Reactions via Hypervalent Silicon Species

The first observation of penta- and hexaeoordinate silicon compounds was reported at the beginning of the 19th century by Gay-Lussac [87] and Davy [88], Subsequent investigation of hypercoordination in silicon compounds stimulated widespread use of nucleophilic activation and catalysis in the application of organosilicon compounds as reactive species in organic synthesis. Synthetic application for silicon-fluorine bond formation can be found in several reviews over the last two decades, and this section focuses on recent advances in the use of hypervalent organosilicon compounds in selective organic synthesis, in particular, selective carbon-carbon bond formation [89]. 

The Lewis acidic character of reactive pentacoordinate silicon compounds has been unequivocally confirmed by Corriu, Sakurai and Hosomi [90]. Allylsilicates prepared from allylsilanes and catechol can undergo allylation reaction with aldehydes in the absence of Lewis acid promoter (Sch. 51). 

The silicon-carbon bonds within the anionic species are activated by KF, NaOMe, CsF or by heating. The reaction between geometrically defined crotylsilicates and benzaldehyde is completely stereoselective (Sch. 52). 

The role of the catecholate group and fluoride is to delocalize negative charge and increase the Lewis acidity of the silicon center, which coordinates a carbonyl oxygen to form a hexaeoordinate silicate. The six-membered cyclic transition state in the chair conformation is consistent with high threo and erythro selectivity similar to that of allyl boronates [91]. It is interesting to see the structure-reactivity and structure-selectivity correlation shown in Sch. 53 [92]. 

There might also be a critical borderline between allyltrifluorosiUcate selectivity and lack of selectivity, i.e. CsF-mediated reaction of allyltrifluorosilane [93] proceeds stereoselectively whereas no selectivity is observed with tetrabutylammonium fluoride because of the generation of the allylic anion-like species [94]. 

---

Non-catalyzed aldol reactions via hypervalent silicon species have also been studied. An aldol reaction between aldehydes and silyl enol ethers of amides was reported by Myers [105]. The reaction can be conducted under mild conditions to produce anti aldol without Lewis acid or base catalysts (Sch. 62). Asymmetric induction was particularly high when the (Z)-silyl ketene A/,0-acetal derived from prolinol was used. 

---