Elimination reactions debromination

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... 

Dealkoxylation (see Hydrogenolysis, Reduction of acetals and ketals) Debenzylation (see Reduction reactions) Debromination (see Elimination reactions)... 

Vicinal dibromides may be debrominated by treating them with certain reducing agents, including iodide ion and zinc. The stereochemical course of such reactions in 1,1,2-tribromocyclohexane was determined using a Br-labeled sample prepared by anti addition of Br to bromocyclohexene. Exclusive anti elimination would give unlabeled bromocyclohexene, while Br-labeled product would result from syn elimination. Debromination with sodium iodide was found to be cleanly an anti elimination, while debromination with zinc gives mainly, but not entirely, anti... 

The use of alumina and bentonite as heterogeneous catalysts for various organic reactions is well documented (Caddick et al., 1995 Gedye et al., 1988 Gutierrez et al., 1989 Smith, 1992). 1,2-Dibromocompounds with KF supported on alumina, or bentonite/EtjN under microwave irradiation leads to an elimination reaction, which produces functionalized alkenes in good yield. Elimination from dibromide carried out over KF-alumina under microwave irradiation led to an isomeric mixture of a-bromoalkenes, and in some cases, debromination occurs to give alkenes (Saoudi etal., 1998). 

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. 

Swartz and Stenzel (1984) proposed an approach to widen the applicability of the cathode initiation of the nucleophilic substitution, by using a catalyst to facilitate one-electron transfer. Thus, in the presence of PhCN, the cathode-initiated reaction between PhBr and Bu4NSPh leads to diphe-nydisulfide in such a manner that the yield increases from 10 to 70%. Benzonitrile captures an electron and diffuses into the pool where it meets bromobenzene. The latter is converted into the anion-radical. The next reaction consists of the generation of the phenyl radical, with the elimination of the bromide ion. Since generation of the phenyl radical takes place far from the electrode, this radical is attacked with the anion of thiophenol faster than it is reduced to the phenyl anion. As a result, instead of debromination, substitution develops in its chain variant. In other words, the problem is to choose a catalyst such that it would be reduced more easily than a substrate. Of course, the catalyst anion-radical should not decay spontaneously in a solution. 

For the cine amination one would normally consider a mechanism involving the formation of thiophyne. However, several pieces of evidence lead to the rejection of the thiophyne mechanism among them, the marked dependence of product distribution on amide concentration, and the non-formation of aminated product in the reaction of 2-bromo-5-methylthiophene under the same conditions. A normal addition-elimination mechanism (2-bromo -> 3-bromo -> 3-amino) is also invalid since the conversion of 3-bromothiophene to 3-aminothiophene under the same conditions is 200 times slower than the conversion 6f 2-bromothiophene to the 3-amino compound. There is also no evidence from NMR of any initial adduct formation. Considering all these facts, the proposed mechanism (Scheme 164) for cine amination involves attack by amide ion at the /8-position of a di- or tri-bromothiophene to form an aminobromothiophene and subsequent debromination of this species. In support of this is cited the fact that the individual polybromothiophenes are converted to 3-aminothiophene by KNH2 in ammonia. Also, 4-amino-2-bromothiophene (485) has been isolated in the reaction of 2-bromothiophene with sodamide. 

Note that log (k,, /k I20) versus X.o, produces a better fit to the experimental data for both the dechlorination and debromination reactions than does log kOH or log kH20 versus X.o,. This is probably because the former approach eliminates steric effects. 

Catalytic hydroboration of vinylic ethers, acetals, and esters with pinacolborane takes place smoothly in the presence of transition metal catalysts. However, a noticeable exception is the catalytic hydroboration of vinyl bromides 59 which do not furnish the expected hydroborated product under these conditions. The reaction of vinyl bromides with pinacolborane initially affords the expected /3-boronoalkylbromide 60. A fast. -elimination ensues to furnish the terminal alkene 61 and 7 -bromopinacolborane 63. The alkene 61 undergoes hydroboration with unreacted pinacolborane to provide the debrominated boronate 62. The intermediate 5-bromopinacolborane 63 cleaves the ethereal C-O bond in the solvent (THF) to provide 4-bromobutyl borate 64 as a side product (Scheme 11) <1996JA909, 2000CSP14505>. 

Evidently, on changing the ethyl ester to methyl ester in bromoacids, the pyrolytic eliminations will be limited only to a debromination process. In this respect, Table 21 describes the results on the pyrolysis of methyl bromoesters under maximum inhibition170 and the data provide further evidence for COOCH3 participation in the elimination from methyl co-bromoesters. By analogy with previous papers16 168,169 the mechanisms of these reactions were also explained in terms of an intimate ion-pair intermediate as for equation... 

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. 

Aryl-substituted vic-dibromides undergo debromination to produce the corresponding E-alkenes when treated with indium metal in MeOH. Since debromination occurs by the usual trans-elimination, meso/erythro- and d,Z-/f/zreo-vic-dibromides would give trans- or cis-alkenes, respectively, as shown in Scheme 4.6. It is thus suggested that in this case the reaction occurs via a common relatively stable radical or anion intermediate, which directly collapses to -alkene. 

A copper-containing oxocarbenoid is assumed to be involved in the formation of both the cyclopropanes 9 and the 4,5-dihydrofurans 10. It is therefore not surprising that the formal cyclotrimers of oxocarbenes, triacylcyclopropanes 12, were obtained in the absence of suitable reagents that could trap the oxocarbenoid. Cyclopropanes 12 are likely to be formed via the formal oxocarbene dimers 11, which in some cases are also found among the reaction products. In addition to the copper-mediated debromination of a,a-dibromo ketones, triacylcyclopropanes can also be obtained from a,a-dihalo ketones by metal-induced a-elimination in other cases, e.g. from a,a-dibromo ketones with Ni(0), Fe(0), or Co(0) complexes,", and from a,a-dichloro ketones with a zinc-copper couple. 

Reductive deconjugation of 2-bromo-2-alkenoates. The process probably involves Michael addition, debromination, and phosphite elimination prior to the kinetic protonation of the ester enolates. Triethylamine is required as base to promote the reaction. 

Elimination. The preparation of the bis[methyl fSHactyl] acetylenedicarboxylate involves alcoholysis of dibromofumaryl chloride and debromination with Zn in refluxing THF. Asymmetric induction in the Diels-Alder reactions of the chiral diester has been probed. 

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