Silicon electrophilic attack

The attack of HF is thought to proceed by a similar mechanism in which there is simultaneous nucleophilic and electrophilic attack on the network silicon and oxygen atoms, respectively, according to ... 

Alkylsilanes are not very nucleophilic because there are no high-energy electrons in the sp3-sp3 carbon-silicon bond. Most of the valuable synthetic procedures based on organosilanes involve either alkenyl or allylic silicon substituents. The dominant reactivity pattern involves attack by an electrophilic carbon intermediate at the double bond that is followed by desilylation. Attack on alkenylsilanes takes place at the a-carbon and results in overall replacement of the silicon substituent by the electrophile. Attack on allylic groups is at the y-carbon and results in loss of the silicon substituent and an allylic shift of the double bond. 

Electrophilic attack at ring silicon, sulfur, selenium, and phosphorus Nucleophilic attack at ring silicon Nucleophilic attack at ring boron... 

Cyclic silicon-containing compounds such as 61 undergo electrophilic attack at carbon with hydrogen peroxide to furnish the stereodefined diol 62 (Equation 12) <2003OL3875>. 

The chemistry of silacyclopentanes resembles closely that of acyclic alkylsilanes. Strong bases are needed to cleave the silicon-carbon bond, while electrophilic attack occurs readily with halogens and strong acids, e.g. sulfuric. In addition, aluminum chloride will induce polymerization of silacyclopentanes unless one of the groups on silicon is carbofunctional (Scheme 154) (67MI12000). If this is chloromethyl, then ring expansion occurs (Scheme 155). 

In spite of the small difference in the electronegativity of silicon and hydrogen (Table 1), most reactions of the Si—H bond are ionic in character, taking place either by electrophilic attack on hydrogen (which is negatively polarized) or/and by nucleophilic attack on silicon (which is positively polarized). Because of the reverse... 

Therefore the polarity of the silicon-carbon bond is Si+C, and silicon-carbon bonds are generally observed to cleave in this direction, either via electrophilic attack at carbon, or nucleophilic attack at silicon. 

In order to determine whether attack anti to silicon was due to steric effects or electronic effects, Fleming and coworkers studied the attack of electrophiles on the pentadienyl-silanes 140, in which the electrophile attacks at the second double bond, as shown in equation 97165. 

The striking contrast between these two reagents was explained by the operation of different mechanisms for the processes. The reaction of tris(2-hydroxyphenyl)amine with acetoxysilane may involve simultaneous nucleophilic attack at the silicon and electrophilic attack upon the leaving acetoxy group via a six-membered transition state. Similar... 

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). 

Eaborn and co-workers28 have reported product isotope ratios for the cleavage of benzyl-silicon, benzyl-tin, and aryl-tin bonds using an equimolar mixture of MeOH and MeOD as the solvent. A free carbanion would not be expected to discriminate between MeOH and MeOD in the fast step (24), and hence28 the product isotope ratio kH/kD should be unity. The values of kJkD in Table 11 indicate that the carbanions are not entirely free, but that some degree of electrophilic attack by the solvent at the benzyl carbon atom takes place, as in (V). 

On the other hand, in the uncatalyzed cleavage by hydrogen chloride, electrophilic attack of a proton on the silicon-silicon bond may play an important role in the four-centered transition state. 

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. 

The selective cleavage of the silicon-silicon bond in the disilacyclopentane and -hexane is probably due to the concentration of internal angular strain at this bond in such smaller polygonal molecules. In case of the disilacyclo-heptane, however, it seems likely that the strain is smaller and uniformly distributed all over the ring, and hence cleavage occurs preferentially at the silicon-carbon bond by the accepted mechanism involving both an electrophilic attack on carbon and nucleophilic attack on silicon by the sulfuric acid molecule(s) 169). 

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). 

Because transition-metal anions can be prepared conveniently in tetrahydrofuran solution, this cyclic ether was often used in earlier attempts to prepare silicon-metal compounds. It is now generally realized, however, that tetrahydrofuran can frustrate these attempts in two ways. First, it promotes electrophilic attack by the silicon compound on oxygen atoms of coordinated carbonyl groups this leads to the formation of products with Si-0 bonds (54, 138, 262, 300, 306, 310, 336,337), e.g.,... 

From these observations, Woerpel and Cleary proposed a mechanism to account for allylic silane formation (Scheme 7.23).85 Silacyclopropane 94 is formed from cyclohexene silacyclopropane 58 through silylene transfer. Coordination of the Lewis basic benzyl ether to the electrophilic silicon atom86-88 generates pentacoordinate siliconate 95 and increases the nucleophilicity of the apical Si-C bond.89 Electrophilic attack by silylsilver triflate 96 forms silyl anion 97. Intramolecular deprotonation and elimination then affords the silylmethyl allylic silane. 

Sommer and Bauman investigated the reaction of optically active a-naphthylphenylmethylsilyl hydride with trityl chloride and found that the stereochemical course was retention in benzene solution, inversion in methylene chloride, and full racemization in methylene chloride (93). The result in CH2C12 was explained as consistent with an exchange in which electrophilic attack on the silicon hydride by a triphenyl cation is of primary importance however, the counterion appears in the transition state [Eq. (16)]. Racemization of the unreacted R3SiH could arise from... 

Electrophilic attack occurs at the a carbon atom and the halide ion released in this step then attacks the silicon atom to release the product and a molecule of Me3SiX, which will be hydrolysed during the work-up. 

The cycloadduct is fragmented with Me3SiBr in acetonitrile. The electrophilic silicon atom attacks the ketone and the furan oxygen atom provides the electronic push. These two groups have the 1,4 relationship necessary for a fragmentation. First of all, we shall draw the product in the same way as the starting material—this is a good tip in a complicated mechanism. The product may look odd but we can redraw it more realistically in a moment. 

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