Issue in Honor of Prof. Eusebio Juaristi ARKIVOC 2005 (vi) 127-135
of the product is constantly drained from the reaction. A particular transesterification methodology, which uses mixed anhydrides, has obtained a new impulse recently. The mixed anhydrides have been planned to contain a sterically hindered part of an acid or a good leaving group in one half of the molecule.3 A former mixed anhydride, the acetic-trifluoroacetic anhydride, has been used without purification because of its sensitiveness to moisture, tendency to disproportionate, and difficulty to establish the end point of its preparation.4
Results and Discussion
The formation of mixed anhydrides has been claimed through microanalysis, IR spectra, depressions of the freezing point, saponification equivalents, and of course, by the obtained acylated products. The common procedure to prepare mixed carboxylic-trifluoroacetic anhydrides is to heat a mixture of several equivalents of the carboxylic acid and trifluoroacetic anhydride (TFAA) during minutes to hours.5 A metathetical alternative, from silver trifluoroacetate and an acid chloride is also available.6 The end point of the preparation of the mixed anhydride has not been clearly defined, so when using heating, the reactions usually become dark and some materials are decomposed. We report herein an easy characterization of carboxylic-trifluoroacetic mixed anhydrides by 1H and 13C NMR.
When 1.2 mole of acetic acid is combined with 1.0 mole of TFAA a clear solution containing the acetic-trifluoroacetic anhydride (ATFAA) is formed in few seconds, as deduced by 1H- and 13C –NMR spectra. In 1H -NMR three methyl singlets are immediately observed in the region around 2 ppm: one for acetic acid (2.19 ppm), a second for Ac2O (2.27 ppm), and a third for ATFAA (2.38 ppm). In the 13C-NMR spectrum four signals are shown in the region for anhydride and acid carbonyls (Equation 1): ATFAA (s, 163.1, and q, 152.5 ppm), acetic anhydride (168.4 ppm), trifluoroacetic acid (q, 160.5 ppm) and acetic acid (179.8 ppm). Typical signals for ATFAA and trifluoroacetic acid are obtained due to their C-F couplings: ATFAA presents two coupling values: 45.5 Hz (for C of carbonyl) and 284.8 Hz (for CF3), and TFA other two: 43.3 Hz (for C of carbonyl) and 283.7 Hz. (for CF3). The chemical shifts for all signals isolated compounds and for their signals in mixtures, as in the above reactions, have a slight variation. The excess of acetic acid and the produced trifluoroacetic acid can be eliminated by distillation, although some ATFAA is lost.
H3C OH O F3C O CF3 O O CDCl3 F3C OH O H3C O CH3 O O H3C O CF3 O O 1.2 equiv 1.0 equiv H3C OH O Equation 1
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