Amines silicon-based

The simultaneous protection of the 3 and S hydroxy groups of nucleosides and the 4,6- or 3,4-hydroxyls of hexopyranoses is a common problem in organic synthesis. In the case of hexopyranoses, we have already seen that benzylidene acetals and, in certain circumstances, isopropyiidene acetals can be used to good effect. An alternative silicon-based group would offer a wider repertoire of conditions for mild deprotection and such a group was devised by Markiewicz the 1,1,3,3-tetraisopropyldisiloxanylidene group (abbreviated TIPDS),22 229 TIPDS groups are stable to water, 0.3 M p-toluenesulfonic acid in dioxane, 10% tri-fluo mace tic acid in chloroform, 5 M ammonium hydroxide in dioxane-H20 (4 1), and tertiary amines in pyridine,... 

Recently silicon-based protection has matured into a new Ix oad area of protecting group chemistry (see Section 3.1.3.2) and amines can also be conveniently blocked with appropriate silicon reagents. In organometallic syntheses, e.g. in the alkylation of amino acid enolates, the so-called stabase adducts (30) serve to temporarily protect the a-amino group (Scheme 21). In the absence of hard nucleophiles, in particular of 0-nucleophiles, the Si—N bonds are stable, but deprotection is readily effected in the presence of water and acids. 

Reliability of the electrical properties of silicone-based isotropic adhesives has been the major difficulty to overcome and has essentially prevented commercialization. Another problem associated with silicones is that the addition polymerization reaction of silicones must be carefully controlled to prevent cure inhibition from various common chemical contaminants such as amines and sulfides. Other concerns include low-molecular-weight silicone polymer migration onto wirebond pads and very high GTE. There has been some activity in the development of hybrid resins that contain silicone blocks as comonomer with epoxies such that the epoxy processing can be maintained with the added stress reduction property of the silicones [52]. 

FIGURE 8.1. Proline-derived catalyst h%l LXII and silicon-based secondary amine catalyst LVIII. 

Figure 4.42 Great Lakes Chemical Corporation high-molecular-weight silicone-based hindered amine light stabilizer, Uvasil 299, CAS no. 182935-99-0.

Alkoxysilanes. Alkoxysilanes are important intermediates in the manufac-tnre of silicone-based materials. The prevalent method for forming the silyl ether linkage is alcoholysis of Si—Cl bonds (eq. 6. Removal of the concurrently formed HCl is typically accomplished by addition of tertiary amines (87,88). Often NaOR is nsed instead of the corresponding alcohol to give the less corrosive and more easily removed NaCl as by-prodnct (89,90). Use of alkyl orthoformates represents an alternative to alcohols or metal alkoxides (eq. 7) (91-93). 

The reagent (188) aminates 0-(trimethylsilyl)aldehyde cyanhydrin anions, to give iV,iV-dimethylamides after hydrolysis, and a multitude of derivatives of (189) have been prepared by various routes, all of which involve the use of MeaSiCN at some stage. The first of two new silicon-based protecting groups reported this year is (190), which protects 1,2-, 1,3-, and 1,4-diols under mild... 

Gas chromatography has been used for the analysis of phenolic and amine type antioxidants. Antioxidants which have high boiling points cannot be directly by GC but they can be analysed as derivatives such as acetates, trifluoroacetates, trimethyl silyl-ethers, methyl ethers, etc. Trimethyl silane based antioxidants, for example, give good separations on standard silicone based columns (a. 10). 

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