Silyl substituent groups

If a Michael reaction uses an unsymmetrical ketone with two CH-groups of similar acidity, the enol or enolate is first prepared in pure form (p. llff.). To avoid equilibration one has to work at low temperatures. The reaction may then become slow, and it is advisable to further activate the carbon-carbon double bond. This may be achieved by the introduction of an extra electron-withdrawing silyl substituent at C-2 of an a -synthon. Treatment of the Michael adduct with base removes the silicon, and may lead as well to an aldol addition (G. Stork, 1973, 1974 B R.K. Boeckman, Jr., 1974). 

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. 

Similar results were obtained by Ogino and co-workers. After the initial cleavage of the CO from the metal, a silyl group migration occurs. It was shown by cross experiments that in this case the silyl substituent migrates and not the silylene. In a subsequent step, the silylene is then displaced by CO. Unfortunately, no trapping experiments of the silylenes in solution have been reported. 

Crystal structure analysis of 43 revealed that the radical is perfectly trigonal planar, implying the sp -hybridization of the central Si atom and, hence, the localization of the unpaired electron on its 3pz-orbital. The remarkable planarity of the radical 43 was explained by the great steric bulk of the voluminous t-Bu2MeSi groups, which prefered to move away from each other as far as possible to avoid the steric repulsion between them. On the contrary, the significant electron donation of the positive silyl substituents... 

Eliminations from cqa -orf/io-disubstituted benzenes can be carried out with various potential leaving groups. Benzylic silyl substituents can serve as the carbanion precursors. 

Allylic silanes show a pronounced tendency to react at the a-carbon in the presence of bidentate ligands.148 This regiochemistry is attributed to the preferential stabilization of cationic character by the silyl substituent. The bidentate ligands enhance the electrophilic character of the TS, and the cation stabilization of the silyl group becomes the controlling factor. 

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 ... 

As discussed in Section II. A, theoretical studies predicted that phospha-silenes with silyl substituents attached to phosphorus should have planar, trigonally coordinated silicon, with the Si—P w-bonds strengthened by the hyperconjugative influence of the silyl group.16 Recently, this was proved by a single-crystal X-ray structure determination of the derivative... 

An early - but mechanistically interesting - construction of a bicyclo[3.1.0]oxa-hexane by a domino radical cyclization was presented by Luh s group [50]. The addition of tributyl tin and AIBN to a solution of bromides 3-111 in refluxing benzene gave 3-114 as single diastereoisomers in acceptable yields via the intermediates 3-112 and 3-113 (Scheme 3.29). It is important that the cyclopropyl carbinyl radical intermediate has the correct stability and reactivity, which is achieved by the a-silyl substituent. 

Silicon and phosphorus generally form rather weak bonds, consequently in phosphorus chemistry, silyl substituents are mainly used as a leaving group. In silicon chemistry, phosphorus-based substituents are less common, though this has changed in the last two decades due to novel concepts in molecular chemistry (prominent examples are [/BuSi-P]4 [1] and R2Si=PR [2]) and in materials sciences. 

Reaction of lithiated allylbenzotriazole 452 with chloromethyltrimethylsilane yields silyl derivative 464 which can be further alkylated to give compound 465 (Scheme 76) <1999JOC1888>. Upon heating, product 465 is readily converted to diene 466 via vicinal elimination of benzotriazolyl and silyl substituents. Additions of lithiated silyl derivative 464 to carbonyl groups of aldehydes lead to alcohols 463 which readily eliminate benzotriazole and silane to furnish 2-(l-hydroxyalkyl)butadienes 466 (R1 = 1-hydroxyalkyl). 

Addition of a silyl substituent into a-position of the a-(benzotriazol-l-yl)alkyl ether brings additional possibilities. Thus, lithiation of silyl ether 770 followed by treatment with an aldehyde or ketone gives unstable P-hydroxy-a-silyl-a-(benzotriazol-l-yl)alkyl ether 771 that spontaneously eliminates silanol to give vinyl ether 772 (Scheme 121). Treatment with ZnBr2 followed by hydrolysis with a diluted acid removes both the benzotriazolyl and the methyl groups to furnish carboxylic acid 773. In this way, in a simple manner, aldehydes and ketones are converted to one-carbon homologated carboxylic acid <1996S1425>. 

The introduction of an additional silyl-substituent in the /i-position to the heteroatom causes a further decrease in the oxidation potential of heteroatom-substituted organosilanes. The introduction of an additional silyl group results in the rise of the HOMO level. Increase in the population of the favorable conformers for the electron transfer also seems to be important. 

We know that reduction potentials of arylsilanes are less negative than those of the corresponding aromatic compounds without silyl substituents (Sect. 2.2.1). The effect of silyl groups to facilitate the electron transfer to the neigbouring aromatic group is explained in terms of p interaction. For example, half wave reduction potentials of naphthylsilanes are less negative than that of... 

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