Sterically hindered silanes

Another approach to improve the stability of bonded phases at acidic pH is the nse of sterically hindered silanes, which use, for example, isopropyl groups as side chains, instead of methyl groups. Due to the somewhat lower surface coverage, this technique leaves a larger amount of silanols on the snrface than a high-qnality bonding with a dimethylsi-lane, but this is balanced against the definite improvement in hydrolytic stability at acidic pH. 

KMn04, oxidation of sterically hindered silanes by, 42 152-153 K5[Nb(CN)81, 40 247 Knight shift, 4 288-289 Knudsen cell, and enthalpies by mass spectrometry, 24 17-19... 

Reduction of aldehydes or ketones.1 This reduction can be effected with hydrosilanes, particularly sterically hindered silanes such as (C6H5)2SiH2 and... 

Summary Treatment of the sterically hindered silanes TsiSiRR H (where RR = H2 or PhMe and Tsi = (MesSilaC) with the hydride abstraction agent Ph3CB(C6Fs)4 affords cationic species TsiSiRR" which are in equilibrium with bridged cationic species. Bridged cations in which Ph or H bridge between 1,3-silicon atoms have been isolated and characterized by X-ray crystallography. 

Cyclic tetrasiloxane with alternating ABAB linkage can be synthesized by the reaction of silanediol with dichlorosilane in moderate to good yield (8—10). Because of its thermal stability, more steric hindered silane should be employed as silanediol counterpart. 

Previous investigations of the photolysis reactions of 5 in the presence of various silanes only showed the products of Si-H activation [20], Metal silyl hydride formation, however, becomes the main reaction path with sterically less hindered silanes [21]. 

Sander et al. [63] investigated the effect of microparticulate silica pore size on the properties of solution-polymerized Cig stationary phases and observed both an increase in bonding density and shape recognition for wider pore (>120 A) silica. A size-exclusion mechanism was proposed, in which the reaction of the silane polymer on the surface is enhanced for wide pores and reduced for narrow pores. Polymeric Ci8 phases prepared on substrates with narrow pores exhibited monomeric-like chromatographic properties. This effect may be the result of an increase in competitive surface linkage with the less sterically hindered monomers that coexist with the bulkier oligomers that have polymerized in the reaction solution (Figure 5.13). 

The reaction of phenylethynyllithium with BTSP affords almost exclusively silane 29, while the reactions with sterically hindered tri(n-propyl)silyl and tri(n-butyl)silyl peroxides afford mixtures of silanes 29 and silyl ethers 30 (equation 47). 

Less reactive silyl metal species such as lithium bis(triphenylsilyl) cuprate react with a variety of acyl chlorides to give the corresponding acyl silanes in moderate to good yields75. Dilithium bis(trimethylsilyl) cyanocuprate is particularly effective for the preparation of sterically hindered acyl silanes (Scheme 8), and appears to provide a good general... 

Tris(trimethylsilyl)silyllithium, which is readily formed upon treatment of tetrakis(trimethylsilyl)silane with methyllithium or triphenylsilyllithium in ether-tetrahydrofuran (63), is useful as the reagent for construction of molecules of an interesting class of highly sterically hindered methyl-polysilanes. The synthesis of hexakis(trimethylsilyl)disilane gives an example (59). 

Concluding, the effect of the substrate structure on chemisorption is controlled by three parameters the number of surface silanols, the presence of strained siloxanes and the average pore size. Chemical bonding in dry conditions is localized on the surface silanols. Only hydroxyl-specific adsorbed silane molecules are chemically bonded during curing. On dehydroxylated silica a anomalous high level of physisorption is found, probably due to interaction with strained siloxanes. In pores smaller than 4.0 nm, the reaction with APTS is sterically hindered. 

The first type of cruciform or cross-like material that we present is based on the so-called tetrahedral approach . The motivation for constructing it-conjugated materials using such tetrahedral cores (e.g. tetraarylmethane or -silane building blocks) stems from the perception that a bulky, sterically hindered architecture should be less prone to self-aggregation in the solid state and hence should lead to more efficient and morphologically stable photo- and electroluminescence. 

Only one case concerning the synthesis of a miktoarm star quaterpolymer has appeared in the literature. It consists of four different branches, namely PS, po-ly(4-methyl styrene) (P4MeS), PI and PB [35]. The reaction sequence for the preparation of this miktoarm star is presented in Scheme 23. The procedure was similar to the one adopted for the synthesis of the ABC-type terpolymers by the chlorosilane method. The characteristic of this method is that two of the arms were incorporated to the linking agent by titration. Consequently the order of addition plays an important role for the preparation of well defined products. PS was chosen to react first with an excess of SiCl4, followed after the evaporation of the excess silane, by the titration with the more sterically hindered P4MeS so that only one arm can be incorporated in the star. The moderately hindered PILi anion was then added by titration, followed by the addition of the fourth arm, which is the least sterically hindered PBLi anion so that complete linking can be achieved. The reaction sequence was monitored by SEC and these results in combination with the molecular and spectroscopic characterization data showed that well defined quaterpolymers were prepared. 

Although partial replacement of the SiH occurs in the more sterically hindered combinations, it is not possible to achieve partial substitution with the less hindered silanes, even using substoichiometric amounts of alcohol and low temperatures. The authors suggested that rapid redistribution of the hydroalkoxysilane intermediates may account for this behavior. This suggestion seems reasonable, since we have observed that the rates of dimethyltitanocene-catalyzed redistributions are strongly dependent on ste-ric encumbrance on the silicon.143... 

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