Silicon effect

Acylsilanes are a class of compounds in which a silyl group is directly bound to the carbonyl carbon, and they have received considerable research interest from the point of view of both physical organic and synthetic organic chemistry [15]. Acylsilanes have a structure quite similar to the structure of a-silyl-substituted ethers a silyl group is attached to the carbon adjacent to the oxygen atom, although the nature of the C-O bond is different. Therefore, one can expect /1-silicon effects in the electron-transfer reactions of acylsilanes. 

Grayson and Whitham have found the regiochemistry of addition of ISCN (from LiSCN and I2) to vinyl silanes to be consistent with the intermediacy of an iodonium ion which is particularly stabilized at one of the carbons due to the -silicon effect.129 The products were a thiocyanate in one case and an isothiocyanate in the other (Scheme 65).129... 

From these data the dependence of the /1-silicon effect on the dihedral angle can be represented graphically as shown in Figure 2, and resembles a cosine-squared curve with a highly flattened left side. 

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. 

Since then the kinetic a-silicon effect has been measured in solvolysis of various systems that are considered to involve ionization in the rate-determining step. The results are compiled in Table 2, which also includes for comparison data for the a-silicon effect... 

The effect of a-silyl substitution on the stability of a carbenium ion was qualitatively unclear for a long time. Early solvolytic studies by the groups of liaborn36 and Cartledge37 suggest a destabilizing effect of a-silyl substitution compared with alkyl. The measurement and interpretation of the kinetic a-silicon effect in solvolysis reactions is, however, often complicated by the fact that steric and ground state effects may play an important role and that, in addition, the rates of ionization often involve a contribution from nucleophilic solvent assistance. 

FIGURE 7. Qualitative dihedral dependences for the inductive, vertical and nonvertical modes of the kinetic -silicon effect. Reprinted with permission from Reference 64. Copyright (1990) American Chemical Society. Experimental data64,66,67 are indicated with solid dots... 

Kresge and Tobin80 investigated the /1-silicon effect on the hydrolysis of vinyl ethers (equation 29) and found a rate acceleration on the hydrolysis of 175 compared with 176, and hence a stabilizing effect of the /1-silyl group on the intermediate -ethoxy carbocation 177 compared with 178. The acceleration is small the rate factor (175) (176) of 129 is equivalent to a free energy of activation difference AAG of 2.9 kcalmol-1,... 

The small /J-silicon effect found by DeLucca and Paquette82 in the solvolysis of the cyclopropylmethyl trifluoroacetate 179 (equation 30) might be a result of the inherent stability of the cationic intermediate 180 and its delocalized structure. The 27-fold rate... 

However, it must be taken into account that the a-phenylvinyl cation 185 is already highly stabilized by the phenyl substituent, leading consequently to a smaller -silicon effect in the vinyl cation 183. Ab initio calculations by Buzek predicted for 184 an additional stabilization of 10 kcalmol-1 by the silyl group7. The thermodynamic stabilization of 183 compared with 185, experimentally determined by Stone and coworkers in the gas phase, is 9 kcalmol 121. Thus, the kinetically determined stabilization of the transition state is only about 6 kcalmol-1 smaller than the /J-silyl effect for stabilization of the ground state carbocation. 

The overall kinetic effect of the disilanyl group is, however, small due to the different solvolysis mechanisms. 224 solvolyses only 3.48 times faster than 226 in TFE at 100 °C. For the k process Shimizu and coworkers calculated the relative rates for 224 225 226 to be 1.60 1.0 10.7. They concluded that these small silicon effects reflect the concerted nature of the k solvolysis involving transition states with a highly delocalized charge distribution88. 

In the adamantyl framework the /-silicon effect was studied by Grob and coworkers96,97. The first-y-trimethylsilyl substituent (cf 245) enhances the solvolysis rate of 1-bromoadamantane 246 by a factor of only 8.6 and the second trimethylsilyl group in... 

In the 2-exo isomer 260 the measured rate enhancement of 23 000 compared with 250 indicates a large /-silicon effect. The intermediate 261 undergoes a fast 2,6 migration and Wagner-Meerwein shifts to give the products via ion 262 (equation 41)100. 

Anodic oxidation of aliphatic aldehydes and ketones is generally difficult because their oxidation potentials are very high (>2.5 V). However, silylation at the carbonyl carbon causes a marked decrease in the oxidation potential as shown in Table 933,40. This silicon effect is much smaller in the case of aromatic carbonyl analogues40. The silicon effect is attributed to the rise of the HOMO level by the interaction between the C—Si a orbital and the nonbonding p orbital of the carbonyl oxygen, which in turn favors the electron transfer. 

The photoionization of benzylic alcohols leading to carbocations has recently been reported197. Taking advantage of the stabilization provided by the /J-silicon effect, flash photolysis of 380 in trifluoroethanol gave rise to the fluorenyl cation 381 which was characterized spectroscopically (equation 46). 

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