Elimination anti and syn

By saturating the co-ordination sites of the cation with a crown ether, the relative contribution of the syn-pathway decreases, as was shown by Zavada et ai (1972) for t-BuOK-promoted eliminations from 5-decyl tosylate. Furthermore, trans/cis ratios in both anti- and syn-eliminations were affected by the presence of dicyclohexyl- 18-crown-6 and the nature of the leaving group (Zavada et ai, 1976). Fiandanese et ai (1973) observed that elimination from fluorosulphonylethanes [177] promoted by potassium phenoxide in dioxan... 

Reaction of l//,2//,3//-heptafluorocyclopcntane 14A and 14B with aqueous alkali gives 2//,3//-hexafluorocyclopent-l-ene (15) by anti and syn elimination, respectively, with only traces of byproducts.70... 

Czech workers have also found several competitive anti- and syn-eliminations (Pankova et al., 1967 Zavada et al., 1967). 

Tatlow and his co-workers conducted an extremely comprehensive programme of syntheses and structure derivations of a series of fluorinated cycloalkanes [24], and concluded that the reactivity of the system, as well as the orientation of the cycloalkene produced, are similarly influenced by electronic factors which have been outlined in the preceding sections of this chapter. Anti elimination is generally the more favourable process but conformational effects may make the synjanti rates nearly comparable. Elimination from the cyclohexanes 6.16A and 6.16B illustrates the balance between electronic and conformational effects [25]. Anti elimination is possible from 6.16A, involving removal of fluoride from >CHF rather than >Cp2 since in this case electronic (the carbon-fluorine bond in CFH is weaker than in CF2) and conformational effects (H and F are anf -periplanar) are in concert (Figure 6.16). In contrast, anti elimination from 6.16B can only occur with elimination of fluoride from the more stable >CF2 position and therefore anti and syn eliminations occur together. 

The reactions of cyclopentanes 6.20A and 6.20B with aqueous alkali both give the same cylopentene by anti and syn elimination respectively, with only traces of by-products arising from syn elimination from 6.20A or anti elimination from 6.20B (Figure 6.20). Inward syn elimination occurs from 6.20B, giving the cyclopentene in 86% yield, and this is a further indication that removal of fluoride from >CHF is easier than from >CF2 (Figure 6.20)... 

In cyclic systems, the extent of anti and syn elimination depends on ring size. Cyclohexyl systems have a very strong preference for anti elimination. See [62, 63]. 

Anti and syn elimination both E and Z stereoisomers are formed. The isomer with the bulkiest groups on opposite sides of the double bond will be formed in greater yield. 

Scheme 731. The elimination of H-L and H-L (where L = halogen) from a threo-2- Hj -3-halobutane diastereomer showing the results of anti- and syn-elimination. As noted in the text, onfi-elimination is apparently preferred since, for the most part, the ds- [or (Z)] 2-butene produced does not contain deuterium ( H), while the tram- [or ( )] 2-butene prodnced does not contain protium ( H).Thus, ignoring a deuterium isotope effect, syn-elimination is apparently avoided.

Attempted dehydrochlorination of (206) by potassium t-butoxide does not generate the expected spirene, but affords (209) instead. Cyclopropenes, a-elimination, and simple substitution have been excluded from the process, and the route is believed to involve (207), reacting from the preferred conformation by anti- and syn-elimination (see also p. 25), or (208). The cyclopentadienyl anion has also been implicated in the nucleophilic addition of 1,2,4-triazines to dimethyl 4,5,6,7-tetrachlorospiro[2,4]-hepta-1,4,6-triene-1,2-dicarboxylate. ... 

Baciocchi E, Ruzziconi R, Sebastiani GV (1983) Anti and syn eliminations from 2,3-dihalo-2,3-dihydrobenzofurans. The role of the substrate structure and the base-solvent system on the reaction mechanism. J Am Chem Soc 105 6114-6120... 

In the El mechanism, there is another kind of selectivity, anti and syn elimination. Syn elimination products are formed when the groups are removed from the same side of the molecule and anti elimination products formed when the groups are removed from the opposite side. Over alumina, menthol yields mainly 2-menthene, and neomenthol eliminates mainly to 3-menthene. ... 

Although there is usually a preference for anti elimination in acyclic systems, syn elimination is competitive in some cases. In acyclic systems, the extent of anti versus syn elimination can be determined by use of stereospecifically deuterated substrates or by use of diastereomeric reactants which will give different products by syn and anti elimination. The latter approach showed that elimination from 3-phenyl-2-butyl tosylate is a stereospecific anti process. ... 

Synthesis of 31 by Method I (107,108) and its conversion to the related anti and syn diol epoxide derivatives (32,33) has been reported (108). The isomeric trans-1,lOb-dihydrodiot 37) and the corresponding anti and syn diol epoxide isomers (38,39) have also been prepared (108) (Figure 19). Synthesis of 37 from 2,3-dihydro-fluoranthene (109) could not be accomplished by Prevost oxidation. An alternative route involving conversion of 2,3-dihydrofluoranthene to the i8-tetrahydrodiol (3-J) with OsO followed by dehydration, silylation, and oxidation with peracid gave the Ot-hydroxyketone 35. The trimethylsilyl ether derivative of the latter underwent stereoselective phenylselenylation to yield 36. Reduction of 3 with LiAlH, followed by oxidative elimination of the selenide function afforded 3J. Epoxidation of 37 with t-BuOOH/VO(acac) and de-silylation gave 38, while epoxidation of the acetate of JJ and deacetylation furnished 39. 

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. 

Then again, many elimination reactions are found to occur much I more readily in that member of a pair of geometrical isomerides inil which the atoms or groups to be eliminated are trans to each other than in the isomer in which they are cis (p. 255). As is seen in the relative ease of elimination from anti and syn aldoxime acetates to yield the same cyanide ... 

For substituents attached to adjacent atoms of a carbon-carbon double bond, the terms els and trans or E and Z are used instead of anti and syn. This is also true of oximes and related molecules as well as to cyclic structures. The terms anti and syn are also used in the context of chemical reactions or transformations, designating the relative orientation of substituent addition or elimination. 

In order for an E2 mechanism to take place a base must approach the proton marked. In C this proton is shielded on both sides by R and R. In D the shielding is on only one side. Therefore, when anti elimination does take place in such systems, it should give more cis product than trans. Also, when the normal anti elimination pathway is hindered sufficiently to allow the syn pathway to compete, the anti — trans route should be diminished more than the anti — cis route. When syn elimination begins to appear, it seems clear that E, which is less eclipsed than F, should be the favored pathway and syn elimination should generally give the trans isomer. In general, deviations from the syn-anti dichotomy are greater on the trans side than on the cis. Thus, trans olefins are formed partly or mainly by syn elimination, but cis olefins are formed entirely by anti elimination. Predominant syn... 

Amides and Related Functions 125 this possibility must be eliminated. These ions can exist either exclusively in the syn or as a mixture of syn and anti isomers. These two possibilities are likely because both predict the same results which correspond to the experimental observation. Indeed, if these ions are syn, it means that the intermediate 59 will first be produced and then, it would give different conformers which would yield the mixture of ester and amine plus amide and alcohol products. If these ions are a mixture of anti and syn, the former will give the ester and amine products whereas the latter would give a mixture of ester and amine plus amide and alcohol products. 

In elimination from the threo compound 285, the situation is different because the stereoelectronic and conformational factors are not cooperative. Since expulsion of the leaving group from the stereoelectronically preferred carbanion 2tH is rendered difficult by severe Ar-ArS02 steric interactions, the anti elimination is slowed down and syn elimination via carbanions 292 and 293 become competitive."... 

In view of the approximate nature of the model, and the complete neglect of nonelectrostatic factors, no great weight should be placed on the absolute values of AW. In particular, comparisons between the anti and syn-cyclic transition states are the most suspect, since the assumptions that have to be made about them are different and are likely to introduce errors which do not cancel. Nevertheless, it appears that the crude figures do indicate that an electrostatic factor could accelerate some eliminations, e.g. 103, and retard others, e.g. in 102. Of course, whatever one might decide about the electrostatic influence of an electronegative substituent, just the opposite conclusions would apply if base-promoted elimination from onium species such as 103 were considered. Ion aggregation would, of course, favor syn elimination. 

When the alkene is cyclic, or the insertion step forms a quaternary center, a substitution product is not obtained. For example, stereospecific syn addition of an arylpalladium halide 13 to cyclohexene generates cyclohexylpalladium(II) intermediate 14 (Scheme 6-3). The C—Pd cr-bond in this intermediate is anti to H and syn elimination to form a substitution product is not possible. However, elimination of cis hydrogen H is possible and generates allylic product 15. This pathway of the Heck reaction is particularly important in complex molecule construction since a new stereogenic center is produced. 

The stereochemistry of the elimination of the p-hydroxysilane at silicon has been investigated. In studies by Larson and coworkers, the 3-hydroxyalkyl(l-iuq)hthyl)phenylmethylsilanes (307) and (309) were isolated and subjected to elimination conditions to ascertain the stereochemistry of the elimination on the silyl groip (Scheme 44). The acid-catalyzed eliminations proceed with inversion of stereochemistry at silicon, while the base-catalyzed elimination occurred with retention. These results are in agreement with the mechanism proposed of anti elimination under acidic conditions and syn elimination under basic. While the optically pure silicon was useful for determining the course of the elimination, it could not be utilized in asymmetric synthesis. Addition of the anion to various carbonyls afforded virtually no diastereoselectivity, and it was not possible to separate the diastereomers formed either by crystallization or by chromatography. 

The steric course of 1,3-elimination reactions has been studied for the anti- and syn-d ia-stereomers of methyl 4-benzoyl-2-chloro-3-phenylbutanoate (1) . 

---