Electrophilic substitution of vinylsilanes

In the examples of electrophilic substitution of vinylsilanes described rove, collapse of the carbon-silicon bond to form a carbon-carbon 7t bond faster than nucleophilic attack on the carbocation by the anionic counterion l the electrophile. This is not always the case. For example, on treatment of -l-trimethylsilylpropene with bromine followed by aqueous ethanol, (Z)--bromopropene is formed almost exclusively (Figure Si5.10). This is pnsistent with anti addition of bromine to the double bond followed by anti limination of trimethylsilyl bromide. 

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. 

Electrophilic Substitution by Addition-Elimination. Electrophilic attack has a parallel series of events, which is best known in the electrophilic substitution of vinylsilanes. The electrophile attacks from above (or below) the u bond in the vinylsilane 5.67, with the creation of an intermediate carbocation 5.68 (with the cationic carbon behind). Rotation about the a bond can take place... 

The hydrosilylation of alkynes provides facile access to a diverse range of vinyl silane products which are versatile building blocks in organic synthesis [68]. The electrophilic substitution of vinylsilanes is one of the most useful methods for the stereoselective synthesis of substituted alkenes [77, 78]. The effectiveness of... 

A short digression is in order at this juncture. The electrophilic substitution of a vinylsilane is a very useful process because it is generally both regio- and stereospecific. The carbon-silicon bond is strongly polarized due to the high electronegativity of carbon (2.35) relative to silicon (1.64).67b A unique and important conse-... 

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). 

Chiral Synthons by Ester Hydrolysis Catalyzed by Pig Liver Esterase Masaji Ohno, Masami Otsuka The Electrophilic Substitution of Allylsilanes and Vinylsilanes... 

The Electrophilic Substitution of Allyisilanes and Vinylsilanes Ian Fleming. Jacques Dunogues, and Roger Smithcrs... 

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). 

Allylsilanes, being homologues of vinylsilanes, undergo a similar regio-controlled attack (I) by electrophiles, this time at the /-position, with resulting loss of the silyl group providing products of substitution with a net shift of the double bond ... 

Vinylsilanes react readily with a variety of electrophiles either by addition to the double bond or substitution of the R3Si group. The addition product 80 often undergoes subsequent elimination to give the substitution product 81, as shown in equation 42. 

Substitution of the carbon-silicon bond in allyl- and vinylsilanes by an electrophile has served as a powerful tool in organic synthesis. Electrophiles ranging from proton, carbon and main group heteroatoms to certain transition metal species have been employed. A... 

Under controlled conditions, vinylsilanes such as 377 undergo regioselective electrophilic substitutions. Thus, acetylation by means of acetyl chloride and aluminum chloride in CH2C12 at —78 °C afforded 380, while bromination led to the vinyl bromide 381. Epoxidation with m-chloroperbenzoic acid efficiently transformed 377 into a 60 40 mixture of the epimers of 382, Eq. (92)82). 

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. 

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... 

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. 

Towards electrophiles, the reactivity of vinylsilanes is similar to that of the corresponding alkene. However, incorporating a silicon substituent at the vinylic carbon of a ir-nucleophile markedly affects the cyclization outcome. Specifically, iminium ion-vinylsilane cyclizations occur cleanly to substitute, preferentially with retention of double-bond configuration, the iminium ion carbon for the silyl substituent. Both endocyclic and exocyclic modes of intramolecular electrophilic substitution have been demonstrated (Scheme 35). 

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. 

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