Stilbene dibromide, elimination

The preference for a fi-stereospecificity is demonstrated experimentally in the acyclic series by use of suitable diastereoisomeric pairs. The we o-stilbene dibromide eliminates to give the cw-bromostilbene and the ( )-dibromide yields the rra j-olefin (77) . ... 

Let us now turn to the experimental results to see if these predictions are borne out in fact. It has long been known that E2H reactions normally give preferentially anti elimination. For example, reaction of ffz fo-stilbene dibromide with potassium ethoxide gives a j-bromostilbene (Reaction 7.39), whereas reaction of the D,L-dibromide gives the trans product (Reaction 7.40).102 A multitude of other examples exist—see, for example, note 64 (p. 355) and note 82 (p. 362). 

Though a choice of paths is available, the additions and eliminations of (154) and (155) are anti stereospecific. Two different eliminations occur with the stilbene dibromides in (156) (Kwok and Miller, 1967b), but they are both anti which path is taken is determined by conformational energy factors to be discussed later. 

A different effect of these product-like transition states is seen in the fact that the overall debromination of df-stilbene dibromide with lithium bromide may proceed in the syn sense. That is, the rate ratio at 59° k(meso)/k(dl) = 50 is composite. In the dl compound, electronic factors which favor anti elimination collide with steric or conformational factors, which favor either bimolecular syn eliminations or some other path to frans-stilbene. The rate ratio for production of trans-stilbene k meso)jk dl) 60, while that for anti elimination is k(meso)lk(dl) 310. 

Several debrominations using triphenylphosphine have been reported. Stilbene dibromides and other vicinal dibromides are debrominated stereospecifically in an fl /i-elimination. 9,9-Dibromofluorene and dibromodiphenylmethane (43) have been converted into the corresponding ethylene, and the compounds (44) gave moderate yields of aroyl cyanides when fused with triphenylphosphine. ... 

The mechanisms of dehalogenations have been reviewed by Miller and in a series of papers , the stereoselectivity of the dehalogenation of the stilbene dibromides with a wide variety of reagents has been discussed. The meso-stilbene dibromide always eliminates to give the thermodynamically more stable alkene, namely tra 5-stilbene which is product of apparent a t/-elimina-tion. However, the J/-stilbene dibromide gives both cis- and rm i-stilbenes, and the ratio of these products can provide useful mechanistic information. One-electron reductants, such as chromous ion, give rise to intermediate radical formation in which rotation about the Ca-Cg bond allows thermodynamic control of the reaction. Two-electron reductants, such as iodide ion in dimethyl formamide, induce highly stereoselective a i-elimination. In protic solvents, carbonium ion intermediates were proposed to explain the trend towards thermodynamic control. Miller has proposed a reaction mechanism which embraces elimination, substitution, and electrophilic addition to alkenes. 

If meso-stilbene dibromide is treated with KOH in ethanol, it is possible to isolate the l-bromo-l,2-diphenylethylene that is formed from the first dehydrobromination. The E2 elimination of the second molecule of hydrogen bromide from this intermediate alkene to give diphenylacetylene has a higher activation enthalpy than the first elimination and thus requires a higher reaction temperature. Explain. 

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). 

Tri- -butyltin hydride debrominated 1,2-dibromopropane at room temperature in 3 hours, pelding 81% of propylene i io-stilbene dibromide required heating to 100° C for 15 hours for complete reaction, giving 85 % of traw5-stilbene. It is not known whether this is the result of a highly selective irons elimination, or simply the consequence of formation of the most stable isomer 32). A dehalogenation mechanism similar to that involved in dehalogenation by metals such as zinc is unlikely because no cyclopropane is formed in the reaction of 1,3-dibromopropane with tri-n-butyltin hydride. 

The bromo-aryl groups are first linked by (5,5 )-stilbene diol to form the dibromide 33. Compound 33 is then dilithiated with t-BuLi at —78°C, followed by addition of CuCN. Intermediate 34 is presumably formed during the reaction. Reductive elimination promoted by molecular oxygen provides compound 35 at 77% yield with 93 7 diasteroselectivity. The final biaryl compound ellagi-... 

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