Debromination iodide

It is not possible to use zinc for reductive debromination in the presence of (x-halo ketones and for transformations involving these intermediates, sodium iodide has been used. ° In some instances, e.g. 5,6-dihalo-3-ketones, iodide does not always give a completely halogen-free product, and zinc does not give clean debromination. The use of chromous chloride has proved advantageous in such cases and is the reagent of choice for vicinal dichlorides, which are inert to iodide ... 

Metljylcoumarone has been prepared by the cyclization of ethyl a-phenoxyacetoacetate followed by hydrolysis and decarboxylation of the resulting ethyl 3-methylcoumarilate,3 4 by debromination and rearrangement of 3,4-dibromo-4-methyl-coumarin to 3-methylcoumarilic acid followed by decarboxylation,4-6 by cyclization of phenoxyacetone with concentrated sulfuric acid,6 and by treatment of 3-coumaranone with methyl-magnesium iodide followed by dehydration of the resulting car-binol.7... 

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),... 

Fig. 12 Relative rates of debromination of v/c/na/-dibromides 27-30 with di-n-hexyl-telluride (26) and tetra-n-butylammonium iodide.

In contrast to the rate of debromination of 30 with telluride 26, the debromination of dibromide 30 with iodide is much more rapid. In general, the rate of debromination with the anionic iodide is several orders of magnitude faster than with the uncharged dihexyltelluride 26. The debromination with iodide most likely proceeds via the E2-like transition state 32 (Fig. 12), which avoids the strain associated with bromonium ion 31." " Diorganotellurides should also be capable of following a similar mechanism. 

Fig. 14 Plausible E2-like transition state for the debromination of 1,2-dibromoalkanes with either diorganotellurides or iodide.

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... 

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. 

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. 

E2 debromination takes place by a concerted, stereospecific mechanism. Iodide ion removes one bromine atom, and the other bromine leaves as bromide ion. 

Predict the elimination products formed by debromination of the following compounds with iodide ion in acetone. Include stereochemistry, and give a correct name for each product. 

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. 

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