Debromination reactions with iodide

Debromination is formally a reduction because a molecule of Br2 (an oxidizing agent) is removed. The reaction with iodide takes place by the E2 mechanism, with the same geometric constraints as the E2 dehydrohalogenation. Elimination usually takes place through an anti-coplanar arrangement, as shown in Mechanism 7-3. Acetone serves as a convenient solvent that dissolves most alkyl halides and sodium iodide. 

H from C0, the commonest probably being 1,2-dehalogenations and, in particular, 1,2-debromination. This can be induced by a number of different species including iodide ion, I , metals such as zinc, and some metal ions, e.g. Fe2. The reaction with I in acetone is found to follow the rate law (after allowance has been made for the I complexed by the I2 produced in the reaction),... 

Vicinal dibromides (two bromines on adjacent carbon atoms) are converted to alkenes by reduction with iodide ion in acetone. This debromination is rarely an important synthetic reaction, because the most likely origin of a vicinal dibromide is frombromi-nation of an alkene (Section 8-10). We discuss this reaction with dehydrohalogenation because the mechanisms are similar. 

In contrast to the dichotomy of reaction pathways observed for the debromination of 27, debromination of 1,2-dibromodecane (29) with either di-/i-hexyltelluride or tetra-/i-butylammonium iodide gave identical (within experimental error) activation parameters (Table The large negative values of ASt (—108 37 and... 

In contrast to the dichotomy of reaction pathways observed for the debromination of 27, debromination of 1,2-dibromodecane (29) with either di-n-hexyltelluride or tetra-M-butylammonium iodide gave identical (within experimental error) activation parameters (Table 3).44 The large negative values of (—108 37 and — 87 12 J K-1 mol-1, respectively) are consistent with a highly-ordered transition state, but the bromonium ion formed from this dibromide would not be particularly favored. The data suggest that both the telluride and the iodide proceed via the E2-like mechanism of Fig. 14 with anti-periplanar transition state 34. 

Various other reagents can be used for this reductive debromination. 1,2-Dipropylcyclopropane was prepared from 4,6-dibromononane using chromium(II) perchlorate in dimethylformami-de/water (yield 93%), lithium amalgam in tetrahydrofuran (75%), lithium biphenylide in te-trahydrofuran (78%), potassium-sodium alloy in tetrahydrofuran (68%), zinc dust and zinc(II) chloride in propan-2-ol/water (95%) and alkyllithiums in tetrahydrofuran (BuLi 16%, i-BuLi 18%, t-BuLi 47%). Ring closure of 1,3-dibromobutane to methylcyclopropane was achieved by treatment with zero-valent copper, which was obtained from reaction of lithium naphthalen-ide and copper(I) iodide/tributylphosphane in tetrahydrofuran (yield 91%) ... 

Analogous trans-a,dditions of perfluoroalkyl iodides to terminal or internal alkynes in the presence of sodium dithionite and mediated by ultrasonic irradiation or catalysed by triethylborane have been reported. 1,2-Dibromotetrafluoroethane reacts with terminal acetylenes in DMF in the presence of the redox system (NH4)2S208/HC02Na.2H20 to yield a mixture of mainly (E)- and some (Z)-olefins 237, the initial adducts having undergone reductive debromination under the reaction conditions... 

Vicinal dibromides may be debrominated by treating them with certain reducing agents, including iodide ion and zinc. The stereochemical features of these reactions... 

Vicinal dibromides may be debrominated by treating them with certain reducing agents, including iodide ion and zinc. The stereochemical course of such reactions in 1,1,2-tribromocyclohexane was determined using a Br-labeled sample prepared by anti addition of Br to bromocyclohexene. Exclusive anti elimination would give unlabeled bromocyclohexene, while Br-labeled product would result from syn elimination. Debromination with sodium iodide was found to be cleanly an anti elimination, while debromination with zinc gives mainly, but not entirely, anti... 

Miscellaneous Processes.—Various types of reaction other than those discussed above have been investigated under PTC conditions, and only some recent instances are cited here. A ready debromination of 1,2-dibromides to alkenes (Equation 7) by aqueous iodide uses catalytic quantities of both a phosphonium salt (to transfer iodide ion) and iodide ion itself. The latter species is regenerated continuously by reduction of the halogen products with sodium thiosulphate in the... 

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