Anti-debromination

The tellurolate-promoted anti-debromination by method G is extended to the preparation of conjugated dienes, starting from 1,2,3,4-tetrabromoalkanes and cycloalkanes, 1, 4-dibromo-2-alkenes and allylic dibromides. ... 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

A good diastereocontrol is obtained for the debromination of Reaction (4.14) and it is attributed to the bulky reducing agent, which approaches the radical intermediate from the less hindered face anti to the two vicinal substituents [35]. 

A different effect of these product-like transition states is seen in the fact that the overall debromination of df-stilbene dibromide with lithium bromide may proceed in the syn sense. That is, the rate ratio at 59° k(meso)/k(dl) = 50 is composite. In the dl compound, electronic factors which favor anti elimination collide with steric or conformational factors, which favor either bimolecular syn eliminations or some other path to frans-stilbene. The rate ratio for production of trans-stilbene k meso)jk dl) 60, while that for anti elimination is k(meso)lk(dl) 310. 

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. 

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. 

E2 debromination of vicinal vic-dibromides is most favoured when the two bromine atoms are periplanar and anti (Scheme 4.6). 

Debromination of 1,2-dibromoalkanes. Bis(trimethylsilyl)niercury de-brominates 1,2-dibromoalkanes to olefins. This reaction has been applied to 1,2-dibromo- and 1,2-diiodoadamantane (2) as a route to adamantene (a). When performed in the presence of 1,3-diphenylisobenzofuran, the hydrocarbon 3 is obtained in 30-35% yield. In principle, this route could be extended to isolation of adducts of other anti-Bredt alkenes. 

Vicinal dibromides can be debrominated by certain reducing agents, including iodide ion. The stereochemical course in the case of 1,1,2-tribromocyclohexane was determined using a Br-labeled sample prepared by anti addition of Br2 to bromo-cyclohexene. Exclusive anti elimination gave unlabeled bromocyclohexene, whereas Br-labeled product resulted from syn elimination. Debromination with sodium iodide was found to be cleanly an anti elimination. ... 

Stereospecific anti elimination in the debromination of meso- (top) and racem/c-2,3-dibromobutane (bottom). 

The relative reactivities of two diastereomeric vicinal dibromide derivatives of cholanic acid also provided evidence for an anti-coplanar orientation of the two carbon-bromine bonds in iodine-promoted dehalogenation. In the lla,12)3-dibromo compound 20, each of the two bromine atoms is held in an axial conformation by the rigid steroid skeleton. On the other hand, each of the two bromine atoms is held in an equatorial conformation in the llj8,12a-dibromo isomer 21. As a result, the conformation of the Br—C—C—Br grouping is anti in 20 but is gauche in 21. When treated with sodium iodide in acetone, 20 underwent debromination readily, while 21 was unreactive under the same conditions. 

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