Allylsilanes, electrophilic substitution reactions

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

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... 

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 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. 

Allylsilanes undergo reactions with a large range of electrophiles, although catalysis by Lewis acids is often necessary. Some recent examples of substitution reactions of allylsilanes are discussed below. 

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 [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. 

Like their carbon counterparts, silylcuprates and stannylcuprates readily participate in SN2 -substitution reaction with allylic electrophiles.43,51 For example, <37z//-diastereoselective allylic substitution of the vinyl oxiranes 147 with the lower-order cyanocuprate (PhMe2Si)Cu(CN)Li cleanly affords the allylsilanes 148 (Scheme 38).106 Products of this type can be converted into the corresponding diols with retention of configuration by Tamao-Fleming oxidation 53,53a,53b,107... 

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). 

Nucleophilic Substitution of Alcohols. The use of alcohols as electrophiles instead of halides and acetate compounds is an ideal method to prevent waste salt formation (Fig. 5). However, cataljdic substitution of the hydroxyl group in alcohols is difficult due to their poor leaving ability, which requires equimolar or greater amounts of reagents. Recently, several homogeneous catalysts, such as NaAuCh, InCla, ZrCh, La, Yb, Sc, Hf triflate, BCCeFsls, BF3, and p-toluenesulfonic acid have been used for nucleophilic substitution reactions of alcohols with amides (33), 1,3-dicarbonyl compounds (34), and allylsilanes (35). However, these catalysts are often limited by low catalytic activity and selectivity, can be difficult to reuse, and require the use of halogenated solvents. 

Nucleophilic substitution reaction is another class of important transformations that organosilicon reagents play key roles in organic synthesis. Electrophiles such as acyl and alkyl halides activated by a Lewis acid in a stoichiometric amount readily react with silyl enol ethers, allylsilanes, and alkenylsilanes. ... 

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. 

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 the presence of the TiCLt catalyst, 6-amino-substituted allylsilanes readily react with isobutyraldehyde to yield piperidine derivatives 88 (equation 60)128. Presumably, an iminium ion intermediate is formed and the Boc group is replaced under the reaction conditions. It is noteworthy that the regioselectivity is different when acid chloride in used as the electrophile (cf Section II.B.5). 

The key element of this protocol is the initial addition of cationic electrophiles such as rerr-alkyl or acyl cations to the double bond of a DCHC complex of the conjugated enyne 118, which results in the formation of the substituted propargylic cation intermediate 119, Subsequent reaction with pre-selected external nucleophiles, for example allylsilanes or silyl enol ethers, leads to the formation of the final adducts 120. The reaction is carried out as a one-pot, three-component coupling and can be used for the creation of two novel C-C bonds. It is a process somewhat complementary to the stepwise Michael addition described earlier (Scheme 2.31), with a reverse order of E and Nu addition. Oxidative decomplexation of 120 yields the product 121. The overall... 

The coupling of an allyl or acyl moiety onto carbon atoms is achieved by anodic oxidation of a-heteroatom substituted organostannanes or Oj -acetals in the presence of allylsilanes or silyl enol ethers. The reaction probably involves carbocations as intermediates that undergo electrophilic addition to the double bond [245c]. 

In the presence of a Lewis acid (such as Et2AlCl), allylsilanes react with electrophiles in a regiospecific manner. The intermediate (3-carbocation is stabilized by (a-Tc)-conjugation with the C-Si bond. The most important feature of this reaction is that the electrophile reacts with the terminus (y-carbon) of the allyl system, and the n-system is relocated adjacent to its original position. Even substituted allylic silanes can be acylated at the more hindered site. Because of this predictability and their high nucleophilicity, allylsilanes are valuable in many synthetic transformations. 

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