Silyl vinyl lithium

Silyl enol ethers, 23, 77, 99-117,128 Silyl enolates, 77 Silyl peroxides, 57 Silyl triflate, 94 Silyl vinyl lithium, 11 (E)-l -Silylalk-1 -enes, 8 Silylalumimum, 8 Silylation, 94 reductive, 26 a-C-Silylation, 113 O-Silylation.99,100 / -SilyIketone, 54 non-cydic, 55 Silylmagnesium, 8 Silyloxydienes, 112 Sodium hexamethyldisilazide, 89 Sodium thiosulphate pentahydrate, 59 Stannylation, see Hydrostannylation Stannylethene, 11 (Z)-Stilbene, 70 (E)-Stilbene oxide, 70 /3-Styryltrimethylsilane, 141 Swern oxidation. 84,88... 

Extension of one conjugation unit [Scheme 5-2 (b)] also gave similar results [32]. The reaction of [2-phenyl-l-(silyl)vinyl]lithium with CO at 15 °C also underwent clean intramolecular transformation of the acyllithium, whereas cyclization to a five-membered ring was observed. Treatment of a THF solution of the vinyl-lithium with CO (1 atm) at I5°C for 1.2 h followed by proton quenching yielded 2-(silyl)-l-indenol in 61% yield together with 3-(silyI)indanone (10%) (Eq. (5.30)). With the prolonged reaction time (24 h), 3-(silyl)indanone was obtained as a sole product in 60% isolated yield (70% GLC yield) after proton... 

Vinylic lithium reagents (26) react with silyl peroxides to give high yields of silyl enol ethers with retention of configuration. Since the preparation of 26 from vinylic halides (12-37) also proceeds with retention, the overall procedure is a... 

Reaction betweeen vinylic lithium compounds and silyl peroxides... 

Reaction betweeen vinylic lithium compounds and silyl peroxides 5-18 Michael-type reaction in the presence of MejSiCl... 

With this end in view, phenyldimcthylsilyl tri-n-butylstannane was added under the influence of zero-valent palladium compound with high regioselectivity and in excellent yield to the acetylene 386 to give the metallated olefin 387 (Scheme 56). The vinyl lithium carbanion 388 generated therefrom, was then converted by reaction with cerium(lll) chloride into an equilibrium mixture (1 1) of the cerium salts 389 and 390 respectively. However, the 1,2-addition of 389 to the caibonyl of 391, which in principle would have eventually led to ( )-pretazettine, did not occur due to steric reasons — instead, only deprotonation of 391 was observed. On the other hand, 390 did function as a suitable nucleophile to provide the olefinic product 392. Exposure of 392 to copper(II) triflate induced its transformation via the nine membered enol (Scheme 55) to the requisite C-silyl hydroindole 393. On treatment with tetrafluoroboric acid diethyl ether complex in dichloromethane, compound 393 suffered... 

Both Gais and Jackson have reported the preparation of a-alkyl and a-trimethyl-silyl vinyl sulfoximines 219 by a-lithiation of vinyl sulfoximines 218 with butyl-lithium or methyllithium followed by treatment with alkyl halides, chloro-trimethylsilane,78 88,107,108 or diphenyl disulfide.109... 

Vinylic lithium reagents (43) react with silyl peroxides to give high yields of... 

The simplest example of such a system is depicted in Eq. (5.28). The lithium enolate of 2-(trimethylsilyl)cyclopropanone is formed by the reaction of [l-(tri-methylsilyl)vinyl]lithium with carbon monoxide (Eq. (5.29)). Treatment of the vi-nyllithium with CO, at atmospheric pressure, in THF at 15 C for 2 h followed by quenching with trimethylchlorosilane at -78 "C afforded a somewhat labile product, which decomposed during the usual hydrolytic work-up. Quenching with tert-butyldimethylchlorosilane/HMPA instead allowed isolation of the products. The major product was a silylated cyclopropane enolate. It is noteworthy that the overall sequence follows a formal [2-1-1 jcycloaddition (Eq. (5.28)). The silylated alle-nolate was also formed as a by-product as the result of a 1,2-anionic silicon rearrangement. 

This compound -66 could be metallated with lithium directly but it would then destroy itself by reaction with its own keto group so reduction and protection first gives the silyl ether -70. This vinyl iodide was converted into a vinyl-lithium -71 and then into a vinyl cuprate -72 for... 

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... 

One problem with the Robinson annulation is the reversible nature of the initial Michael addition. One solution is to use a conjugated system that is particularly prone to Michael addition and forms the product, essentially irreversibly. a-Silyl vinyl ketones have been shown to be powerful Michael acceptors.The lithium enolate of cyclohexanone reacted with conjugated ketone 559 to produce the Michael product. 560.304b jn this case, the initially formed Michael adduct was stabilized by the presence of the silyl group at the a-position, driving the reaction toward the product. Hydrolysis produced 561, which was converted to the Robinson product (562) in 80% overall yield by treatment with NaOMe/MeOH under the requisite thermodynamic conditions.304b Pq,. this sequential process is justified when compared with normal treatment... 

Schemp 6,14 Stereoselective formation of a-suhstituted enol silyl ethers from acylsilanes and vinyl lithium.

The use of a-silylated vinyl ketone is another approach to overcome drawbacks of the standard Robinson annulation conditions such as polymerization of the vinyl ketone. The a-silylated vinyl ketones are stable and can undergo Michael addition in standard aprotic conditions (conditions that induces polymerization for vinyl ketones), as well as protic conditions. Synthesis of the octalone 21 can be used as an example of this variation. The silylated ketone 20 reacts with lithium enolate 13 (generated by methyllithium from its corresponding enol silyl ether in THE) in /-butyl... 

Addition of lithium ynolates generated from eth mylsilyl ethers with silyl vinyl ketenes gave highly substituted benzenes (eq 13). TIPS protection, in contrast to TBDMS, allowed preparation and purification by distillation and silica gel chromatography of these sensitive starting materials. 

Anionic Additions to Aldehydes. The addition of l,3-bis(silyl)-propenes to aldehydes and ketones to yield the vinyl silyl alcohol was explored. This was done using TBAF and good to excellent yields were achieved (eq 11). A further extension of this work was the addition of the (l,3-bis(silyl)allyl)lithium to ketones and aldehydes (eq 12). In this reaction, the substituted silyl diene was isolated in moderate to good yields. These substrates were then explored as ligands for both iron and manganese complexes. 

Palladium-catalyzed bis-silylation of methyl vinyl ketone proceeds in a 1,4-fashion, leading to the formation of a silyl enol ether (Equation (47)).121 1,4-Bis-silylation of a wide variety of enones bearing /3-substituents has become possible by the use of unsymmetrical disilanes, such as 1,1-dichloro-l-phenyltrimethyldisilane and 1,1,1-trichloro-trimethyldisilane (Scheme 28).129 The trimethylsilyl enol ethers obtained by the 1,4-bis-silylation are treated with methyllithium, generating lithium enolates, which in turn are reacted with electrophiles. The a-substituted-/3-silyl ketones, thus obtained, are subjected to Tamao oxidation conditions, leading to the formation of /3-hydroxy ketones. This 1,4-bis-silylation reaction has been extended to the asymmetric synthesis of optically active /3-hydroxy ketones (Scheme 29).130 The key to the success of the asymmetric bis-silylation is to use BINAP as the chiral ligand on palladium. Enantiomeric excesses ranging from 74% to 92% have been attained in the 1,4-bis-silylation. 

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