Electrophilic silicon

Triphenylphosphonium ylide reacts with the silylene complex 93 which has a highly electrophilic silicon center, to give the corresponding cationic adduct 94 [115]. The lengthening of the PC bond indicates a loss of the double bond character of the ylide and corresponds to the formation of a tetrahedral silicon center with four covalent bonds (Scheme 28). 

In addition to the silicon-based in situ activation of hemiacetal donors, there has been a significant body of work that uses electrophilic silicon activation of preformed C-l silyl hemiacetal donors [54—67]. However, this work is outside the scope of this discussion. 

An alternate pathway is possible for systems containing silylamino substituents at phosphorus. This most likely involves attack of the CCl3 anion at the electrophilic silicon resulting in elimination of Me3SiCCl3 as shown in pathway B. In the systems investigated thus far, the reaction pathway preference appears to be influenced by (1) solvent polarity, and (2) steric and electronic effects of the substituents at phosphorus ( ). 

In the particular case of electrophilic silicon derivatives, the exchange reaction is very difficult to achieve. Transfer from Zr to Si is not possible in THF solution at room temperature and the zirconium complex could be recovered unchanged even after heating in neat SiCLj. However, when the zirconacycle 70 was treated with neat SiBr4 for 2 days... 

Another synthetic procedure is based on the reaction of dilithioferrocene with difunctional electrophilic silicon substrates, as shown in equation 52. For example, Me2Si110,... 

With both the Fu and the Denmark catalysts it can be assumed that catalysis is effected by formation of a highly electrophilic silicon cation D from tetrachlorosi-lane and the nucleophilic catalyst C, i.e. by attack of the pyridine N-oxide or of the phosphoramide O-atom on silicon, followed by ionization (Scheme 13.38). The latter cation can then activate the epoxide toward nucleophilic attack by the chloride ion. Exchange of the product silane for another molecule of tetrachlorosilane completes the catalytic cycle [75],... 

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. 

Second only to lithium enolates in usefulness are silyl enol ethers. Silicon is less electropositive than lithium, and silyl enol ethers are more stable, but less reactive, than lithium enolates. They are made by treating an enolate with a silicon electrophile. Silicon electrophiles invariably react with enolates at the oxygen atom firstly because they are hard (see p. 237) and secondly because of the very strong Si-O single bond. The most common silicon electrophile is trimethylsilyl chloride (Me3SiQ), an intermediate made industrially in bulk and used to make the NMR standard tetramethyl silane (Me4Si). 

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. 

The other side of the coin is that the S 2 reaction at carbon is not much affected by partial positive [ charge (5+) on the carbon a tom. The Sn2 reaction at silicon is affected by the charge on silicon. The r most electrophilic silicon compounds are the silyl triflates and it is estimated that they react some 108-109 times faster with oxygen nucleophiles than do silyl chlorides. Trimethylsilyl triflate is, in fact, an excellent Lewis acid and can be used to form acetals or silyl enol ethers from carbonyl compounds, and to react these two together in aldol-style reactions. In all three reactions the triflate attacks an oxygen atom. 

The electrophilic silicon can even bind nucleophilic compounds (ethers, amines, etc.) yielding complexes which initiate polymerizations of vinyl compounds [98]... 

A two-stage reaction pathway might, however, be invoked as an alternative, the first step being a single-electron transfer from the electron-rich nucleophile, Nu-, to the electrophilic silicon atom (eq. [32]). 

Nazarov cyclization. 2-Siloxy-4-aIkenoylfurans fail to undergo Nazarov cyclization in the presence of conventional Lewis acids, but the reaction can be brought forth with addition of an iridium complexHowever, whether the true catalyst is a highly electrophilic silicon species cannot be excluded. 

Where the catalyst is less nucleophilic, e.g. potassium fluoride or solid caesium fluoride, only one fluoride ion is likely to coordinate at all firmly to the silane. The nucleophilic reactant will then also be able to coordinate to the electrophilic silicon atom, itself receiving further activation in the process, and reaction ensues by intramolecular transfer about the hexacoordinate silicon atom as demonstrated in the GTP process. Less nucleophilic substrates such as alkyl halides are unreactive in these circumstances. 

At first, we considered the transformation of the carbamate moiety into more reactive functions. It was shown that Z-O-enecarbamate can be transformed into Z-silyl enol ether 10 by treatment of 8 with methyllithium and quenching of this intermediate lithium enolate 9 by electrophilic silicon reagents. Assuming this lithium enolate intermediate 9 would be able to react with other electrophile reagents, the preparation of more reactive functions including Z-vinyl phosphate and Z-vinyl triflate was considered. The remaining and important question was whether the Z-stereochemistry of this double bond would be preserved. 

Our first assumption was that the formation of 9 and 10 might be due to an intramolecular attack of the Si-K moiety of the monopotassium compound onto the Si(SiMe)3 group. However, this reaction would afford a cyclic structure with four trimethylsilyl groups as in 11, 12 and trimethylsilyl potassium, none of which we were able to observe. Even if the trimethylsilyl potassium reacts immediately with the formed product this does not seem to be very likely, since it would result in the formation of hexamethyldisilane which we also did not observe. In addition we figured out that the new product is formed at the expense of the dipotassium compound and eventually found that the formation was actually caused by partial hydrolysis of the strongly basic dipotassium compound. Once the Si-H bond is formed the now more electrophilic silicon is attacked by the Si-K group and formation of the cyclic product along with trimethylsilane is accomplished. Careful addition of one equivalent of water to a reaction mixture which contained mainly the di-potassium product led to the quantitative formation of the cyclic Si-K species. 

The proposed pathway for the conversion is outlined in Eq. 2. The acid-catalyzed elimination of water from the alcohols 1 affording the carbenium ion is followed by a migration of one trimethylsilyl group from the central silicon atom to the neighboring carbon atom and attack of X", the conjugate base of the acid used as the catalyst, at the electrophilic silicon. The hydrolysis of the intermediates 4 gives the silanols 3 [2]. 

The silyl triflate 8 (Scheme 3) shows an ambireactive behavior of the cation. Reaction with a methylmagnesium chloride or water leads to the methyl-substituted product and to the siloxane, respectively, as expected for an electrophilic silicon center. An oxonium ion reactivity is observed in the reaction with neutral Lewis bases such as triethylamine and trimethylphosphine. 

Thiophene is an aromatic molecule which exhibits a very versatile chemical reactivity. The electron-rich five-membered ring can be easily functionalized at the 2- or 3-positions, and therefore allows the preparation of a variety of substituted derivatives [1]. Silylation of organic molecules often involves nucleophilic or organometallic reactions at the electrophilic silicon... 

---