Silanes resolution

Eigure 3.3 shows the calibration plots for Zorbax PSM columns. (The calibration plots for silanized and unsilanized columns are comparable.) These calibration plots allow the chromatographer to select the appropriate columns for samples. Eor example, the Zorbax PSM 60 column provides resolution of... 

The resolution of these columns for protein mixtures, however, was comparably poor. The peak capacity for human serum albumin was near 3 during 20 min gradient elution. Improvement has been reached by covalent binding of PEI (M = 400-600) onto a 330 A silica of 5 pm particle size [38], The peak capacities of ovalbumin and 2a -arid glycoprotein were 30-40 (tgradienl = 20 min). Enhanced peak capacity and resolution probably were due to the more diffuse structure of PEI coupled to silane moieties than that of strictly adsorbed on silica and cross-linked (see Sect, 2.2). Other applications of covalently adsorbed PEI are discussed in Sect. 4.1. 

Silane and hydrogen show relaxation patterns with the same characteristic time t, however, inverse signs. The fragmentation of silane induced by collisions with electrons, yields molecular hydrogen in an order of magnitude faster than the time resolution of the mass spectrometry setup, i. e. faster than 1 ms. Two possible pathways of silane fragmentation can be regarded ... 

Recently, Schaumann et al. 153,154 an(j Bienz et tf/.155,156 have developed dependable routes for the resolution of racemic functionalized organosilanes with Si-centered chirality using chiral auxiliaries, such as binaphthol (BINOL), 2-aminobutanol, and phenylethane-l,2-diol (Scheme 2). For instance, the successive reaction of BINOL with butyllithium and the chiral triorganochlorosilanes RPhMeSiCl (R = /-Pr, -Bu, /-Bu) affords the BINOL monosilyl ethers 9-11, which can be resolved into the pure enantiomers (A)-9-ll and (7 )-9-11, respectively. Reduction with LiAlFF produces the enantiomerically pure triorgano-H-silanes (A)- and (R)-RPhMeSiH (12, R = /-Pr 13, -Bu 14, /-Bu), respectively (Scheme 2). Tamao et al. have used chiral amines to prepare optically active organosilanes.157... 

Fig. 23 P6 of 28SiH4 recorded on a grating spectrometer. Trace (a) is from a continuous scan of natural-abundance silane at a resolution of 0.020 cm-1. Trace (b) is the same region recorded separately at a resolution of 0.012 cm-1 and is the average of 12 scans.

The purpose of the experiments described below was to study the effect of the pretreatment of the substrate and of the pH of the silane solution on the structure of the film. Only thin films, as described in the previous section for SAAPS on zinc, were prepared and analyzed. Heating experiments were not performed, but will be reported in a forthcoming paper. All spectra were run under conditions of high mass resolution. 

Static SIMS, especially high-mass-resolution TOFSIMS, is a powerful tool for the study of silane films. The accurate mass determination capability of high-mass-resolution SIMS analysis of organic materials has demonstrated that many peaks from organic materials consist of two or more components. Hence ion identification in SIMS with low mass resolution may be ambiguous. 

Surface element concentrations measured at three photoelectron exit angles for water-sized and silane-treated E glass fibers are shown in Table 1. Angular resolved components of high resolution N Is and Si 2p photo peaks are displayed in Tables 2 and 3, respectively. The photoelectron exit angles correspond to penetration depths of approximately 1 nm at 15° to 5 nm at 90° [7]. Aminosilane-treated fibers were analyzed before and after ramping to 300°C in a programmed thermal desorption experiment. 

FIGURE 2. 2D-INADEQUATE spectrum of (Si,Vk j SiMc ) (1) 900 mg silane in 2 ml (Y.I ), containing 30 mg Cr(acac)3 - JSi —1II couplings refocused 256 rows recorded, each 96 FIDs accumulated 4 x 4 K data matrix, spectral width in Fl-dimension is 6000 Hz (2.9 Hz digital resolution) mild gaus-sian multiplication in both dimensions 24 h performance time. Reproduced by permission of Elsevier Science from Reference 11... 

The silanethiyl radical (7) generated from the silane thiol, which is prepared from the silane using first CuCl2 and then LiSH, abstracts hydrogen enantioselectively from silicon in ( )-fraws-2,5-dimethyl-l-phenyl-l-silacyclopentane, to bring about kinetic resolution of the latter. Such reactions are rare (equation 17)28. 

Stereoselective enzymatic hydrolyses of esters represent a further type of biotransformation that has been used for the synthesis of optically active organosilicon compounds. The first example of this particular type of bioconversion is illustrated in Scheme 15. Starting from the racemic (l-acetoxyethyl)silane rac-11, the optically active (l-hydroxyethyl)silane (5)-41 was obtained by a kinetic racemate resolution using porcine liver esterase (PLE E.C. 3.1.1.1) as the biocatalyst7. The silane (5)-41 (isolated with an enantiomeric purity of 60% ee bioconversion not optimized) is the antipode of compound (R)-41 which was obtained by an enantioselective microbial reduction of the acetylsilane 40 (see Scheme 8). 

Enantioselective enzymatic amide hydrolyses can also be applied for the preparation of optically active organosilicon compounds. The first example of this is the kinetic resolution of the racemic [l-(phenylacetamido)ethyl] silane rac-84 using immobilized penicillin G acylase (PGA E.C. 3.5.1.11) from Escherichia coli as the biocatalyst (Scheme 18)69. (R)-selective hydrolysis of rac-84 yielded the corresponding (l-aminoethyl)silane (R)-85 which was obtained on a preparative scale in 40% yield (relative to rac-84). The enantiomeric purity of the biotransformation product was 92% ee. This method has not yet been used for the synthesis of optically active silicon compounds with the silicon atom as the center of chirality. 

Enzymatic kinetic resolutions via enantioselective esterifications have been successfully used for the preparation of optically active (hydroxymethyl)silanes, with the silicon atom... 

Based on these results, attempts have been made to apply enantioselective dehydrogenations with horse liver alcohol dehydrogenase HLADH, E.C. 1.1.1.1), in the presence of NAD+, for kinetic resolution of the isomeric (hydroxyalkyl)silanes rac-105, rac- 107... 

Due to the hydrolytic decomposition of (l-acylalkyl)silanes, enzymatic kinetic resolution of racemic (2-hydroxyalkyl)silanes, catalyzed by HLADH, can only be used for the... 

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