised at the expense of the charge on the ionised bromine or iodine, while the latter... 

Non-metals, typically halogens, can also take part in single displacement reactions. For example, molecular chlorine can replace bromine from KBr, an ionic compound, producing bromine and potassium chloride. 

Many nonmetals displace less active nonmetals from combination with a metal or other cation. For example, when chlorine is bubbled through a solution containing bromide ions (derived from a soluble ionic salt such as sodium bromide, NaBr), chlorine displaces bromide ions to form elemental bromine and chloride ions (as aqueous sodium chloride) ... 

There are some examples of nucleophilic displacement of a suitably activated bromine by fluoride, but there have been instances where such reactions are unsuccessful <93AHC(57)291>. Electrophilic methods do not include direct fluorination, but metallic derivatives provide carbanions which react with fluorine sources. Thus, 2- and 4-fluoroimidazoles have been made from the lithioimidazoles, for example, perchloryl fluoride converts 2-lithio-l-methylimidazole into the 2-fluoro derivative in more than 50% yield. Access to 4- and 5-fluoroimidazoles is even more convenient from the trimethylstannyl derivatives using fluorine or caesium fluoroxysulfate as the electrophile . Mercury groups react in much the same way, but they are more difficult to prepare and purify <86BSF930>. 

The synthesis of thieno[2,3- f]imidazole illustrates again the selectivity in halogen-metal exchange processes in imidazoles, hi this sequence a vinyl was used as A-protecting group, and it includes a nucleophilic displacement of bromine from the 4-position, activated by the 5-aldehyde. 

The chemical reactivity of the thiocyano groups in these chelate rings has not been investigated, but the halogen atoms in the 3-halo metal acetylacetonates have been found to be quite inert and their behavior is different from that of aryl halides, since treatment of the tris(3-bromo-2,4-pentanediono)chromium(III) chelate with magnesium or lithium in benzene or tetrahydrofuran resulted in no reaction. Attempted nucleophilic displacement of the bromine atoms in this chelate by azide, acetate, nitrate, and iodide ions in hot dimethylformamide also failed. In most of these... 

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