Silyl groups, mobility

The relative ease of these transsilylations is because of the mobility of silyl groups, which is a consequence of the coordination number of silicon of 6, as discussed in the subsequent section. 

Tetracoordinate Si+ complexes may undergo transformations in the mobile ligand, since nucleophilic attack of the counterion may be directed toward electrophilic centers located in the ligand group. Some reactions involving these transformations are of importance in synthesis (86,272-274). For example, the silylation of phosphorus alkyl esters has been broadly explored by bioorganic chemists as a convenient method of generation of phosphorus acids [Eq. (57)] (273). The mechanism of the silylation reaction [Eq. (57)] has been well documented for the cases where X is... 

Treatment of the silyl enol ethers of IV-acyloxazolidinones with selected electrophiles that do not require Lewis acid activation similarly results in high induction of the same enolate face (eq 13). The facial bias of this conformationally mobile system improves with the steric bulk of the sUyl group. 

One approach to solving the problem of residual silanol interactions has involved improvements in the synthetic procedures for the production of hydrocarbonaceous stationary phases. One synthetic approach for the elimination of residual silanol groups involves the reaction of the bonded phase with a small silylating reagent such as trimethylchlorosilane which is presumed to have easier access to silanol groups than bulkier, long-chained chlorosilanes. An alternative, synthetic approach involves surface polymerization of the stationary phase, which is believed to reduce the accessibility of surface silanol groups to polar analytes in the mobile phase. Stationary phases produced by the former method are often referred to as end-capped and stationary phases produced by the latter method are sometimes called base-deactivated. ... 

Capillary Columns for SFC-MS. At present, the major limitation to broad application of capillary SFC technology is related to the availability of columns compatible with supercritical fluid mobile phases. The fused silica capillary columns used in this work were deactivated and coated with crosslinked and surface-bonded stationary phases using techniques similar to those reported by Lee and coworkers (40,41). Columns from less than 1 m to more than 20 m in length and with inner diameters of 10 to 200 ym have been examined. Colvimn deactivation was achieved by purging with a dry nitrogen flow at 350 C for several hours followed by silylation with a polymethylhydrosiloxane. Any unreacted groups on the hydro-siloxane were capped by treatment with chlorotrimethylsilane at 250 C. After deactivation, the columns were coated with approximately a 0.15-.25 ym film of SE-54 (5Z phenyl polymethylphenyl-siloxane) or other polysiloxane stationary phases. The coated stationary phases were crosslinked and bonded to the deactivation layer by extensive crosslinking with azo-t-butane (41). The importance of deactivation procedures for elution of more polar compounds, such as the trichothecenes, has been demonstrated elsewhere (42). 

On the other hand, the presence of hydroxyl groups in the tannin molecules can promote interactions by adsorption on the gel or intermolecular linkage by hydrogen bonding, which would lead to incorrect estimation of the MW. These phenomena can be minimized by the use of very polar mobile phases, such as dimethylsulfoxide (DMSO), dimethylformamide (DMF), or tetrahydrofuran (THE) [68-70]. Another procedure is to protect the hydroxyl groups as acetyl derivatives, methyl derivatives, or silyl derivatives [71]. 

---