Silicon allylsilanes

Silicon.— Allylsilanes are outstandingly useful carbon nucleophiles as a result of their ability to react with electrophiles in a controlled and specific manner to form new carbon-carbon bonds. Furthermore, metallation to give a-(trimethyl-silyl)allylic carbanions introduces the possibility of an additional level of control over the mode of attack (either a or y) exhibited by the allylsilane. It is not surprising, therefore, to find the chemistry of these species occupying a prominent position in this year s organosilicon literature, and in addition to the many applications reported some new routes to these compounds have been described. One such example is shown in Scheme 13, in which the starting silyl sulphone is readily prepared by oxidation of the product obtained from reaction of thio-phenol with vinyltrimethylsilane." ... 

Allylsilanes are widely used in organic synthesis1 4. They are stable with respect to 1,3-migra-tion of the silicon indeed high temperatures or catalysts are required for 1,3-equilibration5. The preparation of allylsilanes has been reviewed recently6-7. 

In the intramolecular amidoalkylation of certain (Z)-allylsilanes, piperidine derivatives (n = 2) are formed less stereoselectively than the corresponding pyrrolidines (n = 1). The complete regiocontrol is induced by the /3-effect of the silicon atom. 

So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

In the case of alkylation using allylsilancs in the presence of aluminum chloride as a catalyst, allylsilanes containing one or more chlorine substituents on the silicon react with aromatic compounds at room temperature or below 0 C to give alkylated products. 2-aryl-1 -silylpropanes.- while allyltrimethylsilane did not give the alkylated product but instead dimerized to give the allylsilylation product.. S-itrimethylsilyli-d-itrimethylsilylrnethyl)- 1-pentene (Eq. (1 )). In the alkylation reaction, the reactivity of allylsilanes increased as the number of chlorine... 

The substituent effect of vinylsilanes is similar to that of allylsilanes. The reactivity of vinylsilanes increased as the number of chlorine atoms on the silicon increased, but decreased as the number of methyl groups increased. However, vinyltrimethylsilane does not react with benzene to give alkylated products. " In the aluminum chloride-catalyzed alkylation of arenes with allylsilanes or vinylsilanes, one or more chlorine substituents on the silicon atom of silanes are required. 

The addition reaction of allylsilane to acetaldehyde with BF3 as the Lewis acid has been modeled computationally.95 The lowest-energy TSs found, which are shown in Figure 9.2, were of the synclinal type, with dihedral angles near 60°. Although the structures are acyclic, there is an apparent electrostatic attraction between the fluorine and the silicon that imparts some cyclic character to the TS. Both anti and syn structures were of comparable energy for the model. However, steric effects that arise by replacement of hydrogen on silicon with methyl are likely to favor the anti TS. 

An interesting application of the cydization of alkenyl thioacetals is the stereoselective preparation of olefmic diols. Thus, oxidative cleavage of the silicon—carbon bond [32] in the ring-closed metathesis products, i.e. cyclic allylsilanes such as 35 and 36, affords (Z)-alk-2-ene-1,5-diols 37 and 38 (Scheme 14.18) [33],... 

Pyrrolo[l,2-c][l,3]oxazin-l-one 248 has been obtained by reaction of allylsilanes with a pyrrolidine-lV-acyliminium ion 247 (Scheme 32), formed by addition of a Lewis acid on compound 244. The /3-silyl carbocation formed by the reaction with allylsilane 246 reacted with the oxygen of the N-15OC group followed by the loss of 2-methylpropene. The reaction was not very stereoselective when trimethylsilane was used, whereas with larger group on the silicon the selectivity was increased <1997J(P1)2163>. 

Allylsilanes have been found to be more reactive than vinyl silanes toward electrophiles and they are being studied more intensively in recent years because they hold considerable promise in organic synthesis. In allylsilanes, the geometry of carbon-silicon bond can be more favourably be oriented for efficient stabilization of a developing positive change P to silicon. 

Allylchlorosilanes undergo Friedel-Crafts alkylation with aromatic compounds such as benzene derivatives and ferrocene to give [p-(chlorosilyl)alkyl]arene compounds in the presence of Lewis acid catalyst. Allylsilanes containing two or more chlorine atoms on silicon react smoothly with benzene under mild conditions to give alkylation products in good yields [Eq. (15)]. In alkylations of benzene, the reactivity of the allylsilanes increases as the number of chlorine atoms on the silicon increases, but decreases as the number of methyl groups increases. Because the reactivity of allylsilanes is sensitive to the electronic nature of the substituents on the silicon atom, allylsilane selection is an important factor for alkylation reactions. 

In the reaction with ferrocene, allyldimethylchlorosilane reacts at 0 °C, allyl-(methyl)dichlorosilane reacts at the reflux temperature of methylene chloride, but allylsilanes containing two or more chlorine substituents at the silicon do not give alkylation products. In alkylations of ferrocene, allyldimethylchlorosilane shows the highest activity, allyl(methyl)dichlorosilane is less reactive, and allylsilanes containing two or more chlorine-substituents at the silicon have no activity. Allyl-trimethylsilane reacts with both benzene and ferrocene to give allylsilylation products but no alkylation product. 

The reactivity of allylchlorosilanes for the alkylation of ferrocene varies depending upon the substituents on the silicon atom. Generally, the reactivity increases as the number of alkyl groups on the silicon of allylsilanes increases." Allyl(dialkyl)-chlorosilanes react with ferrocene in the presence of HfCU under mild reaction... 

The class of 3-silyl-substituted reagents provides, upon addition with aldehydes, allylic silanes that offer many options for further derivatization. Oxidative processes are described in previous sections (see the sections on Preparation of 1,2-Diols and 1,4-Diols). If the appropriate silicon substituents are chosen, formal [3+2] cycloadditions with aldehydes can be promoted under Lewis acid catalysis. For example, the mismatched addition of the Z-3-propyl-3-benzhydryldimethyl allylsilane 183 to an a-benzyloxy aldehyde proceeds with low diastereofacial selectivity in favor of product 184 however, after protection of the secondary alcohol, an efficient [3+2] annulation provides the polysubsubstituted furan 185 in good yield and acceptable stereoselectivity (Scheme 24). ° The latter is brought forward to a tricyclic unit found in the antitumor natural product angelmicin B. 

In an approach to direct the alkylation of (l-silyl-2-propenyl)lithium into the a-position by chelating substituents on the silicon atom, chiral pyrrolidinyl-substituted allylsilane 1 was deprotonated and alkylated with iodomethane (94%)51. Regiocontrol was moderate, the a-product 2 dominating (85%). 

Additions to carbon-carbon multiple bonds initiated by electrophiles are generally governed by the Markovnikov rule. However, the rule must be modified to accom-ipodate such substrates as vinylsilanes. The so-called anti-Markovnikov hydro-halogenation [105] is to be contrasted to the Markovnikov addition for allylsilanes. In fact, when one recognizes the acceptor role of the silicon atom and applying the polarity alternation rule, the puzzling results become self-consistent. 

Reaction with Further Electrophiles of Group IVA (Sl,Ge,Sn). IV-Silylated aziridines can be prepared from ethyleneimine by amination of chlorosilanes in the presence of an HC1 acceptor, by dehydrocondensation with an organosilicon hydride or by cleavage of a silicon—carbon bond in 2-furyl-, 2-thienyl-, benzyl-, or allylsilanes in the presence of an alkali metal catalyst (262—266). N-Silylated aziridines can react with carboxylic anhydrides to give acylated aziridines, eg, A/-acetylaziridine [460-07-1] in high yields (267). At high temperatures, A/-silylaziridines can be dimerized to piperazines (268). Aldehydes can be inserted... 

If a substituent is present that can stabilize the cationic intermediate more effectively than the silicon, this may alter the course of the reaction. For example, deuterionation of the allylsilane 113 leads to a mixture of the expected product 114 and an anomalous product 115 (equation 65)135. 

As with vinylsilanes and alkynylsilanes, substitution is favoured over addition for allylsilanes. However, this can be affected by the steric and electronic effects of the silicon substituents. Mayr and Hagen have studied the reactivities of allylsilanes towards the p-methoxy substituted diphenylcarbocation (Scheme 7)136. 37 Relative rate data and observed products are summarized in Table 11 for allylsilanes of the structures 119-122, with various silicon substituents. 

As with allylsilanes, increased steric hindrance at silicon favours annulation (equation 105)172 rather than the Sakurai reaction (equations 103 and 104). 

Allylsilanes 12 react with a-ketoesters 174 to give [3 + 2] cycloadducts 175 in 50-85% yields through 1,2-silyl migration (equation 73)59,228. Use of an allylic trimethylsilane produces an allyl alcohol by-product through a competing silyl elimination process. It can be circumvented by use of a bulkier silicon-containing reagent, such as the... 

A similar transition state 22 is calculated for the reaction of allylsilanes with aldehydes (equation 10)43. In this case, however, the calculations show that the oxygen attacks at an apical site of the silicon centre, while the allyl group departs directly from an equatorial position without causing a pseudorotation, in contrast to the mechanisms previously discussed. 

Since allylsilane can be considered as a very soft nucleophile because of the involvement of o-7i conjugation between the -electrons of the double bond and the o-electrons in the carbon-silicon bond, the addition of allylsilanes to ,/3-unsaturated enone moiety occurs exclusively via 1,4-addition19,20. A typical Sakurai-Hosomi reaction is illustrated in the reaction of allylsilane 66 with enone 67 in the presence of TiCU to give 68 which is used for the synthesis of a cyclic enediyne (equation 45)108. A similar reaction has been used for the synthesis of ewf-herbasolide 70 from enone 69 (equation 46)109. Prenylsilane undergoes 1,4-addition with squaric acid chloride 71 followed by dechlorosilylation to give 72 as the predominant product (equation 47). Interestingly, other simple allylsilanes react in a 1,2-addition fashion to yield 73110. 

A three-component radical coupling reaction involving allylsilanes has been employed leading to 145 (equation 121)213. It is noteworthy that the corresponding carbon-silicon bond remains intact in this reaction. 

---