Silylation steric effect

The DTBS group is probably the most useful of the bifunctional silyl ethers. Dimeihylsilyl and diisopropylsilyl derivatives of diols are very susceptible to hydrolysis even in water and therefore are of limited use, unless other structurally imposed steric effects provide additional stabilization. 

Enantioselectivity can also be based on structural features present in the reactants. A silyl substituent has been used to control stereochemistry in both cyclic and acyclic systems. The silyl substituent can then be removed by TBAF.326 As with enolate alkylation (see p. 32), the steric effect of the silyl substituent directs the approach of the acceptor to the opposite face. 

A number of steric effects on the rate of rearrangement have been observed and can be accommodated by the chairlike TS model.242 The Zf-silyl ketene acetals... 

SENAs derived from secondary AN are not involved in catalytic C,C-coupling reactions with silyl ketene acetals. This is possibly due to a decrease in both the effective concentration of the cationic intermediate (the steric effect) and its lower level of electrophilicity (see the lower entry in Table 3.23). 

The reaction of aryldiazoacetates with cyclohexene is a good example of the influence of steric effects on the chemistry of the donor/acceptor-substituted rhodium carbenoids. The Rh2(reaction with cyclohexene resulted in the formation of a mixture of the cyclopropane and the G-H insertion products. The enantios-electivity of the C-H insertion was high but the diastereoselectivity was very low (Equation (31)). 0 In contrast, the introduction of a silyl group on the cyclohexene, as in 15, totally blocked the cyclopropanation, and, furthermore, added sufficient size differentiation between the two substituents at the methylene site to make the reaction to form 16 proceed with high diastereoselectivity (Equation (32)).90 The allylic C-H insertion is applicable to a wide array of cyclic and acyclic substrates, and even systems capable of achieving high levels of kinetic resolution are known.90... 

It is thought that a large fraction of the allenylsilane product is formed from the solvent-separated ion pair B. The expectedly high reactivity of this species would minimize steric effects associated with the silylation, thus explaining the lack of selectivity observed for Me3Si versus iPrMe2Si in the competition experiment. 

Scheme 19)." Homoallyloxysilanes gave a mixture of five- and six-membered rings, but the intermediate silyl radical underwent predominantly 6-endo cyclization. Pentenyloxysilane gave the 1-endo product only. The stereochemistry of these reactions was found to be determined by steric effects, even in the presence of chiral thiol catalysts. The structures of the radical intermediates were studied by EPR. 

Ion-radical organic reactions of the Sj j l type are less sterically restricted than classical Sj reactions. Generally, the nucleophilic (not Sj j ) reactivity varies with the steric demand at the reaction center. The electron-transfer reactivity does not depend on steric effects. To illustrate this, one can compare electron transfer and nucleophilic reactivity between ketene silyl acetals and cationic electrophiles (Fukuzumi et al. 2001). Nevertheless, space strains may determine the overall results of these reactions if either intermediate radicals or forming products are sterically hindered. 

Library of Congress Cataloging in Publication Data. Main entry under title Organic chemistry, syntheses and reactivity. (Topics in current chemistry 88) Contents Riichardt, C. Steric effects in free radical chemistry. -Birkhofer, L. and Stuhi, O. Silylated synthons. [etc.] 1. Chemistry Organic-Addresses, essays, lectures. 

Finally, steric effects have an important influence on regioselectivity. This is very clearly demonstrated, for example, in the case of allyl carbanions substituted with silyl groups16. Therefore, despite wide investigation of this topic17, understanding of regiocontrol is still very poor, due to the complexity of the situation. 

Surprisingly the benzocyclobutene 6 is stable at 175°, and does not decompose to a benzhydrindane even at higher temperatures, possibly owing to steric effects of the silyl group adjacent to the four-membered ring. 

The influence of both the steric and electronic properties of the silyl group on the rate of epoxidation have been examined experimentally [104], Two rate effects were considered. First, the overall rate of epoxidation of the silyl allylic alcohols was found to be one-fifth to one-sixth that of the similar carbon analogs. This rate difference was attributed to electronic differences between the silicon and carbon substituents. Second, the increase in k[el to 700 for silyl allylic alcohols compared with carbon analogs (e.g., 104 for entry 3, Table 6A.8) was attributed to the steric effect of the large trimethylsilyl group. As expected, when abulky (-butyl group was placed at C-3, k[e] increased to 300 [104],... 

A stereoselective Mukaiyama-type aldol reaction of bis(trimethylsilyl)ketene acetals produces silyl aldols with syn stereoselectivity, predominantly due to steric effects.23... 

Different silyl groups in substrates exert steric influence on organic reactions in various degrees. Evidence for this exists in the following 29 types of important organic reactions, which can be controlled by bulky silyl groups. Their steric effect often dominates the outcome of those reactions. Nevertheless, the electronic or the stereoelectronic effect may simultaneously also play a minor role. 

The size of silyl groups influences the carbon-carbon bond formation between sily-lalkenes and ketones. Treatment of allylsilanes 12 or vinylsilanes 14 with ketones in the presence of MnC>2 and acetic acid at elevated temperature gives a-alkylation products 13 and 15, respectively (equations 9 and 10)18. The steric effect resulting from the silyl groups plays an essential role on the exclusive C—C bond formation at the terminal sp2 carbon of silylalkenes 12 and 14. The yield of the alkylation is inverse to the size of silyl groups and follows the order listed in entry 7 of Table 1. 

Data listed in Table 2 include the substituent constants R1 of trialkylchlorosilanes and the relative rate constants fc(R1Me2SiCl)/A (Me3SiCl) for the reactions of the two chlorides with lithium silanolates and isopropylate (equation 39)57. The reaction rates of silanes are influenced almost exclusively by the steric effects of the alkyl groups attached to the silicon atom. The log(A rei) values of the compounds with various R1 groups give a satisfactory correlation with Taft s Es values151. Thus the steric hindrance of silyl groups follows the order listed in entry 4457 of Table 1. 

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