Vinylsilanes, electrophilic substitution reactions

There is an extensive review of the electrophilic substitution reactions of allylsilanes and vinylsilanes in Organic Reactions and the reactions of allylstannanes have been reviewed briefly. The major books on synthetic organosilicon and organotin chemistry also cover the types of reaction discussed here. 

As mentioned in Section n.B.5, the te/t-butyldiphenylsilyl group attached to the olefinic carbon is a poor electrofugal group and hence does not leave easily in electrophilic substitution reactions. On the otha- hand, the epoxide 172, obtained from the corresponding vinylsilane having such a bulky sUyl substituent, gives the carbonyl compound upon... 

Allenylsilanes may behave as allyl or vinylsilanes. In general, allylsilane behaviour dominates over vinylsilane behaviour. The reactions of allenylsilanes with electrophiles have been comprehensively reviewed by Fleming, Dunogues and Smithers118. Electrophilic substitution of allenylsilanes gives alkynes (equation 106)173. 

Vinylsilanes are proper compounds which have won great appreciation as facile intermediates for many sorts of application. They are prepared either by addition reactions of acetylenes (vide supra) or by elimination reactions of saturated organosilanes. The main reactions, carried out with vinylsilanes, are addition reactions to obtain saturated organosilanes or electrophilic substitutions of the silyl group under Friedel-Crafts conditions where a rapid cleavage of the silyl moiety occurs (Scheme 8). 

Electrophilic substitution of vinylsilanes may be combined with the fazarov cyclization to build five-membered rings, as depicted in Figures i5.11 and Si5.12 (LA = Lewis acid). Note that the reaction depicted in igure Si5.12 is anomalous because the vinylsilane is attacked by the lectrophile at the end of the double bond remote from silicon. This is ecause the sulphur lone pairs stabilize a carbocation more effectively than le silicon P effect. 

A significant advance in the use of Friedel-Crafts acylation of alkenes to prepare divinyl ketones was the employment of vinylsilanes to control the site of electrophilic substitution. Two groups have developed this approach to cyclopentenone annulation using slightly different strategies. In the method described by Magnus the reagent vinyltrimethylsilane (80) is used primarily as an ethylene equivalent (equation 44). The construction of bicyclic systems followed readily as Nazarov cyclization proceeded under the reaction conditions. Tin(lV) chloride was found to be the most effective promoter of the overall transformation. As expected the position of the double bond is thermodynamically controlled. 

Vinylsilanes react with a variety of electrophiles to give substitution or addition products. In substitution reactions, the silyl group is replaced by the electrophile because formation of the carbon-carbon rc-bond is faster than capture of the incipient P-carbocation by the nucleophile. 

Vinylcyclopropanes are also suitable starting materials for the construction of five-membered cycles. The vinylcyclopropane rearrangement first discovered in the late 1950s191 192) immediately became the subject of intense mechanistic investigations 173 193,194) ancj was use(j for tjje CyCiopentene anelation 165.171 195 197> Gr cycloheptane anelation 198 200). This reaction was successfully applied to vinylcyclopropanes bearing a silyl moiety by Paquette 137,138> and was found particularly useful for the synthesis of vinylsilanes as part of a five-membered cycle. These have been in some cases subjected to electrophilic substitution 137,138 201 202) (Schemes 58 b, 65, 82-84). The ring expansion takes place when the thermolysis is conducted between 570° and 670 °C and only occurs with those compounds which do not possess 137)sterie inhibition around the carbon-carbon bond (Scheme 76, compare a and e). Incorporation of the double bond (Scheme 76, compare a and e). Incorporation of the double... 

The stability of the -C -Si< bond has been known for a long time But on the other hand they are reactive compounds which undergo either - as precursors to vinylsilanes - various types of addition reactions or - as only silyl-protected acetylenes - an electrophilic substitution under Friedel-Crafts conditions in presence of catalytic amounts of Lewis acids The — SiR3 moiety has a highly useful protecting and/or activating function. 

Except for this type of reaction, retention of configuration is observed in electrophilic substitution, whether it is direct with a vinyllithium (path a in Fig. 5.8) or indirect with a vinylsilane (path b in Fig. 5.8). It is conceivable that even reactions with vinyllithium reagents take the indirect path, for a concerted attack on... 

Vinylsilanes have been widely recognized as facile intermediates for many sorts of applications39. While vinylsilanes undergo electrophilic substitution at the carbon atom containing the silyl group, the double-bond geometry is quite frequently retained. As a consequence vinylsilanes are well suited as precursors to a variety of reaction products with geometrically defined double bonds. Also feasible is an addition-elimination sequence, however here the reaction conditions have an influence on the stereochemistry of the reaction products. 

A soln. of allyltrimethylsilane in methylene chloride added dropwise to a pre-cooled soln. of dichlorobis(4-chlorophenoxy)methane and SnCl4 in the same solvent at — 78°, and worked up after 72 h - product. Y 78%. With subst. allylsilanes reaction proceeds mainly with double bond shift. F.e.s. H. Mayr et al.. Synthesis 1988, 962-3 review of electrophilic substitution of allylsilanes and vinylsilanes s. I. Fleming et al., Org. Reactions 37, 57-575 (1989). 

More practically, 2-pyridyl-substituted vinylsilanes themselves can be directly subjected to the reactions with various electrophiles. For example, treatment of 2-pyridyl-substituted vinylsilanes with acid chlorides in the presence of aluminum chloride affords the corresponding a, 0-unsaturated enones (eq 5). The reaction of 2-pyridyl-substituted vinylsilanes with bromine and subsequent treatment with sodium methoxide affords the corresponding vinyl bromides (eq 6). ... 

Silylcupration of allene with (PhMe2Si)2CuLi followed by treatment of the intermediate cuprate with H" ", carbon electrophiles, and CI2 affords the corresponding vinylsilanes (eq 9). However, reaction of the intermediate cuprate with I2 affords an allylsilane, from which other more highly substituted allylsilanes are readily obtainable (eq 9). In reaction with substituted allenes, the product ratio (vinylsilane vs. allylsilane) is dependent upon the degree of substitution. ... 

The organolithium reagent (1) also reacts with a wide variety of other electrophiles, including silyl chlorides to provide bis(silyl)methane derivatives, and nitriles to provide -sUyl amines after in situ reduction of the intermediate imine derivative. a-Silyl epoxides are opened to provide the substituted vinylsilane. Reaction of (1) with arenesulfonyl fluorides provides a-silyl sulfones, key intermediates for the preparation of vinyl sulfones. Reaction of the lithium reagent (1) with alu-... 

In analogy to the acylation reactions, vinylsilanes can be alkylated by ipso substitution. In the example in eq 47, the MEM group is activated as leaving group by metal complexation The silyl group in allylsilanes directs the incoming electrophile to the al-lylic 7-carbon (eq 48). TiCU is one of several Lewis acids used for catalysis in acylations. The same reaction is seen in allylsilanes with extended conjugation. eq 49 shows the TiCU-catalyzed alkylation of an allylsilane. ... 

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