Silicon compounds allylation reactions

Although the allylation reaction is formally analogous to the addition of allylic boranes to carbonyl derivatives, it does not normally occur through a cyclic TS. This is because, in contrast to the boranes, the silicon in allylic silanes has little Lewis acid character and does not coordinate at the carbonyl oxygen. The stereochemistry of addition of allylic silanes to carbonyl compounds is consistent with an acyclic TS. The -stereoisomer of 2-butenyl(trimethyl)silane gives nearly exclusively the product in... 

Acetylenic acrylates have been used to reduce side reactions in the preparation of acrylic sil(ox)anes by hydrosilylation [13,14], Allylic acrylates are known to result in addition products with both types of double bonds. Elimination of propene under loss of the allylic group is a major concern, because this path yields acryloxy silicone compounds with SiOC linkages of low hydrolytic stability. 

Quite recently, it was established that the formation of the pentacoordinated germanium and tin compounds EF4(CH2CH=CH2) from EF3(CH2CH=CH2) (E = Ge, Sn) by the addition of F is exothermic148. The nucleophilicity of the allylic y-carbon is much enhanced when the pentacoordinated EF4(CH2CH=CH2) is formed. These results are qualitatively similar to those found for the reaction of the corresponding silicon compound. The pentacoordinated Ge and Sn complexes have a significant Lewis acidity which allows them to form stable hexacoordinated intermediates in the course of the reaction. 

It is possible that such a set of reactions was in force for the related cleavages of allyl-silicon and allyl-germanium compounds studied by Roberts2-3 with mercuric iodide as the electrophile. 

Allylsilanes are known as interesting and useful reagents for organic synthesis.85-88 Hurd first reported the direct reaction of elemental silicon with allyl chloride in the presence of copper catalyst to give a mixture of allylchlorosilanes and polymeric materials.15 Among the volatile compounds, allyltrichlorosilane, formed by the 1 1 1 reaction of allyl chloride, hydrogen chloride and elemental silicon, was observed as the major product instead of the normally expected diallyldichlorosilane. Since diallyldichlorosilane thermally polymerizes at temperatures above 130°C, the product mixture polymerized during the reaction or upon distillation. 

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 similarities between organic tin and silicon compounds, particularly their abilities to stabilize positive charge on a -carbon atom, suggest that vinylic and allylic tin compounds should be useful substrates for Friedel-Crafts acylation reactions. However, very few examples of the acylation of stannanes under the action of Lewis acids have been reported, and this remains a field of Friedel-Crafts reactions not yet fully exploited. The acylation of 1,2-bis(tri-n-butylstannyl)ethylene in the presence of aluminum chloride offers a moderately yielding route to tributylstannyl-substituted enones, useful as precursors to 4,5-dialky lcyclopent-2-enones. 

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. 

Raasch has extended his studies of the reactivity of bis(trifluoro-methyl)thioketen (141). Scheme 8 gives a survey of the reactions that have been carried out. Thus common nucleophilic agents such as alcohols, thiols, primary, and secondary amines, etc., react with (141) with conservation of the C=S double bond, whereas the C==C double bond remains in the products of the reactions of (141) with certain ethers, dimethylaniline, and hydrides of tin and silicon. Compound (141) reacted with a variety of olehnic substances in accordance with the general ene-reaction formalism, yielding allyl vinyl sulphides (148). The formation... 

Alcohol functions have also been introduced via hydrosilylation reactions, for example, the reaction of T8[OSiMe2H]8 with allyl alcohol and allyloxy ethanol (Table 19). In the first case, it has been postulated that the compound T8[OSiMe2 (CH2)30H]8 is not very stable due to back-biting of the -OH groups on the silicon corners (Figure 31). Nevertheless, it reacts with polymers such as polyvinyl pyrrolidone to give polymer hybrids (Table 19, entries 4 and 5). 

Trichloro- and trifluorosilanes introduce another dimension into the reactivity of allylic silanes. The silicon in these compounds is electrophilic and can expand to pentaco-ordinate and hexacoordinate structures. These reactions can occur through a cyclic or chelated TS. 

On the other hand polysilylalkynes with phenyl or allyl substituents are converted with triflic acid into polymeric alkynylsilyltriflates. These polymers react with many acidic element hydrogen compounds or lithium element compounds with formation of silicon element bonds. Thus we found an easy approach to numerous new functional substituted alkynes [12], Eq.(9) shows selected examples of this reaction type. 

Cyclization reactions of vinyl- and alkynylsilanes have been reviewed100. The course of the reaction of the cyclohexenone derivative 184 depends on the catalyst employed ethylaluminium dichloride gives solely the product 185 of 1,6-addition, whereas tetrabuty-lammonium fluoride yields a mixture containing 69% of the 1,4-adduct 186 and 31% of the bridged compound 187 (equation 89)101. Intramolecular addition reactions of allylic silanes102 may also be catalysed by Lewis acids (equation 90) or fluoride ions, and in this case an allyl anion or a pentavalent silicon intermediate may be involved (equation 91). Such reactions are exemplified by the formation of a 1 5 mixture of the diastereomers 189 and 190 when the cyclohexenone derivative 188 is treated with ethylaluminium dichloride (equation 92). In the presence of fluoride anion the ratio of the isomers is reversed103. 

Silicon-based Lewis acids have been known for some time, and the related chemistry in catalysis has recently been reviewed [24]. Most examples in the literature are mainly based on achiral species and will be discussed only briefly in this section. In general, a broad variety of reactions can be catalyzed with compounds like MejSiOTf, MejSiNTf or MOjSiClO. One advantage over some metal Lewis acids is that they are compatible with many carbon nucleophiles like silyl enol ethers, allyl organometallic reagents and cuprates. 

Palladium-mediated substitutions of silylated allylic compounds are not subject to steric hindrances. The silicon atom exerts control over the reaction site snch thaty-substitution results [64]. While the electrophiles are unavoidably disjoint with respect to the acceptor silicon, one of the two limiting forms, i.e., the a-silylcarbenium ion, is much more unfavorable (a-a arrangement) and therefore its population is expectedly low. 

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