Pentacoordinate allylsilicate

Stereoselective reaction with aldehydes. Both (E)- and (Z)-l react with aldehydes to give syn-adducts as the major products (12,146). In an effort to improve the diastereoselectivity, (E)- and (Z)-l have been converted into pentacoordinated allylsilicates (2) by reaction with dilithium catecholate. Reaction of an aryl aldehyde with both (E)- and (Z)-2 shows essentially complete diastereoselectivity. 

Carbonyl allylation using pentacoordinate allylsilicates is valuable for stereospecific synthesis of homoallyl alcohols ... 

The regioselectivity and diastereoselectivity can be interpreted in terms of a six-membered chairlike transition state8(Scheme 3.20. Thus, the nucleophilic attack of a fluoride anion to an allyltrifluorosilane may afford a rather stable pentacoordinate allylsilicate, which then reacts with an aldehyde via a cyclic six-membered transition state. The high level of regioselectivity of the reaction is presumably due to the enhanced nucleophilicity of the ycarbon of the allylsilicate. 

Pentacoordinate Allylsilicates Characterization and Highly Stereoselective Reaction with Aldehydes... 

More recently, Majetich et al. have proposed pentacoordinate allylsilicate intermediates formed from fluoride ion addition to silicon as an ambient nucleophilic species. However, nothing had been known so far about pentacoordinate allylsilicates, before we have initiated the present study, whereas a number of pentacoordinate organosilicates have been well characterized. It is an interesting problem whether the fluoride-catalyzed reactions actually involve free allyl anions or not. The author will discuss this problem in the next section somewhat in detail. In this section the author will describe preparation of well-defined pentacoordinate allylsilanes and their reactions which have been published recently. ... 

The similar method to the synthesis of pentacoordinate hydridosilicates was applied to the preparation of the lithium salts of pentacoordinate allylsilicates (7 and 8). 

Now it turns out that the stereochemical outcome of the present allylation of aldehydes with pentacoordinate allylsilicates is remarkable. Stereochemical control in addition of crotylmetals to aldehydes has been investigated rather extensively from both mechanistic and synthetic view points.Recently, reactions of crotyltrimethylsilanes with aldehydes in the presence of Lewis acids have been found to proceed via acyclic transition states to give erythro products regardless of the stereochemistry of the starting crotylsilanes. The characteristic reactivity of the pentacoordinate allylsilicates was also demonstrated by the diastereoselection of crotylation as shown in Table III. Thus, when a mixture of crotyltrichlorosilanes with an E/Z ratio of 88/12 was used as a starting allyltrichlorosi-lane, a mixture of the threo (9c) and erythro (9d) homoallyl alcohols was obtained in... 

The reaction modes of pentacoordinate allylsilicates including the diastereoselection are very similar to those of the allyl boronates. The stereoselectivity of allylation is thus interpreted by the six-membered cyclic transition states having chair conformation as shown in the following scheme. 

The intermediacy of the pentacoordinate allylsilicates was proved by the isolation of bis(triphenylphosphoranylidene)ammonium (PPN) salts (8a and 8b). 

Reactivity Selectivity Trends in AJlylation of Aldehydes with Pentacoordinate Allylsilicates... 

Now we have examined several different allylsilicates in the allylation of aldehydes. The most reactive allylsilicate is tetrafluorosilicates derived by fluoride addition to allyltrifluo-rosilane. The least reactive one is derived by the Martin s ligand which did not react at all with aldehydes. Bis(diolato)allylsilicates are next inactive. Inversely, bis(diolato)silicates are most selective, since they react only with aryl aldehydes. This interesting reactivity and selectivity relationship may be correlated with the increasing Lewis acidity of pentacoordinate allylsilicates. The steric factors are also important. The most Lewis-acidic tetrafluoroallylsilicates are sterically least hindered. Therefore, it is not easy to distinguish two factors at this moment. However, the importance of the Lewis acidity of the silicon centers is definitely demonstrated by the stereoselection. 

Evidence for a closed transition-state model was gathered on the basis of the diastereoselectivity in reactions of pentacoordinate allylic silicates. Bis(l,2-benzenediolato)allylsilicates la and lb, which can be prepared via the reactions of E- and Z-crotyltrichlorosilanes with dilithium catecholate, react with aromatic aldehydes to give the corresponding homoallylic alcohols 2 in high yields3 (Scheme 3.2a). Unlike allyltrimethylsilane,4 the allylation reactions of... 

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 present reaction may be rationalized by the mechanism illustrated in the following scheme with similar diastereoselection shown in Section 4. Thus, the nucleophilic attack of a fluoride ion to an allyltrifluorosilane may afford rather stable pentacoordinate allyl-silicate which in turn reacts with an aldehyde via a six-membered cyclic transition state. This may be caused by the significant Lewis acidity of the tetrafluorosiliate and the enhanced nucleophilicity of the 7-carbon of the allylsilicate. 

These results suggest strongly that the reaction also proceeds via pentacoordinate diflu-oro(diolato)allylsilicate intermediates, generated in sit% through the six-membered cyclic transition state having a chair conformation. 

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