Threo-dibromides

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). 

The bromination products, dibromide in methylene chloride and methoxybromide in methanol, are a mixture of erythro- and threo-diastereoisomers, obtained in a ratio, Erythro/Threo = 70 / 30, which does not depend on the substituents or on the solvent. As expected, the reaction in the protic solvent is fiilly regioselective, i.e. methanol only traps the intermediate... 

When the bromination of the unsubstituted P-methy 1-styrenes (ref. 19) is carried out in methylene chloride, the two diastereoisomeric dibromides are obtained in ratios of 72 threo/28 erythro and 20 threo/80 erythro for the cis and trans isomers, respectively. This result agrees fairly well with a partially bridged intermediate, since the corresponding benzylic carbocation leads to a 65 erythro/35 threo ratio (ref. 20). When the same reactions are carried out in methanol, the... 

This indicated that retention had taken place. Note that both products are optically inactive and so caimot be told apart by differences in rotation. The meso and d/ dibromides have different boiling points and indexes of refraction and were identified by these properties. Even more convincing evidence was that either of the two threo isomers alone gave not just one of the enantiomeric dibromides, but the dl pair. The reason for this is that the intermediate present after the attack by the neighboring group (17) is symmetrical, so the external nucleophile Br can attack... 

The stereospeciflcity of dehalogenation of vicinal dibromides to olefins was examined for reducing agents including Cr(II), iodide, and Fe° (Totten et al. 2001). For dibromostilbene, the ( )-stilbene represented >70% of the total olefin that was produced, and for threo-dibromopentane reduction by Cr(Il) produced ca. 70% of ( )-pent-2-ene, whereas values for iodide and Fe° were <5% of this. 

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. 

To account for the production of bromo chlorides and bromo nitrates, it is proposed that the reaction of anions (Br, Cl , NO ) in step 5 is very fast. Independent evidence for the existence ot a metastable, cyclic bro-monium ion comes from the observation that the bromine atoms are usually in a threo (trans) orientation in the final product and that in the reverse reaction of acetolysis of dibromides, there is retention of configuration. This would be consistent with the Br-assisted SnI ionization of the dibromide. The assisting Br atom would attack the neighboring C atom from the back side to the leaving Br ion in a typical Walden inversion. Thus from irans-butene-2 we obtain erythro-2,3-dihromo butane ... 

Upon ionic addition of bromine, cii-l-phenyl-1-propene gives a mixture of 17% erythro dibromide and ST/o- three /ra/i5-l-phenyI-l-propene gives 88% erythro, 12% threo and /m/75-l-(p-methoxyphenyl)propene gives 63% erythro, 37% threo. 

First, Winstein and Lucas found (Fig. 28.3) that (racemic) erythro bromo-hydrin yields only the meso dibromide, and (racemic) threo bromohydrin yields... 

This may be illustrated by re-examining a reaction that we have already studied. Earlier, we saw that when a 1,2-dibromide was treated with iodine, the reaction followed a normal anti-E2 pathway to eliminate a bromine molecule, which meant that the threo isomer yielded the trans isomer. If this elimination reaction is performed when either, or both, of the bromine atoms is, or are, attached to a primary carbon (instead of a secondary or tertiary carbon as was the case when we first looked at this reaction), then a different stereochemistry results. When this experiment is performed with deuteriated derivatives, it becomes apparent that the threo isomer gives the cis isomer. Suggest an explanation for this result. 

Tike zinc, the new reagent effects trans-elimination of dibromides however, erythro- and threo-a-hydroxy bromides give the same mixture of cis- and transolefins.5 Likewise reduction of either cis- or fran.v-4-t-butyl-1 -chloro-1 -methyl-cyclohexane (1 or 2) affords predominantly (62 6%) cis-4-t-butyTTmethyl-cyclohexane (3).6 If butanethiol is added to the reduction (method of Barton and... 

In agreement with earlier studies on the chalcone dibromides , the dehydro-chlorinations of 4,4 -dichlorochalcone dichlorides are not stereospecific . The threo isomer yields the trans olefin chiefly (see (30) below), as expected for elimination of HX from an " anti conformation (see Section 2.3), but the erythro isomer gives a mixture of olefins in which the trans form predominates over the cis by a factor of 2 1 (31). 

Though only (-)-e2yt/3ro-dibromide was obtained on the bromination of trans-cinnamic acid, the chlorination gave + )-erythro- and threo-dichlo-rides the e.e. of the trans-adduct reached 37 % (calculated from the JJ IQO value [a]p +67.3°in literature " ) and the... 

In the present case, conjugate addition has the potential to result in two diastereomers. As in the case of the starting dibromide, these stereoisomers can be either erythro or threo. The distribution between these two products is important because it ultimately determines the ratio of products in the next step (third) of the reaction. 

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