Alkenes Peterson alkenation

The Peterson olefination can be viewed as a silicon variant of the Wittig reaction, the well-known method for the formation of carbon-carbon double bonds. A ketone or aldehyde 1 can react with an a-silyl organometallic compound 2—e.g. with M = Li or Mg—to yield an alkene 3. 

The next step of the Peterson olefination allows for the control of the E Z-ratio of the alkene to be formed by proper choice of the reaction conditions. Treatment of /3-hydroxysilanes 5 with a base such as sodium hydride or potassium hydride leads to preferential -elimination to give alkene 3a as major... 

The Peterson oleflnation presents a valuable alternative to the Wittig reaction. It has the advantage to allow for a simple control of the alkene geometry. Its applicability in synthesis depends on the availability of the required silanes.2... 

Peterson and co-workers have carried out a careful investigation of the electrophilic addition of trifluoroacetic acid to a series of aliphatic alkynes (38) and alkenes (39,40). Of particular interest is the behavior of 3-hexyne. At. 1 M concentrations of 3-hexyne, nearly equal amounts of the cis- and trans-3-hexen-3-yl trifluoroacetates are formed in 98% yield, together with about 2%... 

This section deals with reactions that correspond to Pathway C, defined earlier (p. 64), that lead to formation of alkenes. The reactions discussed include those of phosphorus-stabilized nucleophiles (Wittig and related reactions), a a-silyl (Peterson reaction) and a-sulfonyl (Julia olefination) with aldehydes and ketones. These important rections can be used to convert a carbonyl group to an alkene by reaction with a carbon nucleophile. In each case, the addition step is followed by an elimination. 

Trialkylsilyl groups have a modest stabilizing effect on adjacent carbanions (see Part A, Section 3.4.2). Reaction of the carbanions with carbonyl compounds gives (3-hydroxyalkylsilanes. (3-Hydroxyalkylsilanes are converted to alkenes by either acid or base.270 These eliminations provide the basis for a synthesis of alkenes. The reaction is sometimes called the Peterson reaction.211 For example, the Grignard reagent derived from chloromethyltrimethylsilane adds to an aldehyde or ketone and the intermediate can be converted to a terminal alkene by acid or base.272... 

The anti elimination can also be achieved by converting the /5-si lyI alcohols to trifluoro-acetate esters.165 Because the overall stereoselectivity of the Peterson oleftnation depends on the generation of pure syn or anti /5-silyl alcohols, several strategies have been developed for their stereoselective preparation.166 Several examples of synthesis of substituted alkenes in this way are given in Scheme 2.18. 

In the Peterson s synthesis the b-hydroxysilanes are converted to alkenes in either acidic or basic solution. 

Which of the following methods of alkene synthesis appear reasonable a) Witrig b) Homer-WtkIsworth-Emmons or c) Peterson olefination ... 

The stereochemical outcome of the Peterson reaction between unsymmetrically substituted a-silyl carbanions and aldehydes or unsymmetrical ketones is determined by the relative rates of formation of the threo and erythro /3-oxidosilancs. Often the rates are similar, to give a product alkene E Z ratio of 1 1, although some workers report a predominance of cis olefins in the reactions of aldehydes. 

A recent modification of the Peterson reaction involves the use of fluoride ion catalysts. Reaction of bis(trimethylsilyl)methyl derivatives 166 and carbonyl compounds gives the expected alkenes 167 (as shown in equation 137) in high yields, especially for non-enolisable carbonyl compounds, and in some cases with high stereoselectivity209. 

Peterson reaction (5, 724 9, 493). This reaction is limited by the fact that chloromethyl-trimethylsilane is the only commercially available suitable silane. Ager2 has now found two general routes to a a-trimethylsilylcarbanions 3, which undergo the Peterson reaction to give alkenes (4) in satisfactory yield. 

An interesting Fe-catalyzed SN2 -like carbene insertion reaction using diazo compounds and allyl sulfides (the Doyle-Kirmse reaction) was reported by Carter and Van Vranken in 2000 [20], Various allyl thioethers were reacted with TMS-diazomethane in the presence of catalytic amounts of Fe(dppe)Cl2 to furnish the desired insertion products with moderate levels of stereocontrol [Equation (7.6), Scheme 7.14]. The products obtained serve as versatile synthons in organic chemistry, e.g. reductive desulfurization furnishes lithiated compounds that can be used in Peterson-type oleftnations to yield alkenes [Equation (7.7), Scheme 7.14] [21]. 

The Peterson Reaction allows the preparation of alkenes from a-silylcarbanions. The intermediate p-hydroxy silane may be isolated, and the elimination step - the Peterson Elimination - can be performed later. As the outcome of acid or base-induced elimination is different, the Peterson Olefination offers the possibility of improving the yield of the desired alkene stereoisomer by careful separation of the two diastereomeric p-hydroxy silanes and subsequently performing two different eliminations. 

Addition of dimethylsulfonium methylide (122) to various Michael acceptors (121), followed by alkylation, has been reported to produce functionalized 1-substituted alkenes (124), arising via the unprecedented elimination (123), rather than the usual cyclopropanation products. In silyl substituted substrates, where a facile Peterson-type olefination is possible from the adduct, elimination took place instead. Aryl-substituted Michael acceptors (121 R1 = Ar) underwent a similar olefination to give 1-substituted styrene derivatives with moderate yields along with a side product, which arose by nucleophilic demethylation from the adduct of dimethylsulfonium methylide and arylidene malonates. Hammett studies revealed that selectivity for olefination versus demethylation increases as the aryl substituent becomes more electron deficient.164... 

Reaction of DMSB with triphenylsilyl-substituted oxiranyllithium leads to the formation of an olefinic silanol via sequential (1) coordination to the silicon, (2) Si-C bond migration, and (3) Peterson-type Si-O elimination to furnish the alkene. A pentacoordinate siliconate intermediate is presumably involved in this transformation. Therefore, it was reasonable to expect that addition of a nucleophile (methyllithium or lithium t>-propoxide) to an oxiranyl-substi-tuted SCB, which could generate a similar intermediate, would induce the C-Si bond migration to form the same silacyclopentane. Indeed, this alternative order of addition sequence provides the corresponding silanol with better efficiency (84% yield vs. 44%, Scheme 36). 

Recently, Peterson-type reactions have been applied to the syntheses of transient and stable heavy element analogues of alkenes and alkynes. Transient and isolable silaethenes 207331 were prepared by the sila-Peterson reactions121,321,332 340. Similarly to the original Peterson reactions341,342, the sila-Peterson reactions were found to involve a 1,3-silyl migration from silicon to oxygen (equation 135)321. 

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... 

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. 

---