Silyl group size, effect

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... 

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. 

The size of the silyl groups in alkynylsilanes 48 effects their regio- and chemose-lectivity in hydroboration with 9-borabicyclo[3.3.1]nonane125 (9-BBN). Alkynylsilanes... 

The steric effect resulting from trisubstituted silyl chlorides influences their capability in silylation of alkali phenolates65. As the size of the silyl groups increases with the trend shown in entry 53 of Table 1, the activity of the silyl chlorides decreases. 

Triorganosilyl groups can control stereochemistry in organic reactions through a steric, an electronic or a stereoelectronic effect. More than one of these effects may exist simultaneously in some chemical processes. The trend listed in Section II could provide chemists with a clear guideline, yet definitive order, to choose a silyl group with appropriate size in control of reactions based on the steric effect. 

Frey J, Schottland E, Rappoport Z, Bravo-Zhivotovskii D, Nakash M, Botoshansky M, Kafloiy M, Apeloig Y (1994) The effective size of the tris(trimethylsilyl)silyl group in several molecular environments. J Chem Soc, Perkin Trans 2, 2555-2562... 

The use of silyl ethers also provided a good glimpse into the steric effects of the C-H insertion [98], Relative rates were obtained for insertion a to the oxygen atom in silyl protected n-butanol. It was found that the reaction rate increased dramatically as the size of the silyl protecting group decreased, with a 100-fold rate difference between TBDPS and TMS. Complementary steric and electronic effects were observed with the tetralkoxy silane substrates [97,98], hi competition experiments, it was found that the carbenoid derived from diazo ester 99 reacted solely with tetraethoxy silane 123 to form product 126, and not the corresponding tetramethoxy or tetraisopropoxy derivatives 124 or 125 (Scheme 28). Thus, the secondary C-H bonds appear to possess the right balance between steric and electronic requirements for the insertion. 

In this chapter, we have tried to emphasize general aspects of main-group chemical bonding, with particular emphasis on periodic trends. The periodic table remains the most important classification tool in chemistry, and it is crucial to understand even subtle secondary periodicities if one is to make efficient use of the various elements for different chemical applications. The radial nodal structure of the valence orbitals has been pointed out to account for more of the known trends than most practitioners of chemistry are aware of. For example, the inversion barriers of phosphines or silyl anions, the dependence of the inert-pair effect on the electronegativity of the substituents, the stability of carbene- or carbyne-type species or of multiple bonds between heavy main-group elements are aU intricately linked to hybridization defects of s- and p-valence orbitals of disparate sizes. Even the question of hypervalency is closely connected to the effects of primogenic repulsion . 

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