The Peterson reaction is a stereospecific elimination

There arc many reactions in organic chemistry in which an Mc3Si group acts like a proton—Chapter 47 will detail some more reactions of silicon-containing compounds. Just as acidic protons are removed by bases, silicon is readily removed by hard nucleophiles, particularly or RO , and this can promote an elimination. An example is shown here. 

There is another, complementary version of the Peterson reaction that uses base to promote the elimination. The starting materials are the same as for the acid-promoted Peterson reaction. When base (such as sodium hydride or potassium hydride) is added, the hydroxyl group is deprotonated, and the oxyanion attacks the silicon atom intramolecularly. Elimination takes place this time via a syn-periplfl flr transition state—it has to because the oxygen and the silicon are now bonded together, and it is the strength of this bond that drives the elimination forward, this diastereoisomer via this mechanism 

In Chapter 19 you saw that anti-periplanar transition states are usually preferred for elimination reactions because this alignment provides the best opportunity for good overlap between the orbitals involved. Syw-periplanar transition states can, however, also lead to elimination— and this particular case should remind you of the Wittig reaction (Chapter 14) with a four-membered cyclic intermediate. 

Chemists are stilt unsure about the exact mechanism of this reaction, and what we have described here is certainly a very simplified picture of what actually happens. 

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Peterson reactions of a-sUyl carbanions and carbonyl compounds leading directly to the alkenes are generally not stereoselective since the j8-hydroxyalkylsilane or yff-silylalkoxide intermediates are usually formed as a mixture of syn and anti isomers, whereas Peterson elimination from a j8-hydroxyalkylsilane proceeds in an exclusively stereospecific manner. Furthermore, the addition of an a-silyl carbanion to a carbonyl compound proceeds in an irreversible manner [29, 30]. Therefore, when the 8-hydroxyalkylsilane intermediate can neither be isolated nor separated, the diastereomeric ratio of the alkene products of the Peterson reaction is determined in the addition step of the a-silyl carbanion and the carbonyl compound. Stereospecific preparation of the y8-hydroxyalkylsilane is required for the utilization of the Peterson reaction in organic synthesis. 

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. 

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. 

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. 

Nucleophilic substitution of a,/3-epoxysilanes followed by the Peterson elimination is valuable for the stereoselective synthesis of alkenes.3 The reactions with lithium phenylsulfide and diphenylphosphide form alkenyl sulfides and alkenylphosphines, respectively, in a stereospecific manner. 7-Metallo-a,/ -epoxysilanes are isomerized to a-siloxyallylmetals by anionic ring opening and subsequent Brook rearrangement (Equation... 

However, reactions like this are of limited use—their success relies on the base s lack of choice of protons to attack. Logic dictates that only trisubstituted double bonds can be made stereospecifically in this way the reaction must not have a choice of hydrogen atoms or an E alkene wUl result stereoselectively (as in the example on p. 684). The answer is, of course, to move away from eliminations involving H, and we can do this by returning to the Peterson elimination, which you met on p. 671. 

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