Reaction, Peterson elimination

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

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)... 

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. 

Peterson Reaction (Oxysilane Elimination) and Related Reactions... 

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. 

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. 

Anionic pentacoordinated 1,2-oxagermetanide was synthesized quantitatively by deprotonation of the corresponding -hydroxygermane (Scheme 10)63. Upon heating at 150 °C for 30 days, this compound equilibrated with another diastereoisomer and underwent a Peterson-type reaction with elimination of olefins (Scheme 10). 

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. 

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... 

After further reactions in situ (i) Li (ii) D20 (iii) H,0. From dehydration of alcohol. Yield of subsequent reaction. Crude. CeClj added to suppress enolization product formed by Peterson elimination. 

OL-Alkyl-OL, -unsaturated esters. a-Silyl esters can be obtained in >80% yield by reaction of lithium ester cnolatcs with this silane (10, 91 11, 247). Aldehydes and ketones react with the cnolatcs of these a-silyl esters to give adducts that undergo a Peterson elimination to form a-alkyl-a.p-unsaturatcd esters in which the (Z)-isomer predominates. 

In addition to the examples in Table 1, the Peterson methylenation has been used in several interesting natural product syntheses, as the examples in equation (2)-(6) indicate. Danishefsky and coworkers used the Peterson reaction in an approach to mitomycins (4 equation 2). This tq>plication demonstrated the use of unique elimination conditions. The hydroxysilane intermediate was stable to direct Peterson elimination. Therefore, the removal of the silyl protecting group and the elimination of the silyloxy group were carried out with DDQ in quantitative yield. 

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. 

The compound BusSnCHMeCHMeOH shows anti-elimination up to 100 °C, but. yyn-climination above 100 °C, when the reaction occurs through the compound Bu3SnCHMeCHMeOSnBu3 which is formed by reaction with the bis-tributyltin oxide which is eliminated.95 The intermediate 6-1 in the base-induced Un-Peterson elimination of a p-hydroxyalkyltin compound has been isolated as a stable compound. X-Ray crystallography shows that the configuration at the tin is closer to a square pyramid than a trigonal bipyramid.96 97... 

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