Esterification of Carboxylic Acids with Dimethyl Carbonate and DBU 133
dride inlermediale, which in the presence of melhanol gives methyl benzoate 2
(Scheme 20.6).
e
Co) PhC00 9
0 ~J.. .. 0
.... cH3
2
Scheme 20.6
Which of the previous statements is the right one? Comment alt options and discuss the experimental data.
Experimental Data
l. The esteritication of 1 with DMC at 90°C does not work in the absence of base.
2. The reaction rate strongly depends on Lhe concentration of DBU.
3. DBU is recovered unaltered at the end of the reaction.
4. Labeled N-methylated salt 3 was obtained by reaction of DBU and
13 CH 3 1. A
solution of benzoic acid, DBU and salt 3 in acetonitrile was heated to reflux
overnight, but failed to produce any labeled methyl benzoate 4. The total recovery ofunaltered salt 3 was confirmed by NMR (Scheme 20.7).
o
11
13
Ph ............. O_.... CH3
4 (not observed)
Scheme 20.7
5. Carbamate 5 was independently synthesized fi-om methyl chlorofonnate and an
excess of DBU. When a solution ofbenzoate anion was added lo 5, a voluminous evolution of gas started and methyl benzoate 2 was obtained (Scheme
20.8).
-
5
Scheme 20.8