Trifluoroacetic acid-Dimethylphenylsilane

Similar treatment of a trifluoroacetic acid solution of p-tolualdehyde with triethylsilane gives only a 20% yield of /7-xylene after 11 hours reaction time followed by basic workup. Use of 2.5 equivalents of dimethylphenylsilane enhances the yield to 52% after only 15 minutes. This reaction proceeds stepwise through the formation of a mixture of the trifluoroacetate and the symmetrical ether. These intermediates slowly form the desired /7-xylene product along with Friedel-Crafts side products under the reaction conditions (Eq. 192).73 Addition of co-solvents such as carbon tetrachloride or nitromethane helps reduce the amount of the Friedel-Crafts side products.73... 

The diastereoselectivity of the reduction of a-substiluted ketones has been the subject of much investigation. The reagent combination of trifluoroacetic acid and dimethylphenylsilane is an effective method for the synthesis of erythro isomers of 2-amino alcohols, 1,2-diols, and 3-hydroxyalkanoic acid derivatives.86,87,276,375 Quite often the selectivity for formation of the erythro isomer over the threo isomer of a given pair is >99 1. Examples where high erythro preference is found in the products are shown below (Eqs. 218-220).276 Similar but complementary results are obtained with R3SiH/TBAF, where the threo isomer product... 

The consecutive reduction and cyclization of O-benzoyl protected 5-0-methylhexose 0-(terf-butyldiphenylsilyl)oxime (104) with dimethylphenylsilane in trifluoroacetic acid afforded a iV-hydroxypyrrolidine (105) ring system in good yield (equation 44). The mechanism involves a cascade of neighboring group participation steps involving the 0-benzoyl protecting groups " ". ... 

The combination of dimethylphenylsilane and trifluoroacetic acid has been used to reduce protected a-amino ketones of the general form 1 with excellent selectivity for the a /z -isomer (H-isomer)1 2. The stereoselectivity may be explained by assuming that the reaction occurs under chelation control via the proton-bridged conformation 2. 

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