Silicon organometallic nucleophiles

Silicon-based Lewis acids have been known for some time, and the related chemistry in catalysis has recently been reviewed [24]. Most examples in the literature are mainly based on achiral species and will be discussed only briefly in this section. In general, a broad variety of reactions can be catalyzed with compounds like MejSiOTf, MejSiNTf or MOjSiClO. One advantage over some metal Lewis acids is that they are compatible with many carbon nucleophiles like silyl enol ethers, allyl organometallic reagents and cuprates. 

Alkylations are also possible by nucleophilic displacement of a chlorine atom using an organometallic reagent. Substitution occurs faster when the silicon atom is pentacoordinated and the rate of the reaction depends upon the size of the ring, reflecting the strength of the coordination.269... 

Amino, alkoxy, and aryloxy polyphosphazenes are typically prepared by nucleophilic displacement reactions of poly(dihalophosphazenes). Analogous reactions with organometallic reagents, however, result in chain degradation and cross linking rather than in linear, alkyl, or aryl substituted poly(phosphazenes). The thermolysis of appropriate silicon-nitrogen-phosphorus compounds can be used to prepare fully P—C bonded poly(organophosphazenes). The synthesis of two of these materials and their Si—N—P precursors is described here. 

Recently silicon-based protection has matured into a new Ix oad area of protecting group chemistry (see Section 3.1.3.2) and amines can also be conveniently blocked with appropriate silicon reagents. In organometallic syntheses, e.g. in the alkylation of amino acid enolates, the so-called stabase adducts (30) serve to temporarily protect the a-amino group (Scheme 21). In the absence of hard nucleophiles, in particular of 0-nucleophiles, the Si—N bonds are stable, but deprotection is readily effected in the presence of water and acids. 

The dominant influence of the nucleophile on the stereochemistry at silicon cannot be interpreted in terms of the stability of the intermediate on the basis of the apicophilicity rule as stated in phosphorus chemistry (148-153) (see Sect. V-C-l). Such an attempted interpretation fails to explain retention of configuration as the stereochemical outcome. No better explanation can be based on the quasicyclic mechanism proposed by Sommer et al. (1,2). On the other hand, we have presented extensive data obtained with various nucleophiles which all show clearly that the stereochemistry at silicon is controlled by the electronic character of the nucleophile the harder the nucleophile, the closer the stereochemistry is to pure retention, as summarized for organometallics in Scheme 26. 

Nuclear Magnetic Resonance Spectra of Organometallic Compounds, 3, I Nucleophilic Displacement at Silicon Recent Developments and Mechanistic Implications, 20,265... 

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