Silylation chemistries

Ellman used silyl chemistry for the direct linkage of aromatics onto the solid support by converting an aryl bromide to aryl lithium and reacting this with a silyl resin.90 It is the production of the silyl resin that is of interest in the context of this review, since an in situ Suzuki coupling was used to link the allyl silane to bromomethyl polystyrene resin (Scheme 40). 9-BBN is used to carry out the regioselective hydroboration, and this is linked to the resin with palladium catalysis in the usual way. After brief exposure of this... 

Studies of transition-metal silyl chemistry have focused mainly on the later members of the transition series. To some degree this reflects the availability of convenient preparative routes based on oxidative additions to low-valent, electron-rich complexes (e.g. equations 1 and 2). 

Harrod and colleagues88 have recently described an intriguing new entry into early metal silyl chemistry, a rather complex reaction resulting from addition of a primary silane to an alkyl complex (equation 43). X-ray crystal structures of both 14 and 15 were... 

Currently, the 5 -DMT/2 -TBDMS combination constitutes the benchmark for the synthesis of oligoribonucleotides. Although many other methods for the synthesis of RNA have been developed, none have gained the popularity of the TBDMS chemistry. Advances in the use of this silyl chemistry in both synthesis [7,8] and deprotection [7,8,61] strategies have made it an even more viable approach to the production of oligoribonucleotides. 

The silylation literature associated with organic polymer composites and chromatography is far too extensive to review here. As yet, silylation reactions are not widely employed in sol-gel processing of glasses and ceramics however, porous sol-gel-derived oxides are expected to be ideal low-temperature hosts, so the introduction of organic functionality is of much potential interest in the areas of sensors, catalysts, and optics. General reviews of silylation chemistry are found in refs. [110-112]. 

In comparison to the Si—OR bond, the Si—C bond can be considered essentially unreactive if the organic moiety is a simple unsubstituted hydrocarbon. If the organic moiety is substituted as in the case of a trialkoxysilane, the chemistry is more appropriately considered elsewhere (see Silicon COMPOUNDS, SILANES SILICON COMPOUNDS, SILYLATING AGENTS). 

Group-Transfer Polymerization. Living polymerization of acrylic monomers has been carried out using ketene silyl acetals as initiators. This chemistry can be used to make random, block, or graft copolymers of polar monomers. The following scheme demonstrates the synthesis of a methyl methacrylate—lauryl methacrylate (MMA—LMA) AB block copolymer (38). LMA is CH2=C(CH2)COO(CH2) CH2. 

Me3Si0)2S02- This is a powerful silylating reagent, but has seen little application in organic chemistry. 

Trimethylsilyl trifluoromethanesulfonate. This is an extremely powerful silylating agent, but probably is more useful for its many other applications in synthetic 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. 

Synthesis of multisubstituted furan rings using silyl protection 99CSR209. Synthetic applications of furan Diels-Alder chemistry 97T14179. Transformation of furans to N-heterocycles by aza-Achmatovicz reaction 98SL105. 

An interesting example from carbohydrate chemistry is the boron trifluoride-diethyl ether complex catalyzed nucleophilic addition of silyl enol ethers to chiral imines (from n-glyceralde-hyde or D-serinal)22. This reaction yields unsaturated y-butyrolactones with predominantly the D-arabino configuration (and almost complete Cram-type erythro selectivity). 

Early work on the chemistry of organosilyl anions/anionoids has been thoroughly reviewed (/). The most frequently employed preparative routes involve either cleavage of a disilane, when HMPA (CAUTION—CANCER SUSPECT AGENT) is normally required as solvent, or reaction of bulky silyl chlorides with lithium metal. 

Recent progress of basic and application studies in chitin chemistry was reviewed by Kurita (2001) with emphasis on the controlled modification reactions for the preparation of chitin derivatives. The reactions discussed include hydrolysis of main chain, deacetylation, acylation, M-phthaloylation, tosylation, alkylation, Schiff base formation, reductive alkylation, 0-carboxymethylation, N-carboxyalkylation, silylation, and graft copolymerization. For conducting modification reactions in a facile and controlled manner, some soluble chitin derivatives are convenient. Among soluble precursors, N-phthaloyl chitosan is particularly useful and made possible a series of regioselective and quantitative substitutions that was otherwise difficult. One of the important achievements based on this organosoluble precursor is the synthesis of nonnatural branched polysaccharides that have sugar branches at a specific site of the linear chitin or chitosan backbone [89]. 

In Section V, a general discussion of how silicon surfaces can be used to obtain monolayers is presented. The functionalization of silicon surfaces using radical chemistry is an area of intense and active investigation because of the potential for a myriad of practical applications.In order to help those readers who are not familiar with silyl radical chemistry, we discuss some general aspects of silyl radicals in Section II, together with some recent findings. 

The reactions are radical chain processes (Scheme 3) and, therefore, the initial silyl radicals are generated by some initiation. The most popular thermal initiator is azobisisobutyronitrile (AIBN), with a half-life of 1 h at 81 °C. Other azocompounds are used from time to time depending on the reaction conditions. EtsB in the presence of very small amounts of oxygen is an excellent initiator for lower temperature reactions (down to —78°C). The procedures and examples for reductive removal of functional groups by (TMSlsSiH are numerous and have recently been summarized in the book Organosilanes in Radical Chemistry. ... 

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