Iodine atoms, reduction

No satisfactory explanation has been advanced for the reductive removal of one iodine atom. The di-iodination procedure does not appear to be useful with 3jS-hydroxy-A -steroids, but 3-keto-A systems do not interfere. 

In reaction (19) the iodine shown on the left has an oxidation number of zero. After the reaction, some of the iodine atoms have oxidation number +5 and some —1. In other words, the iodine oxidation number has gone both up and down in the reaction. This is an example of selfoxidation-reduction, sometimes called disproportionation. It is a reaction quite typical of, but not at all restricted to, the halogens. 

The homobifunctional photoreactive BASED (Chapter 4, Section 5.1) has two photoreactive phenyl azide groups, each of which contains an activating hydroxyl. Radioiodination of this crosslinker can yield one or two iodine atoms on each ring, creating an intensely radioactive compound. Crosslinks formed between two interacting molecules are reversible by disulfide reduction, thus allowing traceability of both components of the conjugate. 

Reaction graphs are encoded in the same way as static structures. Bonds which change during the reaction are coded as x y where x is the bond type before the reaction and y is the bond type after the reaction. Thus c-c -c represents the reduction of propene to propane and (c-o -cl-c-c-), cl- i represents the formation of tetrahydrofuran and an iodine atom from 4-iodobutan-l-ol. 

The reduction of (2,3)-q - and (2,3)-jS-methylenepenam j6-sulfoxides to the corresponding sulfides using potassium iodide and trifluoroacetic anhydride (TFAA) is found to be much faster than for bicyclic jS-lactam jS-sulfoxides.- In the proposed mechanism, initial attack of the sulfoxide oxygen on TFAA is followed by rate-limiting, nucleophilic displacement of trifluoroacetate by iodide ion nucleophilic attack of iodide on the iodine atom then yields the sulfide and iodine. The rate enhancement is accounted for by the stabilization of the transition state in the rate-limiting step by interaction of the p-like orbital of sulfur and the cyclopropane a orbital. 

The fact that the cyclization is directed to an acetylenic group and leads to formation of an alkenyl radical is significant. Formation of a saturated iodide would be expected to lead to a more complex product mixture because the cyclized product could undergo iodine abstraction and proceed to add to a second unsaturated center. Vinyl iodides are much less reactive, and the reaction product is stable to iodine-atom abstraction. Because of the potential for competition from reduction by the stannane, other reaction conditions have been developed to promote cyclization. Hexabutylditin is frequently used.238... 

The decomposition of the reduced complex is facilitated, most probably, by an electrophilic substitution of I+ by H+. This is a relatively fast step, as no add catalysis or H-D isotope effect could be demonstrated. The presence of Cu(I) in a favorable position close to the iodine atom facilitates the reduction of the ligand. 

Reductive processes are sometimes useful for conversion of polyiodinated pyrroles into compounds with fewer iodine atoms. Sequential action of butyl-lithium and water reduced tetraiodopyrrole to a mixture of 2,3,4-triiodopyrrole (63%) and 2,3,5-triiodopyrrole (3%). Zinc and acetic acid was able to reduce the tetraiodo compound to 3,4-diiodopyrrole which was converted by butyl-lithium and then dimethylformamide into 3-formyl-... 

Reduction of the 7-iodoaminochromes70 with zinc and dilute acid was usually accompanied by virtually complete elimination of the iodine atom,109,155 except in the case of 7-iodonoradrenochrome (42), where, although the main product was 5,6-dihydroxyindole (29), traces of 5,6-dihydroxy-7-iodoindole (56) were also detected.156 Only partial debromination was observed when 7-bromoadrenochrome (57) was reduced with this system 7-bromo-5,6-dihydroxy-.V-methylindole (58) and 5,6-dihydroxy-iV-methylindole (28) were both obtained in significant quantities.155... 

Iodine atom transfer reactions between alkyl radicals and iodocarbonyls are very rapid (107 M-1 s-1 to 109 M-1 s-1).130 This means that, even when these iodides are cyclized by the tin hydride method, iodine atom transfer may supersede hydrogen transfer, and the reductively cyclized product will ultimately be derived from the reduction of a cyclic iodide. Tin hydride cyclizations of halocarbonyls also often require very low concentration to avoid reduction of the initial radical prior to cyclization. For these reasons, reductively cyclized products are best formed by atom transfer cyclization at high concentration, followed by reduction of the product in situ. In a recent full paper, we have described in detail the preparative and mechanistic features of these cyclizations,19 and Jolly and Livinghouse have reported a modification of our reaction conditions that appears to be especially useful for substrates that cyclize very slowly.131 Cyclizations of a-iodocarbonyls can also be promoted by palladium.132... 

Schuchmann H-P, von Sonntag C (1988) The oxidation of methanol and 2-propanol by potassium peroxodisulphate in aqueous solution free-radical chain mechanisms elucidated by radiation-chemical techniques. Radiat Phys Chem 32 149-156 Schwarz HA, Bielski BHJ (1986) Reactions of H02 and 02 with iodine and bromine and l2 and I atom reduction potentials. J Phys Chem 90 1445-1448... 

Treatment of a-iodo lactone (45) with triethylborane under oxygen atmosphere gives the corresponding a-hydroxy lactone (46), via a-lactone radical species. This reaction comprises of SH2 reaction by Ef on the iodine atom of a-iodo lactones, reaction of the formed a-lactone radical with molecular oxygen, and subsequent hydrogen-atom abstraction from the solvent to form alkyl hydroperoxide (ROOH). Finally, by the addition of dimethyl sulfide for the reduction of the peroxide, the corresponding a-hydroxy lactone is obtained (eq. 2.24) [58]. 

The stereoelectronic requirements for ligand coupling in species such as 63 are not clear. However, if the conversion of 63 to 64 is viewed as a concerted process, it seems likely that the vinyl and copper ligands should first assume an apical-equatorial relationship in the i/f-TBP. Thus, if both ligands originally occupy apical sites, pseudorotation about the iodine atom should precede reductive coupling. That pseudorotation can at least occur in 23-iodanes has been demonstrated with a chiral (diacetoxyiodo)binaphthyl system147. 

Polyhalide radical anions have recently been reviewed I4- and I6-have been observed in terf-butanol solution, but they are unknown in aqueous solution (127). The equilibrium constant for formation of I2-[reaction (31)] is the link between the reduction potentials of the iodine atom, the diiodine radical anion, and diiodine. Numerous measurements of this equilibrium constant have been made over the years. There are even two reports of the enthalpy of the reaction, obtained from the temperature dependence of the equilibrium constant (35). Published values for the formation constant of I2- are listed in Table IV (32, 36, 128, 129, 149, 314, 318). As noted in Fornier de Violet s review (127) and in Elliot and Sopchyshyn s paper (109), there is a systematic discrepancy between the flash photolysis results and the pulse radiolysis results. Fornier de Violet suggested that the pulse radiolysis results might be in error because of unrecognized adduct formation... 

Stannyl anions with a highly coordinated tin center are also known. A hydridostannyl anion in the shape of a trigonal bipyramid in which two iodine atoms occupy the apical positions was obtained by oxidative addition of lithium iodide to the corresponding tin hydride (equation 58) . It was characterized by Sn NMR. Since apical iodines are more nucleophilic than the hydrogen, in its reactivity with a-ethylenic carbonyl compounds, attack by iodine precedes reduction by hydrogen, achieving regioselective 1,4 reductions. 

---