P-Silyl cations

Overman15 discusses two conceivable mechanisms for the cyclization. One possibility assumes a direct cyclization of iminium ion 9 via p-silyl cation intermediate 24 to the indolizidinone 10. Cation 24 is stabilized by a p-effect of the silicon atom. Alternatively, iminium ion 9 might first undergo a charge-accelerated cationic aza-Co/fe rearrangement to allylsilaniminium ion 25, which would then cyclize to 10 with loss of a silyl cation. 

The major reaction product formed from the solvolysis of the trans y-silyl ester was cyclohexene formed from the carbocation 138 (Scheme 18) by 1,2-hydrogen migration to give the p-silyl cation 139 followed by loss of the silicon substituent. In contrast to the behavior shown by 136, the cis y-silyl ester 134 exhibited a small inverse p-d4 isotope effect (kH/kn = 0.97) attributed to the inductive effect of the P-deuteriums and implies that there is very little hyperconjugative stabilization of... 

These epoxides react stereospecifically with nucleophiles to give single diastereoisomers of adducts. If a carbon nucleophile is used (cuprates are best), it is obvious from the structure of the products that nucleophilic attack has occurred at the end of the epoxide next to silicon. This is obviously an S>j2 reaction because it is stereospecific in any case an S>jl reaction would have occurred at the other end of the epoxide through the p-silyl cation. 

Now a Lewis-acid-catalysed reaction of the allyl silane via a p-silyl cation, gives the lactone. The This silicon chemistry was double bond in these exo-methylene lactones easily moves into the ring in acid or base so the mild devised by A. Haider, Synthesis, conditions for allyl silane reactions are ideal. 19S5, 271. 

We can reverse this stereoselectivity by using the other isomer of the iminium salt. Evidently, R is larger than CH2. Addition now gives a P-silyl cation that loses silicon to give the syn isomer of the product. 

Allyl silanes are more reactive than vinyl silanes but also react through P-silyl cations... 

Vinyl and aryl silanes react with electrophiles at the same (ipso or a) atom occupied by siUcon. Allyl silanes react at the end of the alkene furthest from silicon (y). In both cases a p-silyl cation is an intermediate. 

Allyl silanes will also attack carbonyl compounds when they are activated by coordination of the carbonyl oxygen atom to a Lewis acid. The Lewis acid, usually a metal halide such as TiC or ZnC, activates the carbonyl compound by forming an oxonium ion with a metal-oxygen bond. The allyl silane attacks in the usual way and the P-silyl cation is desilylated with the halide ion. Hydrolysis of the metal alkoxide gives a homoallylic alcohol. 

However we rationalized that using silicon as a migrating group could result in a unique stabilization of the energy surface of olefin isomerization. This rationalization was based on silicon s well known ability to stabilize both a-carbanions and p-carbocations. Thus a hypothetical "dual-stabilized" zwitterion would be produced by a 90° twist of a vinyl silane, and a following 1,2-shift of Silicon would produce a singlet carbene possessed of the same hyperconjugative stabilization as in a P-silyl cation (Eq. 5). 

The reaction may involve the formation of the P-silyl cation 114 (chapter 12) or in this case participation by the OH group may be involved. In any case the final product is a protected version 112 of the 3-hydroxyaldehyde 116. 

Of course, if the intermediate cation, stabilised by the P-silyl cation effect, has a lifetime long enough to allow rotation about the C C bond, the stereochemical information is lost. Clear-cut examples with retention are often intramolecular and the most valuable are those that create an exocyclic alkene. A famous example is the cyclisation of the two acetals E- and Z-195 with SnCI4 as the Lewis acid to give reasonable yields of the oxepanes 196 with complete retention of configuration at the exocyclic alkene.47... 

Rearrangement of Z.Z-226 now gives a cation 227 that reacts with the allyl silane to close a second ring. Attack occurs at the end of the alkene away from the silicon atom so that a P-silyl cation might be an intermediate. 

The driving force of this reaction is probably formation of the p-silyl cation in 67. A recent variation treated a-hydroxydithianes with red HgO and HBF4, leading to good yields of ring-expanded l,2-diketones.30... 

A thermal, Lewis acid-catalysed [3+2] cycloaddition of allylsilanes giving the dihydrobenzofurans 57 has also been described (equation (3)) (02TL5349). The mechanism has been interpreted in terms of formation of a p-silyl cation by a zinc-benzoquinone complex, followed by cyclisation, and elimination of isobutene when R = Bu. 

Product selectivity in the H+-mont-catalyzed reaction of allyltrimethylsilane with aromatic alkynes is strongly solvent dependent [96]. In nonpolar solvents, such as n-heptane and toluene, allylsilylation occurred (Scheme 6.10i). Arylsi-lylation proceeded in mesitylene (Scheme 6.10ii), while in 1,4-dioxane, alkynes were silylated at the terminal position (Scheme 6.10iii) [96]. These three reaction pathways all include the same P-silyl cation as an intermediate generated by the reaction between the cationic silyl species on the H+-mont surface and the alkynes. 

A general mechanism of this reaction involves globally a regiospecifictranspositionoftheallylicmoiety (Scheme 12.12). The reaction is initiated by the nucleophilic addition of an allylsilane 39 over an electrophilic carbon, which is normally activated by a strong Lewis acid. Thus, a secondary carbocation intermediate (p-silyl cation, 46) stabilized by hyperconjugation is formed. Then it evolves into a compound with a new double bond by elimination of the silyl group. 

The recurrent theme in the above reactions is the activation of a carbonyl compound 4 (or equivalent) by a Lewis acid, followed by the addition of the allylsilane. The reaction proceeds through a P-silyl cation 5 that is stabilized by hyperconjugation, as represented by the resonance form 6. The... 

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