Silyl ethers alkoxysilanes

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

Homolytic substitution reactions including homolytic allylation, radical [2,3]-migrations and stereochemical reactions been reviewed. The review also highlights the possible applications of homolytic substitution reactions. ni reactions at silicon (by carbon-centred radicals in the a-position of stannylated silyl ethers) are efficient UMCT reactions producing cyclized alkoxysilanes. Bimolecular reactions can also be facilitated in good yield (Schemes 32 and 33). ... 

Transition metal complexes have been widely investigated as catalysts for the synthesis of alkoxysilanes via alcoholysis of hydrosilanes. The system provides a convenient method for the protection of hydroxy groups in organic synthesis and the synthesis of silyl ethers. The general reaction is shown in Eq. (59). 

Even if the SMS reaction typically involves allylsilanes, carbonyls and alcohols (or silyl ethers), some transformations can be considered as belonging to the same family. For example, in 2001, Yokozawa et al. described [43] a three-component reaction between aldehydes 6, alkoxysilanes 38 and propargylsilane 88 (instead of allylsilane). Tritylperchlorate was used as the catalyst and a-allenyl ethers 89 were... 

Catalytic alcoholysis of silanes by a variety of transition metal based catalysts is a useful method to form silyl ethers under mild conditions (Scheme 19). The process is atom-economical hydrogen gas is the only byproduct. This mild method has not been fully exploited for the preparation of unsymmetrical bis-alkoxysilanes. A catalytic synthesis using silicon alcoholysis would circumvent the need of bases (and the attendant formation of protic byproducts), and eliminate the need for excess silicon dichlorides in the first silyl ether formation. We sought catalytic methods that would ultimately allow formation of chiral tethers that are asymmetric at the silicon center (Scheme 20). Our method, once developed, should be easily transferable for use with high-value synthetic intermediates in a complex target-oriented synthesis therefore, it will be necessary to evaluate the scope and limitation of our new method. 

There have been a few reports on the controlled catalytic alcoholysis of dialkylsilanes to produce the mono-hydrido silyl ether. 9a,c,15 16 One such report is by Corriu and Moreau who have illustrated the use of either (PPh3)3RJiCl, (PPh3)3RuCl or Raney Ni to execute the controlled alcoholysis of diphenylsilane to give the mono-functional alkoxysilane as the only product.93 Doyle attempted to use the Rh2(pfb)4 catalyst with diphenylsilane and trans- 1,2-cyclohexanediol to prepare the cyclic silyl ketal,16 but this was not successful, perhaps because the trans-fused... 

Ultimately, it was determined that protected allyl alcohols, tetralkoxy silanes, and silyl ethers were the best substrates for these transformations. Alkyl or allyl silyl ethers (120) provided products such as 121 in >85% de and good 74-92% ee with Rh2(S-DOSP)4 as the catalyst (Scheme 27) [98], Extension of the reaction to alkoxysilanes, generated 122 in 88 to >94% de and 81-96% ee. Interestingly, this catalyst did not seem to be compatible with benzyl silyl ethers, and the products were formed with low selectivity (<40% ee). It was found that the use of either a chiral auxiliary or phthalimide Rh2(PTTL)4 resulted in improved results [31]. 

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). 

Rubinsztajn and Celia, looking at the results with alkoxysilanes described above through the lens of a silicone chemist, saw not a strategy for reduction of various functional groups including silyl ethers, but rather an opportunity to convert alkoxysilanes (= silyl ethers [30]) into silicones [11]. The breadth of silicone structures that are synthetically available through the use of this reaction will be discussed below. Here, distinctions are made between the previously described organic mechanistic information and the chemistry that occurs at silicon centers. 

Related to the alkene-to-allylic alcohol conversion is the oxiran to allylic alcohol isomerization. Two recent reports describe the use of trimethylsilyl iodide, either ready-made or prepared in situ from hexamethyldisilane and iodine, with DBU (Scheme 12) the initially formed 2-iodo-alkoxysilane (31) eliminates HI to give an allylic silyl ether that is cleaved during hydrolytic work-up. An alternative procedure (also Scheme 12) employs t-butyldimethyl-silyl iodide, generated in situ according to equation (14), and DBN. In this... 

Because of the instability of silanols, polymers possessing free silanols can hardly be present. In Section 3.18.2.1, Si-O-C linkages in silyl ethers are demonstrated to be stable under the conditions of anionic polymerization and to be quantitatively cleaved by mild acidic hydrolysis. This suggests that alkoxysilane-protected styrene monomers can also be anioni-cally polymerized in a living manner and the resulting polymers are equivalent to free silanol-functionalized polymers. These polymers are useful and versatile functional... 

A radical approach to cyclization is offered by the intramolecular homolytic substitution (ShO reaction at a silicon center. Reaction of phenyl bromoacetate with a stannylated silyl homoallyl ether under atom transfer conditions provides cyclic alkoxysilanes (Equation (119)).2... 

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