Dibromides, debromination

In a modified procedure the free carboxylic acid is treated with a mixture of mercuric oxide and bromine in carbon tetrachloride the otherwise necessary purification of the silver salt is thereby avoided. This procedure has been used in the first synthesis of [1.1.1 ]propellane 10. Bicyclo[l.l.l]pentane-l,3-dicarboxylic acid 8 has been converted to the dibromide 9 by the modified Hunsdiecker reaction. Treatment of 9 with t-butyllithium then resulted in a debromination and formation of the central carbon-carbon bond thus generating the propellane 10." ... 

The 2-deoxy derivative (215) of 202 was prepared from the diacetate 213, obtained by debromination of 34, following a similar sequence starting from the dibromide 214. 

Johnson, C. R. el. al, Tetrahedron Lett., 1964, 45,3327 Preparation of the 4-bromo compound by partial debromination of crude 3,5-dibromo-cyclopentene by addition of its ethereal solution to the aluminate in ice-cold ether is hazardous. Explosions have occurred on 2 occasions about 1 h after addition of dibromide. 

Dodecamethyl[6]radialene (95) is obtained when in situ produced tetramethylbutatriene (93) is trimerized with a Ni(0) catalyst after debromination of the dibromide 151 with Ni(0) (equation 14)65,66,68 (see also Section n.B). This approach was also successfully applied to the dibromides 136a and 136b which yield the electron-rich [6]radialenes 152a and 152b on treatment with [Ni(PPh3)4]/Zn-Cu in DMF (equation 15)98. 

Fig. 10 Plausible mechanism for debromination of v/cma/-dibromides with 2-thienyl-telluride.

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. 

Several other oxidation reactions of selenoxides and telluroxides are summarized in Fig. 23. Ley, Barton, and co-workers discovered that di-4-methoxyphenyltellur-oxide (54) could be used catalytically as an oxidant in the presence of 1,2-dibromotetrachloroethane. After reduction of the telluroxide to the telluride, the di-4-methoxyphenyltelluride (24) debrominated the 1,2-dibromotetrachloroethane to give the tellurium(IV) dibromide, which was hydrolyzed in situ to give the telluroxide 54. This process was used to oxidize phosphines to phosphine oxides and... 

A study of debrominations of vtc-dibromides promoted by diaryl tellurides and din-hexyl telluride has established several key features of the elimination process the highly stereoselective reactions of e/7f/tro-dibromides are much more rapid than for fhreo-dibromides, to form trans- and cw-alkenes, respectively the reaction is accelerated in a more polar solvent, and by electron-donating substituents on the diaryl telluride or carbocation stabilizing substituents on the carbons bearing bromine. Alternative mechanistic interpretations of the reaction, which is of first-order dependence on both telluride and vtc-dibromide, have been considered. These have included involvement of TeAr2 in nucleophilic attack on carbon (with displacement of Br and formation of a telluronium intermediate), nucleophilic attack on bromine (concerted E2- k debromination) and abstraction of Br+ from an intermediate carbocation. These alternatives have been discounted in favour of a bromonium ion model (Scheme 9) in which the role of TeArs is to abstract Br+ in competition with reversal of the preequilibrium bromonium ion formation. The insensitivity of reaction rate to added LiBr suggests that the bromonium ion is tightly paired with Br. ... 

A modification of an earlier procedure for debromination of v/c-dibromides in the presence of catalytic amounts of diorganotellurides has allowed the synthesis of terminal alkenes and cis- and frani-l,2-disubstituted alkenes from appropriate precursors the relative substrate reactivities suggest that, as for the stoichiometric reaction, the catalytic reaction involves intermediate bromonium ion formation. The Te(IV) dibromides formed in the debrominative elimination are reduced back to the catalysts by either sodium ascorbate or the thiol glutathione. 

Several methods achieving the debromination of v/c-dibromides by means of tellurium reagents are well established. These methods are particularly advantageous compared to the conventional ones in terms of the mildness of the experimental conditions, good yields, lack of important side reactions and inermess of several functionalities to the employed reagents. A relevant characteristic of these reactions is the high E2-type stereospecificity demonstrated by the formation of olefins with Z and E geometry from threo-and eryf/iro-dibromides, respectively. 

The tellurolate-promoted anfi -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-aIkenes and aUylic dibromides. ... 

Considering that diaryl tellurides debrominate vic-dibromides, forming diarylteUurium dibromides (see Section 4.1.12.1), which in tnm can be easily hydrolysed to the corresponding tellnroxides by aqueous bases, a catalytic procedure has been provided for the oxidation of thiocarbonyl compounds. 

Strained alkenes, including cyclobutenes 29 [120] and benzcyclobutadiene derivatives 30 [121], can be prepared by this route. Particularly advantageous is the wide range of reduction potentials for 1,2-dibromides, which allows some selective reactions. The dibromide 26 can be purified from any of the isomer 25 by reduction at -0.86 V vs. see when the diequatorial bromide remains unchanged and the diaxial compound is converted to the alkene [110], Bromination of dialkenes followed by selective debromination to recover one alkene leads to protection of the other alkene as the dibromide. Subsequently the second alkene can be recovered by reduction at more negative potentials [122],... 

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... 

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