Cation, silyl-stabilized

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). 

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. 

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. 

Miscellaneous Halonium Ions. The Si-containing ring system 194 analogous to 188 has been reported by Muller and co-workers.422 The silyl cation is stabilized by intramolecular interactions with the F atoms increasing the coordination number of Si and, consequently, Si becomes considerably shielded. In the 29Si NMR spectrum, the resonance at 829 Si 77 indicate that cation 194 has only a small silicenium ion character... 

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... 

Calculations revealed that the parent structure of the silylenium ion H3Si+ is more stable than that of H3C+ by 41-73.5 kcal/mol. However AE tends to decrease strongly on replacement of the hydrogen atom, indicating that substituents, in particular it donors, are decidedly more effective in stabilizing carbenium ions than in stabilizing silylenium ions. Thus, the methyl cation is stabilized by an amino substituent by 93.8 kcal/mol but the silyl cation by only 38.3 kcal/mol (27). +... 

In common with ordinary alkynes, silylated alkynes are nucleophilic towards electrophiles. The presence of the silicon has a dramatic effect on the regioselectivity of this reaction attack occurs only at the atom directly bonded to the silicon. This must be because the intermediate cation is stabilized. 

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. 

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... 

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