Trialkyl silyl groups

A variation of GTP, referred to as aldol GTP, involves polymerization of a silyl vinyl ether initiated by an aldehyde [Sogah and Webster, 1986 Webster, 1987]. Both initiation and propagation involve nucleophilic addition of the vinyl ether to the aldehyde carbonyl group with transfer of the trialkyl silyl group from vinyl ether to the carbonyl oxygen (Eq. 5-82). The reaction has similar characteristics as GTP. The product is a silated poly(vinyl alcohol) (PVA), which can be hydrolyzed by acid to PVA (Eq. 5-83). 

The asymmetric total syntheses of mtamycin B and oligomycin C was accomplished by J.S. Panek et al. In the synthesis of the C3-C17 subunit, they utilized a Mukaiyama aldol reaction to establish the C12-C13 stereocenters. During their studies, they surveyed a variety of Lewis acids and examined different trialkyl silyl groups in the silyl enol ether component. They found that the use of BFs OEta and the sterically bulky TBS group was ideal with respect to the level of diastereoselectivity. The stereochemical outcome was rationalized by the open transition state model, where the orientation of the reacting species was anti to each other, and the absolute stereochemistry was determined by the chiral aldehyde leading to the anti diastereomeric Felkin aldol product. 

Hydrolysis of the trialkyl silyl groups gives poly(vinyl alcohol). Block copolymers in which one block is poly( vinyl alcohol) can be synthesized if a telechehc with an aldehyde terminal group is used to initiate the aldol GTP structures such as polyfsty-rene-h/oc -vinyl alcohol) can be prepared in this way. Alternatively, as silyl ketene acetals can react with aldehydes, block structures can be formed by a coupling process. 

The presence of any residual trialkyl tin groups in the final matrix is not desirable in the context of catalysis. Tin is a catalytically active metal and thus could compHcate and potentially interfere with reactions of other active metals in the matrix. Fortunately we found that all residual tin groups in these matrices can be replaced with tiimethyl-silyl (TMS) groups simply by exposure to TMSCl in the gas phase. This observation also indicates that none of these sites becomes inaccessible to further reaction when crosslinking in the matrix is appropriately adjusted. 

Fluorous trialkyl silyl protecting groups have also been used to simplify the purification of complex reaction mixtures [5] (Scheme 3.16). Separation of the reaction products can be achieved by means of a simple three-phase extraction (aqueous/or-ganic/fluorous) instead of the usual chromatography. In this respect, the concept of using fluorous protecting groups has some parallels with the solid-phase-sup-ported chemistry which also was primarily developed to simplify multiple workup operations. 

However, with aryl(trialkyl)silanes, sometimes for electronic reasons, the reaction does not take place. In order to reduce the electron density on the silicon atom, alkyl groups were replaced by fluorine. The fluorine effect could be explained as follows i) the van der Waals radius of fluorine is roughly comparable to that of hydrogen and hence the fluorine-substituted silyl group is not so bulky than the trimethylsilyl one ii) electronegativity of fluorine favours the formation of the reactive pentacoordinated silicate, enhances the Lewis acidity of the silicate and... 

Similar considerations apply to reactions between tri-l-alkyl alanes and other 1-alkenes. Vinyl and allyl triorganosilanes may also be dimerized in this way. Dimerization of the allyl derivative proceeds normally, i.e., the aluminum atom adds on to the terminal carbon atom of the allyl group, but a trialkyl or triaryl silyl group directly adjacent to the C=C double bond appears to cause reversal of the direction of addition of the A1—C bond (119) ... 

A number of authors have attempted to increase the permeability of PPO by introducing bulky groups like trialkyl-silyl, hydroxyethylene or... 

Asymmetric membranes from PPO with trialkyl-silyl, hydroxyethylene and ehtyleneoxytrialkyl -silyl functional groups... 

A series of ligand displacement reactions has been investigated in which a phosphine replaces a carbonyl group, the ease of reaction depending on both M and the silyl residue. Triphenyl or trialkyl phosphine can remove only one CO molecule in Mn, Re, Co, and Fe complexes, even under forced conditions,... 

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