Debromination, of vicinal dibromides

In some useful synthetic procedures, the carbanionic character results from a reductive process. A classical example of the -elimination reaction is the reductive debromination of vicinal dibromides. Zinc metal is the traditional reducing agent.209 A multitude of other... 

Fig. 13 A plausible mechanism for the debromination of vicinal-dibromides with diorganotellurides.

It was found that the use of a triflate salt of PS-PAPT (0.1 equiv.) in conjunction with sodium hydride (2.5 equiv.) could be a remarkably effective method for achieving dehydrohalogenation of RX or debromination of vicinal dibromides to the corresponding olefins [84] (Scheme 6.23). In this case, deprotonation of a triflate salt of PS-PAPT with sodium hydride may have generated the actual strong base species, since sodium hydride itself did not work well or at all under the same reaction conditions. 

The intermediacy of bromonium ions in the debromination of vicinal dibromides is consistent with the formation of stilbene from stilbene dibromide in 1,2-dichloroethane solution. Bellucci, G. Bianchini, R. Chiappe, C. Brown, R. S. Slebocka-Tilk, H. /. Am. Chem. Soc. 1991,113,8012. See also Zheng, C. Y. Slebcx ka-Tilk, H. Nagorski, R. W. Alvarado, L. Brown, R. S. /. Org. Chem. 1993, 58, 2122. 

In the dehalogenation reactions, ILs used for debromination can be achieved by using a metal like Zn, Mg, or In, in an organic solvent, such as tetrahydrofuran (THF) or methyl alcohol (MeOH). Ranu et al. [21, 22] reported that IL [pmim] [BFJ can be used as a catalyst as well as a reaction medium for the stereoselective debromination of vicinal dibromides to the corresponding ( )-alkenes under MW irradiation. 

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

Organotellurides 1 have been used in debromination of vicinal (vie) dibromides. The reaction takes place by heating the dibromide and the organotelluride in CHCI3 or CH3CN at 90-100°C as shown in Scheme 29.1. 

Fig. 24. Schematic effect of dihedral angle the magnitude of the coupling constant for vicinal protons, Ji,2i or the half-wave potentials for the reduction of cyclic vicinal dibromides (polarographio debromination), A = — JBj/2 (volts vs. S.C.E.) or A = 2D2, the sum of the squares of the displacement of the atoms in dehydrochlorination of ethyl chloride by PLM.

Unlike vicinal dibromides such as 1,2-dibromoethane [124] and //mr-10,11-dibromodibenzosuberone [125] which undergo photochemical loss of bromine followed by facile thermal cleavage of the second C—Br bond, 2-(l,2-dibromo-ethyl)naphthalene (211) and 2-bromo-9-(l,2-dibromoethyl)anthracene (212) [126] require two photons for complete debromination (Scheme 9). 

Mechanism 7-1 Dehydrohalogenation by the E2 Mechanism 304 Mechanism 7-2 Stereochemistry of the E2 Reaction 306 Mechanism 7-3 E2 Debromination of a Vicinal Dibromide 310... 

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. 

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. ... 

Several debrominations using triphenylphosphine have been reported. Stilbene dibromides and other vicinal dibromides are debrominated stereospecifically in an fl /i-elimination. 9,9-Dibromofluorene and dibromodiphenylmethane (43) have been converted into the corresponding ethylene, and the compounds (44) gave moderate yields of aroyl cyanides when fused with triphenylphosphine. ... 

Iwunze, M.O., N. Hu, and J.F. Rusling (1992). Electrocatalysis in organized assemblies—part X Debromination of alkyl vicinal dibromides with nentral metal phthalocyanines in bicontinuous microemulsion. J. Electroanal. Chem. 333(1-2), 353-361. 

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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