Trans-Stilbene 1.2- dibromo

Electrochemical reduction of a,a -dibromoketones affords the unstable cyclo-propanone, which is in equlibrium with the dipolar intermediate 22. The cyclopro-panone hemiacetal is isolated in yields of 40 - 85 % from reaction in acetonitrile and methanol at -20 °C [99], The dipolar form can be trapped in a cycloaddition process with furan [100], Reaction with acetic acid leads to the a-acetoxy-ketone.[101]. Unstable three membered heterocyclic rings are intermediate in the reduction of sulphur and phosphorus linked dibromo compounds 23. In these reactions, the heteroatom is extruded leaving ci - and trans-stilbenes as the isolated products [102,103],... 

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... 

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 trans-stilbene (262) in a DMF-Bu4NCl04-(Pt) system (Scheme 98) [400]. 

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). 

Scheme 7.43. A representation of the use of tin(n) chloride (stannous chloride, SnCl2) in the dehalogenation of racemic f/treo-l,2-dibromo-l,2-diphenylethane [( )-stilbene dibromide] to (mostly) ( )- (or trans) stilbene (94%) and a trace (6%) of (Z)- (or cis) stilbene. There is some evidence that this and related reactions may proceed through a process of one electron transfer reactions.

As versatile synthetic intermediates with a tetramethylindanylindane (stiff-stilbene) core, the cis and trans isomers of 5,16-dibromo-2,2,13, 13 -tetramethylin-danylindanes were synthesized by the Barton-Kellogg coupling [60]. 

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). 

---