2.3- Dibromobutane from 2-butenes

The concept of "selectivity" must be clearly distinguished from the term "specificity" [4][5], Specific, applied to a reaction, means that two (or more) isomeric starting materials give -under the same reaction conditions- different reaction products which are also isomers. Depending upon the isomers we may be considering, we may refer to "regiospecificity" (structural isomers) or to "stereospecificity" (either diastereospecificity or enantiospecificity). For instance, the formation of we5o-2,3-dibromobutane by addition of bromine to ( )-2-butene, in contrast with the formation of the d,l - 2,3-dibromobutene from the (Z)-2-butene, is a case of diastereospecificity. 

Problem 8.33 Show reagents and reactions needed to prepare the following compounds from the indicated starting compounds, (a) Acetylene to ethylidene iodide (1,1-diiodoethane). (b) Propyne to isopropyl bromide, (c) 2-Butyne to racemic 2,3-dibromobutane. (d) 2-Bromobutane to trans-2-butene. (e) n-Propyl bromide to 2-hexyne. (/) 1 -Pentene to 2-pentyne. ... 

Addition of Br2 to 2-butene leads to the formation of 2,3-dibromobutane and to the generation of two chirality centers. What stereochemistry should wc predict for such a reaction Starting with planar, achiral cis-2-butene, Br.> can add to the double bond equally well from either the top or the bottom face to generate two intermediate bromonium ions. For the sake of simplicity, let s consider only the attack from the top face, keeping in mind that every structure we consider also has a mirror image. 

The bromonium ion formed by addition to the top face of cis-2-butene can be attacked by Br ion from either the right or the left side of the bottom face, as shown in Figure 9.17. Attack from the left (path a) leads to (2S,3S)-dibromobutane, and attack from the right (path b) leads to (2i ,3K)-dibromobutane. Since both modes of attack on the achiral bromonium ion are equally likely, a 50 50 (racemic) mixture of the two enantiomeric products is formed. Thus, we obtain ( )-2,3-dibromobutane. 

We can conclude from this representation that the elimination reaction of meso-2,3-dibromobutane to form cis-2-bromo-2-butene proceeds via a trans-elimination. 

Although little is known concerning the identity of the transition state for these iodide induced reactions, the strict adherence to anti stereospecificity indicates the importance of rr-orbital overlap and a high degree of double bond character seems probable. The 6-fold rate difference (almost the maximum predicted from eclipsing of methyl substituents) between the rates of formation of Irani-and c/i-2-butenes from 2.3-dibromobutane supports this hypothesis -. ... 

The addition of deuterium bromide to both cis- and tra s-2-butene proceeds in a stereospecific trans manner at low temperature. The dx-olefin yields three while the trans gives the eryihro bromide. Similarly, the addition of HBr to isomeric 2-bromo-2-butenes is stereospecific at low temperature and in excess of HBr. The stereospecificity decreases as the temperature of the reaction is increased. At room temperature, both olefins yield the same mixture of products. Goering and Larsen first suggested that two different conformations are involved as intermediates from ds- and trons-olefins. The lifetime of these two conformations is so short that they cannot interconvert prior to the chain transfer step, which takes place from the less hindered side. At room temperature, however, these can obtain equilibrium rapidly because of easy C-C bond rotation, which results in the same mixture of meso- and d,l-2,3-dibromobutanes. Another reason may be that the addition of bromine radical (Br ) to noncyclic olefins is often reversible and may lead to nonstereospecific products. The second mechanism assumes a tr-complex formation between olefin and HBr. A bromine atom then collides with the complex leading to its attachment and simultaneous breaking of the HBr bond, which explains the decrease in stereospecificity with rising temperature (Scheme 4.53). 

The stereochemistry of a solvolysis reaction can be affected if the substrate has a substituent that can donate a pair of electrons to the developing carbocation center. For example, treatment of ( )-t/zreo-3-bromo-2-butanol (19) with HBr gave only the racemic 2,3-dibromobutane (20). There was none of the meso compound that would have been expected if the reaction involved protonation, loss of water, and formation of a free carbocation intermediate. Similarly, reaction of ( )-eri/tizra-3-bromo-2-butanol with HBr gave only meso-2,3-dibromobutane. The reaction of 19 seems best explained by nucleophilic participation of the bromine on the adjacent atom in concert with departure of the water. The result is a bridged intermediate (21) that is the same bromonium ion expected from the electrophilic addition of Br2 to cis-2-butene (Figure 8.13). Back-side attack by bromide ion on either carbon atom involved in the three-membered bromonium ring is equally likely, so a racemic mixture results. 

Now, (Scheme 6.16) having begun with cis- or (Z)-2-butene, the net result will be to form racemic or ( )- or a 1 1 mixture of the enantiomers (2R,3R)-2,3-dibromobutane and (25,3S )-2,3-dibromobutane (the original attack occurring with equal probability from the top or bottom face). Interestingly, each of these enantiomers has a twofold rotating axis of symmetry and, although chiral, is thus not asymmetric. They are dissymmetric. 

Elimination of halogen from a,P-dihalides in a variety of circumstances has been observed and can be used to prepare alkenes and alkynes from vicinyl dihalides. The process is usually of limited utility since the dihalides themselves are often prepared from the unsaturated compounds. As shown in Scheme 7.42, both erythro (meso) and threo ( )-2,3-dibromobutane diastereomers undergo stereospecific elimination of bromine to their respective (E)- (or trans) and (Z)- (or cis) 2-butenes on treatment with iodide anion (T) in ethanol (ethyl alcohol, CH3CH2OH). 

The Effl eliminations with leaving groups other than a proton are as stereoselective as the examples cited above. Thus elimination of bromine from 2,3-dibromobutane by iodide ion is stereospecifically trans, as is shown by the exclusive formation of frans-2-butene (44) from m so-dibromide (43) and of cis-2-butene (46) from rfZ-dibromide (45). 

One currently accepted mechanism for the addition of two equivalents of HX to an alkyne involves two steps for each addition of HX addition of (from HX) to form a carboca-tion, followed by nucleophilic attack of X". Mechanism 11.1 illustrates the addition of HBr to 1-butyne to yield 2,2-dibromobutane. Each two-step mechanism is similar to the two-step addition of HBr to cw-2-butene discussed in Section 10.9. 

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