Cyclohexanol silyl ethers

Tsai et al. have disclosed that AIBN-initiated reactions of 5- and 6-bromoalkanoyl-silanes with Bu3SnH give cyclopentanol and cyclohexanol silyl ethers, respectively, in moderate to high yields (Scheme 10.232) [599]. This radical cyclization might proceed via the following radical chain mechanism ... 

Me3SiCH2CH=CH2i TsOH, CH3CN, 70-80°, 1-2 h, 90-95% yield. This silylating reagent is stable to moisture. Allylsilanes can be used to protect alcohols, phenols, and carboxylic acids there is no reaction with thiophenol except when CF3S03H is used as a catalyst. The method is also applicable to the formation of r-butyldimethylsilyl derivatives the silyl ether of cyclohexanol was prepared in 95% yield from allyl-/-butyldi-methylsilane. Iodine, bromine, trimethylsilyl bromide, and trimethylsilyl iodide have also been used as catalysts. Nafion-H has been shown to be an effective catalyst. 

Preparation from hcxamethyldisiloxane and 12/AI powder in detail, followed by cleavage of cyclohexyl methyl ether, to give cyclohexanol (via the intermediate silyl ether). 

In order to determine suitable deprotection conditions, silyl ethers of -butanol, 2-butanol, cyclohexanol, terf-butanol and phenol were synthesized. Table 2 shows various reaction and cleavage conditions that are commonly used in desilylation reactions for secondary alcohols. The progress of the reaction was analyzed using GC techniques and compared to a blank sample. For direct comparison, the corresponding silylethers of too-propyldimethylsilyl (8, IPM2) and TBM2 10 were treated in the same way as the NM2 silyl ether 9. 

Entries III-l and HI-2 show that the much more polar ketones cyclohexanone and 2-cyclohexen-1 -one also possess very high toluene phase affinities. The alcohol cyclohexanol (entry IV-1) exhibits a higher partition coefficient than cyclohexanone, and a phenyl-containing silyl ether derivative (entry XII-2) shows a value even... 

However, when the same protocol was used to protect secondary alcohols, silver nitrate afforded the TBDPS silyl ethers with much shorter reaction times. For example, cyclohexanol was converted to its TBDPS silyl ether in 83% jdeld in 15 min when silver nitrate was the additive (eq 2). In contrast, it took 24 h for ammonium nitrate or ammonium perchlorate to complete the reaction. 

Silylating Agent for Protection of Alcohols. Thexyldimethylsilyl chloride (TDS-Cl) was first introduced as a more easily prepared alternative to the more commonly used f-butyl-dimethylsilyl chloride for the protection of alcohols.Direct silylation of 1° and 2° alcohols occurs upon treatment with TDS-Cl and imidazole in DMF as illustrated by the protection of 1-butanol (1) and cyclohexanol (3) to form the corresponding silyl ethers (eqs 1 and I) Phenols can also be protected using TDS-Cl and imidazole. Typical solvents for the formation of silyl ethers using TDS-Cl include DMF, THF, or CH2CI2. ... 

The formation of ethers such as 1806 by EtsSiH 84b can also be catalyzed by trityl perchlorate to convert, e.g., benzaldehyde in 84% yield into dibenzyl ether 1817 [48]. The combination of methyl phenethyl ketone 1813 with O-silylated 3-phenyl-n-pro-panol 1818, in the presence of trityl perchlorate, leads to the mixed ether 1819 in 68% yield [48] (Scheme 12.15). Instead of trityl perchlorate, the combination of trityl chloride with MesSiH 84a or EtsSiH 84b and sodium tetrakis[3,5-bis-(trifluoro-methyl)phenyl]borane as catalyst reduces carbonyl groups to ethers or olefins [49]. Employing TMSOTf 20 as catalyst gives very high yields of ethers. Thus benzaldehyde reacts with O-silylated allyl alcohol or O-silylated cyclohexanol to give the... 

---