Allylsilanes, Sakurai allylation reaction nucleophilicity

Allyl- and vinylsilane chemistry was one of the first areas of reagent synthesis impacted by CM methodology. Allylsilanes are commonly employed in nucleophilic additions to carbonyl compounds, epoxides, and Michael acceptors (the Sakurai reaction) vinylsilanes are useful reagents for palladium-coupling reactions. As the ubiquitous application of CM to this substrate class has recently been described in several excellent reviews, this topic will not be discussed in detail, with the exception of the use of silane moieties to direct CM stereoselectivity (previously discussed in Section 11.06.3.2). 

Allylsilanes are intrinsically nonnucleophilic. We believe that addition of fluoride ion to an allylsilane forms a silicate anion, which can react at either end of the tc-system. Thus from a mechanistic viewpoint, substitution of an allylsilane under nucleophilic conditions is more appropriately considered an addition-elimination reaction with the terms Se2 and Se2 aptly representing the elimination process. For recent evidence of pentacoordinate silicate intermediates in fluoride ion catalyzed allylation using trimethylallylsilane, see a) M. Kira, M. Kobayashi and H. Sakurai, Tetrahedron Lett.. 28. 4081 (1987) b) M. Kira, K. Sato and H. Sakurai. J. Am. Chem. Soc.. 110, 4599 (1988) c) T. Hayashi, Y. Matsumoto, T. Kiyoi, Y. Ito, S. Kohra, Y. Tominaga and A. Hosomi, Tetrahedron Lett., in press d) H. Sakurai, Lewis Acid Character and Selective Reactions of Pentacoordinate Silicon Compounds, this volume. 

The reaction between allyl nucleophiles and aldehydes or ketones in the presence of titanium Lewis acids gives the homoallylic alcohols that are widely applicable in organic synthesis. The first allylation was reported by Hosomi and Sakurai in 1976 [69]. TiCU was utilized as the promoter to facilitate the addition of allylsilane to aldehydes or ketones. This allylation has the advantage of complete S 2-regiospecificity with regard to the allylic system. Later, Hayashi and Kumada established the acyclic stereochemical outcome of this allylation with a few electrophiles in the presence of TiCU, indicating the anti 8 2 mode of the reaction [70] (Scheme 14.15). 

In conclusion, the Sakurai reaction is a powerful method to introduce a nucleophilic allyl synthon. Similar allylations include the Keck reaction of allylstannanes, and the Roush reaction of allylboranes. What sets the Sakurai reaction apart is the exceptional stability of allylsilanes, which are stable to air and water, and can easily be chromatographed, unlike most other allylmetal species. In addition, allylsilanes are easily accessible by a variety of reactions. These factors have allowed the Sakurai reaction to find widespread use, from basic methodology to total synthesis. 

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