Mechanism Peterson reaction

Because the mechanisms of the Peterson reaction, a synthetic alternative to the Wittig reaction, have recently been reviewed [13, 14], this section will try only to summarize recent developments and trends concerning the Peterson reaction. 

The detailed mechanism of the Peterson reaction has not yet been revealed. When only alkyl, hydrogen or electron-donating substituents are present on the carbon atom bonded to silicon, the /i-hydroxysilane 156 can be isolated, usually as a diastereomeric mixture (equation 124), which can be separated using the usual physical methods. 

When comparing the model of the mechanism of the Peterson reaction with the scheme of the Wittig reaction generated by PEGAS, we also can find in its output synthons which are analogous to synthons generated for the Peterson reaction (Scheme 20). 

The mechanism of the Peterson reaction 2.33.22 Stereochemistry of the Peterson reaction... 

As mentioned in the previous section, the Peterson reaction proceeds by an irreversible addition of the silyl-substituted carbanion to a carbonyl. It has generally been assumed that an intermediate p-oxidosi-lane is formed and then eliminated. In support of this mechanistic hypothesis, if an anion-stabilizing group is not present in the silyl anion, the p-hydroxysilanes can be isolated fixrm the reaction, and elimination to the alkene carried out in a separate step. Recent studies by Hudrlik indicate that, in analogy to the Wittig reaction, an oxasiletane (304) may be formed directly by simultaneous C—C and Si—O bond formation (Scheme 43). The p-hyd xysilanes were synthesized by addition to the silyl epoxide. When the base-induced elimination was carried out, dramatically different ratios of cis- to rranr-alkenes were obtained than from the direct Peterson alkenation. While conclusions of the mechanism in general await further study, the Peterson alkenation may prove to be more closely allied with the Wittig reaction than with -elimination reactions. 

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-periplanar 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 eliminates to give this alkene... 

We have seen that vinyl silanes can be prepared by hydrosilylation of alkynes by three different mechanisms giving good control over geometry of these inevitably terminal vinyl silanes. Vinyl silanes are stable compounds and can be isolated, unlike most of the vinyl metals we have seen so far, and other ways of making vinyl silanes allow the more-or-less controlled synthesis of mono-or trisubstituted compounds with reasonable control over selectivity. These include the Peterson reaction with two SiMe3 groups on the same carbon atom 183 and, more relevant to this chapter, reactions of vinyl lithiums with silyl chlorides.44... 

White, A.F. Peterson, M.L. In Kinetics and Mechanisms of Reactions at the Mineral/Water Interface Sparks, D.L. and Grundl, T.J., Eds. American Chemical Society Washington DC 1998 this volume. 

The mechanism of the formation of the silanol 9 (Scheme 3) is understood as the result of an acid-catalyzed formal elimination of dimethylaminotrimethylsilane generating the silene 7, followed by the addition of H2SO4 at the silene double bond and subsequent hydrolysis of the silylsulfate 8. To our best knowledge, this is the first case of an acid-induced sila-Peterson reaction. 

Stereochemistry and the Reaction Mechanism of the Peterson Reaction of /f-Hydroxyallcylsilanes... 

In contrast to the stepwise addition mechanism, there is a report in which a concerted mechanism is proposed for the addition process, involving a 1,2-oxasiletanide intermediate. Peterson reaction of bis(trimethylsilyl)methyllithium with benzaldehyde gives a mixture of almost equimolar amounts of the E- and Z-isomers of styrylsUane, whereas Peterson eUmination of the corresponding oxidosilane 14 generated by other methods affords the ( )-styrylsilane with very high isomeric purity (Schemes 2.12 and 2.13) [36). The above results indicate that yS-oxidosilane 14 is not formed as a major intermediate and that the Peterson reaction involves nearly simultaneous formation of C-C and Si-O bonds to give a 1,2-oxasiletanide intermediate 13 directly. 

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