Elimination Peterson

The silicon- and sulfur-substituted 9-allyl-9-borabicyclo[3.3.1]nonane 2 is similarly prepared via the hydroboration of l-phenylthio-l-trimethylsilyl-l,2-propadiene with 9-borabicy-clo[3.3.1]nonane36. The stereochemistry indicated for the allylborane is most likely the result of thermodynamic control, since this reagent should be unstable with respect to reversible 1,3-borotropic shifts. Products of the reactions of 2 and aldehydes are easily converted inlo 2-phenylthio-l,3-butadienes via acid- or base-catalyzed Peterson eliminations. 

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

The surprising selectivity in the formation of 4 and 5 is apparently due to thermodynamic control (rapid equilibration via the 1,3-boratropic shift). Structures 4 and 5 are also the most reactive of those that are present at equilibrium, and consequently reactions with aldehydes are very selective. The homoallylic alcohol products are useful intermediates in stereoselective syntheses of trisubstituted butadienes via acid- or base-catalyzed Peterson eliminations. 

The lithium enolate of the oc-silyl-substituted iron-acyl complex 19 reacts with aldehydes, however, products of the Peterson elimination process (E)- and (Z)-22 are usually isolat-ed22- 23,36.37 for t[1js anc other preparations of a,/t-unsaturated iron-acyl complexes see Section I.3.4.2.5.I.3.). 

Optically pure vinylglycine derivatives 3 can be prepared by reacting lithiated (2aldol adducts 212. Hydrolysis of the adduct, followed by heating in 5N hydrochloric acid, effected ester hydrolysis and Peterson elimination to give the vinylglycine derivatives. 

The synthesis of the C(17)-C(24) segment also began with a diastereoselective boron enolate aldol addition. The adduct was protected and converted to an aldehyde in sequence H. The terminal diene unit was installed using a y-silylallyl chromium reagent, which generates a (3-hydroxysilane. Peterson elimination using KH then gave the Z-diene. 

Peterson Olefination Reaction (Peterson Elimination, Silyl-Wittig Reaction) The Reaction ... 

Ally lie amines.1 The trimethylsilylisoxazolidines formed on cycloaddition of vinyltrimethylsilanes with nitrones (12, 566) are converted on reductive cleavage of the N—O bond and concomitant Peterson elimination of (CH3)3SiOH into allylic amines. 

After rotation about the newly formed bond, nothing stands in the way of selective -elimination from 47 - a possible intermediate of a base-induced Peterson elimination - to Z-olefin 48.18... 

Three different routes to the key compounds for the sila-Peterson elimination, the a-alkoxydisilanes 157, are described in the literature, namely A, reaction of silyllithium reagents with ketones or aldehydes B, addition of carbon nucleophiles to acylsilanes C, deprotonation of the polysilylcarbinols. In addition, method D, which already starts with the reaction of 2-siloxysilenes with organometallic reagents, leads to the same products. The silenes of the Apeloig-Ishikawa-Oehme type synthesized so far are summarized in Table 4. 

In another simple procedure, deprotonation of methoxy bis(trimethylsilyl)methane with butyl lithium and addition of the resulting anion to aldehydes induces Peterson elimination (Scheme 27). The product methyl enol ethers could be hydrolysed to the parent acyl silanes with hydrochloric acid-THF or could be treated with electrophiles such as M-halosuccinimides to give a-haloacyl silanes105. Alternatively, treatment with phenyl selenenyl chloride, oxidation at selenium and selenoxide elimination afforded a,/3-unsaturated acyl silanes. 

The Suzuki cross coupling product 151 obtained from 150 and o-bromobenzaldehyde on exposure to caesium fluoride furnished, via in situ capture of the resulting carbanion 152 by the proximate aldehyde group, the Peterson elimination product, the enamine 153 in 54% yield. 

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. 

The series of reactions leading to the 5-silyl-l-pentene - epoxidation, ring expansion, and Peterson elimination -are all stereospecific. Therefore, epoxides with different geometry can be transformed into the corresponding (E)- or (Z)-olefinic silanols <1994BCJ1694, 1991TL4545>. Subsequent Tamao oxidation affords stereodefined pentenols. 

One can also take advantage of the complementary methods for effecting the Peterson elimination to prepare either the (E)- or (Z)-olefinic silanols from a single oxiranylsilacyclobutane via ring expansion followed by a syn- or //-elimination (Scheme 38) <1994BCJ1694, 1991TL4545>. 

Scheme 14.3 Reformatsky reaction/Peterson elimination sequences.15 18...

Reformatsky reaction/Peterson elimination sequence using highly activated Zn(Ag) -graphite preparation of ethyl cyclohexylideneacetate (Structure 14)... 

By introducing a thiomethyl substituent at the terminal position of 5 it was possible to direct attack to this position exclusively (61,103). Introduction of bulky 1,5-trimethylsilyl substituents into a pentadienyllithium provides a way of getting selective attack at C-3 (62). This approach worked with acetone, cyclohexanone, acetaldehyde, and 2-methylpropanal. With diisopropyl ketone, however, addition at C-3 still occurred, followed by Peterson elimination, giving a novel synthesis of 1,3,5-trienes (104) [Eq. (6)]. 

Fig. 4.9. A solution to the regio- and stereoselectivity problems depicted in Figure 4.8 with the help of the Peterson elimination (for the mechanism, see Figure 4.38) as an example of either a syn- or an anti-selective Het /Heh-elimination, depending on the reaction conditions.

A /3-hydroxysilane, like the one shown in Figure 4.43 (top, left), can be prepared stereoselec-tively (see Chapter 10). These compounds undergo stereoselective antislimination in the presence of acid and stereoselective yr -elimination in the presence of a base (Figure 4.43). Both reactions are referred to as Peterson olefinations. The stereochemical flexibility of the Peterson elimination is unmatched by any other Het /HeP-elimination discussed in this section. 

Corey and coworkers reported the reactions of a-siloxyketones 128 with trimethylsi-lyllithiums in the presence of HMPA which gave the corresponding silyl enol ethers 130 (equation 86). The elimination of a-siloxy groups was proposed to occur via the 1,4-silyl migration followed by the Peterson elimination. In accord with this mechanism, the intermediate 129 was trapped by hydrolysis202. 

Fiirstner and coworkers revealed a competition between the Brook rearrangment and the Peterson olefination. The adducts of aroylsilanes 161 with (trimethylsilyl)methyl-magnesium chloride 162 underwent a Brook rearrangement and then a Peterson elimination affording vinylsilane 163 (equation 98). On the other hand, the reaction of cycloalkylcarbonylsilane 164 with 162 gave vinylsilane 165 via the direct Peterson elimination (equation 99)228. 

Ynals can be selectively hydrosilylated in the presence of Pt salts leading to a-triethylsilyl enals, which undergo 1,2-addition of Grignard reagents followed by Peterson elimination from the intermediate alcohol. This method offers a simple synthetic route from ynals to the corresponding allenes.267... 

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... 

A single-step E2 elimination would have to go via an anti-periplanar transition state and would be stereospecific. You will be able compare this stereoselective Julia olefination with the stereospecific Peterson elimination shortly. 

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