2-Silyl groups, reactive

Silyl groups, reactive, in heterocyclics 27, 883 Silyl hemiacetals s. Ketene... 

Silyl groups have found broad appeal as protective groups because their reactivity and stability can be tailored by varying the nature of the substituents on the silicon. Their ability to migrate from one hydroxyl to another is a property that can be used to advantage, but more often, it is a nuisance. The migratory apti-... 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

Addition of diphenyl disulfide (PhS)2 to terminal alkynes is catalyzed by palladium complexes to give l,2-bis(phe-nylthio)alkenes (Table 3)168-172 The reaction is stereoselective, affording the (Z)-adducts as the major isomer. A rhodium(i) catalyst system works well for less reactive aliphatic disulfides.173 Bis(triisopropylsilyl) disulfide adds to alkynes to give (Z)-l,2-bis(silylsulfanyl)alkenes, which allows further transformations of the silyl group to occur with various electrophiles.174,175 Diphenyl diselenide also undergoes the 1,2-addition to terminal alkynes in the presence of palladium catalysts.176... 

In the cathodic reduction of activated olefins, chlorosilanes also act as trapping agents of anionic intermediates. Nishiguchi and coworkers described the electrochemical reduction of a,/ -unsaturated esters, nitriles, and ketones in the presence of Me3SiCl using a reactive metal anode (Mg, Zn, Al) in an undivided cell to afford the silylated compounds [78]. This reaction provides a valuable method for the introduction of a silyl group into activated olefins. 

Pt(PPh3)4, PdCl2(PPh3)2 and Pd(PPh3)4 were much less efficient than H2PtCl6 and RhCl(PPh3)3. With these less reactive catalysts, the enyne 133 could be selectively prepared. Subsequent hydrosilylation of 133 also afforded the allene 134. In this stepwise route, two different silyl groups could be introduced into the allenic product. A typical example is illustrated in Scheme 3.68. 

The influence of the silyl group on the reactivity of the nitronate was found to be negligible (16). Competition studies between trimethylsilyl and triphenylsilyl nitronates showed the two dipoles to be equally effective in performing [3+2] cycloadditions. 

Since in compound 8 the alcohol group at C-24 is unprotected, 2.8 equivalents of the reagent Li-C = C-SMe3 arc required. BF3-OEt2 is again employed as a Lewis acid as a way of increasing the reactivity of the epoxide.8 The silyl group is cleaved with methoxide (3->4). 

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