Methanesulfonates, reductive cleavage

The chloride formed thereby may react as nucleophile or may be oxidized at the anode to chlorine, which leads to unwanted chlorinated side products. This reductive cleavage can be avoided by addition of strong acids, such as trifluoroacetic acid, methanesulfonic acid, or trifluoromethanesulfonic acid [2], These acids effectively suppress the chlorination, because is reduced at the cathode rather than dichloromethane. However, they also decrease the yield of biaryl products and increase the yield of diphenylmethanes. 

With strong protic acids, the regiochemistry of the reductive cleavage reverses to give the benzyl ether of the less hindered alcohol. One of the few metal hydrides that can withstand the harshly acidic conditions of the reaction is sodium cyanoborohydride. When used in large excess, it will reduce benzylidene acetals in the presence of anhydrous HG [Scheme 3.61] -or trifluoro-methanesulfonic acid. The excess is required because the sodium cyanoborohydride is consumed under the reaction conditions at an appreciable rate. More convenient reaction conditions were described by DeNinno and co-workers in which the benzylidene acetal is reductively cleaved using triethylsilane in the presence of trifluoroacetic acid [Scheme 3.62]. ... 

Cleavage of C—O bonds by direct electron transfer from a cathode is usually difficult because of the negative reduction potential of the bond. Therefore, the reduction of aliphatic alcohols (R-OH) to the corresponding hydrocarbons (R-H) is often carried out by the transformation of hydroxyl groups to good leaving groups such as halides (X = Br, I), methanesulfonates (OMs), and... 

Craig and coworkers have reported stereocontrolled polyol synthesis via C-H insertion reactions of silicon-tethered diazoacetates.7 Menthol was treated with diisopropylsilyl bis(trifiuoro-methanesulfonate) and the product was condensed with ethyl diazoacetate to provide the precursor 43 to the C-H insertion reaction (eq 10). The rhodium(II) octanoate-catalyzed decomposition of the diazoacetate 43 gave siloxane 44 as a single stereoisomer. Reduction of the ester, followed by oxidative cleavage of the C-Si bond, gave the desired polyol 46 in 30% yield, along with the elim-... 

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