Allylsilanes and Related Compounds

SCHEME 7.17 C-Glycosidations with allylsilanes and related compounds. 

Table 2 Oxidation Potentials of Allylsilanes and Related Compounds...

Yamamoto et al. have systematically studied the Lewis acid-catalyzed carbosilylation of alkynes with allylsilanes and related compounds [512]. They have disclosed that the EtAlCl2 (cat.J-MeaSiCl (excess) catalyst system or a sub-stoichiometric amount of HfC effectively induces the trans-aUylsilylation of the alkynes (Scheme 10.192) [514]. The HfCLj-promoted method, in particular, is quite useful for stereo- and regioselective aUylsilylation of a variety of aUylsilanes under mild conditions. In sharp contrast to the mechanism proposed by Jung et al., Yamamoto et al. have proposed that EtAlCh or HfCLi adds to an alkyne, the activated alkyne is allylated by an allylsilane, and the resulting vinylmetal becomes an aUylsilylation product by transmetalation. 

For /8-substituted 7t-systems, silyl substitution causes the destabilization of the 7r-orbital (HOMO) [3,4]. The increase of the HOMO level is attributed to the interaction between the C-Si a orbital and the n orbital of olefins or aromatic systems (a-n interaction) as shown in Fig. 3 [7]. The C-Si a orbital is higher in energy than the C-C and C-H a orbitals and the energy match of the C-Si orbital with the neighboring n orbital is better than that of the C-C or C-H bond. Therefore, considerable interaction between the C-Si orbital and the n orbital is attained to cause the increase of the HOMO level. Since the electrochemical oxidation proceeds by the initial electron-transfer from the HOMO of the molecule, the increase in the HOMO level facilitates the electron transfer. Thus, the introduction of a silyl substituents at the -position results in the decrease of the oxidation potentials of the 7r-system. On the basis of this j -efleet, anodic oxidation reactions of allylsilanes, benzylsilanes, and related compounds have been developed (Sect. 3.3). 

Silyl enol ethers are powerful intermediates in organic synthesis. Reactions of silyl enol ethers with various electrophiles provide effective methods for the synthesis of various carbonyl compounds. In this section we will briefly touch on the electrochemical reactions of silyl enol ethers and related compounds. The electrochemical behaviour of silyl enol ethers is expected to be closely related to that of allylsilanes and benzylsilanes because silyl enol ethers also have a silyl group ft to the re-system. 

Tandem (Domino) reactions, in which two or more bond-forming processes result from a single step, are useful for rapid synthesis of complex molecules.138 Functionalized allylsilanes and the related silicon compounds have frequently been used for tandem reactions, because of their ease of synthesis and moderate, controllable reactivity. Recently, much attention has been paid to the use of multifunctional allylsilanes bearing another reactive group on silicon. 

Nishitani, K., Fukuda, H., and Yamakawa, K., Studies on the terpenoids and related alicyclic compounds. Part 42. Diastereoselective cyclization of co-formylated allylsilanes into bicyclic a-methylene-y-butyrolactones. A facile synthesis of /7-menthanolides. Heterocycles. 33, 97, 1992. 

Nishitani, K., Nakamura, Y, Orii, R., Aral, C., and Yamakawa, K., Studies on the terpenoids and related alicyclic compounds. Part 43. Stereoselective intramolecular cyclization of P-alkoxycarbonyl-CO-formylallylsilanes into bicyclic a-methylene-y-lactones, Chem. Pharm. Bull., 41, 822, 1993. Kuroda, C., Ohnishi, Y, and Satoh, J.Y, Intramolecular cyclization of P-(aIkoxycarbonyl)allylsilane with conjugated ketone. A new entry to bicyclo[4.3.0]nonane. Tetrahedron Lett., 34, 2613, 1993. Kuroda, C., Inoue, S., Takemura, R., and Satoh, J.Y, Intramolecular cyclization of allylsilanes in the synthesis of guaian-8,12-olide. Stereoselective formation of trans- and cw-fused methylenelactones, 7. Chem. Soc., Perkin Trans. 1, 521, 1994. 

A related coupling reaction is the condensation of aldehydes with alkyl trichlorotitanium compounds (RTiCl3). When methyltrichlorotitanium (MeTiCl3) was coupled with aldehyde 468 in dichloromethane at -78°C, a 91 9 mixture of 470/471 was formed via the chelated complex 469. Sakurai and co-worker had previously noted the coupling of allylsilanes to aldehydes in the presence of TiCl4.305... 

Other Lewis acid mediated C-C bond forming reactions of silylated C-nucleophiles include the Mukaiyama aldol and Michael additions of enolsilanes ), the related reaction of allylsilanes described by Sakurai ) as well as similar reactions of Me3SiCN ). Other "inert C-nucleophiles such as dialkylzinc compounds also react with carbonyl compounds in the presence of Lewis acids ). We ourselves have utilized many of these processes in performing stereoselective reactions, particularly in achieving diastereofacial selectivity in the reactions... 

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