Carbon-silicon bond stability

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 ... 

It was found that treatment of a mixture of 120 and 121 with tris(diethylamino-sulfonium) trimethyldifluorosilicate [TASF(Et)] resulted in smooth addition-elimination to the naphthoquinone to form the y-alkylation product 125 (85 %). TASF(Et) is a convenient source of soluble, anhydrous fluoride ion [47]. It is believed that exposure of 121 to TASF(Et) results in fluoride transfer to generate a hypervalent silicate anion, as depicted in structure 124. The transfer of fluoride between TASF(Et) and 121 may be driven by stabilization of the anionic species 124 by delocalization of the carbon-silicon bond into the LUMO of the unsaturated ketone. 1,4-Addition-elimination of this species to the naphthoquinone 120 would then form the observed product. 

Allylsilanes have been found to be more reactive than vinyl silanes toward electrophiles and they are being studied more intensively in recent years because they hold considerable promise in organic synthesis. In allylsilanes, the geometry of carbon-silicon bond can be more favourably be oriented for efficient stabilization of a developing positive change P to silicon. 

The diphenyl derivative is more stable than the radical anion of its carbon derivative, 9,9-diphenylfluorene, but, after extended periods of reduction, the spectrum of the biphenyl radical anion begins to grow in intensity. The 5,5-dimethyl derivative appears to be stable under these conditions. The enhanced stability of the silicon derivative might be due to stabilization of the carbon-silicon bonds by delocalization of charge into available d-orbitals 81). Methyl proton hyperfine splitting observed for the anion radicals of the 5,5-dimethyl- and 5,5-diethyldibenzosilole has been cited as evidence for d-7r interaction (56). 

Treatment of aryl silane 10 with the iodonium source IC1 in an iododesilylation reaction yields compound 44, which proceeds by an ipso substitution mechanism.9 Activation towards electrophilic attack arises from stabilization of resonance form 45 by Si. Silicon is arranged / to the positive charge and the carbon-silicon bond can overlap with the empty Jt orbital (hyperconjugation). 

Maximum stabilization only occurs of course if the vacant p orbital and the carbon-silicon bond are in the same plane. Whilst this does not present any problems in acyclic cases, it is not always possible in cyclic systems. 

Dioxolane formation is not observed in allyl silanes as a result of silicons diminished ability in comparison to tin to stabilize a P positive charge. Nevertheless, the propensity of the carbon-silicon bond to donate electron density to electron deficient sites leads to population of a perpendicular conformation (Sch. 10) and ultimately to an unusually high yield of the sterically less stable Z-allylic hydroperoxide (e.g., 17). 

The carbon-silicon bond has two important effects on the adjacent alkenc. The presence of a high-energy filled CT orbital of the correct symmetry to interact with the n system produces an alkene that is more reactive with electrophiles, due to the higher-energy HOMO, and the same ff orbital stabilizes the carbocation if attack occurs at the remote end of the alkene. This lowers the transition state for electrophilic addition and makes allyl silanes much more reactive than isolated alkenes. 

Alkynylsilanes can function as carbon nucleophiles in addition reactions to electrophilic rr-systems. In principle electrophilic addition reactions to alkynylsilanes can occur to produce a- or 3-silyl-substituted vinyl cations, as illustrated in Scheme 37. The a-silyl carbocation is not the most stabilized cation, the reason being that the carbon-silicon bond can achieve coplanarity with the vacant orbital on the -carbo-nium ion, making possible -stabilization through hyperconjugation. Depending on the configuration of the carbocation, the developing vacant orbital can exist as a p-orbital, as in structure (75a), or an sp -hy-brid, as in structure (7Sb). 

Carbanions a to silicon are stabilized by the overlap of the filled 2p orbital on carbon with the vacant antibonding orbital of carbon-silicon bond ((a -p)K conjugation) (Scheme 1 A). For example, the a-carbanions of allylsilanes or similar compounds are easily generated by base removal of a proton (eq (9)) [6]. 

A second new class of MAO mechanism-based inactivators, (aminoalkyl)tri-methylsilanes, have been reported by Silverman and Banik (114). The idea for this class of MAO inactivators is based on the known activation of the carbon-silicon bond toward homolytic cleavage reaction when the silicon atom is /3 to a radical cation (115, 116). The aminomethyl-, aminoethyl-, and (amino-propyl)trimethylsilanes are all pseudo-first-order time-dependent inactivators of beef liver MAO that reduce the flavin cofactor during the inactivation reaction. Since denaturation of the inactivated enzyme allows flavin leoxidation, covalent bond formation might be to an amino acid residue (114). The stabilities of the enzyme adducts from the (aminoalkyl)trimethylsilanes were found to be differ-... 

The stabilization of a carbocationic center by an adjacent carbon-silicon bond (sometimes called the f- ect) can be used to control the course of carbocation rearrangement and carbocation-induced cyclizations. 

The crucial influence on the reactivity pattern in both cases is the very high stabilization that silicon provides for carbocationic character at the p-carbon atom. This stabilization is attributed primarily to hyperconjugation with the C-Si bond (see Part A, Section 3.4.1).85... 

---