A-Silyl group, stabilization

It is all true. Evidence that a silyl group stabilizes the S 2 transition state comes from the reactions of the epoxides of vinyl silanes. These compounds can be made stereospecifically with one equivalent of a buffered peroxy-acid such as m-CPBA. Epoxidation is as easy as the epoxidation of simple alkenes. You will see in a moment why acid must be avoided. 

It is usually difficult to deprotonate alkyl(trimethylsilyl)methanes, Me3SiCH2R, and dialkyl(trimethylsilyl)methanes, Me3SiCH(R)R, bearing only alkyl substituents (R, R = alkyl) at the a-positions [89]. Although a silyl group stabilizes the resultant carbanion, it does not accelerate the deprotonation. 

The effects of silyl groups on the chemical behavior of the anion radicals generated by cathodic reduction is also noteworthy. It is well known that silyl groups stabilize a negative charge at the a position. Therefore, it seems to be reasonable to consider that the anion radicals of re-systems are stabilized by a-silyl substitution. The interaction of the half-filled re orbital of the anion radical with the empty low-lying orbital of the silicon (such as dx-pK interaction) results in partial electron donation from the re-system to the silicon atom which eventually stabilizes the anion radical. 

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. 

The cation radical at the heteroatom generated by one-electron oxidation is stabilized by a silyl group situated at the jS-position. The C-Si a orbital interacts with the SOMO to stabilize the cation radical when they are in the same plane. 

Due to the benzylic p-Tc resonance stabilization the C+-C,pso bond has partial double bond character and the ortho, ortho and meta, meta methine groups syn and anti to the silyl group are non-equivalent. The effect pf the a-silyl group on the positive charge in benzyl cations can be estimated by comparison of the NMR spectroscopic data of the 1 -phenyl-1 -(trimethylsilyl)ethyl cation 1 with those for the 1-phenylethy 1 cation 5 (P) and the cumyl pation 3 (15, 16, 17) (Table 1). 

A fi-silyl group stabilizes (/i-silyl effect) the bicyclo[2.2.1]heptyl cation. For example, endo-2-(4-bromophenylsulfonyloxy)-enc/o-6-trimethylsilylbicyclo[2.2.1]heptane on solvolysis in trifluo-roethanol, in the presence of 2,6-dimethylpyridine, gave predominantly bicyclo[3.1.1]hept-2-ene (norpinene, 29 86% by GC 83% overall yield of solvolysis products).75... 

The different reaction course followed by the silyl- and alkyl-substituted allenes is fully consistent with the hyperconjugation model, which predicts that a /i-silyl group stabilizes carbenium ions better than a /3-alkyl group, and with previous conclusions that a-silyl substitution in carbocations is destabilizing relative to a-methyl substitution. The larger space requirements of the trimethylsilyl groups compared with methyl groups may also contribute to the kinetic stability of 413. 

The fact that a-silyl substitution leads to a significant stabilization of carbanionic species is well-known and has been exploited in synthetic chemistry. On the other hand, silyl anions themselves are in general much more stable than their carbon analogues. The stabilization of carbanions by silyl substituents in the a position has been measured by Brauman and coworkers37. The anions were generated via nucleophilic displacement reactions (equation 2) of a silyl group with F- (see also Section III.B). 

The electron affinities of a number of a-silyl substituted silyl and carbon radicals were determined in photodetachment experiments and confirmed by data obtained from ab initio calculations. The authors conclude in this study that the stabilization a carbanion experiences through a-silyl substitution is approximately 14-20 kcalmol-1 per silyl group that of a silyl anion is approximately 6-14 kcal mol-1. The larger stabilization in the carbanionic systems is readily explained by stronger hyperconjugation of the anionic carbon center with the silyl groups as compared to that of the silyl anion with a silyl group. 

The Norrish type 1 cleavage of cyclic ketones has been found to be strongly influenced by the presence of a silyl group ji to the carbonyl group, where the acyl radical intermediate was believed to be stabilized by the fi effect184. For example, as shown in equation 41 with the ketone 349, Ci— C2 cleavage led to the diradical 350, and thence to the products 351... 

Oxiranyllithium reagents. Epoxides stabilized by a silyl group can be lithiated at the activated position (7, 45-46), but lithiation of nonstabilized epoxides results... 

Trimethylsilyl and other silyl groups stabilize a positive charge on a 3 carbon and are lost very easily. They can be thought of as very reactive protons or super protons . 

Particularly interesting results have been observed from -hydroxy-a-trimethylsilylalkyl selenides since vinylsilanes are exclusively produced under conditions where both the selenyl and the silyl moieties are removed with the hydroxy moiety. Apparently the selenyl moiety seems to stabilize a p-car-benium ion to a better extent than a silyl group. 

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