Silicones — could synthesis

We sought catalytic methods that would allow formation of chiral tethers that are asymmetric at the silicon center. Synthesis of the chiral tether by our method would be advantageous for the reasons we described in the Introduction. Chiral information could be transferred during an intramolecular reaction. This would be an example of substrate-controlled transformation. 

The synthetic approach to silylene complexes (Eq. (2)) is versatile and allows a high variability of both the metals and the substituents at the silicon. A whole series of compounds with bulky substituents like 1 -adamantyloxy, 2-adamantyloxy, neopentyloxy, triphenylmethoxy or f-butylthio could be prepared (Table 1). Compounds with sulfur at silicon are particularly interesting however, their synthesis proved to be very difficult. 

Cancrinites are one of the rarest members of the feldspathoid group, classified as such due to its low silicon content. However, cancrinite is also classified as a zeolite, due to its open pore structure, which confers molecular sieve properties [1], Likewise, variable sodium carbonate and NaOH concentrations in the hydrothermal synthesis of cancrinite could direct the synthesis of the intermediate phase or the disordered cancrinite formation [2], The intermediate phase is described as a phase between cancrinite and sodalite [3], The disordered cancrinite is an intermediate phase which is much closer to the cancrinite structure than sodalite structure [2],... 

The total synthesis of vallesiachotamine (9) and isovallesiachotamine (10) has been completed elegantly by Wenkert and Spitzner (393) by utilizing the addition of a silicon-stabilized anion to pyridinium salt 633 to achieve the properly substituted indoloquinolizidine 634 by cyclization, from which 9 and 10 could be prepared in racemic form by simple reaction steps. 

In a similar manner, Brummond et al. demonstrated the first total synthesis of 15-deoxy-A12,14-prostaglandin J2 (162) that was completed using a silicon-tethered allenic Pauson-Khand reaction to obtain the highly unsaturated cyclopentenone substructure [36]. Treatment of alkynylallene 160 with molybdenum hexacarbonyl and dimethyl sulfoxide affords the desired cycloadduct 161 in 43% yield (Scheme 19.30). Trienone 161 was obtained as a 2 1 Z E mixture of isomers in which the Z-isomer could be isomerized to the desired E-isomer. The silicon tether was cleaved and the resulting product converted to 15-deoxy-A12,14-prostaglandin J2 (162). 

The synthesis of phenylbromosilanes from bromobenzenes and silicon, catalyzed by copper 142) at 411°C, could be correlated by the Hammett equation (series 113, seven points, slope 0.63). 

Substitution of silicon by other atoms like Ti or Al was reported to improve the thermal and hydrothermal stability to some extent [6]. It was also reported that improved hydrothermal stability could be achieved by adjusting the gel pH several times during crystallization process [7], Post-synthesis silylation technique has also been reported to enhance the hydrothermal stability of mesoporous materials by increasing the hydrophobicity of the samples [8,9]. However, it is most desirable to develop a method for preparing hydrother-mally stable mesoporous material by direct synthesis route. 

Starting with this hypothesis, several 6-sila-steroids were prepared127-129. As an example, the syntheses of 6,6-dimethyl-6-sila-oestradiol (197) and 6,6-dimethyl-6-sila-mestranol (198) are described in Scheme 25. Not only the synthesis of the key intermediate 4,4-dimethyl-4-sila-6-methoxy-l-tetralone (7 96) but also the well known organic reactions, leading from 196 to 197 and 198, are strongly influenced by the specific properties of the silicon atom. Although known reactions of steroid chemistry could be used for further transformations from 196 to 197 and 198, conditions were not directly transferable because of the chemical behaviour of the Si-Calkyi and Si-Qryl bond. Important differences in the pH- and solvent profile were neces-... 

Lambert et al. succeeded in developing second-generation dendritic polysilanes [71]. The repetitive steps of this divergent synthesis consist in cleavage of silicon-methyl bonds by trifluorosulphonic acid (CF3SO3H) and subsequent formation of silicon-silicon bonds (Fig. 4.39). Starting from tetrasilanes, up to second-generation starfish-like dendritic molecules with a molecular mass of 1832.9 g/mol could be prepared in this way [72]. 

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