Stereospecific reactions elimination

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

These reactions proceed by alkoxide or fluoride attack at silicon which results in C—Si bond cleavage and elimination of the leaving group from the fi carbon. These reactions are stereospecific anti eliminations. 

Stereospecific reaction (Section 7.13) Reaction in which stereoisomeric starting materials give stereoisomeric products. Terms such as syn addition, anti elimination, and inversion of eonfiguration describe stereospecific reactions. 

The dilithio derivative of A-methanesulfinyl-p-toluidine (29) adds to aldehydes and ketones to give, after hydrolysis, the hydroxysulfinamides (30), which, upon heating, undergo stereospecifically syn eliminations to give alkenes. ° The reaction is thus a method for achieving the conversion RR CO — RR C=CH2 and represents an alternative to the Wittig reaction. ... 

The second reaction is a stereospecifically trans elimination (E2). The marked proton in (25) cannot be lost as it is aia to the leaving group (I). Trans elimination (29) is possible only to give (26),... 

Enol lactones are assumed to form from iV-methylisoquinolinium salts as a result of a Hofmann-type degradation process. This P elimination is a highly stereospecific reaction in which Z isomers are produced from precursors of erythro configuration and isomers from threo diastereomers(5,97). This fact seems to suggest that syn rather than the more usual anti elimination takes place. Examination of models indicates, however, that there is a preferred conformation in which the C-8 hydrogen is in the syn and coplanar position to the quaternary nitrogen. This hypothesis was proved correct in experiments carried out in vitro (5,14,15,91-94). 

This means that if a reaction is carried out on a compound that has no stereoisomers, it cannot be stereospecific but at most stereoselective. The concerted reactions, including SN2 displacements, E2 elimination of alkyl halides, anti and Syn addition to alkenes are all stereoselective. In the case of chiral or geometric substrates the nature of the product depends on the unique stereoelectronic requirement of the reaction. These are examples of stereospecific reactions. 

Selenosulfonylation of olefins in the presence of boron trifluoride etherate produces chiefly or exclusively M products arising from a stereospecific anti addition, from which vinyl sulfones can be obtained by stereospecific oxidation-elimination with m-chloroper-benzoic acid134. When the reaction is carried out on conjugated dienes, with the exception of isoprene, M 1,2-addition products are generally formed selectively from which, through the above-reported oxidation-elimination procedure, 2-(phenylsulfonyl)-l,3-dienes may be prepared (equation 123)135. Interestingly, the selenosulfonylation of butadiene gives quantitatively the 1,4-adduct at room temperature, but selectively 1,2-adducts at 0°C. Furthermore, while the addition to cyclic 1,3-dienes, such as cyclohexadiene and cycloheptadiene, is completely anti stereospecific, the addition to 2,4-hexadienes is nonstereospecific and affords mixtures of erythro and threo isomers. For both (E,E)- and ( ,Z)-2,4-hexadienes, the threo isomer prevails if the reaction is carried out at room temperature. 

Alcohols with organoelement groups listed in Table 13 gave with one exception only small amounts of olefine. But with hydroxyalkyl-selenides (35a, G = -SePh or -SeCHs) stereospecific frans-elimination can be achieved in acidic (for instance excess of perchloric acid ether at room temperature) or basic media to give olefines in good yield So we can state that preparatively useful carbonyl olefination reactions in which epoxides are not a by-product, are allowed not only with phosphorus and silicium containing regents but are possible in the wide area of the periodical table marked in Scheme 55c with little lines. 

The E2 reaction is a stereospecific reaction, i.e. a particular stereoisomer reacts to give one specific stereoisomer. It is stereospecific, since it prefers the anfi-coplanar transition state for elimination. The (R,R) diastereomer gives a cis-alkene, and the (S,R) diastereomer gives a trans-alkene. 

All recent results suggest the second mechanism. The arguments for its validity may be summarised as (1) high stereospecificity of elimination on a number of catalysts (2) existence of both basic and acidic sites on dehydration catalysts (3) the possibility of treating all elimination reactions in a common way from the point of view of mechanism (cf. Sect. 2.1). 

As an illustration of the stereospecificity of eliminations, the meso compound 4 gives the c/s-alkene 5, whereas the d,l isomers 6 give the trans-alkene 7 with ethoxide. Both reactions clearly proceed by antarafacial elimination ... 

In a stereospecific reaction diastereomerically different starting materials give diastereomerically different products. Thus the bromination of the 2-butenes (Equations 2.1 and 2.2) is stereospecific, since one geometrical isomer gives one product and the other isomer a diastereometrically different product. Elimination of TsOH from the two 2-phenylcyclohexyl tosylates, however, is not stereospecific. 

Carbonyl olefination.1 The reaction of 1 with benzaldehyde results in a 1 1 separable mixture of the threo- and eryfAro-adducts (2a and 2b, respectively). The adducts undergo stereospecific ypn-elimination when heated to give /i-phenyl-thiostyrene (3). The (E)-isomer (3a) is formed from 2a, and the (Z)-isomer (3b) is formed from 2b. On the other hand, anfr -elimination obtains on treatment of 2 with perchloric acid in methanol. This carbonyl olefination has one advantage over the Peterson reaction in that intermediate adducts can be isolated and converted as desired to an (E)- or a (Z)-olefin. 

The E2 elimination is stereospecific, with elimination taking place in an antiperiplanar geometry. The diagrams given below show that the four atoms involved in the reaction are in plane with the H and Br on opposite sides of the molecule. 

The E2 is a stereospecific reaction, because different stereoisomers of the starting material react to give different stereoisomers of the product. This stereospecificity results from the anti-coplanar transition state that is usually involved in the E2. We consider more of the implications of the anti-coplanar transition state in Chapter 7. For now, Problem 6-38 will give you an opportunity to build models and see how the stereochemistry of an E2 elimination converts different stereoisomers of the reactants into different stereoisomers of the product. 

The E2 Reaction. / -Elimination, which is usually but not always stereospecifically anti, is the frequent accompaniment to substitution, as we saw earlier [see (Section 4.5.2.5) pages 145-147], We have also already had [see (Section 2.2.3.4) page 81] some discussion about why anti arrangements are preferred in the anomeric effect, where we saw that it is not solely because it allows all the groups to be staggered and not eclipsed. 

The cheletropic elimination of nitrogen from diazenes 8.1 and 8.2 is a stereospecific reaction (for definition of a stereospecific reaction, see section 1.5). 

Other Reactions of Olefinic Steroids.—[6/3- H]Cholest-4-ene reacts with [Pd(PhCN)2Cl2] to give the a-4-617 and /3-4-6t/ PdCl derivatives with stereospecific syn elimination of the 6-H or 6- H, confirming that in the case of the 3-oxo-A -steroids the high proportion of anti elimination must be attributed to... 

Dehydration. Burgess et al. reported that the reagent is useful for dehydration of simple alcohols. The reaction is a stereospecific ds elimination and follows Saytzeflf s rule. Crabbe and Leon have used this procedure with various steroidal secondary and tertiary alcohols. They conclude that the nature of the alcohol group, the configuration, and the environment are the primary factors governing the course of dehydration. The reactions are carried out at room temperature in anhydrous benzene for 2 hr. followed... 

With proton transfer (k ) rate determining, the primary deuterium isotope effect is satisfied as well as the sensitivity of the reaction rates to (C-O) bond rupture. The latter is reflected in the (kjk2) ratio. It was noted that rearrangement in the carbonium ion of the ion pair (e.g., loss of optical activity at the a-carbon center in the reactant ester, Atj > fcj) would substantiate the ion pair mechanism. Results of this kind of experiment in the stereospecific m-elimination that is observed in ester pyrolyses is not compatible with the ion-pair mechanism. For example, the acetate of the erythro-isomsT of 2-deutero-l,2-dipenylethanol gave /runs-stilbene with predominant retention of deuterium (95.6 1.7%). In contrast, the threo-isomer gave Iranj-stilbene with predominant loss of deuterium (26.4+0.6% deuterium) . 

The first is a Wittig reaction with an unstabilized ylid, the second a Julia reaction, and the last two are Peterson reactions under different conditions. Each reaction is described in detail in the chapter. The Wittig reaction is under kinetic control and is a stereospecifically cis elimination. In this case the product is the Z-alkene. 

Double bonds can be created by a number of available stereospecific reactions (E2 elimination, pyrolytic elimination, e.g. Cope, Chugaev reactions), stereoselective Wittig and related reactions, reduction of triple bonds, by cisitrans isomerisation of existing double bonds either photochemically or by wet chemistry, e.g. the Corey-Winter procedure. [17]... 

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