Silyl groups chemistry

The ability to promote /S elimination and the electron-donor capacity of the /3-metalloid substituents can be exploited in a very useful way in synthetic chemistry. Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting intermediates then undergo elimination of the stannyl or silyl substituent, so that the net effect is replacement of the stannyl or silyl group by the electrophile. An example is the replacement of a trimethylsilyl substituent by an acetyl group by reaction with acetyl chloride. 

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... 

Catalytic asymmetric hydrosilylation of prochiral olefins has become an interesting area in synthetic organic chemistry since the first successful conversion of alkyl-substituted terminal olefins to optically active secondary alcohols (>94% ee) by palladium-catalyzed asymmetric hydrosilylation in the presence of chiral monodentate phosphine ligand (MOP, 20). The introduced silyl group can be converted to alcohol via oxidative cleavage of the carbon-silicon bond (Scheme 8-8).27... 

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. 

Silyl ethers, in silicone chemistry, 22 554 Silyl group, in silylation, 22 691 Silyl-terminated polyurethanes (SPU), 22 39-40... 

Syntheses of diastereomerically pure racemates of himachalene derivatives started from cycloheptanone G (Fig. 9). The sequence to I involved dimethyla-tion to yield H followed by bromination/dehydrobromination and conjugate methylation using cuprate chemistry. The sequence furnishing L and M follows a Robinson-annelation type Reaction of I with 3-(trimethylsilyl)but-3-en-2-one yielded K. Refluxing K with potassium hydroxide in ethanol removed the silyl group and cyclized the diketone to form a 97 3 mixture of racemic L and M. Occurring as a volatile in A.flava, L served as a versatile intermediate in the syntheses of other Aphthona compounds. 

Among the various synthetic procedures for polysilanes is the Harrod-type dehydrogenative coupling of RSiH3 in the presence of Group 4 metallocenes (Reaction 8.1) [5,6]. One of the characteristics of the product obtained by this procedure is the presence of Si—H moieties, hence the name poly(hydrosilane)s. Since the bond dissociation enthalpy of Si—H is relatively weak when silyl groups are attached at the silicon atom (see Chapter 2), poly(hydrosilane)s are expected to exhibit rich radical-based chemistry. In the following sections, we have collected and discussed the available data in this area. 

Silicon protection is also commonly used to direct lithiation chemistry in five-membered heterocycles. For example, oxazoles , thiazoles and Ai-alkylimidazoles ° ° lithiate preferentially at C-2, where the inductive effect of the heteroatoms is greatest. If C-2 is blocked, lithiation occurs at C-5, where there is no adjacent lone pair to destabilize the organolithium. Functionalization of these heterocycles at C-5 can therefore be achieved by first silylating C-2, reacting at C-5 and then removing the silyl group. The synthesis of 666 illustrates this sort of sequence (Scheme 258) °. ... 

While the chemistry of the cyclodiphosph(III)azanes is now well founded, few compounds are known in which the P—N skeleton is extended exo to the PjNj-ring. Such compounds would provide examples of multidentate ligands, and present interesting comparisons with trimethylsilylamino-derivatives, where the bulky silyl-groups are known to kinetically stabilise phosph(III)azenes such as (MCjSO NP ... 

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 protection of amines with a silyl group is of wide use not only in organic synthesis but also in chromatographic analysis. Deprotection generally occurs in acidic medium. This has been applied to the chemistry of SMAs.122,177,179,180,186,187,189,191... 

In the chemistry of (3-lactams, the silyl group at the 3-position on the ring, has been used as a good precursor for the introduction of an alkylidene group through a Peterson-type reaction. Protodesilylation of this position is also possible.122... 

A reaction that is related to the Baeyer-Villiger reaction is that associated with the use of a silyl group as a masked hydroxy moiety. This approach allows for the wealth of organosilicon chemistry to be used in a synthesis and provides protection for the hydroxy group.218-219 An example of this... 

To make silicon and germanium chemistry at least somewhat more like carbon we can, for example, use silyl substituents. To wit, H3SiSiH has a reported DSSE of 12.9 kcal/mol, compared to 19.3 for SiH2. The DSSE of HjSiSiH is not negative like CH2, but at least it is smaller than in SiH2. Silyl groups are also known to make amines planar, and this presumably extends to silyl radicals as well. Furthermore, disilylsilylene, (H3Si)2Si, has... 

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