Dehalogenation deiodination

Aryl halides can be dehalogenated by Friedel-Crafts catalysts. Iodine is the most easily cleaved. Dechlorination is seldom performed and defluorination apparently never. The reaction is most successful when a reducing agent, say, Br or 1 is present to combine with the I" or Br coming off." Except for deiodination, the reaction is seldom used for preparative purposes. Migration of halogen is also found," both intramolecular and intermolecular." The mechanism is probably the reverse of that of 11-11." ... 

The inversion in selectivity between the insertion into an H—Si bond favored by the intermediate in the dehalogenation and the addition to a triple bond favored by the intermediate formed upon silirane pyrolysis suggests that different reactive intermediates are formed. Since free silylene is implicated in the silirane pyrolysis, the lithium-induced deiodination probably involves a silylenoid such as a complex of the silylene with Lil or with THF. For a further discussion of this possibility, see Section II.E. 

Another interesting application of microwave irradiation is found in the selective dehalogenation of the iodinated indole alkaloid Plakohypaphorine F, described by E. Fattorusso et al. [117,118] (Scheme 32). The bis-halogenated compound was treated with potassium formate and palladium acetate under controlled microwave irradiation, resulting in selective deiodination. The choice of the solvent, in this case DMSO, was found to be crucial. 

Dehalogenations at saturated carbons are seen quite commonly with cyclic compounds of the type RCH2X. Equations 13.105,197 13.106,198 and 13.107199 are examples of such dechlorination, debromination, and deiodination, respectively, using palladium or platinum as catalyst. Equations 13.108200 and 13.109201 are those using Raney Ni. 

Deiodination. lodopyrroles undergo Pd-catalyzed dehalogenation when HCOONa is used as a hydrogen source. 

The levels of L-triiodothyronine and L-thyroxine are regulated mainly by deio-dination. In the thyroid gland and the peripheric tissues deiodinating enzymes dehalogenate free L-iodotyrosines, L-triiodothyronine and L-thyroxine. 

Three pathways for triiodothyronine degradation have been described oxidation, deiodination, and conjugation. The products of oxidation to acid have been found in the bile of rats after intraperitoneal administration of triiodothyronine. Triiodothyronine may be deiodinated to yield 3,3 -thyronine. This reaction may be of considerable physiological significance because the dehalogenation of Triac provides a means by which a very active compound can be converted to an inactive substance. Glucuronides and sulfates of triiodothyronine have been found in blood, and it is likely that they are synthesized in the liver. 

These reactions are relevant to this review in that they may be photoinduced, but as radical chain processes many of them can be initiated in other ways. Radical chain dehalogenation of aryl halides has been reviewed previously.The chain propagation sequence for the deiodination of aryl iodides with CH5O-/CH3OH is shown in Scheme 1. The species (Arl) is very short lived and deiodinates efficiently. Reactivity diminishes in the order Arl > ArBr > ArCl and is enhanced in cases where there is relief of steric strain. Photochemical dehalogenations of aryl halides with AIH4 and BH4 have been proposed to follow similar radical chain mechanisms. Evidence for electron transfer from BH4 to ArCl has been present by Freeman and Ramnath. ... 

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