Trityl triflate catalyst

Recently Mukaiyama and coworkers introduced the use of trityl salts as efficient catalysts for the aldol reaction. Using a catalytic amount of trityl perchlorate (5 mol %) and t-butyldimethylsilyl enol ethers, the anti aldols were preferentially obtained (anti 73-84%) regardless of the double bond geometry. With trityl triflate (5 mol %) and dimethylphenylsilyl enol ethers, the syn isomers are produced predominantly (syn 63-79% Scheme 1). Several variations of the catalyst system have been developed. Trityl... 

The use of trimethylsilyl-based electrophilic catalysts with organosilicon hydrides also promotes the conversion of aldehydes into ethers and avoids the need to employ the potentially hazardous trityl perchlorate salt.314,334,338 One reagent pair that is particularly effective in the reductive conversion of aldehydes into symmetrical ethers is a catalytic amount of trimethylsilyl triflate combined with either trimethylsilane, triethylsilane, PMHS,334 or 1,1,3,3-tetramethyldisiloxane (TMDO, 64) as the reducing agent (Eq. 179).314 Either... 

The undefined mechanism of the aldol-type Mukaiyama and Sakurai allylation reactions arose the discussion and interest in mechanistic studies [143-145]. The proposed mechanism was proved to proceed through the catalytic activation of the aldehyde and its interaction with the silyl ketene acetal or allylsilane producing the intermediate. From that point the investigation is complicated with two possible pathways that lead either to the release of TMS triflate salt and its electrophihc attack on the trityl group in the intermediate or to the intramolecular transfer of the TMS group to the aldolate position resulting in the evolution of the trityl catalyst and the formation of the product (Scheme 51). On this divergence, series of experimental and spectroscopic studies were conducted. 

Homoallyl methyl ethers.4 Trityl perchlorate catalyzes a reaction of allyltri-methylsilanes with dimethyl acetals or ketals to form homoallyl methyl ethers in 60-90% yield. Diphenylboryl triflate is a somewhat less efficient catalyst. Example ... 

A convenient method for tritylation of carboxylic acids involves reaction of a trimethylsilyl ester with trityloxytrimethylsilane catalyzed by 1. This reaction is limited to tertiary alkyl trimethylsilyl ethers (equation I). The triflate is an excellent catalyst for thioacetalization with ethylthiotrimethylsilane (equation II). A related... 

Although the reaction can be envisaged as catalytic in the Lewis acid, in practice only certain Lewis acids are effective in a catalytic sense. Notable among these are triphenylmethyl (trityl) cation (65), trimethylsilyl triflate (66), and trimethylsilyl chloride-tin(I I) chloride (61 a), which efficiently promote the reaction using as little at 2-3 mol% catalyst. 

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