Allylsilanes protodesilylation

Allylic amide isomerization, 117 Allylic amine isomerization ab initio calculations, 110 catalytic cycle, 104 cobalt-catalyzed, 98 double-bond migration, 104 isotope-labeling experiments, 103 kinetics, 103 mechanism, 103 model system, 110 NMR study, 104 rhodium-catalyzed, 9, 98 Allylnickel halides, 170 Allylpalladium intermediates, 193 Allylsilane protodesilylation, 305 Aluminum, chiral catalysts, 216, 234, 310 Amide dimers, NMR spectra, 282, 284 Amines ... 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

The proton serves as the simplest electrophile to displace the silyl group stereo- and regioselectively. Numerous conditions have been used for the electrophilic protodesilylation of allyl- and vinylsilanes19. ( )-vinyl sulfones 2 are prepared from the silylallylic sulfones 1 in the presence of protic acids in high yield (equation l)34. Diastereoselective protodesilylation of allylsilane 3 gives 4 with excellent control of the geometry of the double bond exocyclic to the ring (equation 2)35. 

Allylsilanes undergo protodesilylation in high yield with the BFj acetic, acid complex at 20° in 5 minutes. 

Protodesilylation, A new preparation of -unsaturated carboxylic acids involves the amide acetal Claisen rearrangement of a 3-(trimethylsilyl)allyl alcohol such as 1 to allylsilanes (2). Desilylation of 2 by usual methods results in formation of stereoisomers. However, use of liquid HF at low temperatures results only in (E)-3 in 88% yield. The HF-pyridine complex is not so stereoselective. Amides such as 3 can be converted into carboxylic acid esters by Meerwein s trialkyloxonium salts (—70% yield). 

The exclusively obtained adduct (183c) has been converted to monoteipenes by simple reaction sequences involving protodesilylation of the allylsilane unit either by HCl/MeOH (-> 185) or by fluoride/DMSO (- IM) (Scheme 44). 

The addition of Si and H across a ir-bond is formally a reduction, since Si is more electropositive than carbon. However, unambiguous reduction is only complete when the silicon is replaced by hydrogen. This is easily possible by the protodesilylation of allylsilanes and vinylsilanes. 

Allylsilanes are easily protodesilylated, completing the reduction process. In the presence of Lewis acids, allylsilanes also react with electrophiles like aldehydes, ketones and acid halides through an Se2 mechanism involving anti stereochemistry. These reactions are extensively discussed in Volumes 2-4. 

In fluoride ion catalyzed reactions of allylsilanes, aldehydes are the only carbon electrophiles that work well protodesilylation is unavoidable with most other electrophiles, even in the most rigorously dried media. The most commonly used fluoride ion source, which only needs to be present in catalytic amounts, is TBAF in THF at room temperature or under reflux, benzyltrimethylammonium fluoride may be better, and cesium fluoride in DMF, potassium fluoride with 18-crown-6 in THF, and TASF in polar... 

These cycloadditions are more sensitive to the quality of the catalyst, the major side reaction being protodesilylation of the allylsilane subunit. Since this can not be measured readily either in the case of the tetrakis(triphenylphosphane)palladium(0) or the palladium acetate/triisopropyl phosphite methods, an improved method for generating the palladium(O) species has been developed22. This involves in situ preparation of tetrakis(triisopropyl phos-phite)palladium(O) by direct reduction of palladium acetate with butyl lithium. This method is illustrated by the addition of the methyl-substituted TMM-Pd complex to eyelopentenone. 

The very general picture of the mechanism given in the introduction needs to be modified somewhat. It is fairly clear that allylsilanes react in the manner illustrated as (3) — (5), except that the acid catalyst is almost invariably a Lewis acid not a proton. Protic acid is apt to induce protodesilylation in competition with carbon-carbon bond formation, but Lewis acids are less likely to attack the carbon-carbon double bond. The intervention of a cationic intermediate (4) has not been proved for this type of reaction, but there is good circumstantial evidence that cations are involved by analogy with protodesilylation,7 from the occasional formation of oxepins, without loss of the silyl group,8 and from the stereochemistry of the reaction. 

The protodesilylation of the two allylsilanes were not equally stereoselective with respect to double bond geometry. Protodesilylation... 

The most popular promoters are BF3 OEt2, TiCU, and SnCU. They are followed, arguably, by AICI3, AlC Et, and TMSOTf. Protic acids are used more rarely since they tend to cause an unwanted protodesilylation. BF3, TiCU, and SnCL have somewhat different properties. BF3 does not react with allylsilanes directly, and has only one coordination site. TiCU is also inert towards allylsilanes, even at room temperature, and has two coordination sites. TiCU forms 1 1 or 1 2 adducts with aldehydes, depending on the stoichiometry of the reagents. It can also form chelates with a- and P-alkoxyaldehydes, which influences the stereochemistry of the reaction (chelation control). TiCU is best used at low temperatures (-78 °C) to prevent the formation of unwanted chlorination products that appear at room temperature. The major characteristic of SnCU is that, in the presence of an allylsilane, it undergoes a transmetallation within minutes at -80 °C. Hence,... 

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