Silyl-stabilization

Metalated Epoxides and Aziridines in Synthesis 5.2.4.2 Silyl-stabilized Lithiated Epoxides... 

A jS-silyl stabilized vinyl cation, the l-bis(trimethylsilyl)methyl-2-bis(trimethylsilyl) ethenyl cation (32) was investigated by dynamic 13C NMR spectroscopy (Fig. 6).55... 

Conjugated ketones and esters react with allenylsilanes to yield acylcyclopentenes (Eq. 9.60) [63]. These products are formed by initial 1,4-addition to the conjugated double bond to afford a silyl-stabilized vinyl cation intermediate. 1,2-Silyl migration gives rise to a second silyl-stabilized vinyl cation which cyclizes to the acyl cyclopen-tene (Scheme 9.14). 

While there are few examples of conjugate additions of either a-oxygen- or a-silyl-stabilized carbanions, Tamao and Posner have reported two hydroxymethyl synthons ["ClfcOH] (246 and 247) which show synthetic promise. Additions with the silicon-based synthon (246) is restricted to 2-cyclohexen-l-ones and work-up requires a successive acid and base procedure that is incompatible with sensitive molecules,188a-b while the tin-based synthon (247) is more versatile and the hydroxyl group is obtained under neutral conditions (Scheme 83).,88c... 

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. 

A variety of other methods have been used to measure the /3-silicon stabilization of car-bocations. From gas-phase studies, Hajdasz and Squires50 derived a value of 39 kcal mol-1 for the stabilization of the cation Me3SiCH2CH2+ relative to the ethyl cation. This is in agreement with calculations by Ibrahim and Jorgenson39. Siehl and Kaufmann51 have used carbon-13 NMR spectroscopic data to give an indication of the /1-silyl stabilizing effect in some aryl vinyl cations. 

Compared with hydrogen (54 R = R = H), one trimethylsilyl group (54 R = H, R = SiMc3) accelerated the ethanolysis by a factor of 8.6 and two trimethylsilyl groups gave a further acceleration by a factor of 3.8, giving a total acceleration of 33, and forming the doubly /-silyl stabilized ion 55. 

Stone and coworkers determined the /3-silicon effect in w-alkyl- and aryl-substituted carbenium ions20 and vinyl cations21 by measuring in a high-pressure mass spectrometer the thermodynamic data for the association of various alkenes and alkynes with trimethylsilylium ion. From their measured thermodynamic data they calculated, by using equations 13 and 14, the /S-silyl stabilization energies listed in Table 1. 

Similar comparisons between the thermodynamic /J-silyl stabilization measured in the gas phase20,21 and the kinetic -silicon effect81,83 found in protonation experiments in solution are possible for the acetylenes 186 and 188 and for the alkene 190. The data for both solution study and gas phase equilibrium measurements are summarized in Table 5. 

TABLE 5. Comparison between the kinetic /6-silyl effect (AAG ) derived from protonation experiments and thermodynamic /3-silyl stabilization (All") of small carbocations20,21,81,83 ... 

The 1 -(para-methoxyphenyl )-2-(triisopropylsilyl)vinyl cation 131 has also been generated, characterized, and studied323,333 to elucidate the importance of a-7t aryl and (3-0 hyperconjugative stabilization. A comparison was made with the para-methoxyphenylvinyl cation 124, which has a higher demand for a-aryl 7t-stabilization. The para carbon of cation 131 is 7 ppm more shielded compared to that of cation 124, indicating that the (3-silyl stabilization effect is operative. The experimentally determined rotation barrier of the para-methoxy substituent for 131 (<8 kcal mol-1) and 124 (9.0 kcal mol-1) shows a less significant double-bond character of the MeO-C(4)... 

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. 

This gentle variant of the Claisen Rearrangement employs the allyl ester of a carboxylic acid instead of an allyl vinyl ether. The ester is converted to its silyl-stabilized enolate (silyl ketene acetal), which rearranges at temperatures below 100 °C. 

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>. 

Oximes derived from p-silyl and P-stannyl ketones undergo unusual Beckmann rearrangement, giving rise to nitriles as predominant products. For example, Beckmann rearrangement of 3-trimethylsilylcyclohexanone oxime 88 (Scheme 15) proceeds to the formation of the acyclic alkenyl nitrile 89 as the major product.90 The nitrile is believed to be formed by fragmentation of the protonated oxime, giving the P-silyl—stabilized cation 90, followed by desilylation. Our... 

Fig. 4. Variable demand for p-silyl stabilization m a-aryl vinyl cations A5 of C para C shift for Ar-C =C(H)R for R = HandR = Si(jPr)3...

P-Silyl stabilization in this type of benzyl cations is about as elBcient as the effect of an a-phenyl group or an a-cyclopropyl ring. This is in accord with a comparison of the C NMR chemical shifts of the para-carbon in l-/ -anisylmethyl cations An-C (H)R, with R = CH3, CH2CH3, and CH2Si(iPr)3, which are cpara 185.84 ppm, 181.93 ppm, and 178.84 ppm, respectively. 

Recently Yoshida et al. have employed silyl-stabilized a-alkoxy organolithium reagents for the synthesis of a variety of carbonyl compounds. Methoxy(trimethylsilyl)methane and methoxybis(trimethyl-silyl)methane, when deprotonated with Bu Li and Bu"Li respectively, give anions which can be alkylated with a variety of electrophiles. Electrolysis of a solution of the alkylated product in methanol yields, by virtue of the reduced oxidation potential of ethers a-substituted with silicon, either the dimethyl acetal or in the latter case the orthoester. The mildness of the electrolytic process recommends the method for the preparation of a variety of carbonyl compounds. 

Note that the preferred products have the olefin syn to the carbonyl group. If the initial adduct of the conjugate addition (the silyl-stabilized cation/Lewis... 

These results can be compared with the calculations of Hopkinson and Lien27 on CH4 and CH3SiH3 which complete the set. Although the calculations used different basis sets, both sets of results given in Table 3 are without d-orbital functions, and the trend of successive silyl stabilization of a-carbanions is well reproduced. 

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