Silicon substitution

The assay of ethyleneamines is usually done by gas chromatography. Compared to packed columns, in which severe tailing is often encountered due to the high polarity of the ethyleneamines, capillary columns provide better component separation and quantification. Typically, amines can be analyzed using fused siUca capillary columns with dimethyl silicones, substituted dimethyl silicones or PEG Compound 20 M as the stationary phase (150). 

Silicon-substituted derivatives may require the use of lithio or sodio reagents, e.g. ... 

Another method, starting from the 4-amino-l-azabutadiene system, e.g. 19, involves initial reaction with dichlorodiphenylsilane to give 1,3,2-diazasilincs, which can normally be isolated. Further reaction with dimethyl acetylenedicarboxylate provides the silicon-substituted 1,5-di-azocines.40... 

In the presence of catalytic amounts of Pd(0), silicon-substituted vinyloxiranes can rearrange into the corresponding ot-silyl- 3,y-unsaturated aldehydes (Scheme 9.34) [151]. Treatment of 80 with Pd(OAc)2 and P(OPh)3 results in the formation of 7t-allylpalladium complex 81. Bond rotation to give 82, followed by migration of the silyl moiety, affords aldehyde 83, which is trapped in situ to provide the Felkin-Anh product 84. The reaction proceeds with retention of configuration and the ee of the starting material is retained in the product. The size of the silicon substituents is critical for the outcome of the reaction, as is the choice of ligands on palladium. 

Hydrolysis of diphenyl phosphorochloridate (DPPC) in 2.0 M aqueous sodium carbonate is also believed to be a two-phase process. DPPC is quite insoluble in water and forms an insoluble second phase at the concentration employed (i.e. 0.10 M). It seems highly significant that the hydrophobic silicon-substituted pyridine 1-oxides (4,6,7) are much more effective catalysts than hydrophilic 8 and 9. In fact, 4 is clearly the most effective catalyst we have examined for this reaction (ti/2 < 10 min). Since derivatives of phosphoric acids are known to undergo substitution reactions via nucleophilic addition-elimination sequences 1201 (Equation 5), we believe that the initial step in hydrolysis of DPPC occurs in the organic phase. Moreover, the... 

The silicon substituted nitrogen atom is the more appropriate ligand for metals... 

Vallet-Regi, M. and Arcos D. (2005) Silicon substituted hydroxyapatites. A method to upgrade calcium phosphate based implants. Journal of Materials Chemistry, 15, 1509—1516. 

Pinkerton and Thames have carried out a new, more extensive study of pd-rc-bonding in silicon substituted furans and have slightly modified the earlier, Soviet conclusions. The aromatic protons indicate by their chemical shifts at infinite dilution that there are simultaneous but opposing +1 and... 

Several reports have been made of a successful catalyzed addition/ substitution reaction resulting in direct attachment of phosphorus to aromatic rings. The preparation of mixed triarylphosphines has been accomplished by the reaction of tin- or silicon-substituted diphe-nylphosphines with aryl halides catalyzed by palladium reagents.74 A similar transformation has also been reported using nickel catalysis.75 The addition/substitution of diphenylphosphine to triflate functionalized phenolic linkages has been of use for the preparation of substances as analogues of tyrosine-related amino acid derivatives, accomplished with catalysis by palladium acetate (Equation 4.29).76... 

Table 4. Oxidation potentials of silicon-substituted nitrogen, phosphorous, and sulfur compounds [35]...

S. Hoger, J. McNamara, S. Schricker, and F. Wudl, Novel silicon-substituted, soluble poly(phe-nylene vinylene)s enlargement of the semiconductor bandgap, Chem. Mater., 6 171-173, 1994. 

Silicon substitution often increases the rates of reactions which proceed quite slowly with the unsubstituted compound. 

Organosilicon compounds are widely used in organic synthesis. The understanding of the structure and properties of the intermediates involved in their reactions is a prerequisite for further development and optimization of useful synthetic transformations involving silicon substituted compounds. Trialky lsilyl-substituted carbocations are particularly... 

In doped silicon (an extrinsic semiconductor) the doping element has either three or five valence electrons (one electron less or one electron more than the four valence electrons of silicon). Substituting an arsenic or phosphorus atom (five valence electrons) for a silicon atom in a silicon crystal provides an extra loosely-bound electron that is more easily excited into the CB than in the case of the pure silicon. In such an n-type semiconductor, most of the electrical conductivity is attributed... 

A variety of localized lithiated carbanions, such as aryllithiums and sulfur- and silicon-substituted alkyllithiums, have been found, by application of C, Li, and Li NMR techniques, to form triple ions in THF-HMPA solution. Thus, change to triple ion structures (18a-g) could be discerned as HMPA (2-5 equiv.) was added to solutions of monomeric structures (17a-g) in 4 1 THF-diethylether. The amount of triple ion is sensitive to ortho substitution monomeric (17a) and (17b) form 65-80% triple ion in presence of 1-3 equiv. HMPA whereas (17c) and (17e) form less than 20% at 5 equiv. HMPA. Pyridylthio-substituted carbanion (19) forms bis-chelated triple ion (20). 

Besides the conventional zeolites, several novel zeolite analogues such as the ALPOs (aluminophosphates), MeALPOs (divalent-metal (Me) substituted aluminophos-phates), SAPOs (silicon substituted aluminophosphates) and so on have been synthesized (Davis Lobo, 1992). Wilson et al. (1982) first reported the synthesis of microporous ALPOs. ALPO synthesis differs from zeolite synthesis in that it involves acidic or mildly basic conditions and no alkali metal ions. Some members in the ALPO... 

Alkynyl(diisopropylsilyl)oxy-diazoacetates (295) undergo intramolecular 1,3-dipolar cycloaddition in good yield when R1 = H (isolation of silver pyrazolide 296 was possible) and R2,R2 = Me,Me or (CH2)5, but no reaction occurred when R = H, R2 = H or R1 = Me (340). The silicon substitution is apparently crucial. Replacement of the Si(i-Pr)2 in 295 (R1 = R2 = H) by Si(f-Bu)2 allowed an uncatalyzed intramolecular [3 + 2] cycloaddition to take place [xylene, 140-160°C, 11% yield (340)], while Ag(I) catalysis led to decomposition. A diazoacetic acid (2-propyn-l-yl)oxysilyl ester also produced a bicyclic pyrazole, but in low yield. On the other hand, the same diazo compound 295, which reacted intra-molecularly under silver ion catalysis, underwent dimerization by an mfermolecular... 

Many silicon-substituted rings have been prepared by pyrolyzing small silanes, but the method has little synthetic value. It is, however, included with many of the specific methods wherever appropriate. 

It has been reported that by heating in the presence of water vapor, silicon can be incorporated into the lattice of y-Fe203 (55,64). It is unlikely that this effect occurs under the butene oxidation conditions since a physical mixture of large size iron oxide and silica gel retains its selectivity for butadiene after a prolonged reaction time without any indication of changes in the catalyst. If silicon substitution takes place, the water from the oxidation reaction could catalyze the substitution. In fact, deliberate pretreatment of such a physical mixture with water vapor before reaction does not change the activity or selectivity (54). 

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