Silicon reaction with electrophiles

The second proposal by Speckamp and co-woikers used an allylsilane as an excellent nucleophile for an A -acyliminium cyclization reaction (Esch et al. 1987) (Scheme 7.13). The 3-effect of the silicon atom is a powerM determinant of the regiochemistry of allylsilane reactions with electrophiles, so the new carbon-carbon bond is formed at the vinyl caibon distal to silicon, that is, at the y-position (Hiemstra et al. 1984,1985). 

The most useful of all allyl anion equivalents are the allyl silanes.20 This is because it is easy to make them regioselectively, because they do not undergo allylic rearrangement (silicon does not do a [1,3] shift) and because their reactions with electrophiles are very well controlled addition always occurring at the opposite end to the silicon atom. Symmetrical allyl silanes can be made from allyl-lithiums or Grignards by displacement of chloride from silicon. A useful variant is to mix the halide with a metal, e.g. sodium, and Me3SiCl in the same reaction, rather after the style of the silicon acyloin reaction,21 as in the synthesis of the acetal 80. 

The silyl group directs electrophiles to the substituted position. That is, it is an ipso-directing group. Because of the polarity of the carbon-silicon bond, the substituted position is relatively electron-rich. The ability of silicon substituents to stabilize carboca-tion character at )9-carbon atoms (see Section 6.10, p. 393) also promotes ipso substitution. The silicon substituent is easily removed from the c-complex by reaction with a nucleophile. The desilylation step probably occurs through a pentavalent silicon species ... 

The carbon-carbon bond formation is accomplished by the reaction of the silicon-stabilized carbanions with electrophiles. Magnus and Roy have reported that methoxy(trimethylsilyl)methane is deprotonated with sec-butyllithium in... 

Another application of [Bu4N][Ph3SiF2] 826 involves the silicon-carbon bond cleavage of allyl-, benzyl-, and alkynylsilane derivatives 827-829.826 Subsequent reactions of the generated carbanions with electrophiles (Scheme 112) and alkyl halides (Scheme 113) provide high yields of carbon-carbon coupled products. 

Electrophiles such as halogens may cleave the transition metal-silicon (or germanium) bonds. We have already seen that in the case of the hydrido complexes they induce deinsertion reactions with retention of configuration at silicon (cf. Sect. 3.1). 

Metalloaldimines 57 bearing a silicon linked to a carbon can be alkylated with a range of carbon electrophiles. When 57 is treated with an aldehyde, the resulting adduct 58 undergoes the Brook isomerization providing a new lithioaldimine intermediate 59. Reaction of this lithioaldimine with electrophiles such as chlorotrimethylsilane provides a new imidoylsilane 60 in 62% yield (equation 21). ... 

These lithiooxiranes can be trapped by various electrophiles with retention of the configuration. The addition to aldehydes occurs with a low diastereoselectivity [but this can be enhanced by adding ClTi(OPr-/)3]. The reaction with enones occurs in a 1,2 fashion only. Intramolecular 1,4-silicon shift has also been reported. The reaction of the enantiomerically pure TMS-stabilized lithiooxirane 189 (Scheme 80) with an aldehyde has been used in a total synthesis of (-l-)-cerulenine. It must be noted that protodesi-lylation using TBAF (tetrabutylammonium fluoride) occurs with conservation of the oxirane stereochemistry. 

Reaction with Further Electrophiles of Group IVA (Sl,Ge,Sn). IV-Silylated aziridines can be prepared from ethyleneimine by amination of chlorosilanes in the presence of an HC1 acceptor, by dehydrocondensation with an organosilicon hydride or by cleavage of a silicon—carbon bond in 2-furyl-, 2-thienyl-, benzyl-, or allylsilanes in the presence of an alkali metal catalyst (262—266). N-Silylated aziridines can react with carboxylic anhydrides to give acylated aziridines, eg, A/-acetylaziridine [460-07-1] in high yields (267). At high temperatures, A/-silylaziridines can be dimerized to piperazines (268). Aldehydes can be inserted... 

With halogen electrophiles both retention and inversion of stereochemistry have been observed. In this case the addition of the electrophile may lead to the -silicon cation, or a cyclic halonium ion. Scheme 5 shows a generalized mechanism for the reaction of vinylsilanes with electrophilic reagents120. 

Other examples of Si—C(Cp) bond cleavage reactions deal with pentamethylcyclopen-tadienyl (Cp ) substituted silicon compounds. Scheme 11 shows a collection of different kinds of reactions with (pentamethylcyclopentadienyl)trimethylsilane as the substrate. Nucleophilic as well as electrophilic attack is the basis for Si—C(Cp) bond fission97. Reactions with various element halides involved in pentamethylcyclopentadienyl transfer are very useful in synthetic chemistry88-91 (see also Sections II.B and HE). 

It is possible to selectively cleave a Si—C(Cp )bond even in the presence of silicon-transition metal bonds as shown in reactions with nucleophiles, electrophiles and chlorinated hydrocarbons (equations 41 and 42)92,93. 

In view of the facts described above, a possible reaction mechanism seems to involve a four-centered transition state with electrophilic attack of bromine on one silicon assisted by nucleophilic attack of nitrogen on another. 

Reduction of 2-chloro-l,3-diaza-2-silacyclopentanes with lithium metal in THF gives the corresponding lithium compound which was shown to be configurationally stable by NMR spectroscopy up to temperatures of 333 K in THF or 388 K in diglyme <20020M1319>. Reaction of the lithium compound with electrophiles can be used to functionalize the silaheterocycle at the silicon atom <20020M1319>. 

Aside from substitution reactions on silicon, interesting reactivity on SCB-substituted methylstannane has been described <2005CC3047>. Upon treatment of the mixed SCB/stannane substrate with -BuLi, transmetallation predominates over ring-opening and ambiphilic SCB-methyllithium can be generated (product formed after quenching with electrophile was isolated in 50-60% yield, Scheme 64). 

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