Carbocations silicon-stabilized

Effects for the tertiary butyl carbocation were also calculated and, as would be expected, the value for -silicon stabilization was smaller than in the secondary system, having a value of 15.9 kcalmol-1. For comparison, the /)-methyl stabilization was 5.0 kcalmol-1. 

Nucleophilic attack in a 1,6 fashion by the allylsilane double bond at the doubly activated Michael acceptor produces A containing the silicon-stabilized carbocation. Loss of the trimethylsilyl group generates the exocyclic methylene moiety. 

An unprecedented one-pot stereoselective synthesis of 2-azetidinone P-chlorinated allylic alcohols 23, which can also be considered as functionalized allylsilanes, has been developed, by tin(IV) chloride-mediated reaction of propargyltrimethylsilane and 4-oxoazetidine-2-carbaldehydes <02CEJ1719>. An explanation for the formation of P-chlorovinyl alcohols involves a stepwise process with the chlorination proceeding via a silicon stabilized carbocation. 

Protonation of the hydroxyl and trimethylsilyloxy groups of A and D, respectively, generates carbocations B and E. In B, attack of the methyl-substituted triple bond leads to formation of the preferred linear vinyl cation. Its trapping by the formate ion furnishes the substituted bicyclo[2.2.2]octene C. On the other hand, attack of the silicon-substituted triple bond in E occurs to form the silicon-stabilized p-carbocation, which, on trapping by formate ion, yields the substituted bicyclo[3.2.2]nonene F. 

Scheme 5. Postulated mechanism for the conversion ofbenzylsilane cis-diol to an ortho-substituted phenol through a silicon stabilized carbocation.

Lewis Acid-catalyzed Addition to Epoxides. The Lewis acid-catalyzed addition of l,3-bis(silyl)propenes to epoxides has been studied for both inter- and intramolecular reactions. In the inter-molecular reaction, monosubstituted epoxides give higher yields (66-53%) than bis-substituted epoxides (33-19%) (eq 5). It is proposed that the epoxide opens in a Sn 1 fashion, followed by nucleophilic attack of the bis(silyl)propene on the resultant cation. This leads to a silicon-stabilized carbocation, which undergoes C O silyl migration 3uelding the final alkene. 

West and coworkers [28] reported the efficient formation of bicyclic bridgehead ketones 110 via the electrocyclization/formal intermolecular [3+2]cycloaddition of dienones 108 with allyltrimethylsUane 109 (Scheme 3.24). LA-mediated electrocyclization of 108 provides oxyallyl intermediate 111, which reacts with allylsilane 109 to give intermediate 112. At this point, ring closure provides the bridgehead ketone 110. In some cases, a minor product 113 was observed, the result of the elimination of the TMS group from the same silicon-stabilized carbocation 112. 

Other, removable cation-stabilizing auxiliaries have been investigated for polyene cyclizations. For example, a sdyl-assisted carbocation cyclization has been used in an efficient total synthesis of lanosterol. The key step, treatment of (257) with methyl aluminum chloride in methylene chloride at —78° C, followed by acylation and chromatographic separation, affords (258) in 55% yield (two steps). When this cyclization was attempted on similar compounds that did not contain the C7P-silicon substituent, no tetracycHc products were observed. Steroid (258) is converted to lanosterol (77) in three additional chemical steps (225). 

The methoxymethyl cation can be obtained as a stable solid, MeOCH SbF Carbocations containing either a, p, or y silicon atom are also stabilized, relative to similar ions without the silicon atom. 

It should be recognized that the stability of cation radicals generated by anodic oxidation is also affected by jS-silyl substitution. Stabilization of car-bocations by a silyl group situated at the -position is well known as the / effect . The interaction of the C Si a orbital with the empty p orbital of the carbon stabilizes the carbocation. Therefore, we can expect similar effects of silicon for cation radical species. The interaction of the filled C-Si a orbital with the half-filled orbital of the carbon may stabilize the cation radical. 

A recent theoretical study has nicely addressed the question of mechanism on the silicon surface. Minary and Tuckerman carried out an ab initio molecular dynamics (MD) study of the [4 + 2] cycloaddition reaction on Si(100)-2 x 1 [251]. Because the previously reported ab initio DFT models were static , these were not able to address in detail the mechanisms by which the [4 + 2] product was formed. The results of the MD study indicate that rather than being concerted, the dominant mechanism is a stepwise zwitterionic process in which an initial nucleophilic attack of one of the C=C bonds by the down atom of the dimer leads to a carbocation. This carbocation exists for up to 1-2 ps, stabilized by resonance, and depending on which positively charged carbon atom reacts with which Si surface atom, can form... 

In this section we concentrate mainly on mechanistic aspects of reactions involving ft-silylcarbocations. The structure and properties of /J-silylcarbocations are discussed in detail in Chapter 12 on Silicon-substituted Carbocations in this volume. The strong stabilization of /J-silylcarbocations is of particular importance in relation to activating and directing effects in organic syntheses using silicon compounds, and is still an important area for mechanistic, theoretical and synthetic studies. 

A variety of methods have been used to determine the energy of stabilization of a carbocation by a /J-silicon substituent. Li and Stone45 studied the association of the trimethylsilicenium ion with alkenes in a mass spectrometer and have calculated the yS-silyl stabilization energies for the carbocations produced as shown in Table 2. 

The effect of the substituents at silicon to stabilize a /J carbocation has also been investigated by Brook and Neuy57, who studied the degree of syn addition to ( )-/ -silylstyrenes. Addition of bromine to vinyltrimethylsilanes normally proceeds in an anti sense as shown in Scheme 1, the trans vinylsilane 37 giving the cis product 38. 

In addition to the stabilization of /1-carbocations, the /1-effect of silicon can also be observed in the ground states of neutral molecules. Lambert and Singer58 studied compounds of the type 42 where hyperconjugation should be enhanced by increasing the electron-accepting properties of the substituent X (MeO < Me < H < CN). o-tt overlap in this system gives the resonance structure 43 shown in equation 19. 

However, the directing influence of silicon can be overcome if the vinylsilane contains another substituent that can stabilize a carbocation more strongly than silicon. For example, when the silyl group is attached to C-2 of a terminal alkene, reaction occurs to give the more substituted carbocation 82 (equation 44)107. Similarly, if the silicon is bound to the same carbon atom as a phenyl group, reaction occurs via the benzyl cation to give the product shown in equation 45108. 

Provided that the silicon-carbon bond can be coplanar with the vacant p orbital, the /J-silyl substituted carbocation should be stabilized by hyperconjugation, and this has been demonstrated by Kresge and coworkers47,49. 

The increased stabilization by the /J-silicon of the carbocation a to the cyclopropyl ring presumably disfavours the isomerizations in which the charge is located on other carbon atoms in the molecule. 

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