Ethers triethylsilyl, alcohol

Although some methods for reductive etherifications of carbonyl compounds have been reported [152-162], the iron-catalyzed version possesses several advantages (1) fairly short reaction times are needed, (2) not only trimethylsilyl ether but also triethylsilyl and butyldimethylsilyl ethers and alcohols are adaptable, and (3) a broad substrate scope. 

Doyle and co-workers have employed Rh2(pfb)4 as a highly selective catalyst for the room temperature synthesis of silyl ethers from alcohols and triethylsilane.159 The selectivity of the catalyst is demonstrated by reactions of olefinic alcohols, in which hydrosilylation is not competitive with silane alcoholysis when equimolar amounts of silane and alcohol are employed. High yields (>85%) of triethylsilyl ethers are obtained from reactions of alcohols such as benzyl alcohol, 1-octanol, 3-buten-l-ol, cholesterol, and phenol. Tertiary alcohols are not active in this system. 

Three of the most common procedures for the formation TES ethers have been selected for their large scale preparative value. Triethylsilyl ethers are prepared by the reaction of the alcohol with chlorotriethylsilane in the presence of a catalytic amount of imidazole [Scheme 4.23]30 or DMAP31 Triethylsilyl triflate in the presence of pyridine [Scheme 4.24]32 or 2,6-lutidine [Scheme 4.25]23 can be... 

In the second step, the resulting alcohol is protected as triethylsilyl ether (TES). 

Mukaiyama aldol reactions using a catalytic amount of a Lewis acidic metal salt afford silylated aldols (silyl ethers) as major products, but not free aldols (alcohols). Three mechanistic pathways which account for the formation of the silylated aldols are illustrated in Scheme 10.14. In a metal-catalyzed process the Lewis acidic metal catalyst is regenerated on silylation of the metal aldolate by intramolecular or intermolecular silicon transfer (paths a and b, respectively). If aldolate silylation is slow, a silicon-catalyzed process (path c) might effectively compete with the metal-catalyzed process. Carreira and Bosnich have concluded that some metal triflates serve as precursors of silyl triflates, which promote the aldol reaction as the actual catalysts, as shown in path c [46, 47]. Three similar pathways are possible in the triarylcarbenium ion-catalyzed reaction. According to Denmark et al. triarylcarbenium ions are the actual catalysts (path b) [48], whereas Bosnich has insisted that hydrolysis of the salts by a trace amount of water generates the silicon-based Lewis acids working as the actual catalysts (path c) [47]. Otera et al. have reported that 10-methylacridinium perchlorate is an efficient catalyst of the aldol reaction of ketene triethylsilyl acetals [49]. In this reaction, the perchlorate reacts smoothly with the acetals to produce the actual catalyst, triethylsilyl perchlorate. 

As above described, Bu-P4 base was found to catalyse the coupling reaction of aryl fluorides with silyl ethers. The use of silyl ether was required for these reactions, and direct arylation of unsilylated alcohols using superbase catalysed coupling reactions has been a challenge. Therefore, the combination of triethylsilyl hydride (Et3SiH) and catalytic Bu-P4 is considered to represent an attractive hydride generating system with which to carry out sequential deprotonation and Sj-jAr reaction [55] (Figure 5.5). 

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