Replacement bromine with hydrogen

Hypohahtes replace the acetylenic hydrogen with chlorine, bromine, or iodine (203). 

Further evidence for a bromine-bridged radical comes from radical substitution of optically active 2-bromobutane. Most of the 2,3-dibromobutane which is formed is racemic, indicating that the stereogenic center is involved in the reaction. A bridged intermediate that can react at either carbon can explain the racemization. When the 3-deuterated reagent is used, it can be shown that the hydrogen (or deuterium) that is abstracted is replaced by bromine with retention of stereochemistry These results are also consistent with a bridged bromine radical. 

Replacement of a hydrogen with bromine in the polyfluoroalkyl group of a ketone or acyl fluoride can be carried out with phosphorus pentabromide [i50] (equation 45)... 

In very reactive halogen derivatives such as a-bromo ketones [234] and a-bromothiophene [235] the halogens are replaced by hydrogen with hydrogen bromide in acetic acid provided phenol is added to react with the evolved bromine and to affect favorably the equilibrium of the reversible reaction. Hydrogen bromide in acetic acid also reduces azides to amines [232],... 

In some esters, bromine trifluoride selectively replaces the tertiary hydrogen atom with fluorine.127 The reaction is carried out in CFC-113 at 25-35 C with 60-80% excess bromine trifluoridc. The products are obtained in 40-50% yield. 

Evidently, hydrogen facilitates the nucleophilic replacement of fluorine by the methoxy group more efficiently than the halogens fluorine < chlorine < bromine < iodine < hydrogen.83,84 1,2,4,5-Tetrafluorobenzene when treated with sodium methoxide in methanol yields 1,2,4-tri-fluoro-5-methoxybenzene (7).83... 

In bromopentafluorobenzene and pentafluoroiodobenzene the main reaction with methoxide ions is the replacement of fluorine in the 4-position relative to the halogen atom (bromine and iodine) that gives products 8 and 9.85 Replacement at the 2-position occurs to a smaller extent and decreases along the series chlorine > bromine > iodine hydrogen.85,86... 

The reactions of 2,6-dibromopyridine with the carbanions formed from a-phenylbutyronitrile or a-ethyl-a-phenylbutyronitrile give mixtures of products in which one or both of the bromines have been replaced together with products in which one bromine has been replaced by hydrogen and the other by the carbanion.104... 

Replacement of the iodine or bromine substituent of a perfluoroalkyl iodide or bromide with hydrogen is a process which is a side reaction in most of the reduc-tively-catalyzed perfluoroalkylation processes described earlier. If one wishes to carry out such a reaction synthetically, it may be accomplished easily by use of any of a number of hydrogen atom transfer agents such as n-Bu3SnH, -Bu3GeH, (TMS)3SiH, or Et3SiH. 

Radical bromination with NBS can result in the replacement of a sugar ring hydrogen by bromine.18 These reactions can be highly regioselective... 

The bromine atom in the methyl 2-desoxy-2-bromo-hexosides can be replaced with hydrogen by sodium amalgam, to give the 2-desoxy-D-glucoside mixture represented by XIII. A homogeneous methyl... 

Several replacement reactions at C-4 in sydnones may be carried out but aqueous bases must be avoided. Butyllithium can be used to displace bromine from a 3-phenylsydnone the resulting organolithium salt can be carbonylated, will add to ketones, and forms a silyl derivative (80CB1830). A sydnone Grignard derivative can also be made and will add ketones in the normal way (80JCS(Pl)20). Sodium borohydride will reduce a sydnone sulfone, formed by oxidation of a thioether (Table 5) with hydrogen peroxide, back to the unsubstituted sydnone (74T409). 

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