Carbanions silylated

Stereoselective C —Si bond formation is also observed in intramolecular carbanion silylation. 

The ex- and y-silylaled allylphosphonates, prepared by the carbanionic silylation of allyl- and vinylphosphonates and a subsequent double bond migration, undergo the Friedel-Crafts reaction with acyl chlorides in ClhCl, al room temperature in the presence of AlCljto give high yields of... 

In this section we will discuss various reactive intermediates that contain silicon. In particular, the silicon analogues of the classic organic reactive intermediates such as carbenes (silylenes), carbenium (silicenium) ions and carbanions (silyl anions) have attracted the interest of experimentalists and theoreticians. In light of the elusive nature of these species it is not surprising that much of what we know about their properties comes from theory, in particular ab initio molecular orbital calculations. 

Keywords carbanions, silyl-stabilization, hydrogen migration, silacyclobutanes... 

The increased stability of the valence s orbital also accounts for the relative stability of silyl anions relative to analogous carbanions. Silyl anions provide powerful tools for the assembly of complex polysilane frameworks. 

The carbanions derived from thioacetals, however, are typical -synthons. Most frequently used are 1,3-dithianes and C -silylated thioethers (see p. 33f. D. Seebach, 1969, 1973 B.-T. Grobel, 1974,1977). In these derivatives the proton is removed by butyllithium in THF. 

The phosphorus ylides of the Wittig reaction can be replaced by trimethylsilylmethyl-carbanions (Peterson reaction). These silylated carbanions add to carbonyl groups and can easily be eliminated with base to give olefins. The only by-products are volatile silanols. They are more easily removed than the phosphine oxides or phosphates of the more conventional Wittig or Homer reactions (D.J. Peterson, 1968). 

The Peterson reaction has two more advantages over the Wittig reaction 1. it is sometimes less vulnerable to sterical hindrance, and 2. groups, which are susceptible to nucleophilic substitution, are not attacked by silylated carbanions. The introduction of a methylene group into a sterically hindered ketone (R.K. Boeckman, Jr., 1973) and the syntheses of olefins with sulfur, selenium, silicon, or tin substituents (D. Seebach, 1973 B.T. Grdbel, 1974, 1977) illustrate useful applications. The reaction is, however, more limited and time consuming than the Wittig reaction, since metallated silicon derivatives are difficult to synthesize and their reactions are rarely stereoselective (T.H. Chan, 1974 ... 

The silyl enol ethers 209 and 212 are considered to be sources of carbanions. and their transmetallation with Pd(OAc)2 forms the Pd enolate 210. or o.w-tt-allylpalladium, which undergoes the intramolecular alkene insertion and. 1-elimination to give 3-methylcyclopentenone (211) and a bicyclic system 213[199], Five- and six-membered rings can be prepared by this reaction[200]. Use of benzoquinone makes the reaction catalytic. The reaction has been used for syntheses of skeletons of natural products, such as the phyllocladine intermediate 214[201], capnellene[202], the stemodin intermediate 215[203] and hir-sutene [204]. 

SynttMSH ol alkenes from a silyl carbanions and cartMnyl compounds In cases where separation ol sitylaloohol diastereomers (e g 4] can be achieved, pure Z or E oleflns can be Isolaled... 

Intramolecular cyclization of 2-phenysulfonylmethyl lactam 3 took place upon reaction with lithium hexamethyldisilazan via generating its a-sulfonyl carbanion to give a cyclized postulated intermediate that can be quenched with trimethylchlorosilane to afford the stable silyl ketal 4. The later ketal was desulfonylated by Raney-Ni and desilylated through treatment with tetrabutyl ammonium fluoride (BU4NF) to afford the carbacephem 5 (94M71) (Scheme 1). 

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

Some ylides 92 among them C-silylated ones have been synthesized in order to compare the stabilization influence on the carbanionic center of various C-substituents (I, SiMej, Ph3P ) [ 113]. It appears from this study that the stabilization due to the electron-withdrawing C-substituents (R or R ) is not so negligible by comparison with the hyperconjugative stabilization between the ylidic carbon and the phosphonium group [114]. This is particularly true for the iodine substituent. 

A recent paper [44] shows that the treatment of silyl thioketones 68 with lithium diethylphosphite proceeds via a thiophiUc attack followed by a thio-phosphate mercaptophosphonate (69 70) carbanionic rearrangement and the migration of the silyl group from the carbon to the sulfur atom leading to the S-silylated sulfanylphosphonate carbanion 71. The last step represents the first example of the thia-Brook rearrangement (Scheme 18). 

Several examples of conjugate addition of carbanions carried out under aprotic conditions are given in Scheme 2.24. The reactions are typically quenched by addition of a proton source to neutralize the enolate. It is also possible to trap the adduct by silylation or, as we will see in Section 2.6.2, to carry out a tandem alkylation. Lithium enolates preformed by reaction with LDA in THF react with enones to give 1,4-diketones (Entries 1 and 2). Entries 3 and 4 involve addition of ester enolates to enones. The reaction in Entry 3 gives the 1,2-addition product at —78°C but isomerizes to the 1,4-product at 25° C. Esters of 1,5-dicarboxylic acids are obtained by addition of ester enolates to a,(3-unsaturated esters (Entry 5). Entries 6 to 8 show cases of... 

Eliminations from cqa -orf/io-disubstituted benzenes can be carried out with various potential leaving groups. Benzylic silyl substituents can serve as the carbanion precursors. 

The domino reaction is initiated by the chemoselective attack of the carbanion 2-458 on the terminal ring carbon atom of epoxyhomoallyl tosylate 2-459 to give the alkoxides 2-460 after a 1,4-carbon-oxygen shift of the silyl group. The final step to give the cyclopentane derivates 2-461 is a nucleophilic substitution. In some cases, using the TBS group and primary tosylates, oxetanes are formed as byproducts. 

The intermediate formation of betaines with the carbanionic center is also postulated in the reactions of permethylsilirane, sila- and disilacyclobutanes with phosphorus ylides. For data on these betaines isomerized in situ to silylated phosphorus ylides, see Section 5.4. 

It is most likely that silylation of AN with silyl derivatives of amides, like the processes considered in Section 3.2.3.2, involve the formation of a-nitro-carbanions as the key step. It is also possible that only aci forms of AN can react with DPSU. This is evidenced by a comparison of the results of entries 8 and 9 in Table 3.3. 

Classical C,C-coupling reactions of AN anions (Henry, Michael, and Mannich) involve complex systems of equilibria and, consequently, generally not performed in protic solvents. The introduction of the silyl protecting group allows one to perform these reactions in an aprotic medium to prepare or retain products unstable in the presence of active protons. In addition, the use of nucleophiles which are specifically active toward silicon (e.g., the fluoride anion) enables one to design a process in which the effective concentration of a-nitro carbanions is maintained low. 

Under the action of a base, the first step (Ki) reversibly produces the a-nitro carbanion A, which then rapidly reacts with Si X (K2) to form SENA. The latter is reversibly silylated by the second equivalent of SiX (K3) to give the cationic intermediate B, whose deprotonation with the base (K4) affords the target nitroso acetal (or BENA). 

This reaction was easily performed with malonic ester derivatives using approaches described above for nitro carbanions. It should be noted that the anion of malonic ester can be prepared not only by the reactions of bases with malonates but also by desilylation of silyl ketene acetal (449) with fluoride anion. 

A carbon-substituted five-membered ring, 24, was also successfully ring-opened via a [Pg.568]

These P elimination reactions have been used in an olefine synthesis called the Peterson olefination reaction which is analogous (and sometimes superior) to the Wittig reaction. The Peterson olefination reaction involves the addition of an a-silyl carbanion to an aldehyde or ketone to give P-hydroxysilane, followed by P-elimination to give the olefine. 

Some important reactions of silyl anions are as follows and they have emerged on the basis of the carbanion chemistry. 

Silylation using the silylacetate (3.1.14) [49] involves the initial formation of the acetate carbanion, which abstracts a proton from the carbonyl compound or alcohol (Scheme 3.2, Table 3.5). When the reaction with a ketone is conducted in the presence of an aldehyde, crossed aldol products are obtained (see Chapter 6). 

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