Elimination stereospecific

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

Eor the elimination of trimethylamine and water from the erythro- and threo-isomer of trimethyl-1,2-diphenylpropylammonium iodide 6 and 7 respectively, by treatment with sodium ethoxide, a stereospecific -elimination has been found to take place thus supporting a E2-mechanism. Erom the -isomer... 

Direct chemical evidence for the head-to-tail structure was furnished in 1959 by Applequist et al. using a stereospecific elimination/21 ... 

Tetraalkylstannanes with a /i-acyloxy group undergo highly stereospecific elimination to yield Z - or E -unsaturated products, depending on whether the organotin compound has erythro- or threo-configuration, as depicted in reactions 72 and 73, respectively324. 

The unusual stereospecific elimination via a carbocation was rationalized by suggesting that extensive C-metal a-ir conjugation is present in the carbocation intermediate (Figure 3). The awP -periplanar stereochemistry is found because the a-jt stabilization to the SnMe3 group is strongest when the metal is involved in a vertical stabilization of the carbocation. 

Stereospecific eliminations can give pure single isomers of... 

You met a stereospecific elimination in Chapter 19. The requirement for the H and the Br to be anti-periplanar in the E2 transition state meant that the two diastereo isomers of this alkyl bromide eliminated to alkenes with different double bond geometries (p. 491). 

How can the Z selectivity in Wittig reactions of unstabilized ylids be explained We have a more complex situation in this reaction than we had for the other eliminations we considered, because we have two separate processes to consider formation of the oxaphosphetane and decomposition of the oxaphosphetane to the alkene. The elimination step is the easier one to explain—it is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state (as in the base-catalysed Peterson reaction). Addition of the ylid to the aldehyde can, in principle, produce two diastere-omers of the intermediate oxaphosphetane. Provided that this step is irreversible, then the stereospecificity of the elimination step means that the ratio of the final alkene geometrical isomers will reflect the stereoselectivity of this addition step. This is almost certainly the case when R is not conjugating or anion-stabilizing the syn diastereoisomer of the oxaphosphetane is formed preferentially, and the predominantly Z-alkene that results reflects this. The Z selective Wittig reaction therefore consists of a kinetically controlled stereoselective first step followed by a stereospecific elimination from this intermediate. 

In the last chapter, we looked at some stereospecific eliminations to give double bonds, and you know that E2 elimination reactions occur best when there is an anti-periplanar arrangement between the proton and the leaving group. 

Ley and cowoikers have done studies with phenyl (trimethylsilyOmethyl sulfones (320 equation 73). The lithio anion was generated with Bu"Li in DME to form vinyl sulfones (321) in good to excellent yields as isomeric mixtures. There is some indication that the reaction should best be carried out in DME at -78 °C, rather than in THF as in the initial Ager work. Trapping the intermediate alkoxide as the acetate, followed by attempts at stereospecific elimination did not prove to be successful in forming a single alkene isomer. 

Isotopic 0 labeling experiments suggested that the formation of 5-HPETE occurs via radical trapping of oxygen. The 5-HPETE is then transformed into the (5S, 6S)-epoxide LTA4 by a loss of water, involving stereospecific elimination of a C-10 hydrogen from 5-HPETE. ... 

These two versions of the HWE are close to stereochemical control the formation of either isomer (E or Z) at will from (more or less) the same starting materials. The next two reactions achieve this aim. By purification of Wittig-type intermediates the stereospecific elimination gives a single isomer of the alkene. 

If the E-al kcnc is required, reduction of the ketones 127, prepared by acylation of 119 or by oxidation of mixtures of isomers of 122, with NaBH4 in alcoholic solution gives anti-122 selectively. Stereospecific elimination then gives exclusively -126. The reduction is controlled by Felkin-Anh selectivity (see chapter 21). 

A few more vinyl halides can be made stereospecifically by halogenation and base-catalysed elimination. One example is the vinyl bromide E-28 available by stereospecific lruns bromination of crotyl alcohol 26 followed by stereospecific elimination.4 Various regioselectivities are available in the elimination reaction so the formation of that particular alkene is in a way more surprising than the stereopecificity of the reaction. Presumably the bromine atoms increase the acidity of nearby Hs (H-2 and H-3 in anti-21) so that one or other of the vinyl bromides will be formed. One explanation is an intramolecular elimination through an anti-peri-pimsa transition state in a chair like conformation using OLi as an internal base 26. It can reach H-3 in a five-membered cyclic array. 

Support for this mechanism came from an unexpected quarter. In a synthesis of sterpuric acid 215, Paquette planned to put in the alkene at the end by a Julia olefination.35 Addition of MeLi to the ketone 216 and non-stereospecific elimination (chapter 15) looked a good strategy. The 1,2-relationship between OH and ketone in 216 is awkward and the decision was made to try a hydroxylation on the silyl enol ether of the parent ketone 217. 

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