Silyl stannanes

In Pd(0) mediated addition of trialkyl[dimethyl(phenyl)silyl]stannane to acetylenes <89S218> the product obtained from l-hydroxybut-3-yne after normal workup contained the substituted 1,2,5-oxasilastannolene (354) in addition to the expected 2-tributylstannylalk-l-ene (Equation (32)). 

Palladium-mediated addition of silyl stannane reagents to alkynyl ethers has been employed for the synthesis of aliphatic acyl silanes in very good yields via the intermediate a-alkoxy-/J-stannyl vinyl silanes (enol ethers of acyl silanes)82. In a second palladium-catalysed step, the vinyl stannane moiety could be coupled to suitable halides before hydrolysis to the acyl silanes with trifluoroacetic acid (Scheme 11). 

Ethyl enol ethers of acyl silanes have been prepared by the palladium-mediated addition of silyl stannanes to alkynyl ethers. Hydrolysis using trifluoroacetic acid gave very high yields of acyl silanes (vide supra, Section m.A.4)82. 

The methodology was adapted to the synthesis of a simplified fragment of bryostatin. As in the previous example, an aldehyde (130) was reacted with (2-chloromethyl)allylstannane 131 using BINOL/Ti(Oi-Pr)4 catalysis to obtain 132. In parallel, aldehyde 133 was subjected to a Keck reaction employing the silylated stannane 134. The resulting allylsilane 135... 

Silyl stannanes have been prepared by trapping tin anions with TBDMSCl or other silyl chlorides. AUcynes treated with silyl stannanes and catalytic tetrakis(triphenylphosphine)palladium(0) give cw-silyl stannylalkenes in good yields. ... 

Alkenyl silanes and stannanes have the potential for nucleophilic delivery of vinyl groups to a variety of electrophiles. Demetallation also occurs in these reactions, so the net effect is substitution for the silyl or the stannyl group. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

Comparable treatment of the allenic stannane resulted in a similar outcome, producing a mixture of regioisomers from the in situ silylation and near exclusive formation of the allenylsilane from the sequential addition (Eq. 9.8). 

Scheme 9.7 Mechanistic pathways for reactions of stannane-derived allenyl- and propargyllitihum intermediates with silyl chlorides.

Use of di-M-butylstannyl dichloride along with an acyl or silyl halide leads to addition of allylstannanes to the aldehydes.106 Reaction is also promoted by butylstannyl trichloride.107 Both SnCl4 and SnCl2 also catalyze this kind of addition. Reactions of tetraallyl-stannane with aldehydes catalyzed by SnCl4 also appear to involve a halostannane intermediate. It can be demonstrated by NMR that there is a rapid redistribution of the allyl group.108... 

Details of the Janda-Chen synthesis were as follows. A tetrahydropyran (THP) linker was attached to the NCPS support enabling attachment of alcohols via THP ether formation.13 The THP-NCPS resin 1 is derivatized with / -(+)-4-hydroxy-2-cyclopentanone 2, giving the THP ether-based resin 3, followed by coupling of the C13 20 fragment by enone-cuprate addition. The cuprate required was generated from the corresponding E-vinyl stannane 4. The resulting enolate was trapped as the silyl end ether... 

Initial reports on the use of simple enolates as nucleophiles in TT-allylpalladium chemistry met with only limited success.77 106 The enolate of acetophenone reacted with allyl acetate in the presence of Pd(PPh3)4, but gave predominantly dialkylated product.106 The use of the enol silyl ether of acetophenone gave only monoalkylated product with allyl acetate and Pd° catalysis, but substituted allyl acetates did not function in this reaction.106 Enol stannanes, however, have been found to give monoalkylated products with a wide variety of allyl acetates (equation 19).106 In situ generation of enol stannanes from lithium enolates and trialkylstannyl trifluoroacetates followed by Pd°-catalyzed allylation has been demonstrated.107... 

Mikami et al. also investigated the addition of ketene silyl acetals. They found that addition of the silyl enol ether of acetone and allylic silanes did not result in the synthesis of substituted l,2-dihydro[60]fullerenes [218a,220], In 1997, Mikami et al. [221] reported the photoaddition of allylic stannanes that leads to monoallylation of C6o (Scheme 13). 

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