To clarify some mechanistic aspects of the oxidation, we have conducted some additional runs. The course of the oxidation does not change using
catalytic alkaline salts (NaBr, NaI, LiF) other than NaCl or using Bu4NBr. The use of only 1 equiv of TCCA causes a very slow reaction: in the case of
oxidation of benzyl alcohol, the reaction requires 15 h at 25 °C for completion. The reaction is very slow even when NCS (2 equiv) is used instead of
TCCA. The oxidation to carboxylic acid does not occur in the absence of water in the reaction mixture, and the reaction stops with the formation of
the aldehyde. The reaction can be conducted both in acetone and MeCN, under the reported experimental conditions, with the same identical results,
while only carbonyl compounds are formed using dry dichloromethane.11 Increasing the amount of water increased the formation of chlorinated products;
under these conditions, benzyl alcohol is oxidized slowly to a mixture of chlorosubstituted benzoic acids.
It is noteworthy that the oxidation of the intermediate aldehyde does not require TEMPO as catalyst. In fact, the aldehyde is rapidly oxidized to the
corresponding carboxylic acid by treatment with 1 equiv of TCCA in acetone/water, without addition of the catalyst. |
) but that's been one of the ones I've been trying to
get my hands on for a while, without much success. I'd prefer to trade something I have since I'm pretty much broke, but could possibly purchase a
small amount.