Dibromo-l,2-diphenylethane

The Co(III) complexes with the 1,4,8,11-tetraazacyclotetradecane (cyclam) ligand are found to behave as redox mediators for the indirect electroreduction of meso-1,2-dibromo-l,2-diphenylethane (261) leading to tr ns-stilbene (262) in a DMF-Bu4NCl04-(Pt) system (Scheme 98) [400]. 

TABLE 6. Z E ratio for the reductive debromination of 1,2-dibromo-l,2-diphenylethane as a function of potential... 

Some papers have appeared that deal with the use of electrodes whose surfaces are modified with materials suitable for the catalytic reduction of halogenated organic compounds. Kerr and coworkers [408] employed a platinum electrode coated with poly-/7-nitrostyrene for the catalytic reduction of l,2-dibromo-l,2-diphenylethane. Catalytic reduction of 1,2-dibromo-l,2-diphenylethane, 1,2-dibromophenylethane, and 1,2-dibromopropane has been achieved with an electrode coated with covalently immobilized cobalt(II) or copper(II) tetraphenylporphyrin [409]. Carbon electrodes modified with /nc50-tetra(/7-aminophenyl)porphyrinatoiron(III) can be used for the catalytic reduction of benzyl bromide, triphenylmethyl bromide, and hexachloroethane when the surface-bound porphyrin is in the Fe(T) state [410]. Metal phthalocyanine-containing films on pyrolytic graphite have been utilized for the catalytic reduction of P anj -1,2-dibromocyclohexane and trichloroacetic acid [411], and copper and nickel phthalocyanines adsorbed onto carbon promote the catalytic reduction of 1,2-dibromobutane, n-<7/ 5-l,2-dibromocyclohexane, and trichloroacetic acid in bicontinuous microemulsions [412]. When carbon electrodes coated with anodically polymerized films of nickel(Il) salen are cathodically polarized to generate nickel(I) sites, it is possible to carry out the catalytic reduction of iodoethane and 2-iodopropane [29] and the reductive intramolecular cyclizations of 1,3-dibromopropane and of 1,4-dibromo- and 1,4-diiodobutane [413]. A volume edited by Murray [414] contains a valuable set of review chapters by experts in the field of chemically modified electrodes. 

A simple and efficient method for the debromination of vzc-dibromidcs to ( )-alkenes utilized the Sm-TMSC1-H20 catalytic system (Scheme 8.19). For example, franj-stilbene was produced from 1,2-dibromo-l,2-diphenylethane within 5 h in good yield at room temperature. The benzylic vz odibromides similarly gave the corresponding ( )-alkenes in a high yield. In the case of 1,2-dibromocyclohexane, a longer reaction time was needed to obtain cyclohexene. This is perhaps because the radical or anion intermediate of an aliphatic... 

To a solution of mew-1,2-dibromo-l,2-diphenylethane (2mmol) in benzene (5 ml) was added a 30% aqueous Na2CS, solution (6 mmol) and trioctylmethylamroonium chloride (0.08 mmol). The mixture was stirred... 

Dehalogenation of vicinal polyhalides with nickel was then investigated to establish that the second step in Equation 7.11 proceeds as shown. 1,2-Dibromo-l,2-diphenylethane (meso) reacted with metallic nickel at room temperature to yield stilbene trans) in 89% yield and a trace amount of cw-stilbene. Similarly, ethyl 2,3-dibromo-3-(4-nitrophenyl)propanoate (erythro) underwent debromination to give ethyl 4-nitrocinnamate (tram) in 76% yield. Dechlorination of l,2-diphenyl-l,l,2,2-tetrachloroethane at 85 C afforded 1,2-dichloro-l,2-diphenylethene (97%, cis trans = 70/30). 

On treatment with the aromatic base pyridine (Sec. 31.8), racemic 1,2-dibromo-1,2-diphenylethane loses HBr to yield /rfl/w-l-bromo-l,2-diphenylethene in contrast, the meso dibromide loses Bri to yield /ra/w-l,2-diphenylethene. (a) Suggest a mechanism for the reaction of each stereoisomer, (b) How do you account for the difference in their behavior ... 

Methyl eryr/zA o-2,3-dibromo-3-phenylpropanoate Methyl threo-2,3-dibromo-3-phenylpropanoate Methyl erythro- 2,3-dibromo-4-oxophenylbutanoate Methyl threo- 2,3-dibromo-4-oxophenylbutanoate Meso-1,2-dibromo-1,2-diphenyle thane c ,/-l,2-dibromo-l,2-diphenylethane... 

Fig. 4. Catalytic reduction of mew-l,2-dibromo-l,2-diphenylethane (DBDPE) at poly-/J-coated Pt. cyclic voltammograras in 0.1 M TBAP/acetonitiile at 0.1 V s- scan rate. A 16 at bare Pt B DBDPE at poly-/J (F = 1 x 10" mol cm" ) C poly-f5 in clean el trolyte (from ref.

Elimination of HBr from m 50-l,2-dibromo-l,2-diphenylethane gave cis-2-bromostilbene, while the ( + ) or ( —) isomer gave the trans alkene. This stereospecific result, which was obtained in 1904, demonstrates that in this... 

The stereochemistry of addition reactions of Br2 to olefins is difficult to control. For example, the reaction of ( )-stilbene (1) with Br2 in CH2CI2 gives a 84 16 mixture of meso- (2) and rac-l,2-dibromo-l,2-diphenylethane (3) in 98%... 

When diphenylacetylene dissolved in cold chloroform reacts with iodine monofluoride suspended in trichlorofluoromethane, the iodine atoms in the primary addition product are easily replaced by fluorine to give l,1,2,2-tetrafluoro-1,2-diphenylethane (60%) along with benzil (10%). Since the C —Br bond is stronger than the C —I bond, the reaction of bromine monofluoride with diphenylacetylene gives 1,1-dibromo-2,2-difluoro-1,2-diphenylethane (65%) and benzil (15%). 

Stilbene adds bromine relatively slowly, trans-Addition of bromine, which is usual with olefins, occurs with cw-stilbene in cold CS2 in the dark, affording 83% of (db)-l, 2-dibromo-1,2-diphenylethane (meso-isomer, m.p. 236°. The former product is converted into the latter by, e.g., bromine or iodine in CC14 in diffuse daylight.39 The meso-compound arises as main product on bromination of trans-stilbene in CS240 or ether.41 ( + )-m, .p. 93-94°, and meso- 1,2-di-chloro-l,2-diphenylethane (a.a -dichlorobibenzyl), m.p. 191-193°, are both formed in the reaction of trans-stilbene with a saturated solution of chlorine in anhydrous ether under the influence of ultraviolet or sun-light.39,41... 

Most studies of bromine addition to alkenes have presumed that the intermediate proceeds to product and does not revert to alkene and bromine. Brown and co-workers determined that bromonium ions generated from the solvolysis of the fra s-2-bromo-l-brosylates of wclohexene or cyclopentene could react with added Br to produce Br2- Furthermore, erythro-2-hTomo-l, 2-diphenylethanol was foimd to react with anhydrous HBr (in 1,2-dichloroethane or chloroform) to produce both frans-stilbene and meso-l,2-dibromo-l,2-diphenylethane. The reaction of the erythro diastereomer can be explained by a mechanism involving anchimeric assistance in departure of water, which leads to a bromonium ion that reverts to the stilbene, as shown in Figure 9.10. ... 

The first example of Table 7.13 illustrates that preferential anti- (trans or antarafacial) elimination is encountered with ethoxide in ethanol from an acyclic substrate. Only one product is reported from the (shown) racemic threo-dibromide [( )-l,2-dibromo-l,2-diphenylethane (m-stilbene dibromide)] as well as from the (not shown) diastereomeric cryf/iro-dibromide [me50-l,2-dibromo-l,2-diphenylethane (trans-stilbene dibromide)] in the first step of the eUmination. Interestingly, both isomers also undergo a second elimination [shown from (Z)-l-bromo-l,2-diphenylethene] to 1,2-diphenylethyne (diphenylacetylene). 

Once Mechanism 1 has been disearded Mechanism 2 is the alternative of choiee. The formation of a bromonium ion intermediate seems to justify very well all the experimental data we have cheeked. However, we cannot decide that Mechanism 2 is the right option without diseussing a final point of evidenee The odd stereoselectivities obtained in the debromination of l,2-dibromo-l,2-diphenylethane 2. The debromination is stereoseleetive in the case of the erythro-isxivast but the threo- yields a mixture of cis/trans-o Qfvas,. 

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