V/c-Dibromides

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 electrochemical reduction of v/c-dibromides is a highly irreversible two-electron process in which two bromide ions are expelled leaving the correspond-... 

Problem 6.26 (a) What does each of the following observations tell you about the mechanism of the addition of Br2 to an alkene (i) In the presence of a Cl salt, in addition to the v/c-dibromide, some v/c-bromochloroalkane is isolated but no dichloride is obtained, (ii) With cis-2-butene only rac-2,3-dibromobutane is formed, (iii) With trans-2-butene only meso-2,3-dibromobutane is produced, (b) Give a mechanism compatible with these observations. M... 

Bisdehydrobromination of vic-dWromides." A route to N-acyl-l,2-dihydropyri-dines (3) involves bromination of an N-acyltetrahydropyridine followed by bisdehydrobromination of the v/c-dibromide (2). Usual methods, including DABCO or DBU in DMF,... 

The reagent is also useful for dehydrohalogenation of sec-alkyl bromides and of v/c-dibromides, for some Michael additions, and for aldol condensation.3... 

The sodium diethyl tellurolophosphates also catalyzed the conversion of v/c-dibromides to alkenes and the condensation of aromatic aldehydes with phenacyl bromides to 1 -benzoyl-2-arylethenes. ... 

The reaction of sodium 4-ethoxybenzenetellurolate with dichloromethane gave products of indefinite composition. However, the reactions of lithium 2-benzothiophenetellurolate with dichloromethane and lithium benzenetellurolate with diiodomethane yielded his[aryltelluro]methane, and those of lithium ethynetellurolates with equimolar amounts of chloroiodomethane gave chloromethyl ethynyl tellurium compounds. v/c-Dibromides and sodium 4-ethoxybenzenetellurolate reacted to produce ethenes and bis[4-ethoxyphenyl] ditellurium. ... 

Diaryl ditellurium compounds and v/c-dibromides, aryl tellurium trihalides diaryl tellurium dihalides, tellurium tetrahalides or sulfur tetrafluoride react to give diaryl tellurium dihalides and elemental tellurium. All of these reactions might proceed via aryl tellurium halides that subsequently disproportionate to the observed products ... 

It is known that pyridinium tribromide converts olefins with specific r/7 t/-dibromina-tion. Therefore, the v/c-dibromides thus obtained can be cathodically reduced to the original olefins without geometrical changes of molecular structure [218]. This stereospecific bromination-debromination may be useful as an olefin protection-deprotection method. 

Dehalogenation a-Bromo or a-chloro ketones are reduced by nickel boride in DMF to ketones in 70-95% yield. v/c-Dibromides are reduced to alkenes in 80-90% yield. 

In certain cases, dimsylsodium (24) will undergo selective nucleophilic substitution with bromides thus, v/c-dibromides (29) and gem-dibromides (30) are preferentially debrominated to the products (31) and (32) (Scheme 13). 

Considering that diaryl teUurides debrominate v/c-dibromides, forming diaryltellurium dibromides (see Section 4.1.12.1), which in turn can be easily hydrolysed to the corresponding telluroxides by aqueous bases, a catalytic procedure has been provided for the oxidation of thiocarbonyl compounds. 

Eliminations. Alkenes are formed rapidly from v/c-dibromides, )3-chloro-ethers, jS-iodoethyl esters," and /3-acetoxy sulfones" on treatment with SmL. The formation of furan derivatives from epoxypropargyl esters by reaction with... 

Debromination Ho and Wong have reported that some v/c-dibromides are debrominated by this very hindered base (DMF, reflux) in surprisingly high yields. On the other hand, indene dibromide does not undergo this reaction. 

The debromination of activated v/c-dibromides by trivalent phosphorus compounds has been demonstrated. The process is an E2 type (38, 39). The more potent dicyclohexylphosphide anion is capable of debrominating 1,2-dibromoethane (40). It is of interest to note that the slightly harder diphenyl-phosphide elects to attack on the carbon atom (Sn2). 

A bromide anion attacks at the back side of one carbon (or the other) of the bromonium ion in an S,j2 reaction, causing the ring to open and resulting in the fomnation of a v/c-dibromide. 

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