Silicon halides Lewis acidity

Silicon exhibits Lewis acidity in most oxidation states, including the pervasive oxidation state +4. For the purposes of this review, Si(IV) coordination complexes will not be discussed, but we do note that some of the earliest reports were made by Kuhn et al. who explored the coordination chemistry of NHCs with a variety of Si(IV) halides. An example of such a complex is shown in Figure 15.33 [175]. 

The silicon atom in Wiberg s silenes is a Lewis acidic center and can be coordinated with donors. These donors may be halide ions [like, e.g., in 104 LiX(T2-C-4)831, ethers or nitrogen bases84. Coordination with the donor stabilizes the silenes and, when the basicity suffices (LbOcBr, THF, NMe3, F ), the adducts serve as stock compounds from which the silenes may be liberated (equation 32). 92 decomposes at —100 °C, but its trimethylamine adduct is stable at room temperature85. Noticeable amounts of 97 are available from 97-LiBr already at — 78 °C, whereas from 97 LiF the temperature has to be raised to +30 °C. Structures of such silene adducts will be discussed in Section I.C.l. The silene adduct 124-TIIF has been discussed in Section I.A.2. 

Polysilanes have also been modified by removal of aryl groups under acidic conditions, to give Si-functional polymers. Phenyl or other aryl groups on silicon can be selectively replaced by halogens, upon treatment with a hydrogen halide and a Lewis acid. 

Lithium derivatives of allyl silanes react in the y-position with alkyl halides, epoxides, and carbonyl compounds. The lithium derivative 110 of allyl silane 109 gives only the y-adduct 111 with ketones.31 Vinyl silanes such as 111 are usually converted into carbonyl compounds via epoxides which rearrange with Lewis acid catalysis and loss of silicon to give protected versions of ketones or aldehydes 112. 

Although the mechanism of the Mukaiyama reaction is not yet fully understood, several points have now been firmly established (a) a Lewis acid enolate is not involved (b) the Lewis acid activates the carbonyl group for the nucleophilic addition and (c) the Si—O bond is cleaved by nucleophilic attack of the anionic species, generally halide, on silicon. Point (a) has been established by the use of INEPT- Si NMR spectroscopy. Moreover, trichlorotitanium enolates have been synthesized, characterized and shown to give a completely different stereochemical outcome than the TiCU-mediated reactions of silyl enol ethers. Complexes between Lewis acids and carbonyl compounds have been isolated and characterized by X-ray crystallography and recently by NMR spectrometry. On the basis of these observations closed transition structures will not be considered here open transition structures with no intimate involvement between the silyl enol ether and the Lewis acid offer the best rationale for the after the fact interpretation of the stereochemical results and the best model for stereochemical predictions. 

Nucleophilic carbenes have been reported to react with Lewis acidic group 14 complexes with the isolation of neutral or ionic compounds via simple adduct formation or displacement of a halide ion. The range of carbene complexes of silicon, germanium, tin, and lead, as well as some related cyclopropenylidene complexes are described below. 

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