Allylsilanes protonation

Hodgson and coworkers extended this concept to epoxides of unsaturated cyclic ethers 128 [5] and amines 130 [46, 47] (Scheme 5.28). It is interesting that the use of trimethylsilylmethyllithium as the organolithium in this case resulted in substituted allylsilanes 129 and 131 (R = CH2SiMe3) presumably the epoxide ring protons of 128 and 130 are more acidic than those of a simple terminal epoxide (see Scheme 5.26). 

The proton serves as the simplest electrophile to displace the silyl group stereo- and regioselectively. Numerous conditions have been used for the electrophilic protodesilylation of allyl- and vinylsilanes19. ( )-vinyl sulfones 2 are prepared from the silylallylic sulfones 1 in the presence of protic acids in high yield (equation l)34. Diastereoselective protodesilylation of allylsilane 3 gives 4 with excellent control of the geometry of the double bond exocyclic to the ring (equation 2)35. 

A highly selective synthesis of homoallylic alcohols has been reported by Tietze et al.,917 who reacted methyl ketones, the chiral norpseudoephedrine derivative 285, and an allylsilane in the presence of a catalytic amount (0.2 mol%) of triflic acid [Eq. (5.340)]. The transformation was interpreted as an SN2 attack of the allylsilane to the protonated mixed acetal 286. The obtained ethers were then cleaved to the final product, homoallylic alcohols. 

Protiodesilylation. The complex of BF, and icetic acid serves as both a proton source and an efficient nucleophile for silicon (9, 492). The protonation of allylsilanes containing a carboxyl group or hydroxyl group is intramolecular and can result in 1,3-and 1,4-asymmetric induction. Thus treatment of 1 with BF,-HOAc results in 2 with greater than 8 1 diastereoselectivity. 

Phen and the radical anion of the alkene. Secondary electron transfer from allylsilane to Phen produces the radical cation of allylsilane and neutral Phen. The radical cation of allylsilane is cleaved by assistance of acetonitrile to generate an allyl radical. The allyl radical adds to the radical anion of the alkene to give the allylated anion which is converted into the product upon protonation. Alkyl and arylmethyl radicals can be generated in a similar manner from tetraalkyl tin compounds and arylmethylsilanes, respectively [124]. These radicals add regioselectively to the -position to the cyano groups in the radical anions of alkenes. 

Desilylation reactions are based on the well-known stabilization of a,/3-carbenium centers by silicon. The conversion of allylsilanes into alkenes by use of the BF3. (AcOH)2 complex has been studied in considerable detail. The reaction occurs by protonation, followed by nucleophile-induced desilylation (Eq. 61) [107]. The stereochemical implications have also been considered in detail, and in the deuteration-desilyation sequence shown in Eq. (62) the structure shown is the major product [108]. Protonation-desilation of alkynes to give allenes in yields ranging from 70 to 97 % can evidently be considered to involve a /3-vinyl cation (Eq. 63) [109]. 

Enantioselective protonation of prochiral allyl anion derivatives is a very simple and attractive route for the preparation of optically active olefins. The acid-promoted hydrolysis of allyltins or allylsilanes is an interesting alternative the enantioselectivity of which has not yet been investigated. Allyltrialkyltin, a synthetic equivalent of allyl anion, is more reactive than the corresponding allylsilane and can be isolated. The... 

The hydroalumination-protonation of l-trimethylsilyl-2-alkynes and the Wittig reaction using silylated ylides are other good approaches for the synthesis of allylsilanes. 

For the introduction of TMS groups under acidic conditions, trimethylsilyltriflate, hexamethyldisila-zane and also allyltrimethylsilane (45) - have been successfully applied. The allylsilane offers the advantage of being stable to hydrolysis. The mechanism proposed for its reaction with alcohols is shown in equation (12). It involves an initial protonation of the double bond. The 1-silylisopropyl cation formed is then attacked by the alcohol at the silicon to produce the silyl ether and propene. 

If the reduction system I proton source the allylsama Thus, (Z)-allylsilanes are aa Pinacol formation fipom i chemists. In the presence of cyclitol synthesis the diasta a-substituents of the aldehyd effect on the reactivity of S pinacol coupling of ketones reduced to a few minutes). 

If the reduction system for allylic phosphates is supplied with McjSiCl instead of a proton source the allylsamarium intermediates would be captured to afford allylsilanes. Thus, (Z)-allylsilanes are accessible by this method. ... 

Carbanions a to silicon are stabilized by the overlap of the filled 2p orbital on carbon with the vacant antibonding orbital of carbon-silicon bond ((a -p)K conjugation) (Scheme 1 A). For example, the a-carbanions of allylsilanes or similar compounds are easily generated by base removal of a proton (eq (9)) [6]. 

Even when isomeric mixtures of the allylsilane are used, complete stereochemical control is observed for all the stereocenters established in the conjugate addition. The only breakdown in selectivity in the process occurs with the stereocenters adjacent to the carbonyl carbon, which result from protonation on work-up. The less stable cis-fused decalone is readily converted to the trans-fused isomer by treatment with base. Hence, with two steps, complete control over the relative configuration of four contiguous stereogenic centers is achieved. 

In addition to their use in Mannich (and variant) reactions, iminium ions are useful for other cationic type cyclizations. Corey employed a novel tandem iminium ion cyclization as part of an elegant cascade used for the synthesis of aspidophytine. The reaction of tryptamine 292 and dialdehyde 293 in CH3CN at ambient temperature afforded the pentacyclic skeleton of the alkaloid (296 Scheme 54) (99JA6771). Condensation of the free amino functionality of 292 with the dialdehyde produced a dihydropyridinium intermediate 294 that then cyclized onto the indole n-bond to give 295. The iminium ion so produced underwent a second cyclization with the tethered allylsilane moiety to give 296. Protonation of the enamine in 296 provided still another iminium ion (297) that was then reduced with NaCNBH3 to furnish 298 in 66% yield. All of the above reactions could be made to occur in a single pot. 

The reaction proceeds smoothly with styrene, -substituted styrenes, and /3-vinyl-naphthalene to give the corresponding ( )-vinylsilanes in 83-100% yield. However, isomerization of the double bond gives an allylsilane when a substrate having allylic proton(s) is employed ... 

The very general picture of the mechanism given in the introduction needs to be modified somewhat. It is fairly clear that allylsilanes react in the manner illustrated as (3) — (5), except that the acid catalyst is almost invariably a Lewis acid not a proton. Protic acid is apt to induce protodesilylation in competition with carbon-carbon bond formation, but Lewis acids are less likely to attack the carbon-carbon double bond. The intervention of a cationic intermediate (4) has not been proved for this type of reaction, but there is good circumstantial evidence that cations are involved by analogy with protodesilylation,7 from the occasional formation of oxepins, without loss of the silyl group,8 and from the stereochemistry of the reaction. 

Thus B is higher in energy and since both have an identical intermediate for electrophilic attack by the proton, the activation energy for protiodesilylation of the allylsilane would be expected to be smaller than that for the vinylsilane. Similarly 37 (see equation 69) reacts as an allylsilane, not a vinylsilane208. 

RS form 42 the addition of the electrophile (e.g. H+) was always anti, however the RR form 43 gave a mixture of products. Deuteriodesilylation of the RR form 43 indicated that syn addition occurred via an indirect pathway, whereas anti addition occurred as expected. It was concluded that the cyclohexyl ring had a preference for axial protonation and when this axial preference opposes the anti selectivity of the allylsilane the molecule finds another pathway whereby stereospecificity is lost. By contrast, epoxi-dation with MCPBA gave anti selectivity both for the RR and RS forms214. 

A detailed NMR analysis of a cyclic allylsilane, [(2R, 3S )-l,l-diphenyl-2-vinyl-l,2,3,6-tetrahydrosilin-3-ol],has been performed by Mahieux et yielding a set of proton-proton couplings including the allyhc Vhh and homoallylic Jhh ones. These values have been used as NMR constraints to calculate the 3D structure of the compound. 

The protonated acid site of H+-mont could generate cationic organosilane intermediates that undergo nucleophilic addition of alkenes (Scheme 6.8) [95]. After the C-Si bond formation, nucleophilic addition of an allylsilane to the... 

The stabilization of a 3-carbonium ion is also involved in the reaction of an allylsilane with an electrophile (Eq. 14.16). An advantage of silicon over other metals in this context is that it does not undergo 1,3 shifts, and so the point of attachment of the electrophile can be reliably predicted (Eq. 14.17). - This 3 stabilization of the carbonium ion also has stereochemical implications Eq. 14.18 shows how the stereochemistry of a vinylsilicon reagent can be retained on protonation. A TMS group on carbon has been described as a... 

Let us look first at the allylsilane 46 that does not give good stereocontrol the conformation 51, which will lead to the E-isomer 49, must either be protonated syn to the silyl group or endo on the bicyclic system. Since neither of these pathways is likely to be favourable, it is not surprising that this diastereoisomer does not lead... 

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