Silanes vinyl, reactions with electrophiles

Scheme 9 demonstrates the further synthetic application of the thus obtained N,0-acetals. Substitution of the alkoxy or acyloxy group by nucleophiles like enol ethers, enol esters, enamines, other electron-rich olefins, CH-acidic compounds, electron-rich aromatics, isocyanides, trimethylsilyl cyanide, organometallics, vinyl and allyl silanes, hydroxy functions, or trialkylphosphites either catalyzed by Lewis acids or proton acids leads to the product of the amidoalkylation reaction (path a). In the presence of stereocenters as control elements, diasteroselective amidoalkylation reactions can be performed as shown in a large number of examples. On the other side, as Nyberg showed for the first time [196], elimination with formation of enecarbamates [208] and enamides [196,208,209] followed by reaction with electrophiles or nucleophiles (path b) also is possible. 

Stereochemical information is built into reagents 9 that have a nucleophilic vinyl-metal bond in the plane of the alkene. This bond already has a stereochemistry - it is cis to R1 and trans to R2 -and this information can be passed to the products providing that the reaction with electrophiles is stereospecific. In most of the methods in this chapter vinyl metals 9 M = Li, Mg, Cu, Sn, Al, Zr, or compounds closely related to them such as vinyl boranes 10 or vinyl silanes 11, react with electrophiles E+ with retention of configuration 12. 

Vinyl silanes react with electrophiles in a highly regioselective process in which the silicon is replaced by the electrophile at the ipso carbon atom. The stereochemistry of the vinyl silane is important because this exchange usually occurs with retention of geometry as well. Consider the reaction of the two vinyl silanes derived from phenyl acetylene with the simple electrophile D+. Deuterons are chemically very similar to protons but are, of course, distinguishable by NMR. 

It can be quenched with electrophiles in the usual way. In this example prenyl bromide is the first electrophile giving the stable lithiated hydrazone 50 the Shapiro reaction gives Z-51 trapped as the vinyl stannane7 E-52. The vinyl silane 39 and stannane 52 products from these reactions can be used to make more elaborate alkenes with control over geometry as we shall see later. 

Vinyl silanes resemble alkenes in reactivity they combine with reactive electrophiles such as bromine without catalysis but need Lewis acid catalysis for reaction with carbon electrophiles. Reaction usually occurs 189 at the silyl end of the alkene so that the intermediate 190 enjoys the P-silyl stabilisation of the carbocation. The silyl group is removed by a nucleophile, usually a halide ion.45... 

The explanation resembles that for the retention of stereochemistry in the reaction of vinyl silanes with electrophiles (chapter 16). To make this clearer, we have drawn the diagrams as nearly the same as those in that chapter as possible. It does not matter which surface of the alkene is attacked by the nucleophile as long the bromine atom continues to rotate in the same direction. Once it reaches the vertical it can be eliminated by the carbanion and the result is retention (<5% of the other isomer). 

Interactive mechanism for reaction of vinyl silanes with electrophiles... 

A-Acyliminium ions are suitable partners for electrophilic addition reactions of allylstannanes. Although the corresponding silanes have been more widely studied in these cases/ allylic stannanes are fully competent reaction participants, as illustrated by formation of the l-azabicyclo[3.1.0]pentane 159 via intra-molecular cyclization of the fi-allylic stannane 157 (Scheme 5.2.33). The reaction produces the vinyl cyclopropane under protic acid conditions with complete stereocontrol. s... 

If the electrophile also benefits from Lewis acid catalysis, as in the aliphatic Friedel-Crafts reaction, good yields of products are generally found. The anion of the allyl phosphonate 192 can be silylated to give the vinyl silane 193 and this reacts in turn with the classical combination of acetyl chloride and A1C13 to give the enone -194 that can be used in HWE reactions to make dienones46 (chapter 15). 

The Pd-catalyzed cross-coupling reactions of metal nucleophiles with carbon electrophiles are of considerable value for the regio- and stereocontrolled synthesis of functionalized organometalhc compounds, in particular, silanes, stannanes, and boranes, which are important reagents for Pd-catalyzed carbon-carbon cross-coupling as shown in Sects. in.2.2-in.2.4. Symmetrical bimetallic compounds such as disilanes, distannanes, and diborons are usually used as metal nucleophiles. The present metallation is applicable to aryl, benzyl, vinyl, acyl, and aUyl (Sect. V.2.3.3) electrophiles. 

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