Silicon nucleophiles

Attack on oxiranes by trivalent phosphorus (64HC(19-l)43l) provides a method of deoxygenation to alkenes with inversion (c/. Section 5.05.3.4.3(hY)) and this makes possible the interconversion of (Z)- and (f)-alkenes (Scheme 58) (B-74MI50505). Silicon nucleophiles behave analogously (76JA1265, 76S199). 

While trifluoro and other halosilanes function by increased electrophilicity at silicon, nucleophilic reactivity of allylic silanes can be enhanced by formation of anionic adducts (silicates). Reaction of allylic silanes with aldehydes and ketones can... 

In 1976 W.C. Still (J. Org. Chem. 41, 3063-41) discovered Me3SiLi and in the same year RB. Dervan and MA Shippey (J. Awer Chem. Soe, 98, 1265-1267, 1976) discovered Me3SiK which are silicon nucleophiles and both of which promised to be very useful for preparing organosilicon compounds. 

For the addition of silicon nucleophiles to activated enynes, silyl cuprates can be utilized. For example, treatment of ethyl 5-phenyl-2-penten-4-ynoate with (MejSi CuLi gave the 1,4-addition product with 76% yield (equation 77)38. 

Another reaction type for which EGA catalysis has been thoroughly explored is the reaction between organo-silicon nucleophiles and acetals or unprotected aldehydes and ketones [31-33]. The reaction types are aldol condensation, allyla-tion, cyanation, and hydride reductions depending on which of the nucleophiles (16) to (20) is used. 

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

Silicon nucleophiles have also been investigated with uracil derivatives, and thus the addition of dimethylphe-nylsilyllithium to 5-substituted 1,3-dimethyluracils 459 was found to give stable products of addition at the 6-position 460, although products arising from addition at the 5-position were also observed with some 6-substituted derivatives <1998H(48)2601>. 

Solvolysis of 15 in 97% trifluoroethanol gave a secondary isotope effect of 1.17, which indicates a vertically stabilized transition state. Thus the highly unsymmetrical dihedral dependence of silicon participation can almost entirely be attributed to the hyperconjugation model with little non-vertical involvement of the silicon nucleophile. 

These studies show the simultaneous lengthening of the silicon-leaving group bond as the silicon-nucleophile interaction increases. Molecular orbital calculations at the STO-3G level by Deiters and Holmes29 also show that as the nucleophile approaches from an apical position, the trans axial bond to the leaving group increases in length. However, there is little conclusive evidence as to whether a pentacoordinate intermediate with a finite lifetime is involved in such invertive processes. 

The results in Table 3 were explained as shown in Scheme 4. From the fact that no kinetic isotope effect was observed in the reaction of phenyl-substituted disilenes with alcohols (Table 1), it is assumed that the addition reactions of alcohols to phenyltrimethyl-disilene proceed by an initial attack of the alcoholic oxygen on silicon (nucleophilic attack at silicon), followed by fast proton transfer via a four-membered transition state. As shown in Scheme 4, the regioselectivity is explained in terms of the four-membered intermediate, where stabilization of the incipient silyl anion by the phenyl group is the major factor favoring the formation of 26 over 27. It is well known that a silyl anion is stabilized by aryl group(s)443. Thus, the product 26 predominates over 27. However, it should be mentioned that steric effects also favor attack at the less hindered SiMe2 end of the disilene, thus leading to 26. 

Substitution at Silicon Nucleophilic or One-Electron Transfer Process ... 

Substitution at silicon nucleophilic or a single electron transfer process . 334... 

Furthermore the addition of allyl tin and -silicon nucleophiles to aldehydes is accelerated in LiClOVether. Ipaktschi et al. [19] found that for instance allyl tributyl tin 17a adds to the a-epoxy aldehyde 16 with high iyn-selectivity (Scheme 5). Typically Lewis... 

Tributyl(trimethylsilyl)stannane (Me3SiSnBu3) as well as (Me3Si)2 can work as a silicon nucleophile. The reaction with aryl bromides gives arylsilanes exclusively (Scheme 3). It has also been reported that the silylstannane behaves as a tin nucleophile... 

Additions of silicon nucleophiles are similar to conjugate borylation. Hoveyda showed that Cu complexes with chiral carbene ligand 240 are excellent catalysts... 

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