Replacement of silyl groups

The desilylation of C-silylpyrazoles can be achieved selectively the replacement of silyl groups in 4-positions by hydrogen is smoothly effected by concentrated sulfuric acid. If concentrated nitric acid is used instead, 4-nitro-3(5)-TMS-pyrazole (344) can be isolated (Scheme 47)205 ... 

Replacement of silyl groups. Ethyl diloroformate added dropwise with stirring at 20° under dry N2 to 2-trimethyl-silylbenzothiazole, and heated 3 hrs. at 90° with distillation of the resulting tri-methylchlorosilane -> ethyl 2-benzo-thiazolecarboxylate. Y 90%. F. e. s. 

We suggest that the ejected thiyl radical undergoes a fast 1,2-migration of silyl group from silicon to sulfur (Reaction 85), affording a new silyl radical that either reacts with (TMSlsSiH (Reaction 86) which completes the reaction cycle, or replaces the (TMSlsSi radical in the above described reaction sequence. 

A different approach to selective O-substitution involves preferential replacement of existing groups. When tris-0-(tert-butyldimethylsilyi)-D-glucal is treated with phosphorus oxychloride-dimethylformamide complex in pyridine, the 0-6 silyl ether is selectively and efficiently replaced by a formate ester (Vilsmeier-Haack reaction).85... 

Thioacylsilanes are potential synthetic equivalents of unstable thioaldehydes, related to their well-known ease of replacing the silyl group with a proton by means of a fluoride ion at the stage of the reaction products. This synthetic equivalence was explored in a large variety of reaction products of thioacylsilanes. 

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. 

Replacement of an aromatic/heteroaromatic proton with a trialkylsilyl group can confer a variety of synthetic advantages. The silyl moiety can mask a potentially acidic proton, and it can be readily removed by electrophiles, normally resulting in a process of ipso desilylation ... 

Two significant communications indicate the considerable potential of transition metal complexes as multifunctional homogeneous catalysts in the silane field (5, 53). Here the same catalyst activates silanes toward different substrates and it is probable that all proceed via a common metal hydrido intermediate. Both Co2(CO)8 and (Ph3P)3CoHX [X = H2, N2, or (H)Si(OEt)j] catalyze 0-silylation and hydrosilylation the hydrogen on Si may be replaced by R O, R COO, R CONH, or R3SiO [e.g., Eqs. (117)-(120)], and excellent yields of silylated product result. Phenolic groups do... 

Aryltrimethylsilanes has been found to be a useful complement to direct thallation in the preparation of arylthallium(III) intermediates. The thallium(III) replaces the silyl substituent and the scope of the reaction is expanded to include some EWGs, such as trifluoromethyl. How does the silyl group function in these systems ... 

The synthesis of luminescent organoboron quinolate polymers (21) (Fig. 15) via a three-step procedure starting from a silylated polystyrene has been communicated. The synthesis was initiated by the highly selective borylation of poly (4-trimethylsilylstyrene) (PS-Si), followed by the replacement of the bromine substituents in poly(4-dibromoborylstyrene) (PS-BBr) with substituted thienyl groups (R = H, 3-hexyl, 5-hexyl). In the final step, the 8-hydroxyquinolato moiety was introduced. The hexyl-substituted polymers efficiently emitted light at 513-514nm upon excitation at 395 nm.40... 

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