Substitution at silicon

In silicon derivatives, X-ray studies of compound 27 were consistent with a covalently bonded trigonal bipyramidal molecule <2000CC565>. In addition, nucleophilic substitution at silicon for similar compounds was modeled either by NMR or X-ray techniques and both methods correlate in the calculation of % Si-O bond formation <2003JOM66, 2003JOM154>. 

Although substitutions at germanium and tin bonded to transition metals are well known, only a few reports describe substitutions at silicon - . 

However, manganese hydrides undergo substitution at silicon without cleavage of the silicon-manganese bond Scheme 13) ... 

Substitution at silicon in an optically active silicon-manganese hydride has also been observed ... 

It should be pointed out that substitution at silicon takes place after hydrogen abstraction, since the anion, prepared on reaction with NaH, undergoes substitution when treated with RLi. 

Si H M agostic interactions in silylamido complexes have been extensively studied to date. The earlier examples were prepared by halide displacement in the coordination sphere of a metal by a silylated amide, which puts severe limitations on the nature of the substituents at silicon (usually, robust methyl groups are used). More recently, a new route to p-agostic silylamides based on the direct coupling of silanes with imido ligands was discovered that allows one to trace the effect of substitution at silicon on the extent of the Si-H bond complexation (vide infra). 

The dependence of the acid-lability of trialkylsilyl and related ethers on the type of substitution at silicon is illustrated by the formation302 of l-0-(tert-butyldiphenylsilyl)-2,3 4,5-di-0-methylene-D-mannitol (47) in 96% yield on treatment of 6-0-(terf-butyldimethylsilyl)-l-0-(fert-butyldiphenylsilyl)-2,3 4,5-di-O-methylene-D-manni-tol (48) with 80% acetic acid. 

Iron enneacarbonyl reacts smoothly with 1,1-dimethylsilacyclobutane to insert into the ring Si—C bond with complete regiospecificity (76JCS(D)910). The ferrosilacyclopentane (54) is thermally stable but reacts with HCl, and can also be prepared from 3-chIoropropyI-dimethylchlorosilane and Fe(CO)42. Carbonyl anions will substitute at silicon if this atom bears chlorine, and platinum will insert into the Si—H bond (Scheme 79) (72CC445, 78JOM( 144)317). 

In recent years, mainly from the 1960s on, a great number of stereochemical studies of substitution at silicon by nucleophiles have been carried out. The earlier work, up to 1964, is the subject of an important monograph by Sommer (/) more recent studies have also been summarized by him (2) and others [see Prince (2) and Fleming (4)] in comprehensive surveys. Recently, we collected the main known features in this area (5). The intent of this chapter is to focus upon the latest developments concerning nucleophilic displacement at silicon and to survey the following topics ... 

In the following sections we shall examine the factors controlling the stereochemistry of substitution at silicon. 

Hydrolysis and condensation reactions of silanes may be considered in the broad category of nucleophilic substitutions at silicon. The common nomenclature for these reactions is SN.V-Si, where A represents the kinetic order or molecularity, Si indicates that silicon is the reaction center, and SN indicates that the reaction is a nucleophilic substitution. Nucleophilic reactions at silicon have been reviewed thoroughly and have been the subject of fundamental studies by several laboratories over the last three decades [33]. The literature is not as voluminous as the literature on the corresponding reactions at carbon. A general mechanistic view of these reactions has, however, emerged. There are many parallels to carbon-centered reaction mechanisms. One distinction from carbon-centered reactions is clearly apparent. Silicon is able to form relatively stable higher coordinated (pentavalent) intermediates carbon is not [33]. 

Of the four possible isomers, substitution at silicon produces the most stable isomer (except for COOH), as shown in Table 1. The authors concluded that the degree of stability... 

In this review we have chosen to limit the scope to nucleophilic substitution at silicon. A short overview is given of material covered in detail in our previous review in The Chemistry of Organic Silicon Compounds 1, with recent advances covered in greater depth. 

Recently, the stereochemistry of nucleophilic substitution at silicon has been reviewed by Holmes2, and the role of pentacoordinate silicon compounds as reaction intermediates has been reviewed by Corriu and coworkers3. 

As in substitution at carbon, stereochemical and kinetic data provide the means of differentiating between these two possibilities. The stereochemical data are examined first. Substitution at silicon leads to both retention and inversion of configuration and the stereochemical outcome depends upon the nature of the leaving group, the nucleophile, solvent, complexing agents and whether or not the silicon is part of a ring. 

Sidorkin and coworkers28 have used quantum chemical (MNDO) methods to model the pathway of intramolecular Sjv2 substitution at silicon, as shown in equation 3. 

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