Silicon Stabilization

This Li-phenyl interaction is similar to the r/ -Ph contact observed in the silicon-stabilized carbanion complex [Li C(SiMe2Ph)3 (THF)] (140). Addition of tmeda to 49 yields the monomeric compound [Li C(SiMe2Ph)(PMe2)2 (tmeda)l in which the lithium is bound by the two P atoms of the phosphinomethanide ligand and the two N atoms of the tmeda in a distorted tetrahedral geometry (139). There... 

The total synthesis of vallesiachotamine (9) and isovallesiachotamine (10) has been completed elegantly by Wenkert and Spitzner (393) by utilizing the addition of a silicon-stabilized anion to pyridinium salt 633 to achieve the properly substituted indoloquinolizidine 634 by cyclization, from which 9 and 10 could be prepared in racemic form by simple reaction steps. 

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... 

As one can notice, the monoUthiated allenic isomer seems to be preferred in almost aU cases in hexane. However, West and Gomowicz reported that the IR spectrum of monolithio-I,3-bis(trimethylsilyl)-3-phenylpropyne exhibits two bands at 2000 and 1850 cm and attributed them to the acetylenic and allenic forms ". In light of the new interpretation, Priester and coworkers ascribed the band at 2000 cm to the propargyl anion, where both phenyl and silicon stabilized the charge at carbon atom 3 . 

Some reports concerning the reaction of lithiated thioallylethers with oxiranes have been published. A slow reaction was observed with a terminal oxirane. However, with cyclopentadiene oxide, the reaction occured smoothly with an excellent regioselectivity in favor of the Sjv2 displacement in the ally lie position. Other examples involving sulfur, selenium and silicon stabilized organolithium reagents have been reported . 

Only one example of electrophilic behavior of silicon-stabilized lithiooxiranes is reported. Intermolecular C—Li insertion followed by Li20 elimination occurs by raising the temperature, and ( ) vinylsilanes are obtained stereoselectively (Scheme 80). Reaction of lithiooxiranes with aluminum , zirconium and silicon reagents leads to the corresponding ate complexes, which undergo 1,2-metallate rearrangements. 

This reaction was successfully applied to various hydrazones derived from acyclic or cyclic ketones and extended to a variety of unsubstituted vinylsilanes as acceptors (trialkyl, triphenyl and alkoxy vinylsilanes). In all cases, the regioselectivity of the addition reaction was consistent with the formation of a silicon-stabilized organozinc reagent. 

The internally stabilized silyl cation (36), formed by hydride transfer as shown, has a 2-silanorbomyl cation structure, and is not coordinated to die solvent or the counterion.78 NMR chemical shifts calculated on the basis of die bridged structure shown are in agreement with the experimental values. The autiiors describe it as free but internally re-stabilized .78 The silicon-stabilized oxonium ion (37) shows considerable stereoselectivity in its reactions (38) is the preferred product isomer by a 92 8 ratio, and (39) by 98 2.79... 

The ring opening of the (1. y-epoxysilane (33) with/vnitrobcnzoic acid in chloroform is regiospecific and gives the two esters (35) and (36).33 The mechanism involves the -silicon-stabilized carbenium ion (34), which is captured by the /vnitrobcnzoatc counter-ion. 

Effects for the tertiary butyl carbocation were also calculated and, as would be expected, the value for -silicon stabilization was smaller than in the secondary system, having a value of 15.9 kcalmol-1. For comparison, the /)-methyl stabilization was 5.0 kcalmol-1. 

A variety of other methods have been used to measure the /3-silicon stabilization of car-bocations. From gas-phase studies, Hajdasz and Squires50 derived a value of 39 kcal mol-1 for the stabilization of the cation Me3SiCH2CH2+ relative to the ethyl cation. This is in agreement with calculations by Ibrahim and Jorgenson39. Siehl and Kaufmann51 have used carbon-13 NMR spectroscopic data to give an indication of the /1-silyl stabilizing effect in some aryl vinyl cations. 

The stability of an a-silyl carbanion is responsible for the unproved synthetic utility of the Stork annulation over other annulations195,196. These reactions involve the Michael addition of an enolate ion to an enone, and in the absence of a a-silyl substituent suffer drawbacks due to the reversibility of the Michael reaction. However, the addition of enolate ions to a-trimethylsilylvinyl ketones is not reversible, owing to a-silicon stabilization of the canonical form 152 shown in equation 122. 

Since silicon stabilizes an a-carbanion by d jr-p jr interaction, silicon should promote electron transfer to a carbon-halogen bond which generates the a-carbanion. In fact,... 

Nucleophilic attack in a 1,6 fashion by the allylsilane double bond at the doubly activated Michael acceptor produces A containing the silicon-stabilized carbocation. Loss of the trimethylsilyl group generates the exocyclic methylene moiety. 

An unprecedented one-pot stereoselective synthesis of 2-azetidinone P-chlorinated allylic alcohols 23, which can also be considered as functionalized allylsilanes, has been developed, by tin(IV) chloride-mediated reaction of propargyltrimethylsilane and 4-oxoazetidine-2-carbaldehydes <02CEJ1719>. An explanation for the formation of P-chlorovinyl alcohols involves a stepwise process with the chlorination proceeding via a silicon stabilized carbocation. 

Panek, J. S. Silicon Stabilization. In Comprehensive Organic Chemistry, Trost, B. M., Fleming, I., Eds. Pergamon Oxford, 1991 Vol. 1, Chapter... 

The mechanism of electrophilic desilylation is the same as that for electrophilic aromatic substitution except that the proton is replaced by trimethylsilyl. The important difference is that the silicon stabilizes the intermediate cation, and hence the transition state leading to it, to a dramatic extent so that the rate is much faster. This is the first step with bromine. 

Some aldehyde-derived diaziridines are unstable and hamper the Schmitz synthesis of the corresponding diazirines <1975J(P2)686>. A one-pot derivative of the standard Schmitz reaction has been developed to circumvent such problems, in which an intermediate diaziridine is stabilized by a trimethylsilyl group <2000TL795>. Treatment of the aldehydes 68a-c with LiHMDS, then HOSA, gave the silicon-stabilized diazirines 69a-c. In situ oxidation with f-butyl hypochlorite generated the diazirines 70a-c in moderate yields (Scheme 24). 

The use of silicon is illustrated by Scheme 3.50 involving a Grignard reagent containing a trimethylsilyl group in which the silicon stabilizes the carbanion. Elimination of the silyl group from the P-hydroxysilane leads to the formation of the alkene. 

Very few pericyclic reactions of carbene complexes have been studied that are not in the cycloaddition class. The two examples that are known involve ene reactions and Claisen rearrangements. Both of these reactions have been recently studied and thus future developments in this area are anticipated. Ene reactions have been observed in the the reactions of alkynyl carbene complexes and enol ethers, where a competition can exist with [2 + 2] cycloadditions. Ene products are the major components firom the reaction of silyl enol ethers and [2 + 2] cycloadducts are normally the exclusive products with alkyl enol ethers (Section 9.2.2.1). As indicated in equation (7), methyl cyclohexenyl ether gives the [2 -t- 2] adduct (84a) as the major product along with a minor amount of the ene product (83a). The t-butyldimethylsilyl enol ether of cyclohexanone gives the ene product 9 1 over the [2 + 2] cycloadduct. The reason for this effect of silicon is not known at this time but if the reaction is stepwise, this result is one that would be expected on the basis of the silicon-stabilizing effect on the P-oxonium ion. 

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