Allylsilanes, electrophilic substitution

Electrophilic substitution reactions of allylsilanes have been reviewed in detail by Fleming, Dunogues and Smithers118. 

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. 

The chiral allylsilanes (3b) were used to determine the stereochemistry of the electrophilic substitution reactions (SE1). Typical results are shown in equations (II) it rid (III). The (Z)-allylsilanes give products of (R)-configuration and the (E)-allylsilanes give the (S)-isomers. In each case, the electrophile attacks the double bond anti to the leaving group.6... 

The [3 + 2] reaction of an allylsilane with an enone was proposed to proceed via a regiospecific electrophilic substitution (cf 80) of the enone at C-3 of the allylsilane followed by a cationic 1,2-silyl migration (equation 57). At the yyw-clinal transition state (81), the carbonyl group of the unsaturated ketone was assumed to occupy an endo orientation in relation to the allylsilane142. The transition state was thought to be favorable due to minimization of the charge separation and to possible secondary orbital stabilization. 

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

Open-chain SE2 reactions are special cases of electrophilic attack on a C=C double bond in the sense 5.132, with attack specifically at C-3, and followed by the loss of an electrofugal group from the stereogenic centre. The most studied of these are the electrophilic substitution reactions of stereodefined allylsilanes,... 

There thus exists a preference for anti (or antara) hydroxylation in these cyclohexenylstannanes, where electrophilic substitutions are known to proceed faitiifully with allylic rearrangement. A more likely padiway is shown in Scheme 4, which is supported by results with optically active allylsilanes, whidi require anti attack by MCPBA on the silane conformation maximizing C—Si tr nr interaction. 

As the ipso effect is sometimes ineffective with strong-electron donating groups, advantage of the possible electrophilic substitution of the allylsilane intermediate was taken to perform the acylation of phenols in the meta position according to Scheme 6 (refs. 15 - 17) ... 

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. 

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). 

Optically active (Z)-l-substituted-2-alkenylsilanes are also available by asymmetric cross coupling, and similarly react with aldehydes in the presence of titanium(IV) chloride by an SE process in which the electrophile attacks the allylsilane double bond unit with respect to the leaving silyl group to form ( )-s)vr-products. However the enantiomeric excesses of these (Z)-allylsilanes tend to be lower than those of their ( )-isomers, and their reactions with aldehydes tend to be less stereoselective with more of the (E)-anti products being obtained74. 

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 ... 

Knolker and coworkers also used a domino [3+2] cycloaddition for the clever formation of a bridged tetracyclic compound 4-172, starting from a cyclopentanone 4-168 and containing two exocydic double bonds in the a-positions (Scheme 4.36) [57]. The reaction of 4-168 with an excess of allylsilane 4-169 in the presence of the Lewis acid TiCLj led to the spiro compound 4-170 in a syn fashion. It follows a Wag-ner-Meerwein rearrangement to give a tertiary carbocation 4-171, which acts as an electrophile in an electrophilic aromatic substitution process. The final step is the... 

In general, allylsilanes undergo electrophilic addition or substitution via an intermediate /1-silylcarbocation (equation 64). 

---