Silyl-stabilized carbocation

Finally, returning to the main group series, the diborapropellane 64 (R = Me) reacts with hydrochloric acid to give boracyclobutene 88 (Equation 29), presumably by initial protonation of the alkene to generate the /3-silyl-stabilized carbocation <1999AGE2936>. 

When the attacking nucleophile is carbon-centered, intramolecular epoxide ring-opening reactions provide an entry intocarbocyclic systems. For example, epoxy allylsilane 70 cyclizes in an overwhelmingly 5-exo fashion under Lewis acid catalysis to form a putative silyl-stabilized carbocation intermediate (71) which then undergoes... 

This approach allows one to functionalize the j3-C atom of AN. For this purpose, AN are initially subjected to double silylation to prepare BENAs, which are then coupled with various stabilized carbocations (512, 513), as well as with sulfenyl and episulfonium cations (514). Molecules containing good leaving groups (e.g., arenesulfenyl chlorides (514)) are used as sources (or precursors) of... 

Removable cation-stabilizing auxiliaries have been investigated for polyene cyclizations. For example, a silyl-assisted carbocation cyclization has been used in an efficient total synthesis of lanosterol. Other conditions for the cyclization of polyenes and of ene-ynes to steroids have been investigated. Oxidative free-radical cyclizations of polyenes produce steroid nuclei with exquisite stereocontrol Besides the aforementioned A-ring aromatic steroids and contraceptive agents, partial synthesis from steroid raw materials has also accounted for the vast majority of industrial-scale steroid synthesis. 

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. 

These data show a decrease in the extent of /J-silyl stabilization with successive methyl substitution. The methyl (and phenyl) substituents stabilize the carbocation by polarization and inductive effects, resulting in a delocalization of positive charge away from the carbocation, and therefore a reduction in hyperconjugative interaction with the fi-substituent bond. 

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. 

TABLE 1. Experimental stabilization energies (kcalmol ) of j6-silyl-substituted carbocations Me3SiCHR R2-CR3R4 and Me3Si-CR =CR2 according to equations 13 and 14, respectively... 

The large difference between the AAG = 5.7 kcal mol 1 found for the cyclohexene system 190/19181 and the AH = 34 kcal mol 1 for the similar ions 196/19720 point to drastic differences in the mode of stabilization of the transition state for the protonation in solution and the free silyl-substituted carbocation in the gas phase. 

Ab initio DFT calculations were performed using the B3LYP/6-31G(d) hybrid method for the analogous l-p-anisyl-2-SiH3-substituted ethyl cation 324, which is a close model for the experimentally observed cation 323 with alkyl groups at silicon. It has been shown that different alkyl groups at silicon have no significant impact on the electronic stabilization in a-aryl-/l-silyl-substituted carbocations. 

To study the possible stabilizing effect of [3-silyl cations, Olah and co-workers334 prepared the 2- [(1 -trimethylsilyl)vinyl]-2-adamantyl cation 132 [Eq. (3.43)] as well as the parent silicon-free carbocation. In contrast to the above observations, NMR data [the (Cl ), (C2), and (C2 ) carbons are more deshielded in 132 than in the parent ion] showed that cation 132 is destabilized compared with the silicon-free analog. Furthermore, at — 100°C the C(l) and C(3) carbons were found to be equivalent, whereas in the parent ion they were nonequivalent. This indicates a rapid rotation about the C(l)-C(3) bond in 132, which can be rationalized by assuming the intermediacy of the [3-silyl-stabilized cation 133. The difference between cation 132 and those having [3-silyl-stabilization discussed above may be the orthogonal arrangement of the [3-C-Si bond and the p-orbital of the carbocation center. 

Cyclobutadiene iron tricarbonyl complexes also stabilized carbocations on an adjacent carbon. The cation reacts with silyl enol ethers to afford alkylated complexes such as (127) (Scheme 187). A samarimn diiodide -mediated intermolecular radical cychzation of iron tricarbonyl complex (128) is depicted in Scheme 188. An excellent stereocontrol at three contiguous centers is observed. 

Coupling of silyl enol ethers or boron enolates with Co2(CO)6-stabilized carbocations, generated via Lewis acid treatment of the appropriate propargyl ethers or aldehydes (aldol reaction), via the Nicholas reaction has been used to obtain large, highly strained, ring ketones. 

Both silyl enolates and allylsilanes are excellent nucleophiles for alkylation by other stabilized carbocations such as the tertiary alkyl cations 111 or 112 (Scheme 2.42). Similarly, Michael-like additions, for example, the coupling of 113 with silyl ketene acetal 114, can be also achieved.Owing to the high electrophilicty of the enone system, this reaction proceeds smoothly in polar solvents, even in the absence of Lewis acids. 

Lewis Acid-Mediated Reactions. The highly nucleophilic double bond of allylsilanes reacts with a vaiiety of electrophiles provided that the electrophiles are activated by a Lewis acid, such as TiCl4, BFj, or AICI3. The y-position of the allylsilane attacks the electrophile to generate a stabilized carbocation 3 to silicon. Subsequent loss of the silyl group results in the transposition of the double bond. ... 

A similar stereoselectivity has been described for the Lewis acid catalyzed pinacol rearrangement of vinyl-24 and cyclopropyl-substituLed25 hydroxysulfonates 16 (via mesylation) and 17. Of note is a dramatic rate enhancement observed for the [1,2] rearrangement of silyl-substituted substrates 16, an observation consistent with the well-documented ability of silicon substituents to stabilize / -carbocations. 

The ability to promote 6-elimination and the electron-donor capacity of the 6-metalloid substituents can be exploited in a very useful way in synthetic chemistry. Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting intermediates then undergo elimination of the stannyl or silyl substituent, so that the net effect is replacement of the stannyl or silyl group by the electrophile. The silyl and stannyl substituents are crucial to these reactions in two ways. In the electrophilic addition step, they act as electron-releasing groups that promote addition and control the regio-chemistry. A silyl or stannyl substituent strongly stabilizes carbocation character at the 6-carbon atom and thus directs the electrophile to the a-carbon. 

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