Brominations, using iodine reagents

This reaction could be of interest for substituting chlorine for bromine or for iodine in certain aromatic compounds. Indeed, in the halogenation by aromatic electrophilic substitution, the chlorinated reagents are more effective [2] and cheaper than brominated and iodinated reagents. However, bromo or iodo compounds are generally more interesting if the haloaromatic compounds are intended to be used in subsequent reactions because they are generally more reactive [2],... 

Synthetic procedures are available for the preparation of fluoro, chloro, bromo and iodo compounds from the corresponding lithio derivatives. Perchloryl fluoride (FCIO3), N-chlorosuccinimide, bromine and iodine are examples of reagents which can be used to introduce fluorine, chlorine, bromine and iodine, respectively. 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

Besides new insight into the reactivity of free radicals, methods for die production of carbon-centered free radicals have also seen major improvements in die last several years. One very common new mediod is to use tin-based reagents as radical chain carriers. Trialkyltin radicals readily abstract bromine or iodine from carbon to produce a carbon-centered free radical. Placement of a bromide or iodide substituent on a substrate dius permits formation of a carbon-centered free radical at diat position using tin-based mediodology. This process was initially developed for die reduction of alkyl halides, and it remains an excellent synthetic method for diat purpose. The complete chain mechanism for die reduction is shown. 

Phenylselenyl chloride adds to the double bond of the glycal 26 to give a phenyl-selenoxonium ion which is tra/w-opened by an approaching nucleophile to the glycoside 27. Other electrophiles have also been used likep-toluenesulfonic acid, bromine, or iodine. Acid conditions, however, may cause destruction of labile 2-deoxy sugar products and the value of such reagents will be diminished by this evident disadvantage. 

In addition to the direct conversions shown above, electrochemistry is often used in an indirect fashion, e.g. for the in situ generation of (harmful) reagents such as bromine or iodine by oxidation of bromide and iodide ion, respectively, or of Ce4+ by oxidation of Ce3+ [28]. Also, the regeneration of oxidation products such as dichromate, Equations 6.1a and 6.3, has been put to use [28] ... 

A modification of GSR has been reported by Classon and co-workers.190 The idea remains the same create a covalently bound phosphorus cation and displace with a nucleophile—in this case, a halogen. Both bromine and iodine have been used.190 Three different systems were evaluated (1) chlorodiphenylphophine, iodine-bromine, and imidazole (2) p-(dimethylamino)phenyldiphenylphosphine, iodine-bromine, and imidazole or (3) polymer-bound triphenylphosphine, iodine-bromine, and imidazole. The last two were found to be very similar to just triphenylphosphine itself, and displayed reactivity inferior to the first system. The polymer-bound reagent does allow for easier removal of triphenylphosphine oxide produced in the course of the reaction. As with the original procedure, and consistent with a Sn2 mechanism, inversion of configuration occurred. Again, as with the original method vicinal diols were readily converted into alkenes.191 This... 

Bromination and iodination of indoles in DMF at 25 °C result in a highly selective formation of 3-haloindoles in almost quantitative yield1051. This procedure is a considerable improvement in comparision to former methods where reagents such as pyridinium or dioxane perbromides1052 and 2,4,4,6-tetrabromocyclohexane-2,5-dienone1053 have been used. 

Numerous modifications of the direct zinc insertion procedure can be found in the hterature. For example, simple diaUcylzincs can be used as reagents instead of metallic Zn, bnt in this case the reaction is accelerated by catalytic qnantities of zinc salts or transition metal see Transition Metals) salts. Whereas the Cu -catalyzed iodine-zinc exchange reaction provides diorganozincs, the Pd, Mn and nF catalyzed iodine- or bromine-zinc exchange leads to organozinc halides. 

---