Reactions of Allylsilane Anions

Reactions of Allylsilane Anions Generally, deprotonated silanes react in a y-regio-selective manner with electrophiles to give vinylsilanes. 

Silyl-l,3-dienes undergo anodic methoxylation in methanol to give 1,4-addition products with an allylsilane structure as intermediates. Therefore, they are further oxidized to give l,l,4-trimethoxy-2-butene derivatives as the final products. The products are easily hydrolyzed to provide the corresponding y-methoxy-a, /t-unsaUirated aldehydes. Since 1-trimethylsilyl-l,3-dienes are readily prepared by the reaction of the anion of l,3-bis(trimethylsilyl)propene with aldehydes or ketones, l,3-bis(trhnethylsilyl)propene offers a, /i-formylvinyl anion equivalent for the reaction with carbonyl compounds (equation 15)16. 

There are two types of reactions of allylsilanes with electrophiles (1) Lewis acid-mediated reactions, and (2) reactions involving allyl anions. 

Stereoselective reactions of crotylsilicates (97)-(100) and aldehydes have been described (Scheme 17). These reactions readily proceed at room temperature in the absence of a Lewis acid catalyst, and type I diastereoselection is clearly evident. Evidence supporting a cyclic transition state has been provided through studies of optically active crotylsilicates such as (101 Scheme 18). The absolute stereochemistry of the products requires that the reaction is suprafacial with respect to the allylsilane moiety, in contrast to the anti stereochemical outcome of 5e reactions of allylsilanes. Cyclic transition states are also implicated in the reactions of the crotyltrifluorosilanes and CsF, but the crotyltrifluorosilane/BuiNF reaction apparently proceeds via an uncomplexed allyl anion species. ... 

Allylsilanes and allylboranes are allyl anion equivalents, which are stable enough to be included in subsequent allylation reactions of aldehydes under... 

Whereas an allylsilane can serve as an allyl anion synthon, the reaction of 1,3-bis(silyl)propene with electrophiles can afford the 1,3-disubstituted propene. Thus, treatment of a mixture of (If)- and (Z)-2-aryl-l,3-bis(trimethylsilyl)propenes 5 with 2 equivalents of NBS at —78 °C stereoselectively yields the corresponding (Z)-2-aryl-l,3-dibromopropene 6. When 1 equivalent of NBS is employed, the monobromo product 7 is obtained (equation 5). The reactions apparently proceed via the pattern of sequential displacement of allylsilane moieties40,41. 

The use of silylallyl anion in organic synthesis has been extensive24,29. The regio-and stereochemistry of these reactions can be controlled. For example, alkylation of the anion generated from the corresponding allylsilane with an electrophile E+ takes place selectively at the /-position due to steric hindrance (equation 197)352. 

In 1982, Sakurai [7] described a catalytic version of this reaction (Scheme 13.4). The addition of small quantities of fluoride anions to the allylsilane 1 generates the pentacoordinated silicon species 10, probably in equilibrium with the starting materials 1 and 11. This activated species can react with the carbonyl derivative 6 to yield the alkoxide 12 which is trapped by fluorotrimethylsilane. This last step not only furnishes the silylated compounds 13 but also regenerates the fluoride catalyst 11. Acidic work-up then leads to the desired homoallylic alcohol 7. 

Since the preparation of anionic species or organosilicon compounds is largely restricted by its reaction conditions, the synthetic utility of silyl anions for the synthesis of organosilicon compounds is limited. However silyllithiums possessing one or more phenyl groups on a silicon atom are readily prepared by the reductive metallation of the corresponding halosilanes with lithium. Allylsilanes are synthesized by the reaction of allyl acetates with the cuprate prepared from such a reagent (eq (47)) [43]. 

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. 

Allylsilanes are readily deprotonated as the anion generated is stabilized not only by conjugation with the adjacent double bond but also by the neighbouring silyl group. The anion may react with electrophiles through either its a-carbon atom or its y-carbon atom. The regiochemical and stereochemical outcome of these reactions depends on several factors of which the most important is probably the identity of the counterion (Equations Si6.9— 12). 

As we mentioned before, a classical Grignard reaction is formally described by the coupling of a covalent (albeit polarized) electrophile with an anionic nucleophile. Reactions shown in Scheme 2.41 (opposite) exemplify the alternative approach involving an interaction between cationic intermediates generated from carbonyl compounds (or their derivatives) under the action of Lewis acids and a purely covalent nucleophile, an allylsilane such as I09a or 109b. Similar electrophiles used in reactions with covalent silyl enolates such as 110 result in the formation of the aldol-Iike products (the Mukaiyama reaction ). 

The allylation of electrophiles with allylsilanes can also be carried out in the presence of fluoride ion as the catalyst. In this case the reaction is initiated by attack of the fluoride ion on silicon to produce an allyl anion, or at least something akin to it. Two such examples are shown below, one an intermolecular reaction76 (equation 71) and the other intramolecular one77 (equation 72). 

---